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Production of Refractory Metal Powders
Chapter 23
Production of Refractory Metal Powders Oleg D. Neikov*, Stanislav S. Naboychenko†, Irina B. Murashova† and Nicholas A. Yefimov* *Frantsevich Institute for Problems of Materials Science (IPMS), Kiev, Ukraine,
†
Ural State Technical University (UPI), Yekaterinburg, Russia
Chapter Outline Introduction Production of Tungsten and Tungsten Carbide Powders Tungsten Metal Powders Tungsten Carbide Powders Tungsten Carbide Electrical Contacts Diamond Tools Hardfacing Cemented Carbides Production of Cemented Carbide Powders Cemented Carbide Compaction Sintering and Secondary Operations Recycling Molybdenum Metal Powders Production of Molybdenum Powders
685 686 686 707 712 714 714 717 717 719 721 721 723 723
Properties of Molybdenum and Molybdenum Alloy Powders Workplace Atmospheres Safety Niobium and Tantalum Niobium and Niobium Alloys Tantalum Tantalum Carbide Workplace Atmosphere Safety Rhenium Production of Rhenium Powder Rhenium Alloys Applications References
725 733 734 734 742 744 747 747 747 750 750 751
INTRODUCTION Tungsten, molybdenum, niobium, tantalum, and rhenium, classified as the refractory metals, are distinguished by several general characteristics, including high melting point, high density, and superior resistance to wear and corrosion. With the exception of two of the platinum-group metals, osmium and iridium, they have the highest melting temperatures and lowest vapor pressures of all metals. Tungsten has a melting point of 3683 K, the highest of all metals, and has a density more than twice that of iron. However, they are also subject to heavy oxidation above 773 K, and they must be protected for service at elevated temperatures using coatings or non-oxidizing atmospheres. These metals have body-centered cubic crystal structures (except for rhenium, whose crystal form is hexagonal). Powder metallurgy is the only production route for commercial tungsten, rhenium, and their alloys, because melting technology is costly and results in a coarse-grained microstructure, which negatively affects both further processing and the properties of the final product. Selection of a specific refractory metal alloy is often based on fabricability rather than strength or corrosion resistance. Special attention must be taken during the processing and use of refractory metals because of their ductile-to-brittle transition temperatures (DBTTs) and their reactions with gases [1]. Niobium, tantalum, and their alloys are the most easily fabricated refractory metals. They are ductile in the pure state and have high interstitial solubility for carbon, nitrogen, oxygen, and hydrogen. Both niobium and tantalum adsorb hydrogen and form hydrides at moderate temperatures. Niobium and tantalum begin oxidizing at 473 K and 673 K, respectively. Below 1643 K, the oxides are non-volatile, but the ductility of the parent material is reduced because of oxygen absorption. Molybdenum, molybdenum alloys, tungsten, and tungsten alloys have limited solubility for carbon, nitrogen, oxygen, and hydrogen. Molybdenum begins to oxidize at approximately 773 K. At temperatures above 1053 K, the eutectic temperature of MoO2 and MoO, oxides become partly volatile, and the oxidation rate accelerates rapidly with increasing temperature. Extrusion (or hot forging) is used for transformation the arc-cast refractory metal ingots into rounds or rectangular bars. These bars, as well as PM-sintered products, are processed into sheets, plates, foil, tubing, and rods. When the refractory Handbook of Non-Ferrous Metal Powders. https://doi.org/10.1016/B978-0-08-100543-9.00023-3 © 2019 Elsevier Ltd. All rights reserved.
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products are involved and cannot be fabricated from standard mill products, they require machining or pressing and sintering to near-net shape. Many refractory metals and alloys are available as wire. Tungsten wire, which is produced in diameters as small as 0.01 mm, is used extensively for lighting, electronic devices, and thermocouples. Tantalum wire is used widely in capacitor manufacturing and in surgical applications. Molybdenum mill product forms, such as sheets and foil, can be stamped into simple shapes. Molybdenum is also machined with ordinary tools and can be gas tungsten arc and electron beam welded or brazed. The refractory metals are extracted from ore concentrates, transformed by processing into intermediate products, and then reduced to the metal. Except for niobium, they are produced exclusively as metal powders, which are consolidated by sintering and/or melting. The process for niobium differs only in that the metal is generally reduced by aluminothermic reduction of oxide.
PRODUCTION OF TUNGSTEN AND TUNGSTEN CARBIDE POWDERS Tungsten was adopted in industry at the beginning of the 20th century due to its application as an alloying element in highspeed steels and as filament wire in incandescent lamps. In both applications, pure tungsten was manufactured in powder form. At the end of the 19th century, very hard tungsten carbides, WC and W2C, were studied by Henri Moissan [2,3]. However, for a long time, all attempts to compact tungsten carbide powders resulted in porous and brittle products. In 1922, researchers of the OSRAM study group combined the brittle tungsten carbide with a ductile metal binder. After sintering such powder mixtures, a material of both high strength and toughness was obtained. Sintered hardmetals, based on tungsten carbide bonded with cobalt, were first commercially produced by WIDIA in Germany in 1926. The global hardmetal industry is estimated to have totaled 32,250 mt in 2002 with a sales value of over $8.5 billion followed by further rises to 51,900 mt in 2006, with sales at an estimated $43 billion. Of these production quantities, around 60% is for sintered cutting tool materials used for machining all types of metallic components, for woodworking, mining tools, drilling tools used in oil exploration, etc., and the remainder is used for components such as rolls used in steel mills, metal forming dies, and structural components for severe wear resistance applications [4]. Western Europe once led the market in this sector, but its share diminished to 39% in 2006, while China’s market share increased to 33%, the United States’ share is at around 13%, and Japan accounts for 12%, with others at 3%. Since 1993, the global production of tungsten, which is the main raw material used in sintered hardmetal products, has fluctuated in a narrow range between 30,000 mt and 35,000 mt. China is the predominant tungsten producer accounting for almost 85% of global output in 1999. The usages of tungsten scrap as a raw material is estimated to be as high as 30% in some countries, with the incentive to use scrap increasing as tungsten prices increase. As a result, annual global consumption of tungsten has been in the range of 40,000–50,000 t [5], and over half of the tungsten consumption is for tungsten carbide powder for hardmetal production. Along with this, important products are based on tungsten alloy powders, such as: l l l
Heavy metal alloys (W-Fe-Cu-Ni-Co) typically comprising 90–98 wt% tungsten Electrical contacts (W-Cu, W-Ag) Ductile tungsten, including lamp filaments.
Tungsten, as an alloying element for steels and superalloys, is predominantly used as ferro-tungsten, or as melting base alloys, which are prepared from either ore concentrates or tungsten-bearing scrap. An insignificant part of the tungsten consumption is used in the form of various chemical products, such as catalysts, pigments, solid lubricants, and heavy liquids.
Tungsten Metal Powders Production of Tungsten Powder by Hydrogen Reduction Feed to Process: Industrial raw materials to process tungsten-bearing minerals are wolframite ([Mn, Fe]WO4), scheelite ubnerite (MnWO4). The lower limit of workable tungsten concentration ranges from 0.1 (CaWO4), ferberite (FeWO4), and h€ to 0.3 wt% WO3. Concentrations of >2 wt% are infrequent. The average WO3 content is usually about 0.5 wt% [6,7]. Recently, Wolfram Bergbau and H€ utten GmbH (WBH) in Austria, a world-leading producer of tungsten oxide, tungsten metal, and tungsten carbide, mined about 400,000 t of scheelite ore, with a 536,306 t peak in 1986, with an average WO3 content of 0.55 wt% [8]. Depending on the mineral type, different techniques, such as gravity methods and flotation, are
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used to concentrate the ore to reduced WO3 contents of about 40–60 wt%. Tungsten-bearing scrap, which usually has a high tungsten content, is a considerable alternative tungsten source to the natural mineral resources. Process: The main process steps from concentrates or scrap through the formation of high purity ammonium paratungstate (APT: (NH4)10 (H2W12O42)2H2O) intermediate to tungsten metal powder [6] are shown schematically in Fig. 23.1. In the dissolution step, the tungsten-bearing feed materials (wolframite concentrate and oxidized tungsten scrap) are treated with NaOH, or scheelite concentrate is treated with Na2CO3 to form water-soluble sodium tungstate NaWO4. The purification steps include: l l
l
Removal of aluminum, arsenic, fluorine, phosphorus, and silicon by precipitation. Substitution of sodium by NH4 forms an ammonium solution (NH4)2WO4. This is ordinarily done by solvent extraction, which is a liquid ion exchange process, in which tungsten is extracted by means of an aliphatic amine dissolved in isodecanol and kerosene. This process is followed by re-extraction into an aqueous ammonia solution. Crystallization of APT by evaporation of ammonia and water. APT is the intermediate product of highest purity for metallic tungsten and tungsten carbide powder production.
The calcination step includes heating the APT to 673–1173 K, predominantly in rotary furnaces, for ammonia and water elimination. If accomplished in an excess of air, the resulting oxide is WO3 yellow tungsten oxide. Under exclusion of oxygen, a slightly reduced, blue-colored tungsten oxide is formed (WO3-x tungsten blue oxide). WO3 is the most stable a-oxide with a homogeneity range of WO3-WO2.96; it has three crystalline modifications: monoclinic modification stable at temperatures lower than 1000 K, tetragonal modification in the range from 1000 K to 1370 K, and cubic modification at temperatures higher than 1370 K. It sublimes noticeably at 1070–1120 K and exists in the form of polymer molecules in the vapor phase. Reduction step with hydrogen reducer: The reduction of the oxide to metal proceeds through several intermediate oxide phases. Generally, two furnace designs for the reduction process are used industrially: a multitube pusher type and rotary furnaces. During the reduction process, the sequence in which oxides occur depends on the applied temperature and on the
Scheelite concentrate
Wolframite concentrate Dissolution
Purification Precipitation: Si, P, F, Al, Solvent extruction Crystalization
Amonium paratungstate
Calcination
Tungsten oxide WO3–x
Reduction
Metallic tungsten powder
Tangsten-bearing scrap
FIG. 23.1 Tungsten metal powder production flowsheet.
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moisture within the powder bed, accordingly, which are static in pusher-type popular furnaces and dynamic in rotary furnaces. Tungsten reduction by hydrogen includes four steps: WO3 + 0:1H2 $ WO2:9 + 0:1H2 O, Keq ¼ 3792:0=T + ð4:8286Þ WO2:9 + 0:18H2 $ WO2:72 + 0:18H2 O, Keq ¼ 142:5=T + 1:684 WO2:72 + 0:72H2 $ WO2 + 0:72H2 O, Keq ¼ 801:7=T + 0:8615 WO2 + 2H2 $ W + 2H2 O, Keq ¼ 2219=T WO3 + 3H2 $ W + 3H2 O The numerical value of summary equilibrium constant Keq,sum may be calculated as a composition of four previous equilibrium constants for preceding stages. It measures for temperature T, K Keq:sum ¼
9:61 1011 1:26 1010 2:67 107 1:55 104 + T4 T3 T2 T
In the pusher furnace, tungsten oxide is loaded into flat metal “boats,” which are pushed through the horizontal furnace tubes (Fig. 23.2). Increased demand for tungsten products made it possible for WHB Company to modernize the reaction stage by replacing the manually operated pusher furnaces by automated furnaces (Fig. 23.3). The powder layer preserves its static state throughout the whole reduction process. Typical reduction temperatures range from 873 K to 1373 K. The boats are pushed at a rate usually in the range of 5 to 30 mm/min. The hydrogen flow is commonly in the countercurrent direction. Water vapor formed during reduction is carried away by the hydrogen flow. The cooler is located on the discharge end of the tube. Electrolytic hydrogen (collected in gas holders and freed from oxygen) and steam are used. As the hydrogen supply is greatly in excess of that required for the reduction (the efficiency is not >20%), the outgoing gas passes through a regeneration plant and returns to the process after drying. The boats are shifted in the direction of higher temperatures and less-damp water concentrations. Reduction rate is controlled by water vapor diffusion through the layer of WO3 powder, and then, transformation throughout the all layer section WO3 ! WO2.9 ! WO2.72 ! WO2 ! W is accomplished. The pusher-type furnace is very adaptable to the manufacture of different particle size powders and can quickly be adapted to produce the desired powder properties. By variation of reduction parameters such as temperature, powder layer depth in the boats, and boat speed, the particle size of the metallic tungsten powder can be controlled. Grain size of the WO2 particles depends significantly on the tungsten
FIG. 23.2 Multitube push type furnace.
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FIG. 23.3 Automatic push type furnace. (Courtesy: Wolfram Bergbau- und Hutten-GmbH.) €
particle size. Increase of the tungsten powder particle size is encouraged by high temperature and quick temperature increase along the furnace tube, high push rate, large depth of the powder layer, low hydrogen flow rate, and its high humidity. By using rotary furnaces, the tungsten oxide is directly introduced into the rotating furnace tube. An overall view of a rotary furnace used at the refining plant of WBH Company is shown in Fig. 23.4. The rotating furnace tube has, as a consequence, not a static but a dynamic powder layer. The depth of the layer is influenced by the feed rate, rotational speed, incline, and lifters inside the tube. The temperature range is comparable to that of pusher-type furnaces, and the hydrogen flow direction is also usually countercurrent. With respect to changes in particle size, the rotary furnace is not as flexible as the pusher furnace. A characteristic of application is the continuous production of powders, with medium size ranging from 1 to 3 mm.
FIG. 23.4 Rotary furnace. (Courtesy: Wolfram Bergbau€ und Hutten-GmbH.)
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FIG. 23.5 Morphological transformations of tungsten oxides during reduction at 1273 K. (Source: Spross M. Berg- und Huettenmaennische Montashefte. J Min Metall Mater Geotech Plant Eng 1996;141 (8):359–362.)
The intermediate oxides differ as regards both color and morphology. The latter is illustrated in Fig. 23.5. The cause of these morphological changes is the transport of tungsten via the gas phase, that is, chemical vapor transport (CVT) during the reduction sequence. Tungstic acid, WO2 (OH)2, has been identified as the most volatile compound in the system W-O-H and is responsible for the CVT. If the partial pressure ratio pH2O/pH2 is low enough, the reduction from WO2 to tungsten starts. From a thermodynamic point of view, WO2(OH)2 may form from both WO2 and tungsten, but the equilibrium pressure is lower above tungsten, which causes the CVT of WO2(OH)2 from WO2 to tungsten, where this vapor deposits as metallic tungsten, thus leading to growth of the tungsten crystal. The CVT mechanism permits the control of the metallic tungsten particle size through the interaction of temperature and humidity (partial pressure ratio pH2O/pH2), which is shown schematically in Fig. 23.6 for the pusher furnace [6,9,10].
Reduction by Solid Carbon Reduction follows the relationship shown in Fig. 23.7 and can be written: WO3 + 3CO ¼ W + 3CO2 3ðCO2 + C ¼ 2COÞ ________________________ WO3 + 3C ¼ W + 3CO, Keq ¼ P3 CO where Keq is equilibrium constant.
Relation temperature/humidity
Process parameters in push furnace
Low temperature
• Low temperature
Low humudity
High temperature High humidity
Tungsten transport via gas phase
• High H2 flow rate • Low oxide boatload
WO2(OH)2
Tungsten deposition W powder formation WO2(OH)2 + 3H2 = W + 4 H2O
Fine tungsten metal powder
• High temperature • Low H2 flow rate • High oxide boatload
FIG. 23.6 Control of the metallic tungsten particle size in the push furnace.
Coarse tungsten metal powder
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FIG. 23.7 Equilibrium compositions of gaseous mix of CO and CO2 depending on temperatures for CO–CO2 and CO–CO2–WO2–W systems. 1: C + CO2 $ 2CO; 2: WO2 + 2CO $ W + 2CO2.
FIG. 23.8 Push type furnace for tungsten anhydride reduction by carbon.
At temperatures lower than 1000 K, the oxide is not reduced because of insufficient partial pressure of CO. The temperature necessary to accelerate the process is above 1470 K. Reduction is accomplished in pusher type furnaces with carbon tubes (Fig. 23.8). Maximum temperature in the furnace is 2100 K; consumption power is 25 kW, daily yield is 350–380 kg [11]. To produce fine powders, the temperature is maintained in the range from 1670 K to 1770 K, the boats are moved through the high temperature zone rather faster, or some excess of black is injected. The powder produced contains inclusions of carbides and is friable. These powders are used for the production of tungsten carbide powders.
Precipitation from Gaseous Phase of Tungsten Hexafluoride and Tungsten Hexachloride The powder is produced from gaseous substances: l l l
at low temperature by dissociation of tungsten hexafluoride or tungsten hexachloride in hydrogen; by decomposition of tungsten hexachloride in inert gas (argon or nitrogen); by high temperature decomposition of tungsten hexafluoride in hydrogen fluoride.
The melting point of tungsten hexafluoride is 276 K, and its boiling point is 290.1 K. The melting and boiling points of tungsten hexachloride are 554.5 K and 623 K, respectively. Reduction of tungsten hexafloride takes place according to the reaction (HHal) formation: WHalx + 0:5 xH2 ¼ W + xHHal,
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FIG. 23.9 Flow diagram of the reduction process by precipitation from gaseous phase of tungsten hexafluoride and tungsten hexachloride in fluidized bed.
Keq ¼ pxHHal = p0:5x p MHal x H2 A flow diagram of the reduction process by precipitation from the gaseous phase of tungsten hexafluoride and tungsten hexachloride is shown in Fig. 23.9. The plant consists of a halide evaporator, reactor of fluidized bed and devices for regeneration and gaseous product collection, and a porous bottom made of sintered coarse tungsten powder, replaced sometimes by a layer of tungsten small fragments (1–5 mm) or a metal grate placed in the lower part of the reactor. Tungsten halide vapor from the evaporator, together with hydrogen and argon, is supplied by pump to the lower part of the preheated reactor where, at the bottom, tungsten fine powder (20–60 mm) as seed is found. During reduction, the seed is converted into granules which are discharged periodically or continuously. The hydrogen chloride or fluoride is collected in a scrubber. After discharge of the hydrogen and argon mixture from the desiccant, the hydrogen is absorbed by heated titanium sponge, and argon is returned to the cycle. The sponge is dehydrogenated periodically, and purified hydrogen is also returned to the cycle. The temperature in the reactor influences the degree of metal precipitation. However, increasing the temperature above 1170 K leads to eventual sintering of seed particles and, as a result, to the failure of the fluidized bed. Tungsten reduction from tungsten hexachloride is carried out at 1020–1070 K and from tungsten hexafluoride at 820–870 K. In the first case, the molar ratio of hydrogen to tungsten hexachloride in the gas-vapor mixture is maintained at approximately 30–40 to 1; while the molar ratio of hydrogen to argon is approximately 1.75 to 1; tungsten recovery amounts to 90%–99% and degree of precipitation on powder is 96%–97%. In the second case, the molar ratio of hydrogen to tungsten hexafluoride, maintained at 15–18 to 1, provides full recovery of tungsten. The gaseous flow rate must be in the range of 5–10 cm/s for fluidized bed formation. For the production of tungsten powders with a spherical particle shape, the metal is reduced from concentrated solutions of salts, oxides, or hydroxides. Spheroidizing takes place in the high temperature zone where powder is transferred by carrier gas. High purity tungsten nanopowders are produced from fluorine-containing compounds according to the following scheme: tungsten metal ion extraction from aqueous solutions (fluorine organic acids, ethers, amines, carbonyl acids, ketones and others), re-extraction by solutions of fluorine compounds (hydrogen fluoride, ammonium fluoride, ammonium hydrogen fluoride), and reduction of tungsten fluoride and tungsten oxyfluoride in hydrogen current. Metal powder particle size is approximately 30 nm. Similar powders are also produced by evaporation of volatile chlorides and hydrides in a reaction gas atmosphere (hydrogen, ethane, ammonia), heating being done by high-frequency induction or laser beam.
Production of Tungsten Powder by a Plasma Process The plasma process is realized at temperatures much higher than the boiling point of the basic materials. Tungsten is produced in a vaporous state. Powder is formed by condensation, coagulation, or crystallization accompanied by polymorphous transformations. Complete reduction to tungsten is achieved by processing tungsten trioxide in an argonhydrogen plasma (by the molar ratio of argon to hydrogen at 1 to 1) at temperatures ranging from 4270 K to 5270 K. A two-stage scheme is used for the production of powder with a narrow range of granulometric composition (0.6–2 mm) preserving its properties by storing in air. In the first step, the tungsten trioxide is preliminarily reduced at 2270 K to a
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FIG. 23.10 Electric arc device for tungsten and molybdenum oxides reduction by hydrogen.
residual oxygen content in the powder of 8%; in the second step, annealing at 1020–1070 K in reduction furnaces is carried out. Plasma plant (Fig. 23.10) contains a consumable tungsten electrode inside a chamber, in which is a direct current arc (200 A, 30 V). The powder particles formed with average size about 0.05 mm are carried to a bag filter by a stream of hydrogen-argon mixture. The capacity up to 60 kg/h of tungsten powder with particle size 0.03–0.05 mm is provided for by control of jet temperature and reaction area geometry. Tungsten powder produced in compound current of arc and radio-frequency plasmas has average particle size about 10 nm and impurities content not >33 ppm Fe, 23 ppm Mo, 17 ppm Si and 280 ppm carbon.
Amalgam Method Tungsten and molybdenum powders can be produced by reducing their pentachlorides by 2–5 wt% zinc amalgam (Fig. 23.11). The reduction reactions may be represented as follows: 2MoCI5 + 5ZnðHgÞ ¼ 2MoðHgÞ + 5ZnC12 + ðHgÞ 2WCl5 + 5ZnðHgÞ ¼ 2WðHgÞ + 5ZnCl2 + ðHgÞ Amalgam consumption is 110%–200% of stoichiometry reactions. The process temperature is a little bit lower than the boiling point of the zinc chloride formed (591 K). After metal reduction, the temperature is increased to 623 K and maintained for 30 min for growth of the crystals. Molten zinc chloride is floated on the amalgam surface, cooled, and then a
FIG. 23.11 Flow sheet of molybdenum and tungsten powder production by amalgam method.
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solidified cake of zinc chloride is separated from the amalgam surface. Excess zinc and mercury are distilled off in a vacuum by increasing the temperature to 1073 K.
Carbonyl Technique Tungsten and molybdenum carbonyl powders are produced by carbon monoxide action on their compounds (salts) at high pressures and elevated temperatures with metal carbonyl formation, and subsequently, its thermal decomposition with emission of dispersed metal. This process has been developed on enlarged laboratory and semi-industrial scales. Synthesis of carbonyls is done in liquid media. Fig. 23.12 shows a flow diagram of this process. The basic materials for this process are the following: higher metal chlorides (MoCI5 and WCl6), produced from ore concentrates, zinc powder as reducer, and dichlorethane or ether as solvent. After solvent cleaning, basic materials are fed into an autoclave in the following sequence: dichlorethane, metal chloride, zinc, and ether. For tungsten carbonyl production, the reaction components are charged in a weight ratio of tungsten hexachloride to zinc powder of 1 to 0.667 and volumetrical ratio of tungsten hexachloride to ether of 1 to 25. The process is carried out at 320–350 K and under 4–6 MPa of carbon monoxide pressure. On completion of the synthesis, carbon monoxide is removed from the autoclave, and the reaction mass is taken to the still where first, the solvents are distilled off, and then, carbonyl is evaporated off by live steam. Solvents then go to a regeneration unit and, after purifying, are returned to the process. Carbonyl is supplied to a water spraying capacitor, and the suspension is delivered to a nutsche filter and the filtrate exhausted to the collector by vacuum pump. Squeezed carbonyl is fed to the topping still, cleaned from iron pentacarbonyl (Fe(CO)5) by 1% aqueous solution of sodium hydroxide, and is distilled once more using live steam. Cleaned carbonyl is filtered in the nutsche filter, washed with distilled water and dried in a vacuum oven. The plant capacity is 200–500 kg per day; autoclave volume is 1–15 L, with carbonyl efficiency amounts to 80%–90%. There is also a method of dry synthesis from chlorides. The process uses iron swarf as the reducing agent and is conducted under carbon monoxide at a pressure up to 28 MPa and a temperature of 550 K. The advantage of this process is the absence of inflammable solvents. Tungsten and molybdenum powders are also produced by carbonyl decomposition under atmospheric pressure and at temperature ranges from 620 K to 1470 K in a gas current (hydrogen, nitrogen, and others). Sometimes, it is decomposed at reduced pressure without using a carrier gas. Powder efficiency is about 90%. The flow sheet of powder production in hydrogen is shown in Fig. 23.13. Crystalline carbonyl is charged into a horizontal sublimator of the “pipe-in-pipe” type with exterior heating. After sealing, the system is cleaned out using pure nitrogen. In the sublimator, carbonyl is heated up to temperatures ranging from 330 K to 400 K, and its vapor is carried by preheated and purified hydrogen to a decomposition unit provided with electroheating. Carbonyls are decomposed at temperatures ranging from 620 K to 670 K. The powder is collected in a container. Tungsten powder efficiency is 94%; molybdenum powder efficiency is 86%. More detailed information on the carbonyl process is contained in Chapter 6. FIG. 23.12 Diagram of plant for tungsten and molybdenum carbonyl synthesis in solvent media.
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FIG. 23.13 Flow sheet of powder production by carbonyl decomposition in hydrogen current.
Properties of Tungsten Powders Tungsten has two outstanding properties: the highest melting point of all metals at 3683 K and a very high density of 19.25 g/cm3 at 293 K. It cannot be shaped using traditional casting techniques. Consolidation of tungsten powders is based almost exclusively on powder metallurgical methods [12,13]. Therefore, definite and detailed powder specifications are very important in order to obtain the required final properties of a consolidated tungsten product. These specifications include physical and chemical properties. The basic physical properties are particle size, flow rate, and different powder density values. All the powder sizes, from ultrafine to hundreds of microns, are produced on an industrial scale, but the bulk of the powder used at present is in the size range 2–5 mm. Particle sizes in the range of 1–10 mm are measured by Fisher subsieve sizer (FSSS) (ASTM B 330 and MPIF 32 standards) [14] and correspond to particle sizes observed directly by optical microscopy (OM) (MPIF E 20 and GOST 23402-78 standards) and by scanning electron microscopy (SEM). SEM and OM are also the best ways to estimate the overall particle morphologies. Particle size distributions can be measured by light scattering techniques (ASTM B 822 standard) or by sedimentation based on X-ray monitoring of gravity sedimentation scattering (sedigraph) (ASTM B 761 standard). Controlling the reduction parameters make it possible to produce either narrow or wide size distribution and to provide any specific requirement on tungsten powders for various applications. Apparent density (ISO 3923 standard, and in accordance with it ASTM B 212, MPIF O4, and GOST 19440 standards), tap density (ISO 3953 standard, and in accordance with it, is accepted in ASTM B 527, MPIF 46, and GOST 25279 standards) as well as green density and green strength have great importance for determining the behavior of a powder during subsequent consolidation steps, including pressing and sintering. A feature of tungsten powder is a relatively high purity, in comparison with most other metal powders used for powder metallurgical products. Table 23.1 contains analytical data of typical tungsten powders produced by hydrogen reduction. Trace elements, including molybdenum, aluminum, arsenic, silicon, phosphorus, sodium, potassium, calcium, magnesium, and sulfur, are removed during ammonium paratungstate manufacture. The contaminants, such as nickel, chromium, iron, and cobalt, characteristic alloying elements of reduction boats or furnace tubes, have to be avoided. Physical analyses of typical medium sized tungsten metal powders are presented in Table 23.1. The characteristic morphology of the tungsten powders produced by hydrogen reduction is shown in Fig. 23.14. The particles have angular prismatic shape in either coarse or submicron sizes. In the powders reduced by solid carbon with stoichiometric carbon black consumption, the carbon content is decreased to 0.5 wt% and oxygen to 0.10–0.15 wt. Apparent density of these powders ranges between 3 and 7 g/cm3 by 1–10 mm powder particles size. Such powders may be used in hard alloy manufacture to produce tungsten carbide. Tungsten powders obtained by precipitation from the gaseous phase of tungsten hexafluoride and tungsten hexachloride (reduction of highest halides) with particle sizes between 0.05 and 0.80 mm and roundish shape are of high purity (Table 23.2) and differ significantly from tungsten powder reduced from oxides. Tungsten powders produced from WF6 in hydrofluoric medium and by WOx decomposition in hydrogen current are finer than particles produced by high-temperature WF6 dissociation or by high-temperature treatment of preliminary chlorinated wastes. High oxygen content in some “gas-phased” powders is caused by their high absorption properties in wet air.
<10
<20
<5
…
800
<10
<5
<50
<5
<5
<5
<20
<5
<5
<5
…
<5
<20
<10
<20
<5
…
800
<15
<10
<100
<5
<5
<5
<25
<5
<20
<5
…
<10
<25
Guar. Virgin
4.4 0.3
2.5 0.3
0.8 0.1
1.0 0.1
W 10
<20
<30
<5
…
800
<5
<10
<150
<5
<5
<5
<20
<10
<100
<5
…
<10
<20
Typ. reclaimedb
99.95
4.8 0.3
2.8 0.3
1.4 0.2
1.5 0.2
W 15
<20
<30
<10
…
800
<15
<15
<200
<5
<5
<5
<30
<15
<100
<5
…
<15
<20
Guar. Reclaimed
5.2 0.4
3.3 0.3
2.4 0.3
2.5 0.3
W 25
b
Tungsten powder made from tungsten ore. € Tungsten powder made by recycling sintered carbide scrap; Companies: WBH: Wolfram Bergbau- und HuttenGmbH; OSRAM: OSRAM Brunta´l spol. s r. o.
a
<5
<5
ppm
Sulfur
<10
ppm
Silicon
<10
ppm
Potassium
…
ppm
Phosphorus …
ppm
Oxygen …
<5
<5
ppm
Nickel
…
…
…
ppm
Sodium
250
<20
<20
ppm
Molybdenum
2650
…
…
ppm
Manganese
…
…
ppm
Magnesium
…
…
ppm
Lead
<5
<5
ppm
Iron
…
…
ppm
Copper
<5
<5
ppm
Cobalt
…
…
ppm
Chromium
…
…
ppm
Carbon
<2
<2
ppm
Calcium
<5
<5
ppm
Aluminum
Typ. Virgina
99.98
3.8 0.3
2.0 0.2
0.5 0.1
0.6 0.1
W 06
OSRAM
6.0 0.4
4.0 0.3
3.3 0.3
3.5 0.3
W 35
6.3 0.5
4.5 0.4
4.2 0.3
4.5 0.3
W 45
6.5 0.8
4.7 0.5
3.5 1.0
6.5 1.0
W 65
SECTION
Trace impurities:
wt%
Tungsten min. Excluded oxygen
…
…
g/cm
Tap density
…
…
3
g/cm
Scott density
…
… 3
mm
Lab milled
48
1 mm Coarse
1
mm
Unit
As supplied
Particle sizes FSSS
Property
WBH
Source: Standard Grades
TABLE 23.1 Properties of Typical Tungsten Powders Produced by Conventional Calcination/Reduction Process 696 D Production of Non-Ferrous Metal Powders
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FIG. 23.14 Scanning electron micrographs of coarse and submicron tungsten powders produced by hydrogen reduction.
TABLE 23.2 Composition of Typical Tungsten Powders Produced by Reduction of Highest Halides Technique [7] Property
Unit
LTF
LTHL
HTF
Average particle size
mm
0.8
0.1
0.2
Aluminum
ppm
…
…
14
Arsenic
ppm
…
…
80
Calcium
ppm
30
…
12
Chromium
ppm
…
7
…
Fluorine
ppm
130
…
900
Iron
ppm
1500
600
130
Magnesium
ppm
10
20
…
Manganese
ppm
30
5
…
Molybdenum
ppm
…
320
1300
Oxygen
ppm
200
6500
…
Silicon
ppm
10
60
300
Titanium
ppm
100
…
…
Trace impurities:
LTF: Low temperature dissociation by reduction of tungsten hexafluoride in hydrogen; LTHL: Low temperature dissociation by reduction of tungsten hexachloride in hydrogen; HTF: high temperature decomposition of tungsten hexafluoride in hydrogen fluoride medium.
Properties of carbonyl powders are shown in Table 23.3. Tungsten and molybdenum powders produced at decomposition temperature 620–670 K in hydrogen are represented by 60%–70% of particles with round shape with rough and sharp spurs on the surface with mean particle sizes of 2–4 mm (Fig. 23.15). Tungsten powders contain 1.4–2.2 wt% carbon and up to 2.0 wt% of oxygen, while molybdenum powders contain 3.4–3.9 wt% carbon and up to 5.8 wt% of oxygen. Particles of tungsten and molybdenum powders reduced at temperatures in the range from 670 K to 1170 K in an argon current are spherical and non-uniform in size, especially at low decomposition temperatures. The surface of molybdenum particles is comparatively smooth, while the surface of tungsten particles is uneven and includes some separate spheroids <1 mm in size. The particles become significantly smaller in size, and specific surface increases on increase of the
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D Production of Non-Ferrous Metal Powders
TABLE 23.3 Variation of Properties With Decomposition Temperature for Carbonyl Tungsten Powders Decomposition Temperature in Current of Argon (K)
Color of Powder
Average Particle Size (mm)
Specific Surface Area (m2/g)
Carbon Content (wt%)
Lattice Spacing (nm)
673
Gray
1.2
0.18
1.08
0.4178
773
Gray
1.0
0.21
1.00
0.4172
873
Dark-gray
0.74
0.30
1.50
0.4167
700
Black
0.12
1.56
1.83
0.4163
973
Black
0.07
3.00
1.82
0.4159
1073
Black
0.03
8.20
…
…
2273–4243
Black
0.01–0.03
…
0.50–1.00
…
(a) SHF: discharge in nitrogen current.
FIG. 23.15 Scanning electron micrographs of carbonyl tungsten (A) and molybdenum (B) powders.
decomposition temperature from 670 K to 1170 K; for example, for tungsten powders, the average size decreases from 1.2 mm to 0.03 mm, and the specific surface increases from 0.18 m2/g to 8.2 m2/g, respectively. Carbon content in tungsten powders obtained in argon is 1.9 wt%, while in molybdenum powders, it amounts to 3.5 wt%. The addition of an oxidizing component to the carrier gas decreases the carbon content in the powder. Sintering temperatures of carbonyl tungsten and molybdenum powders are some hundreds of degrees lower than that of powder produced by other methods. They are used for the production of porous and compact parts by powder metallurgy. Metal carbides, for example, ZrC and other abrasive materials such as boron nitride, can be coated with tungsten or molybdenum by thermal decomposition of their carbonyls resulting in improved of service performance. For example, the hardness of cutting wheels of cubic boron nitride metallized by tungsten is increased twofold. Diamond grains metallized by tungsten increase their cohesion in instrument assembly, preventing diamond chipping. Other methods of abrasives metallization are not effective enough because they do not provide good coating cohesion. The quantity of carbon and oxygen in the powder depends, first of all, on the process conditions in its production. Peak of carbon content is observed at about 970 K connected with free amorphous carbon formation because of catalytic decomposition of the carbon oxide. Carbon content can be decreased by a process of carbonyl W(CO)6 decomposition in superhigh-frequency discharge at temperatures ranging between 3043 K and 4000 K in a nitrogen current. The particles have a
Production of Refractory Metal Powders Chapter
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spherical shape and their size ranges from 12 nm to 30 nm; nitrogen content is 0.3 wt%, and oxygen content is not >1 wt%. The content of other impurities does not exceed the value 1.0 104 wt%. Carbonyl tungsten powders are metastable carbides with a face-cantered cubic lattice. Their properties change with increasing temperature; thus, lattice spacing decreases (see Table 23.3). Powders obtained by plasma-chemical reduction are characterized by high dispersivity; particles of an average size of about 0.07 mm form weak conglomerates with a developed specific surface and a very low apparent density of 0.25 g/cm3. When first produced, a large quantity of hydrogen chloride is absorbed on the powder surface, which volatilizes during storage in air, while the oxygen concentration is increased to 4%–8%. Nanopowders of approximately 20 nm average particle size are produced by melt atomization in arc argon plasma. Tungsten particles of 510 nm sizes have face-cantered or hexagonal close-packed lattice in comparison with typical body-centered for solid state. Tungsten powders obtained through high-temperature reduction in argon-hydrogen plasma represent double component systems consisting of a-W (body-centered lattice) and b-W (A-15) with main peaks (210) of b-phase and (110) of a-phase.
Tungsten Powder Applications Ultrahigh purity powders are used for manufacturing semiconductors and high-performance electrodes of high-intensity discharge (HID) lamps to avoid outgassing of impurities and guarantee a consistent lamp quality with increased lifetime. In particular, the incorporation of alpha emitters such as uranium and thorium is extremely crucial and must be held below 1 ppb [15]. The mobile alkali ions (for instance, sodium and potassium) have to be held below 0.1 ppm. Purification of APT by means of multiple extraction and crystallization steps ensures the required purity, so all production steps must be carried out in clean room conditions [6,16]. In the work [17], a method based on ammonium paratungstate (APT) row material, used for the production of ultrahighpurity tungsten powder (6N W), was developed. The APT was reduced by hydrogen to high-purity tungsten metal powder (Fig. 23.16). Two types of high purity tungsten material for electrodes in HID were prepared. One was a tungsten rod with a diameter in the range of 12–40 mm, which was processed by powder compaction with cold isostatic pressing (CIP), sintering at a temperature higher than 2273 K in an induction heating furnace under a hydrogen protection atmosphere, and hot forging. The other was a tungsten wire with a diameter in the range of 0.02–12 mm, which was fabricated by powder compaction by CIP, self-resistance sintering at a temperature approximately 2970–3070 K, swaging, and wire drawing. The Fisher subsieve sizer (FSSS) particle size of the produced 6N W powder is in the range of 2.2–3.2 mm, and the apparent density is approximately 2.5 g/cm3. The SEM image of the powder morphology is shown in Fig. 23.17. The d10, d50, and d90 of the tungsten powder (analyzed using a laser particle size distribution analyzer) are 2.027 mm, 5.161 mm, and 15.816 mm, respectively. The flowablility of the 6 N W powder is suitable for producing tungsten rods, wires, and targets. The purity (excluding the gaseous ingredients) of the tungsten rods and wires is 5 N or higher (Table 23.4). Tungsten is currently the most promising material for the first wall in demonstration fusion reactors (DEMO) and as a plasma facing material (PFM) in future fusion reactors [18]. Feasibly utilizing tungsten in the next generation of fusion reactors will require components with a thin tungsten layer. Coating technologies such as vacuum plasma spray (VPS), physical vapor deposition (PVD), and chemical vapor deposition (CVD) have been studied in recent years [19–21]. High purity (>99.9999%), full density (19.23 g/cm3) tungsten parts and coatings were manufactured with chemical vapor deposition (CVD) method with a deposition rate higher than 0.6 mm/h [22]. The thermal conductivity of the CVD-W material with a columnar grain structure is higher than that of forged tungsten, but the coefficient of thermal expansion (CTE) of both types of tungsten is very similar. Thermal shock tests show that the threshold energy of CVD-W for crack initiation is between 1.5 and 2.25 MJ/m2, while that of forged tungsten is lower than 1.1 MJ/m2, because the pyramidal shapes of the surface of CVD-W coating can withstand a greater level of distortion, preventing the appearance of cracks. The high melting point (3683 K) of all metals, low tritium retention, high thermal conductivity, and the high low erosion rate under plasma loading makes tungsten a suitable PFM. Pure tungsten powders: Pure tungsten powders are made into rods by pressing and sintering. In order to achieve high densification, a high temperature in the range of 2273–3373 K is required [2,6]. The sintered tungsten bars thus obtained are forged or rolled at elevated temperatures in order to produce rods (which are subsequently drawn to manufacture wires) or sheets. Such pure tungsten materials are widely used for high temperature applications, for example, in high-temperature furnace parts [2,6]. For these materials, powders produced by the highest halide reduction can be used. Non-sag tungsten wires: To prevent sagging of tungsten filaments in incandescent lamps during high temperature operation, doping elements—aluminum, silicon, and potassium—are included in the powder [3,23]. The present technique consists of adding an aqueous solution of compounds containing aluminum, silicon, and potassium into the tungsten oxide
700
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.16 Flow sheet of the process of ultrahigh purity tungsten products.
FIG. 23.17 SEM micrograph of the ultrahigh purity tungsten (6N W) powder.
Production of Refractory Metal Powders Chapter
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TABLE 23.4 Purity of the High Purity Tungsten products Product
Tungsten Powder
Tungsten Rod
Tungsten Wire
Purity (tungsten element content, wt%)
99.999931
99.99989
99.999922
powder before reduction to tungsten metal. During this reduction, a portion of the above trace elements is incorporated into the tungsten metal powder particles. During the sintering of the pressed tungsten powder, aluminum and silicon evaporate, while the potassium atom is too big to diffuse through the tungsten lattice. This process results in the incorporation of potassium droplets as inclusions. During wire drawing, the small potassium droplets are elongated and finally break into a multitude of little tiny potassium bubbles in lines along the wire axis, which results in an elongated interlocked grain structure. During recrystallization at elevated temperatures, the tiny potassium bubbles effectively stabilize through an effect similar to dispersion hardening, the movement of dislocations, and thus, the migration of the grain boundaries perpendicular to the wire axis. Large inclusions in the metal particles generate large potassium pores in the sintered ingot from which long bubble rows are formed in the wire. This characteristic structure explains the non-sag properties of the tungsten filaments in incandescent lamps [2,6,23–26]. Spheroidization of tungsten powders: The spherical tungsten powders exhibit outstanding properties such as excellent flowability, high bulk density, and low sintering shrinkage, making them suitable for abroad range of applications such as thermal spraying, additive manufacturing, etc. The thermal plasma technique has been proven effective for preparing spherical tungsten powders because of its high temperature and high enthalpy [27–29]. Plasma spheroidization of tungsten powders consists of two main processes: first, the raw tungsten powder, injected into the high temperature region of radio frequency (RF) thermal plasma, is heated and melted to form spherical droplets (due to surface tension), and then, the melted droplets solidify into spherical particles due to rapid quenching when they spurt out of the plasma. However, not all powder particles can be melted completely, despite the plasma temperature being much higher than the melting point of tungsten. Possible reasons for this incomplete melting could be that the powder particles cannot absorb sufficient plasma energy due to the non-uniform temperature inside the plasma torch, the local cooling effect of plasma by the cold carrier gas and flow of powder particles [28], or insufficient optimization of experimental parameters. The paper [30] describes a study of the spheroidization of tungsten powders by radio frequency thermal plasma, including the melting, solidification, and growth behavior of the tungsten powder particles during the spheroidization process. The flight time and melting time of tungsten powder particles in the plasma were estimated, and the growth behavior of the particles, specifically, the change in the average tungsten powder particle size before and after plasma spheroidization, was analyzed. The flight time and melting time for tungsten powder particles with radius of 7.8 mm were calculated to be 9.3 ms and 2.9 ms, respectively. The particle size distribution before and after spheroidization were determined (Table 23.5). It can be seen (Fig. 23.18A) that the raw powder particles are irregular, and the particles after RF processing (Fig. 23.18B) possess a spheroidal shape with few satellites present on the rough particles’ surfaces. The authors reported [30] that the change in powder particle size during the process showed that growth of tungsten powder particles was mainly caused by the coalescence of droplets in the thermal plasma system. The experimental results demonstrated that the spheroidization rate could reach up to 95% under the operating conditions used. Tungsten composite materials: Tungsten metal powder blended with other powders, such as copper, iron, cobalt, and nickel, form so-called “tungsten heavy alloys” (WHAs), which typically contain 90–98 wt% W and can be liquid phase sintered at about 1773 K. Their properties include high density (17–19 g/cm3) and ductility, and energy penetration and
TABLE 23.5 Particle Size Distribution of the Tungsten Powders Before and After Spheroidization Undersize (mm) of Cumulative Distribution Powder Condition
d5
d10
d50
d90
Raw powder
5.78
7.09
15.60
29.02
In-flight particles
…
…
20
…
Spheroidized powder
4.52
7.11
28.79
68.02
702
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.18 Field emission SEM images of tungsten powders: (A) raw powder, (B) spheroidized powder.
radiation shields are the application fields of these tungsten materials [6]. WHAs are typically formulated from elemental powders with mean particle sizes in the range of 1 mm to 7 mm. Because of exclusive high melting temperature of tungsten, PM manufacturing methods are predominantly used for such materials. Infiltration (INF) is used exclusively for making tungsten-based composite contact materials. Refractory metal powder is first blended to the desired composition with or without a small amount of binder to impart green strength, then it is pressed and sintered into a skeleton of the required shape. Silver or copper is next infiltrated into the pores of the skeleton. This technique produces the highest density composites, generally 97% or more of theoretical density. Because of the presence of some closed pores in the sintered skeleton, complete densification is not possible. The press-sinter method can be used for small refractory metal parts (not exceeding about 25 mm in diameter). A high density material is obtained by pressing a blended powder of the exact final composition into shape and then sintering it at the melting point of the low-melting-point component (liquid-phase sintering). In some cases, an activating agent such as nickel, cobalt, or iron is added to improve the sintering effect on the refractory metal particles. For this process, powders of much finer particle size are preferable to enable more bonding surface. However, material formed by this process is weaker compared that formed via the infiltration process. The press-sinter-re-press (PSR) process is used for all categories of contact materials, especially silver-based materials. Blended powders are compacted to the desired shape and then sintered. The materials are further densified by a second pressing (re-pressing). Sometimes, the properties can be modified by a second sintering or annealing. The flexibility of this process makes it applicable to parts of any configuration and of any material. Material thus produced may also have weaker bonding between particles. The first heavy alloy developed was a W-Ni-Cu alloy. Alloys of this ternary system are still used occasionally, mainly for applications in which ferromagnetic character and resistance must be minimized. However, the present industry standard W-Ni-Fe exceeds the above alloy in corrosion resistance and mechanical properties. Most of the current purposes for WHAs are best satisfied by the W-Ni-Fe system. Alloys such as 93W-4.9Ni-2.1Fe and 95W-4Ni-1Fe represent common compositions. Mechanical properties depend on the nickel-to-iron ratio. They are sensitive to hydrogen embrittlement due to the body-centered cubic (bcc) structure of the principal tungsten phase [31]. By means of the nickel-to-iron ratio selection and post-sinter heat treatment, optimal mechanical properties are obtained. There are two common post-sinter heat treatments for WHAs. Vacuum heat treatment is useful for the reduction of hydrogen embrittlement, as is resolution and quench for reduction of segregation-induced embrittlement. If alloys are subjected to vacuum heat treatment, nickel-to-iron ratios in the range of 2.3–4 should be chosen, as better mechanical properties are generated [32,33]. For example, 91W-6.3Ni-2.7 Fe alloy ensures an ultimate tensile strength of 940 MPa, elongation of 35%, and hardness of 29 HRC. The addition of cobalt to a W-Ni-Fe alloy is a common approach for a slight increase of both strength and ductility. The presence of cobalt provides solid solution strengthening of the binder and slightly enhanced tungsten-matrix interfacial strength. Even higher mechanical properties are attainable from the W-Ni-Co system with nickel-to-cobalt ratios ranging from 2 to 9 [34]. These alloys provide high static tensile strength, ductility and hardness, for example, for 91W-6Ni-3Co alloy, ultimate tensile strength is 960 MPa, elongation is 40%, and hardness is 31 HRC. The use of such alloys is generally limited to defense applications requiring advanced mechanical properties. A number of other special WHAs are known, too. An example is an addition of molybdenum to the W-Co-Ni-Fe quaternary alloy to restrict tungsten dissolution and spheroid growth, resulting in higher strength, but reduced ductility, in the sintered state [35]. The addition of boron in the form of NiB and FeB to W-Ni-Fe system alloys leads to a small increase of
Production of Refractory Metal Powders Chapter
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hardness in some cases, in particular, for W-7NiB-3Fe and W-7NiB-3FeB compositions [36]. The sintering temperature selected for these alloys was 1773 K. The structural and mechanical properties of 90W-7Ni-3Fe alloy compacts produced using a high-voltage electric pulse sintering technique were studied [37]. High-voltage electric pulse consolidation (HVEPC) involves passing a high-voltage discharge passage (30 kV) with a high current density ( 100 kAcm2) and short pulse duration (<300 ms). This current pulse is passed through a powder filling during the application of external pressure. The rapid increase in temperature at the areas of contact of the powder particles leads to their instantaneous welding. In this study, the average particle size of selected heavy alloy was equal to 6.03 mm; powder density was equal to 17.13 g/cm3. The value of the applied voltage varied from 4.5 kV to 5.8 kV. The pressure applied to the filling was equal to 45 MPa. It was reported [37] that the relative density of the sintered compacts is in the range of 94.9%–95.9% of theoretical density. The highest density increase values and increased microhardness of compacts are achieved at the maximum value of discharge 5.8 kV, although stabilized density values were obtained at 5.6 kV. Further increase in the microhardness by increasing discharge voltage leads to embrittlement of the material. The microstructure of the cylindrical shape samples is shown in Fig. 23.19. The average grain size is approximately 10 mm; that is, high-voltage sintering contributes to maintaining a fine-grained structure, more uniform distribution of iron-nickel binder, and almost total absence of porosity. Porous skeletons of pressed tungsten metal powder infiltrated with copper alloys or silver forming W-Cu as well as W-Ag composites are widely used as electrical contacts [38–40] due to their high electrical conductivity, low thermal coefficient of expansion, and their resistance to damage caused by electrical arc formation. Many researchers have tried to obtain W-Cu alloys of full density without sintering additives by use of fine tungsten powder and increasing the homogeneity of the W-Cu powder in order to facilitate rearrangement of tungsten particles in liquid copper during liquid-phase sintering [41,42]. Through mechanical alloying of W/Cu or tungsten oxide and copper oxide, homogeneous W-Cu composite powders with nanosized tungsten particles can be produced, and fully densified specimens can be fabricated. W-(25/30)Cu composite powders were fabricated by the sol-spray drying and subsequent hydrogen reduction, then pressed and directly sintered [43]. The authors reported that when sintering time (ranging from 15 min to 120 min) and temperature (between 1573 and 1693 K) are increased, relative densities of W-(25/30)Cu increase. When sintered at 1693 K for 120 min, relative densities of W-25Cu and W-30Cu composite materials are 98.09% and 99.13%, respectively. According to kinetic characteristics during sintering, the first stage of W-Cu liquid sintering finishes at some time from 0 to 30 min into the process. During the sintering process, spherical tungsten grains grow and uniformly distribute into the matrix Cu phase. According to liner fitting of W grain sizes, grain growth of W-25/30Cu composite materials conforms to the dissolve-precipitation mechanism during liquid phase sintering. BE-SEM images of W-30Cu sintered at 1693 K for 30–120 min show (Fig. 23.20) that nearly spherical tungsten particles are uniformly distributed by copper phase, and with sintering going on, tungsten particles grow. In the work [44], W-30 wt% Cu alloys were fabricated using cold pressing and infiltration sintering methods from two types of powders, that is, mixed copper-tungsten (abbreviated name: M-Cu-W) powders and newly developed coppercoated tungsten composite (abbreviated name: Cu@W) powders. The latter were prepared using the thermo-electroless method [45]. Tungsten-copper mixed powders with an average particle size of tungsten 6–8 mm and copper-coated tungsten powders with an average particle size 4–10 mm were used to fabricate bulk W-30Cu alloys through infiltration sintering at FIG. 23.19 Microstructure of the cylindrical shape sample of 90W7Ni-3Fe alloy compacted by high-voltage electric pulse consolidation.
704
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.20 SEM images of W-30Cu composite sintered at 1653 K for 30–120 min: (A) 30 min, (B) 60 min, (C) 120 min.
1623 K for 90 min under a hydrogen atmosphere. The polyhedral structures of tungsten particles are observed (Fig. 23.21A), while the surface of roundish copper particles appears rough (Fig. 23.21B). The cross-section image of the coated powder (Fig. 23.21C) shows that the copper phase is coated onto the surface of tungsten particle with coating thickness in the 2–6 mm range. Microstructures of W-30Cu alloys fabricated by the infiltration sintering method are shown in Fig. 23.22, in which the gray-white particles are tungsten phase, and the black ones are copper phase. It can be seen in Fig. 23.22A that the copper
FIG. 23.21 The morphology of copper coated tungsten composite powders: (A) pure tungsten powders, (B) copper coated tungsten powders, (C) cross section of the coated powder. (Modified from Chen W, et al. Infiltration sintering of WCu alloys from copper-coated tungsten composite powders for superior mechanical properties and arc-ablation resistance. J Alloys Compd 2017; 728(25 Dec): 196–205.)
FIG. 23.22 Microstructures of W-30Cu alloys fabricated by the infiltration sintering method: (A) mixed copper-tungsten powder; (B) copper-coated tungsten composite. (Source: Chen W, et al. Infiltration sintering of WCu alloys from copper-coated tungsten composite powders for superior mechanical properties and arc-ablation resistance. J Alloys Compd 2017; 728(25 Dec): 196–205.)
Production of Refractory Metal Powders Chapter
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phase in the (M-Cu-W) alloy has a certain segregation (denoted by arrow). The copper particles are distributed among the tungsten particles with size distribution in the 40–100 mm range. In contrast, in the (CuW) alloy (Fig. 23.22B), the tungsten particles are distributed more homogeneously, and no obvious agglomerates can be observed. Chen et al. [44] reported that the arc-erosion resistance of the CuW alloy is better than that of the (M-Cu-W) alloy. The ablation area of the former was larger, and its ablation crater was shallower, and the arc erosion is mainly attributed to the evaporation of the low melting point of copper. For the (M-Cu-W) alloy, arc erosion was mainly attributable to the sputtering of copper. A mechanothermochemical process using the liquid solution as starting material and a grinding process using salt-free oxide powder enables the production of ultrafine, homogeneous W-Cu composite powders [46]. The spheroidal composite W-Cu oxide powder is produced by spray drying an aqueous solution of tungsten and copper salts, and subjecting it to subsequent oxidation. The spheroidal structure of the oxide powder is destroyed during ball milling, and W-10 wt% Cu composite with nanosized particles is produced through reduction of the ground oxide powder at 974 K in hydrogen. But as the copper content in W-Cu composite powder decreases, densification of W-Cu powder compacts is difficult due to pore formation by local densification in agglomerated powders. W-10 wt% Cu-based oxide powders with tungsten trioxide addition before reduction sintered at 1673 K in hydrogen can achieve a sintered density of 17.05 g/cm3, which amounts to 98.5% of theoretical density. The thermal and electrical conductivities of such a composite are 203 W/ (mK) and 36.5%IACS, respectively [46]. To fabricate tungsten-copper nanocomposite powder, the tungsten-copper oxide mixture ball-milled in argon was reduced at a low temperature of 573 K [47]. The reduced W-Cu nanocomposite powder was consolidated at 1473 K for 1 h with a heating rate of 10 K/min. It has been shown recently that porous tungsten can be obtained at approximately 1473 K, a much lower temperature than the 2273 K used in the conventional process. This has been achieved by employing reactive sintering. Reactive sintering method makes use of the addition of aluminum, the low melting phase of which decreases the process temperature while acting as a potential sintering aid. As a result, higher homogeneity and uniform porosity distribution have been obtained [48]. High-energy ball milling is a well-known technique for producing oxide-dispersion strengthened materials. Due to activation of a material during grinding without oxide and metal powder admixtures, this method has essential advantages [49–51]. It makes the sintering stage easier, resulting in both higher rate and lower temperature. The reduced size of mechanically ground particles and their high density in lattice defects promote the sintering transport phenomena. It also leads to finer and more homogeneous oxide dispersion. This latter feature, together with the nanocrystalline structure of the ground powders, results in a small grain size after sintering. As reported [52], long milling times on the order of 30 h of W-1 vol% Y make it possible to reach a relative density of 96.5% after sintering for 4 h in vacuum at 2073 K, while within a milling time of 80 min, the relative density amounts only to 76.5%. Recent developments [53,54] define the production of W/Cu composite powders which can be directly pressed to shape and sintered, being produced without infiltration or machining, which is of particular importance with respect to their application as heat sinks in the semiconductor industry. Tungsten with oxide dispersions is manufactured with the addition of thorium-containing compounds, such as thorium oxide or thorium nitrate. The mixtures consisting of tungsten metal powder and the above additives are pressed, sintered, and further processed into wires or sheets. The main application of this material is welding electrodes, which, due to the high electron emission ability of thorium, ensure easier arc formation, and offer stabilization and improved physical properties arising from dispersion hardening in sheets and wires, particularly for vibration-resistant automotive lamps [2,6]. Because of the radioactive danger of thorium, alternatives have been developed. Cerium and lanthanum have been tried as potential candidates to replace thorium in welding electrodes. Properties of typical tungsten based composites are shown in Table 23.6.
Recovery of Metallic Tungsten Virtually almost all types of tungsten products, if available in large enough amounts and workable, are presently recycled [4,6]. Even unselected, low-quality scrap containing enough tungsten can be used as a feed material for the chemical ammonium paratungstate production process, electrochemical recovery, or for alloying in steel. But any possible, practicable use for selected tungsten scrap is, of course, desirable for economic reasons. Typical kinds of tungsten scrap include: l
Selected manufacturing scrap of pure tungsten can easily be crushed in high energy mills and directly re-used for applications with lower quality demands.
INF
INF
44Cu-56W
30Cu-70W
PSR
91W-6Ni-3Co
… …
…
13.85–14.18
12.76
11.90–11.96
17.25
14.20–14.77
12.10–12.60
…
14.45
12.87
11.39
17.81
12.84
…
…
36–51
55
45–63
29–35
49
47–50
45–53
60–65
72–80
85–90
Electrical Conductivity (% IACS)
31 HRC
29 HRC
86–96 HRB
79 HRB
60–81 HRB
95–105 HRB
90 HRB
55–65 HRB
80–93 HRB
50–60 HRB
40–47 HRB
25–38 HRB
Hardness
… 827 572
… … …
960d
[34] [34]
…
C, A
C
A
C
C
C
C
A
A
A
Data Source
…
1000
…
940c
827
…
758
434
…
379
896
…
…
483
…
Modulus of Rupture (MPa)
…
Tensile Strength (MPa) b
Defense applications
Arcing tips, vacuum interrupter, oil circuit breakers Circuit breaker runners, arcing tips, tap change arcing tips
Motor governor, semiconducting material
Motor starters, aircraft equipment, circuit breakers, contactors, computers, arcing tips
Automotive starting switches, circuit breakers
Current-carrying contacts in circuit breakers’ lightduty contactors
Application Examples
INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. c Elongation 35%. d Elongation 40%.
a
PSR
91W-6.3Ni2.7Fe
Ternary alloys
INF
50Cu-50W
Tungsten-copper
PSR
10Ag-90W
15.56
…
PSR
INF
14.92
INF
35Ag-65W
27.5Ag-72.5W
14.65–14.74
14.92
PSR
12.00
60Ag-40W
12.16
PSR
10.60–11.30
Typical
70Ag-30W
11.27
Calculated
PSR
Manufacturing Methoda
85Ag-15W
Tungsten-silver
Nominal Composition (%)
Density (g/cm3)
TABLE 23.6 Properties of Typical Tungsten Base Composites
Production of Refractory Metal Powders Chapter
l
l
l
l
l
23
707
Chips of heavy metal, obtained during the shaping operations, can be transformed into re-usable powder by means of oxidation and subsequent reduction. Large solid pieces can be machined into smaller pieces to enable subsequent recycling. The oxidation step leads to a total disintegration of the chips into oxide powder containing tungsten trioxide and tungstates of iron, nickel, or cobalt. Hydrogen reduction results in a metal powder, in which all the components (W/Fe/ Co/Ni) of the initial heavy metal are presented. Composite scrap mixtures, such as W/Cu, are difficult to oxidize, and are, therefore, not suitable for chemical recycling. Also, re-using in the steel industry cannot be realized due to the deleterious effect of copper in steel. A possible option is salt melt digestion with sodium nitrate, sodium nitrite, and sodium carbonate mixtures. However, this process presents environmental problems due to the nitrogen oxide formation. An alternative would be the electrochemical recovery of the tungsten. The slimes recovered from grinding high-speed tool steel, such as those containing up to 6 wt% tungsten, are de-watered by hot air and oxidized in a tube furnace in oxygen (0.05 m/s) with self-heating up to 1220 K. The oxides are vaporized at a temperature higher than 1670 K and condensed on cold screens. Tungsten extraction yield reaches up to 86% after 6 h at 1620 K. The condensate from tungsten and molybdenum oxides is reduced in hydrogen for 2 h at 1173 K. This results in powders with an average particle size of 15 mm and the following element content: 50 wt% tungsten, 50 wt% molybdenum, 0.02 wt% vanadium, 0.02 wt% chromium, and 0.1 wt% iron. The recovery process for tungsten bearing wastes includes the following steps: oxidation in hot air; milling of formed mixture of tungsten oxide and cobalt-tungsten oxide; and reduction at T < 947 °C in endogas or dissociated ammonia with an addition of natural gas. The recycling of composites of tungsten with thorium-containing compounds has problems due to radioactivity. Therefore, direct recycling in the steel industry is impossible. Chemical recovery in the conventional process leads to inadequate separation of thorium, and consequently is also inapplicable. At present, the only possible way to recycle this material is by electrolysis, such as through the method based on the electrolytic dissolution of tungsten in ammonia [55]. The difficulty is carrying out the process in such a way as to prevent dissolution of any of the radioactive products.
Accident Prevention When Working with the Tungsten Powder Metallic tungsten dust is not toxic and is ranked in the fourth category according to State Standard 12.1.005-88 under the legislation in the Commonwealth of Independent States (CIS) [56]. Tungsten inspirable dust takes a predominantly fibrous effect. The limit for dust in the atmosphere of the work place unhealthy is 6 mg/m3. Under the Workplace exposure limits in the European PM industry, the long- and short-term (15-min reference period) workplace exposure limits (WELs) for tungsten and insoluble compounds (as W) in the workplace atmosphere are 5 mg/m3and 10 mg/m3, respectively [57]. Tungsten in drinking water is also unhealthy and falls into the second category. A limit of 0.05 mg/L is specified in CIS State Standard 4630-88 [58]. Fine oxidizable powder mixed with air can constitute an explosion hazard, but the risk with tungsten powder is minimal even with powder fraction <74 mm; a minimal ignition temperature of powder deposits (self-ignition temperature) is 583 K. The products with such characteristics of inflammability and explosion risk are relegated to the “low explosion hazard” class of danger according to the Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used for pollution prevention and environmental control during powder manufacture and its processing. Detailed information on health and environment protection measures and on the prevention of inflammability risk in such conditions can be found in the Chapter 27.
Tungsten Carbide Powders Tungsten carbide powder is the basis for the production of cemented carbides, the vast class of hard, wear-resistant, refractory materials, in which the hard carbide particles are bound together, or cemented, by a soft and ductile metal binder. Conventional manufacturing of tungsten carbide powders accounts worldwide for the bulk of the tungsten carbide powder produced. This technique permits the production of tungsten carbide powders with particle sizes ranging from 0.15 mm to 50 mm [60]. This field offers all industrial applications and is described below, along with other methods.
Production of Tungsten Carbide Powders Conventional technique: There are two methods [15,61] by which tungsten carbide powders are produced from pure tungsten powder. In the first method, fine tungsten powders are blended with high purity carbon black, filled into graphite boats, and pushed through a high temperature furnace under hydrogen at temperatures between 1673 K and
708
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.23 Single tube push type furnace for tungsten carbide powder production. (Courtesy: Wolfram Bergbau€ und Hutten-GmbH.)
€ FIG. 23.24 Scanning electron micrographs of coarse and ultrafine tungsten carbide powders. (Courtesy: Wolfram Bergbau- und Hutten-GmbH.)
1773 K. Compared to the reduction step for tungsten, temperatures are much higher, and single tube pusher type furnaces (Fig. 23.23) are used. Each particle is composed of numerous tungsten carbide crystals. The particle size of the tungsten carbide powder (Fig. 23.24), for the most part, preserves the particle size of the submicron tungsten metal powder (see Fig. 23.14). Further, cooled caked tungsten carbide powder is de-agglomerated. This step just separates particles from sticking to each other and is not used to change the particle size. This process, including the ore-processing steps, is also called the conventional calcination-reduction-carburization (CRC) process and offers the potential to manufacture commercial WC ultrafine powders with median grain sizes below 0.5 mm. Strict process control and a high degree of automation permits greater uniformity and batch-to-batch reliability [62]. This process is unsuitable for the production of coarse-grained powder. Techniques of the former conventional tungsten carbide production are usually limited during carburization to temperatures around 2270 K. At these temperatures, carburization with tungsten powder with particles <100 mm can be achieved, but the resulting product is polycrystalline. By the latter, the increase of tungsten carbide single crystals is realized by the use of a liquid phase, which is able to dissolve both tungsten and carbon. The formation of tungsten carbide within the melt of solvent metal, such as iron, is the
Production of Refractory Metal Powders Chapter
23
709
basis of the Menstruum process, also called “macro process.” [63] The powder mixtures of tungsten ore concentrates, iron oxide, aluminum, calcium carbide, and/or carbon react exothermally (after ignition by a detonator) to generate tungsten carbide, iron, calcium oxide, and aluminum oxide. A high temperature exothermic reaction (2Al + 3FeO $ Al2O3 + 3Fe) at about 2770 K produces a molten mass that, when solidified, consists of tungsten carbide crystals dispersed in iron and a slag containing impurities. This technique permits complete reduction and carburization of a batch of about 22 t of tungsten carbide, which is achieved in 60 min. To get the tungsten carbide in powder form, the solidified solvent metal is dissolved, which is ordinarily done with hot hydrochloric acid. The obtained tungsten carbide powder is separated according to desired particle size by sieving. Menstruum tungsten carbide powders have a high yield of well-faceted single crystal particles with sizes of 400 mm and below. Production of fine grained powder: Recently, the demands for tungsten carbide powders finer than 1 mm are increasing, as material for precision tools such as drills and end mills for metal cutting, printed circuit boards, cutters and precision pins, and so on. This increased demand, together with conventional techniques’ efficiency limitations in producing fine-grained metal powders [64], creates a strong motivation to develop alternative techniques. Tokyo Tungsten Co., Ltd. has developed [65] the direct carburizing process, in which tungsten trioxide and carbon black react in the following steps: WO3 ! ðWO2:9 Þ ! ðWO2:72 Þ ! WO2 ! W ! W2 C ! WC At the (WO2.72) ! WO2 step, ultrafine particles are generated and then transformed into tungsten carbide without grain growth. The starting material is prepared in the form of pellets from a mixture of tungsten oxide with carbon black. Subsequent reaction is accomplished in rotary furnaces in two steps: the direct reduction of the WO3 to tungsten by carbon under nitrogen, followed by carburization under hydrogen at temperatures up to 1873 K to tungsten carbide powder. This method permits the production of ultrafine powder with particle sizes ranging from 0.1 mm to 0.7 mm. Vickers hardness, HV (50 kg), decreases from 2150 to 1900 as particle size increases from 0.1 mm to 0.3 mm. Another direct carburization process has been developed by Dow Chemical Company [66,67]. A mixture of tungsten trioxide and carbon black falls through a vertical reactor at temperatures of approximately 2270 K. Accordingly, the reaction time is a few seconds and results in the formation of an intermediate mixture of W, W2C, WC, and C. This product must be subjected to a second, more conventional carburization step. Particle sizes were reduced by controlling the milling time, carbon content, inhibitor addition, and sintering temperature. The characteristic particle size range for this process is between 0.1 mm and 0.8 mm, which is identical to the lowest limits of the conventional process [64]. The spray conversion process has been developed for Nanodyne Inc. [68] to produce nanopowders. Aqueous solutions of tungsten and cobalt salts are spray dried, then subsequently reduced and carburized with a gas mixture such as hydrogen and ammonia or carbon dioxide and carbon monoxide in a fluidized bed reactor. The product obtained is relatively large (about 75 mm) hollow WC-Co composite powder particles that consist of tungsten carbide crystallites with a grain size of about 30 nm. This product is milled in order to obtain a powder suitable for consolidation. Another technique to produce tungsten carbide nanopowders is a chemical vapor reaction process [69]. This process is based on the gas phase reaction of metal halides with different gas mixtures to produce nanosized powder ranging from 5 nm to 50 nm. Production of tungsten carbides is possible in principle; however, the process seems better adapted to the production of nitrides or carbides of other group IV and V refractory metals.
Properties of Tungsten Carbide Powders High hardness is one of the most important properties of pure tungsten carbide and is the basis for the application in hardmetals, also called “cemented carbides.” As of 2014, >50% of the 30,000 tons of tungsten produced annually was used in the production of cemented carbides [70]. A number of physical and technological characteristic values are quite similar to those for tungsten metal powders. Tungsten carbide powders may have powder particle structures which are either single or polycrystalline. SEM and bright field TEM images (Fig. 23.25) show the typical morphology and physical constitution of ultrafine tungsten carbide [62]. The grain boundaries within a single particle are revealed by dotted lines. Recent developments in the field of standard ultrafine grades were driven, above all, by the need to control the sinter activity and grain growth tendency of ultrafine carbides. Table 23.7 shows the basic characteristics of standard ultrafine powder grades. Abbreviation CRC symbolizes the milestones of conventional process from tungsten concentrate to tungsten carbide through calcination, reduction, and carburization. The values obtained using a Fisher subsieve sizer (FSSS), based on the air permeability method of measuring the average particle size of powders, are not absolute and irreproducible in the range of ultrafine powders. It could be
710
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.25 Morphology and physical constitution of ultrafine tungsten carbide. (A) Scanning electron micrograph of powder particles; (B) transmission electron microscopy image of single particle. (Courtesy: Wolfram Bergbau - und Huttengesellschaft mbH NfG KG, Austria.)
TABLE 23.7 Properties of Ultrafine Tungsten Carbide Powder Grades Standard Grades Unit
CRC025
WC O6
CRC020
WC 05
CRC015
mm
0.63
0.64
0.55
0.56
0.49
2
m /g
1.71
1.52
2.03
1.95
2.43–2.83
mm
0.22
0.25
0.19
0.20
0.16–0.14
mm
…
0.30
…
0.25
0.18
Total carbon
wt%
6.12
6.17
6.09
6.19
6.11–6.15
Free carbon
wt%
0.03
0.03
0.03
0.02
0.01–0.05
VC
wt%
Undoped
0.1–0.5
Undoped
0.3–1.0
0.3–1.0
Cr3 C2
wt%
Undoped
0.1–0.5
Undoped
0.3–1.0
0.3–1.0
Oxygen
ppm
900
1030
1080
1450
1810–2800
Property a
FSSS grain size BET specific surface area (SBET) b
BET calculated particle size FESEM particle size dSEM
c
a
Fisher Sub-Sieve Size. 6 BET particle size calculated, based on measurement 300 particles, according equation: dBET ¼ 15:6S mm. Pd ofPminimum Pd Pd BET d FESEM particle size according equation: dSEM ¼ dji d= dji N mm, where dji d is sum of diameter of all particles in range from di to dj; dji N is total number of particles in range from di to dj. VC, vanadium carbide.
b c
demonstrated using scanning electron microscopy that obtained FSSS particle sizes are overstated by two to three times in comparison with reality [64,71]. With field emission secondary electron microscopy (FESEM), particle sizes are slightly higher than calculated by BET (Table 23.7) because the surface roughness may give a high surface area measurement, leading to the wrong conclusion that they are finer. In turn, an arithmetic mean FESEM particle size, calculated according to Eq. (c) in Table 23.7 used for particle size distribution: dj X
jðdÞ ¼
d
di dN X d1
d
Production of Refractory Metal Powders Chapter
23
711
FIG. 23.26 Scanning electron micrographs of standard tungsten carbide powder grades CRC025 (A) and CRC015 (B).
P where ddN1 d is sum of particle diameter of all measured particles N (mm) does not represent reliable weight particle distribution. In consequence of cubic dependence of particle volume on its size, the weight distribution will be leveled by FESEM particle size distribution calculated by the sum of particle diameters. In reality, for example, if powder particle size boundaries ranged from 0.05 to 1.0, the linear particle sizes would differ by 20 times, while their volumes would differ by 8000 times. Thus, the average FESEM particle size may be understated. A higher fines yield can be advantageous in order to increase the sinter activity/densification rate (at low sintering temperatures during solid state sintering) or to enable a slight rise in hardness due to maximum grain growth inhibition. Special applications (in particular, in the area of wear parts) set the demand for carbide grades with BET specific surface areas higher than 2.4 m2/g. Developed CRC 015 grade satisfies these requirements (see Table 23.7). These carbides with calculated BET particle sizes of 0.15 mm are produced in full-scale units by the conventional CRC process. Powder grades CRC 025 and CRC 015 differ appreciably in their particle sizes (Fig. 23.26). Added chromium plays an important role in grain growth inhibition during sintering. Energy dispersion analysis provides evidence [61] that chromium added during powder processing is preferentially present at WC/WC grain boundaries, even in the interior of polycrystalline particles or in necks between carbide particles. 10 wt% Co alloys produced by conventional liquid phase sintering at 1693 K and doped with chromium carbide (0.3–1.0 wt% Cr3 C2) and vanadium carbide (0.3–0.6 wt% VC) resulted in coercitivity values ranging from 440 Oe to 560 Oe with a corresponding increase in BET specific surface area from 1.5 to 2.8 m2/g. Additionally, the increase in maximum coercitivity is directly proportional to the BET surface area. The most important chemical component of tungsten carbide powder is its carbon content. Stoichiometric pure tungsten carbide has a total carbon content (Ct) of 6.135 wt%. From the point of view of thermodynamic equilibrium, WC has a very narrow stability range [2]. Thus, not even a small carbon shortage leads to W2C to the formation of free carbon (Cf). In commercial WC powders, there is always a small portion of free carbon present, about 0.03% at a stoichiometric Ct level. Even when the total carbon is below the stoichiometric level, the coexistence of traces of W2C and Cf can occur. The carbon balance is important for hardmetal manufacturing, because a shortage of carbon leads to the formation of the brittle phases, for instance, Co2W3C, and any excess in carbon leads to graphite precipitation. Thus, both cases result in a detrimental effect on the mechanical properties of the final product [2,61]. The role of carbon source in the production of WC powders starting from metal tungsten (W) and tungsten oxide (WO3) has been studied [72]. Those conclusions follow. In the W + C ! WC process conditions, inert atmospheres are recommendable for the synthesis of WC when metallic tungsten powders are used as precursors. In these conditions, fine WC powders can be obtained at 1373 K using either graphite or carbon black powders. The resulting WC powders consist of agglomerates of submicron particles with irregular platelet morphology. In the WO3 + 4C ! WC + 3CO process conditions, is possible to synthesize WC directly from WO3 powders. In this case, atmospheres containing hydrogen are needed to activate reduction of oxides at lower temperatures, whereas, at higher temperatures, reduction is promoted by the presence of carbon. Carburization in Ar-50%H2 of mixes containing WO3 + carbon black is complete at 1373 K, whereas, for WO3 + graphite powders, complete transformation to WC is
712
SECTION
D Production of Non-Ferrous Metal Powders
achieved at higher temperatures (1573 K). High energetic milling of WO3 + graphite allows the complete carburization at 1373 K. The resulting WC powders have spherical morphology, submicron particle size, and crystalline grain sizes below 30 nm. Nanoscale WC-Z composite powder was synthesized by in situ reaction of tungsten oxide, cobalt oxide, and carbon black mixtures for fabricating a nanostructured WC-based coating in which the conventional Co matrix was substituted for Z phases (Co6W6C, Co3W3C, Co2W4C, etc.) [73]. WC-Z nanocomposite powder with a particle size of 50–200 nm was prepared by in situ reaction of metal oxides and carbon, based on which, the nanostructured, WC-based coating was then fabricated by high-velocity, oxy-fuel thermal spraying. Song, et al. [73], based on fulfilled studies, reported the following: (a) Above 1170 K, the synthesized composite powder contains only WC and Z phases. At lower temperatures, the CoWO4 phase preferentially forms in the synthesized powder and then transforms into W2C, W, Co6W6C, and Co3W3C phases as the temperature increases. (b) The amount of Z phases of the obtained WC-Z composite powder depends largely on the carbon addition of the raw materials. The appropriate carbon content was estimated as 3.9%–4.2% for the WC-Co thermal spray feedstock powder resulting in no metallic Co phase. (c) The thermal spraying feedstock prepared using the WC-Z nanocomposite powder has a homogeneous WC particle size distribution with a mean size of about 260 nm. In contrast, the conventional powder consists of WC particles of 3 5 mm (Fig. 23.27). Elements such as vanadium, chromium, tantalum, and niobium, due to their grain-growth inhibiting effect during hardmetal sintering, are added to tungsten carbide powder in amounts around 0.1–1.5 wt%. Such elements can be introduced as carbide to the tungsten carbide powder, or as oxide or carbide prior to the carburization step. Trace elements similar to nickel, iron, and cobalt can be admitted in amounts around 100 ppm. At the same time, there are other impurities that should be limited in the very low ppm range because of their detrimental action on hardmetal properties. Calcium, sulfur, silicon, and phosphorus are typical of such elements [74,75]. Compositions and properties of several standard tungsten carbide powders are given in Table 23.8.
Tungsten Carbide Electrical Contacts Tungsten carbide analogous to other refractory metals and their carbides is distinguished by high melting and boiling points and very high hardness at both room and elevated temperatures, but also by poor electrical and thermal conductivities and poor oxidation resistance. Forming a composite can compensate for these drawbacks. The development of composite contact materials involving silver with tungsten or molybdenum or their carbides has resulted in materials that can withstand higher currents and more arcing than other contact materials, without experiencing sticking or rapid erosion. Refractory metals, the content of which can vary from 10% to 90%, offer good mechanical wear resistance and resistance to arcing. Silver provides good electrical and thermal conductivities.
FIG. 23.27 Cross-sectional microstructures of the feedstock particles prepared with the WC-Z composite powder (A) and (B) the conventional powder. (Source: Song X, Wang H, Yang T, Liu X, Wang X. In situ synthesis of nanoscale WC- composite powder and its application in cermet coating. In: Compiled by European Powder Metallurgy Association. Proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
TABLE 23.8 Properties of Typical Tungsten Carbide Powders Source: Standard Grades WBH
OSRAM
Property
Unit
WC 05D [3]
WC 06
BC0 4 U
BC1 0 U
BC45S
BC65S
BC75H
WC-TiC 70/30a
WC-Ti 50/50a
FSSSb
mm
0.53
0.64
0.5–0.7
0.91–1.20
4.31–5.00
11.01–15.00
> 30
2.0 0.3
2.0 0.3
2
m /g
1.90
1.52
…
…
…
…
…
…
…
Total carbon
wt%
6.23
6.17
6.13 0.05
6.13 0.05
6.13 0.05
6.13 0.05
6.13 0.05
9.7–10.2
12.6 0.2
Free carbon
wt%
0.04
0.03
max 0.08
max 0.08
max 0.06
max 0.06
max 0.06
max 0.25
max 0.4
VC
wt%
0.32
0.1–0.5
…
…
…
…
…
…
…
Cr3C2
wt%
0.64
0.1–0.5
…
…
…
…
…
…
…
Aluminum
ppm
10
<5
…
…
…
…
…
…
..
Calcium
ppm
3
<2
…
…
…
…
…
max 0.01
max 0.01
Chromium
ppm
…
…
100
100
50
50
75
…
…
Cobalt
ppm
<5
<5
…
…
…
…
…
…
…
Copper
ppm
<2
…
…
…
…
…
…
…
…
Iron
ppm
46
20
350
350
500
500
500
max 0.05
max 0.05
Magnesium
ppm
<1
…
…
…
…
…
…
…
…
Molybdenum
ppm
<20
<20
…
…
…
…
…
…
…
Nickel
ppm
<5
<5
150
150
100
100
100
…
…
Oxygen
ppm
1440
1030
3000
2500
500
500
500
…
…
Phosphorus
ppm
<20
…
…
…
…
…
…
…
…
Potassium
ppm
<4
…
…
…
…
…
…
max 0.01
max 0.01
Sulfur
ppm
6
<5
…
…
…
…
…
max 0.01
max 0.01
Silicon
ppm
<10
<10
…
…
…
…
…
max 0.005
max 0.005
BET specific surface area Nominal composition:
Trace elements:
Density
g/cm
14.72
14.91
14.94
14.94
14.94
…
…
Hardness
HV30
2000
1650
1380
1220
<1050
…
…
Coercivity
Oe
500
260
122
80
<60
…
…
3
€ Fine-grained Grade; WBH: Wolfram Bergbau- und Hutten Company (Sankt Martin im Sulmtal, Austria); OSRAM: OSRAM Sulvanias, North America (Wilmington, MA, United States). Fisher Sub-Sieve Size.
a
b
714
SECTION
D Production of Non-Ferrous Metal Powders
Silver does not alloy with tungsten, molybdenum, or their carbides. Therefore, PM processes are used in their manufacture. Depending on the composition, contact materials are accomplished either by pressing and sintering or by the pressinterinfiltrate method. At temperatures rise above the melting point of the infiltrant, the liquid metal penetrates and fills the interconnecting voids of the pressed and sintered compact. Densities that are 96%–99% of the theoretical density can be achieved by this technique. Infiltrated contact materials find use as current-carrying contacts in air- and oil-immersed circuit breakers, heavy-duty relays, automotive starters, and switches. Copper infiltrant, which costs less but has very poor corrosion resistance, is used for parts that operate in non-corrosive environments such as oil, vacuum, or inert atmospheres. Table 23.9 contains data on typical tungsten and carbide electrical contact grades published by manufacturers, which usually include density, hardness, and electrical conductivity.
Diamond Tools Tungsten carbide powder, together with certain metal powders, can be used to adjust the characteristics of alloys in which diamonds are embedded [76].
HARDFACING Hardfacing is the application of hard, wear-resistant material to the surface of a component by welding, thermal spraying, or a similar process for the main purpose of reducing wear. Carbide/metal mixtures very like hardmetal compounds are applied to machine parts to improve their resistance. This can be realized by flame or plasma spraying of powders or by welding techniques. The detonation gun spray process is also used. The extremely high particle velocities achieved in this process result in coating with higher density, greater internal strength, and superior bond strength than can be achieved with conventional plasma spraying or single-step flame spraying. Another category of hardfacing techniques is cladding. Hardfacing materials can be clad onto substrates by furnace fusing prearranged layers of loosely bonded hardmetal onto the substrate. Thus, the Conforma Clad Process (Conforma Clad, Inc.) uses a flexible cloth made of polytetrafluoroethylene and a hardfacing powder of the desired composition. The main benefit from this technique is that the cloth can be cut into any required size and placed on the surface to be coated, regardless of the profile of the workpiece. During subsequent furnace sintering, polytetrafluoroethylene is vaporized, and the hardfacing powder forms the deposit layer [77]. Sintered tungsten carbide powders are widely used for thermal spray coatings and torch welding hardfacing. Powder can be a product of crushing bulk WC-Co alloys or specifically made spherical shaped pellets. Tungsten carbide powders for thermal spray coatings usually contain from 6% to 18% cobalt. Particle sizes of WC-Co for thermal spray coatings range from 5 mm to 150 mm. Powder particle sizes for torch welding hardfacing range from 20 mm to 100 mm. Macrocrystalline tungsten carbide powders are a special kind of tungsten carbide powder manufactured by a hightemperature thermit process during which ore concentrate is converted directly to tungsten carbide. This product maintains the correct stoichiometric carbon content of 6.13 wt%. Macrocrystalline tungsten carbide is grown in crystals ranging from 1 mm to 5 mm. Coarse mesh sizes of macrocrystalline carbide are extensively used in abrasion erosion protection applications. The finer size powder is employed as wear rate modifier. Coarse tungsten carbide powder can also be obtained by conventional carburization method. Commercial grade powders are manufactured by H. C. Starck GmbH (Munich, Germany), Kennametal India Ltd. (Bengaluru, India), etc. Cast tungsten carbide is another category of carbide that also finds wide applications in hardfacing. Cast carbide refers to eutectic of WC and W2C that can range in carbon content from 3.5 to 4.5 wt%. It is manufactured by melting compounds of metallic tungsten, tungsten carbide, and carbon at a temperature above 3270 K. The melt is cast into billets, which are then crushed to the size range of powder. The cast is crushed to powder <850 mm and as fine as below 45 mm. Cast carbide, mostly used in torch welding hardfacing, is often used in combination with sintered tungsten carbide and other carbides to enhance wear resistance. At the same time, the addition of cast carbide tends to produce coatings with lower toughness. A relatively new spherical cast tungsten carbide product has all the basic characteristics of crushed cast carbide. However, this material has extremely high wear resistance and higher resistance to chipping and cracking in comparison with crushed cast carbide due to its spherical shape. Spherical cast carbide is manufactured by a technique ensuring rapid solidification of molten eutectic carbide droplets [78]. Diamond-like carbon (DLC) coatings technology due to their surface smoothness, hardness, and chemical inertness in combination with a low friction coefficient against most metals make DLC films interesting for wear-protective purposes. However, DLC films present high compressive internal stress and low chemical interaction between films and their metal
TABLE 23.9 Properties of Typical Tungsten Carbide Electrical Contact Grades Density (g/cm3) Nominal Composition (%)
Manufacturing Methoda
Calculated
Typical
Electrical Conductivity (% IACS)
Hardness
Tensile Strength (MPa)
Modulus of Rupture (MPa)
Data Sourceb
Application Examples
Tungsten carbide-silver INF
11.86
11.53–11.85
55–60
50–65 HRB
272
483
C, A
60Ag-40WC
PSR
12.09
11.40–11.92
46–55
60–70 HRB
…
…
A
50Ag-50WC
INF
12.56
12.12–12.50
43–52
75–83 HRB
276
793
C, A
Aircraft contactors, lighting relays, low-voltage switches, Circuit breakers
40Ag-60WC
INF
13.07
12.0–12.92
40–47
90–100 HRB
379
827
C, A
Heavy-duty
38Ag
INF
13.18
12.92–13.29
35–38
90–100 HRB
552
…
C
Circuit breakers
20Ag-80WC
PSR
14.23
13.2
19
400 HV (c)
…
…
M
Semiconducting material
Arcing contacts in oil, wiping shoes in power transformers
Tungsten carbide-copper 50Cu
INF
11.39
11.0–11.27
42–47
90–100 HRB
…
1103
C, A
44Cu
INF
11.77
11.64
43
99 HRF
…
1241
C
30Cu
INF
12.78
12.65
30
38 HRC
…
…
…
a INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy, Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. M: Metz Degussa, South Plainville, NJ.
Production of Refractory Metal Powders Chapter
65Ag-35WC
23 715
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D Production of Non-Ferrous Metal Powders
FIG. 23.28 Schematic of the magnetron sputtering system.
substrates. Now, the most general approach to solve the adhesion problem is to use metal or silicon interlayers between steel and DLC. In work [79], a physical vapor deposition (PVD) method for the deposition of tungsten carbide and DLC multilayer coatings by using a single sputtering target comprised of two equal halves (one half carbon, the other tungsten) was realized. A set of coatings were deposited by means of a RF (radio frequency) (13.56 MHz) magnetron sputtering system (Fig. 23.28). The different layers are obtained by the variation of the argon and methane/argon sputtering gas composition. The pressure of both argon an argon/methane mixture was 4 Pa. Using this deposition method, researchers prepared multilayer coatings onto steel substrates. The coatings are formed by a multilayer stack of different WC layers and an upper DLC layer. The DLC thickness was approximately 0.3 mm, and the total thickness was approximately 1.0 mm. Fig. 23.29 shows the SEM cross-section morphology of a WC/DLC multilayer. As reported [79], the structure of this multilayer stack consists of cubic WC1x polycrystalline layers separated by carbon rich narrow interlayers. The upper DLC layer has been obtained in the same process by increasing the methane concentration in the gas mixture. The experiments have demonstrated that multilayer coatings improve the adhesion and wear resistance of the DLC coatings on steel substrates while keeping their low friction properties. FIG. 23.29 SEM micrograph of cross-section of a WC/DLC multilayer coating. (Source: Rinco´n C, Zambrano G, Carvajal A, Esteve J. Tungsten carbide/diamond-like carbon multilayer coating on steel for tribological applications. Surf Coat Technol 2001; 148(2–3): 277–83.)
Production of Refractory Metal Powders Chapter
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The paper [80] presents the results of abrasive wear resistance, microhardness, and adhesive wear resistance tests conducted on AISI 1020 commercial steel when it is standardized, heat treated through a process of quenching-tempering, and when a coating of tungsten carbide in powder form of nanometric size is deposited on it. This had been reported as follows: Knoop microhardness tests allowed researchers to conclude that thermally treated specimens with a quenchingtempering process improve this property by 5.5%, while the WC coatings have a more significant increase, reaching a value of 124.2% compared to standard steel. The average loss of material per unit of covered length during abrasive wear resistance tests on AISI 1020 standard steel, thermally treated with a quenching-tempering process and coated with nanometricsized WC is of 0.0382 mm3/m, 0.0291 mm3/m, and 0.0151 mm3/m, respectively. The average loss of material per unit of covered length during adhesive wear resistance tests on AISI 1020 standard steel, thermally treated with a quenchingtempering process and coated with nanometric-sized WC is of 0.0827 mm3/m, 0.0736 mm3/m, and 0.0271 mm3/m, respectively.
CEMENTED CARBIDES Production of Cemented Carbide Powders Optimal interfacial bonding in hardmetals is achieved by means of optimization of balance between the high hardness of the tungsten carbide and the high toughness of a ductile binder metal, such as cobalt, nickel, or iron [76,81]. The steps of hardmetal manufacture include: blending of grade powders, milling, powder consolidation, liquid phase sintering, postsintering operations, and post-sinter forming. The excellent wetting of tungsten carbide by cobalt is one of its key properties. By variation of the tungsten carbide particle size and the percentage of cobalt, desired properties of the hardmetals can be obtained [76,81,82]. Fig. 23.30 illustrates, in diagram form, that tungsten carbide particles with equal particle size may differ concerning the size of the individual crystals within the particles, which, depending on the powder manufacturing process, can either be single or polycrystalline. This characteristic has a strong influence on the behavior of the powder during the manufacturing of the cemented carbide. Generally, the higher the number of subgrains per WC particle, the finer the microstructure of a hardmetal can be [3]. A general rule, moreover, is the finer the carbide and the lower the cobalt content, the harder, but less tough, the composite is. But, higher binder content at a certain hardness (combined with a lower average WC grain size) does not necessarily mean a higher toughness. In particular, in the lower hardness range, this relationship does not hold. At the higher hardness range, however, a higher binder can reduce the carbide contiguity, and thus, improve the hardness to toughness relationship. However, the improvements which can be obtained at a high hardness are obviously limited for hardness above 2000 HV30 [83], while the basic WC-Co material has been modified to produce a variety of hardmetals, which are used in a wide range of applications, including metal cutting, mining, construction, rock drilling, metal forming, structural components, and wear parts. FIG. 23.30 Influence of crystalline size of tungsten carbide particles on the microstructure forming of hard metal.
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Additions of other carbides such as titanium, tantalum and niobium, or titanium carbonitrides are used in steel cutting grades and are produced from metal oxides of titanium, tantalum and niobium. These oxides in particulate form are mixed with metallic tungsten powder and carbon black. The oxides are reduced under a hydrogen atmosphere or vacuum at elevated temperatures and form solid solution carbides WC-TiC, WC-TiC-TaC, or WC-TiC-(Ta, Nb)C. The Menstruum process can be used to produce a WC-TiC solid solution. In this method, both carbides are dissolved in liquid nickel, which functions as a contributory metal. Solid solution carbides are then precipitated during cooling. The cemented carbide powders may consist of tungsten carbide mixed with a finely dispersed metallic binder (cobalt, nickel, or iron) or with additions of other above-named cubic carbides, depending on the desired properties and application of the tool. Intensive milling is necessary to disintegrate the initial carbide crystallites and to blend all components so that every carbide particle is coated with binder material. This process is accomplished in ball mills, vibratory mills, or attritors using carbide balls. The mills are usually lined with carbide sleeves; however, mills lined with low-carbon steel or stainless steel are also used. Grinding is carried out under an organic liquid such as heptane or acetone to prevent oxidation of the powder. After milling, the liquid is distilled off, and the powder is dried. A solid lubricant, such as paraffin wax, is added to the powder mix in the final stage of the milling or subsequently in a blender. The lubricant protective coating prevents or greatly reduces the oxidation of the powder and imparts strength to the powder mix during its consolidation. In the spray drying process commonly used in the hardmetals industry, a hot inert gas such as nitrogen impinges on a stream of carbide particles. Thus, a product in the form of spheroidal powder aggregates is produced. The favorable mechanical properties of WC-Co-based cemented carbides are significantly deteriorated at elevated temperatures or in corrosive environment [84]. In order to overcome these shortcomings, the introduction of ordered fcc_L12g0 precipitates [85] into a Co-binder is a way to strengthen the binder phase and improve its high temperature properties or corrosion resistance, similar to that in cobalt-base superalloys [86]. Thus, it offers new possibilities for development of novel WC-Co-Ni-Al cemented carbide that possesses both high-temperature strength and environmental resistance. Long et al. [87] recently developed a type of WC-Co-Ni-Al cemented carbides with novel g + g0 Co-based binder. The abnormal WC grain is much larger than average, and this greatly worsens the mechanical properties of the alloys because these abnormal grains act as initiation points for cracking and breakage [88]. Thus, suppression of abnormal grain growth (AGG) becomes an important issue for the preparation of ultrafine or nanocrystalline WC-Co cemented carbide [89]. However, the mechanism for AGG is not fully understood. Park et al. [90] considers the AGG behavior to be related to two-dimensional (2-D) nucleation. Recently, Long et al. [91] considered the 2-D nucleation of WC grains in ultracoarse WC-Co-Ni-Al alloys in connection with solid-liquid interfacial energy and solubility of tungsten in the binder phase. In the work [92], the growth behavior of WC grains in WC-Co-Ni-Al alloys with different Co-Ni-Al binder phase compositions was studied. Powder mixtures of WC-18(Co-Ni-Al) samples were milled in ethyl alcohol in a ball mill at the ballto-powder ratio of 6:1 for 48 h. Powder mixtures of WC-8Co like WC-8Ni3Al were milled in ethyl alcohol in a ball mill at the ball-to-powder ratio of 15:1 for 96 h. These powder mixtures were then dried under vacuum at 358 K. These samples were uniaxially cold pressed into bars of 25 8 6.7 mm at a pressure of 150 MPa and then sintered at 1723 K under the 6 MPa pressure for 1 h and cooled with a furnace. Then, WC grains in the as-sintered samples were extracted by removing the binder matrix with saturation hydrochloric and Fe3Cl solution in supersonic cleaner. Fig. 23.31 shows the morphology of cemented carbide grains extracted from the WC-18(Co-Ni-Al) alloys sintered at 1723 K for 1 h. It can be seen that, as the Ni3Al content increases, WC grain size is significantly reduced, especially the size of the coarse WC grains.
FIG. 23.31 Morphology of cemented carbide grains extracted from the WC–18 (Co-Ni-Al) alloys obtained by various Ni3Al content: (A) without Ni3Al, (B) 5.3 wt% Ni3Al, (C) 11.3 wt% Ni3Al.
Production of Refractory Metal Powders Chapter
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FIG. 23.32 Microstructure of binder jet printed and sintered parts with 12 wt%Co (A) and 17 wt%Co (B).
It was concluded [92] that with the increase of Ni3Al content in Co-Ni-Al binder, the increasing interface energy slows the growth rate of WC; therefore, with the increase of Ni3Al content in Co-Ni-Al binder, the AGG of the alloy is appreciably inhibited. Additive manufacturing (AM) of polymer, metal, and ceramic materials has become more and more common. For hardmetals however, very little work has been reported so far. Yet, the implementation of additive manufacturing in the field of hardmetals would enhance the possibility of producing complex shaped parts, which cannot be produced by conventional means, and allowing the production of parts within hours instead of days because of the tool-free production technology. Binder jetting of cemented carbide green parts was first published by Kelley in 1998 [93]. Like select selective laser sintering (SLS), it is a powder-bed-based AM technique. In contrast to SLS, the particles are not selectively “sintered” but “glued” together by organic binder, which is applied layer-by-layer using a special print head. Thus, the printed parts are green parts, and their green density is mainly based on the powder density of the used starting material. During a postprinting debinding step, the organic materials used during printing must be removed, and the samples must be sintered. Using this technique, in the work [94], the samples with cobalt contents between 12 wt% and 20 wt% were produced by sintering temperatures between 1673 K and 1743 K. Densities achieved are above 99.8% of theoretical density and hardness as well as fracture toughness are comparable to standard hardmetal grades with the same composition. The 17wt%Co samples showed homogeneous microstructure, while samples with 12wt%Co showed some larger WC grains, which can be avoided by using a composition with grain growth inhibitors such as Cr3C2 or vanadium carbide (Fig. 23.32). A suspension-based method of thermoplastic 3D printing (T3DP) has been developed as an AM technology within the last 10 years. As reported [94], using T3DP showed promising results for cemented carbides. WC-10wt%Co samples were produced with nanosize WC powders of very fine cobalt powder. The hardmetal content in the organic suspension was 67 vol% which results in a quite high green density compared to conventional hardmetal fabrication. Samples were debindered under standard conditions with hydrogen and sinter-HIPed at 1623 K with 6 MPa argon HIP pressure. The results showed that by T3DP, total density of parts was 14.23 0.01 g/cm3 above full density, and the properties were comparable to conventional (via uniaxial pressing) produced samples. Uhlmann et al. [95] showed that cemented carbide with a content of 17 wt% Co with a density of 96% theoretical density and different geometries could be produced by additive manufacturing selective laser melting process.
Cemented Carbide Compaction A variety of techniques is used to compact hardmetal powders to the desired green density and shape before they can be sintered to full density. Powder consolidation methods include uniaxial pressing, cold isostatic pressing (CIP), extrusion, and injection molding. Processes such as uniaxial pressing and CIP are performed using powders directly after their creation, whereas extrusion and injection molding require extra processing by binders. Consolidation and shaping of cemented carbide powders are described in several publications [96–100].
Pressing Pressing involves compaction of powder under pressure by constraining it between die walls and rams to increase green density to a required level. In uniaxial pressing, powder in a die is pressed between top and bottom rams. Pressing is a onestep process, combining consolidation of the powder to required green density and achieving required shape. Compaction force ranges from 165 MPa to 331 MPa. Pressing is often used to produce near-net shape carbide parts, which requires
720
SECTION
D Production of Non-Ferrous Metal Powders
optimization of various processes and powder properties, like controlling the relationship between pressing pressure and shrinkage factor (green dimension/sintered dimension or green-sintered dimension/green dimension). Simple metal-cutting insert shapes are typically round or polygonal with straight walls, with or without a central hole. Rods are also produced in straight-walled dies with curved upper and lower punches by length pressing, with the length perpendicular to the pressing axis. These are used to make blanks for drills and end mills. Parts with clearance angles are also pressed in single-action tooling with an angled die wall. Multiplaten tooling is used to create shapes with significant steps in height or changes in cross-section. For geometries that contain undercuts, cross holes, and other geometries that prevent removing the part from the die, moving side die elements are used in side pressing.
Extrusion Rods, end mills, boring bars, and wires are formed by extrusion. Components having internal structures, such as internal holes for delivering coolant to the cutting edge, are also made using extrusion. The extrusion process is accomplished in an extrusion press, using either a piston-type or screw-type mechanism to push a powder-binder mix through a nozzle. Batch and continuous extrusion presses are used. The most common nozzle shape is round, but a variety of cross-sections can be extruded. Optimal powder-binder mix content and good mixing are required. Generally, powder properties, binder type and amount, as well as extrusion temperature, determine extrusion pressure and speed. Powder compaction is required to remove porosity and achieve sufficiently high green density so that full density is achieved on sintering. Poor mixing of powder and binder results in binder pools, which create imperfections and microstructural defects. Binder content and temperatures that are too high can cause a high rate of extrusion without sufficient powder compaction. Too low a binder content results in excessively high pressing pressures, which can cause surface cracks and damage to tooling and press.
Cold Isostatic Pressing Cold isostatic pressing (CIP), in which the powder is subjected to equal pressure from all directions, followed by machining, is also in general use for wear and metal forming tools. Cold isostatic pressing is commonly used for parts that are too large to be pressed in uniaxial presses and that do not require high precision in the sintered state. Dry-bag and wet-bag cold isostatic presses are used. In this process, an impervious moldable bag placed in pressure chamber is filled with powder and subjected to an isostatic pressure of 69 MPa to 207 MPa using a water-based liquid medium at ambient temperature. Shaped bags with cores are used to obtain near-net shapes. Powders with no or low wax are used in cold isostatic pressing. Dimensional tolerances for parts produced by CIP are relatively large, so size and shape control are not as critical as in uniaxial pressing. Parts are machined in the green, or presintered, state, and after sintering.
Injection Molding Injection molding of carbide powders is similar to injection molding metal powders and is used for high-volume production of relatively complex-shaped parts. Fig. 23.33 shows a schematic representation of the injection molding process. Carbide injection molding is achieved by mixing additional waxes, polymers, and surfactants with a dried powder to attain the
FIG. 23.33 Schematic of injection molding process.
Production of Refractory Metal Powders Chapter
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721
desired solids loading, injecting the thermoplastic feedstock into a die, removing from the die, and debinding in one or more stages prior to sintering. Large parts with wall thicknesses less than about 15 mm are produced by low-pressure systems, while smaller geometries with higher dimensional control and surface finish requirements are generally produced by highpressure systems. The differences in high- and low-pressure systems are usually the polymer versus wax content and solids loading. Due to the angular shape of carbide grains, the attainable solid loading in high-pressure systems is usually limited to 53%–58% by volume. Binder systems must minimize shrinkage on cooling in the die, and avoid carbon and oxygen pickup. Polymers must be selected for their clean burning during thermal debinding to avoid addition of impurities into the system. Surfactants are required to reduce molding pressure and to reduce tool wear. It is important that binder components are compatible with each other and bonded to powder to avoid separation. Mixing of additional lubricants into the powder to form a feedstock is performed in sigma and continuous mixers. Carbides are more sensitive to porosity than metallic systems; therefore, avoidance of air traps in the molding process is very important. The additional steps of mixing and debinding, as well a longer cycle time in molding, add to the cost as compared with traditional die pressing.
Sintering and Secondary Operations Hardmetals (cemented tungsten carbide, cemented carbide) are two-phase materials consisting of hard carbide phases and binder materials. The binder phase is primarily cobalt, but can also contain other binders including nickel, iron, vanadium, chromium, and molybdenum, or a combination of these. Successful application of these materials in metal cutting, oil drilling, mining, construction, metal forming, and shaping dies depends on achieving low levels of residual porosity with uniform, homogenous microstructure and desired chemical composition. Generally, there are two major sintering methods: pressureless sintering and pressure-assisted sintering. Pressureless sintering techniques include vacuum and partialpressure, hydrogen, and microwave sintering. Pressure-assisted techniques include overpressure sintering (sinter-HIP), sintering followed by postsinter HIP, hot pressing, and several rapid hot consolidation techniques. The sintering of carbide materials is comprehensively considered in several publications [98,101–103]. The first step in the sintering process is the removal of the lubricant from the powder compact. Then, by the pressureless sintering method, the compacts are usually set on graphite trays coated with graphite paint. Using a semicontinuous or batch type graphite furnace, the compacts are first heated to about 770 K in a hydrogen atmosphere or vacuum. Subsequent to lubricant removal, the compacts are sintered in a vacuum of 0.1 Pa (or 103 Torr) at a final sintered temperature ranging from 1623 K to 1873 K, depending on the content of the cobalt binder and the desired microstructure. This leads the cobalt to melt and binds the carbide particles together. Shrinkage of the compact ranges from 17% to 25% on a linear scale and provides a practically pore-free, fully dense material. The sinter-HIP process was developed in the early 1980s [104]. In this process, low-pressure hot isostatic pressing (up to about 7 MPa) is combined with vacuum sintering, and pressure is maintained at the sintering temperature while the metallic binder is still molten. This technique facilitates the production of pore-free products at costs only slightly higher than those of vacuum sintering green compacts. In the 1970s, the hardmetal industry took advantage of hot isostatic pressing (HIP), in which vacuum-sintered material is re-heated under a gas (argon or helium) pressure of 100–150 MPa. The temperatures of this process are 278–323 K below the sintering temperature. Sintering plus HIP is used to further reduce porosity and improve part durability. Acceptable levels, distribution, and sizes of porosity depend on the applications, and their measurement method is described in the article [105]. The high temperatures and pressures employed in the HIP autoclave improve the nearly perfect properties of hardmetal parts by removing any residual internal porosity and pits. A large number of hardmetal parts are machined after sintering because of surface finish, geometry and tolerance requirements. This operation is both time-consuming and expensive. The sintered parts can undergo many secondary operation including processing by means of metal-bonded diamond or silicon carbide wheels, turned with a single-point tool, or lapped with diamond-containing slurries, electrical discharge machining, and brazing to achieve desired condition, such as shape, specified size, edge state, and surface finish.
RECYCLING Hardmetal scrap containing tungsten carbide can be divided into two groups: l
Soft scrap, such as hardmetal grinding sludge, filter dusts, broken green parts, and floor sweepings
722
l
SECTION
D Production of Non-Ferrous Metal Powders
Hard scrap, which represents sintered parts consisting of either used parts or fragments.
The contamination of the scrap with impurities is another criterion for classification. While particulate scrap contains impurity levels that cannot be admitted for direct reuse, hard scrap, even if heavily contaminated, can be classified, thus making direct reuse easier. Hardmetal recycling processes can be subdivided into the following kinds: l l
l
l
l
Direct conversion of sorted hard scrap into graded powder ready for pressing and resintering. “Zinc process” is the most significant process used [106]. It includes the following main processing steps: sorted, cleaned, and crushed hardmetal scrap; reaction in molten zinc at 1173–1273 K in argon or hydrogen atmosphere; vacuum distillation at 1273–1323 K; obtained cake is consecutively crushed, and ball milled, and the resultant powder is screened; then, oversize product (>75 mm) is returned to melting, undersize product (<75 mm) is ball milled, and blend carbon adjusted, resulting in graded hardmetal powder. Also used on a commercial scale is the so-called “coldstream process,” a high-velocity process. The cleaned, sorted, and previously crushed hardmetal scrap is entrained in a gas stream (7 MPa) and projected against a stationary baffle plate. After the material has struck the target and shattered, it is removed from the impact chamber by suction. Material is then subjected to screening. The oversize product is returned into a feed vessel for subsequent impact against the baffle plate. Further classification of undersize material and mixing of different size fractions result in graded hardmetal powder ready for use. Removing the binder by means of leaching is another method of hardmetal recycling. The composition of the hardmetal scrap used for the leaching process determines the quality of the resulting carbide residue. The chemical conversion process can be used for all contaminated scraps, either soft or hard, to recover primary powders of the hardmetal components. Fig. 23.34 schematically shows the process from the contaminated and unsorted scrap via the conventional WC powder manufacturing route, which involves calcination, reduction, and carburization intermediate to tungsten carbide powder.
Due to the intrinsic value of the hardmetal components, and due to the increasingly more severe legislation concerning waste disposal and the preservation of natural resources, today, recycling has considerable significance, which will increase in the future.
FIG. 23.34 Flowsheet for the chemical conversion process. (Source: Lux B, Zeiler B. Production of tungsten and tungsten carbide powders. In: ASM handbook, vol. 7. ASM International Publishers; 1998 p. 188–201.)
Production of Refractory Metal Powders Chapter
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MOLYBDENUM METAL POWDERS Molybdenum powders find application in different fields of technique. The major use for molybdenum is as an alloying element for alloy and tool steels, stainless, and nickel- or cobalt-based superalloys to increase hot strength, toughness, and corrosion resistance. Molybdenum ingots, produced by melting PM electrodes, are extruded, rolled into sheets or rods, and subsequently worked to other mill product shapes, such as wire and tubing. Molybdenum has outstanding electrical and heat conducting capabilities. The latter is approximately 50% higher than that of steel, iron, and nickel alloys. It finds wide usage as a heat sink. The coefficient of thermal expansion of molybdenum plots almost linearly with temperature over a wide range. This characteristic, in combination with its heat conducting capabilities, accounts for its use in bimetal thermocouples. Methods of doping molybdenum powder with potassium aluminosilicate to obtain a non-sag microstructure comparable to that of tungsten have also been developed [107]. In the electrical and electronic industries, molybdenum is used in cathodes and cathode supports, for radar devices, as current leads for thoria cathodes, and cores for winding tungsten filaments. Molybdenum plays a significant role in the rocket industry, where it is used for high-temperature structural parts. Molybdenum alloys satisfy the requirements for use in airframes because of their retention of mechanical properties after thermal cycling due to high recrystallization temperatures and good creep strength. Molybdenum has been also effective in the nuclear, chemical, glass, and metallizing industries.
Production of Molybdenum Powders Molybdenum powder is produced by a technique similar to that used for tungsten powder. Molybdenum disulfide (MoS2) is the major source of raw material. Molybdenum disulfide is concentrated by flotation and transformed to an impure technical molybdenum trioxide (MoO3) by means of roasting, which oxidizes the sulfur and removes it as gaseous sulfur dioxide [108]. Subsequent purification takes advantage of the sublimation characteristics of molybdenum trioxide. Above 820 K, it sublimes easily and can be distilled from its impurities and condensed again as pure molybdenum trioxide. Solvent extraction processes are also acceptable. Additional purification can also be achieved by dissolving molybdenum trioxide in ammonia to form ammonium molybdate, (NH4)2MoO4. The pure molybdenum trioxide or ammonium molybdate is reduced in tube push type or rotary furnaces similar to those used for tungsten reduction. Typical purification reduction reactions can be written: MoS2 + 7=2O2 ¼ MoO3 + 2SO2 endothermic ðreactionÞ MoO3 + 3H2 ¼ Mo + 3H2 O MoO3 + 2NH4 OH ¼ ðNH4 Þ2 MoO4 + H2 O ðNH4 Þ2 MoO4 + 3H2 ¼ Mo + 2NH4 OH + 2H2 O Reduction of molybdenum trioxide by dry hydrogen involves two steps: MoO3 ! MoO2 ! Mo [11]. The bond strength metal-oxygen in MoO3 is less than in WO3, and in MoO2, is larger than in WO2. The magnitudes of the equilibrium constants for the two steps of Mo reduction (Keq ¼ pH2O/pH2) are given in Table 23.10. The first step is fulfilled at temperatures ranging from 670 K to 920 K to realize slow increase of temperature in order to accomplish the formation of MoO2 at 770–820 K and not to form molten material at 820–870 K (low-melting eutectic Mo4O11-MoO2). The second reduction step is carried out at 920–1220 K. The reduction parameters like temperature, reduction time, and powder layer thickness (oxide load of the boats in push type furnace) are chosen so that the residual oxygen content does not exceed 1000 ppm after the second reduction step.
TABLE 23.10 Magnitudes of the Equilibrium Constants for the Two Steps of Molybdenum Reduction Temperature (K)
670
700
870
915
1070
1200
Keq, 1st step (n-d)
5.0 107
…
1.7 106
…
1.58 105
…
Keq, 2nd step (n-d)
…
0.076
…
0.234
0.389
0.55
n-d, Non-dimensional.
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SECTION
D Production of Non-Ferrous Metal Powders
TABLE 23.11 Properties of Typical Molybdenum Metal Powders Produced by Hydrogen Reduction Sources [3]
SIC Standards
Property
Unit
1–6 mm
TU48-19-316
TU48-19-313a
Particle sizes
mm
1–6b
<5
40–150
wt%
99.9c
99.5
99.9
Aluminum
ppm
5–25
50
…
Calcium
ppm
3–15
70
70
Carbon
ppm
10–50
…
…
Chromium
ppm
5–25
…
…
Cobalt
ppm
…
…
…
Copper
ppm
5–10
…
…
Iron
ppm
10–100
140
…
Lead
ppm
5–10
…
…
Magnesium
ppm
1–10
30
30
Nickel
ppm
5–50
50
200
Oxygen + water
ppm
500–1000
3000
…
Potassium
ppm
…
500
…
Silicon
ppm
…
50
…
Sodium
ppm
…
150
150
Tin
ppm
15–50
…
…
Tungsten
ppm
100–300
400
…
NVM (summation of Ca, Na, K, S)
ppm
500–1000
…
…
Nominal composition Molybdenum Trace elements:
a
Coarse powder for plasma spraying, CIS: Commonwealth of Independent States’ standards. Particle size FSSS. c Metallic molybdenum content, exclusive of gases. b
Properties of typical molybdenum metal powders produced by reduction of molybdenum trioxide by hydrogen are shown in Table 23.11. To produce high-purity molybdenum powder, an aqueous solution of ammonium molybdate is mixed with a solution of iron chloride (III); formed precipitate of iron molybdate is separated from the solution containing some of the impurities. A solution is acidified to make molybdic acid (H2MoO4); that is separated and then dissolved in an aqueous solution of ammonia (NH4OH). At the second stage of purification, the obtained solution of refined ammonium molybdate is mixed with a solution of iron chloride (III), and the cycle is repeated; formed precipitate of iron molybdate is separated, washed, and, after acidifying the molybdic acid, is dissolved in an aqueous solution of ammonia. The solution is separated by filtration, and finally, pure ammonium molybdate is cooled. The crystals of pure ammonium molybdate are separated and reduced in a hydrogen atmosphere at 1170–1220 K. Reduction of low boiling point molybdenum chlorides and fluorides also allows production of high-purity molybdenum powder. The melting point of molybdenum pentachloride (MoCI5) is 467 K, and its boiling point is 541 K. A fluidized bed unit for reduction is described in Chapter 4, where molybdenum pentachloride is being supplied at temperatures ranging from 470 K to 570 K (MoCI3 is decomposed at higher temperature). Optimal conditions providing metal precipitation of
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88% and reduction degree of 98% are as follows: temperature range from 870 K to 1070 K, MoCI5 concentration in gas vapor phase of 4–5 vol%, and a molar ratio of hydrogen to molybdenum pentachloride NLE 30 to 1. Obtained fine powder contains: 40–60 wt% fraction minus 0.6 mm, 20–23 wt% fraction 0.6–1.2 mm, 10–20 wt% fraction 1.6–1.8 mm, and 1–3 wt% fraction 1.8–2.4 mm. If a rotating reaction chamber is used, where ammonia paramolybdate is reduced by hydrogen, porous conglomerates of dispersed powders are obtained which are narrow in size and shape. Powders are characterized by high bulk mass and good flow, and their dispersivity depends on temperature condition. Highly dispersed (3–5 mm) powders of rhenium and molybdenum alloy (1:1) are produced by treatment of ReOMoO(OCH3)7 in two steps: calcinations in oxygen (523–573 K, 1–2 h) and reduction by hydrogen (1073–1173 K, 2–3 h). The carbonyl process for molybdenum powder production is based on the decomposition of carbonyl molybdenum (Mo (CO)6). The powder particle sizes are around 1 mm, and the surface of the particles is rather smooth. Having increased the temperature of carbonyl decomposition, there have been changes in powder color, carbon content, apparent density, and lattice spacing (Table 23.12). Carbonyl molybdenum powders enriched by carbon represent the metastable carbides with face-centered lattice. The increasing decomposition temperature leads to the extension of lattice spacing. Impurities of carbon and oxygen are typical for molybdenum powders, and the content of sulfur, phosphorus, and arsenic does not exceed 1 ppm. By execution of the carbonyl process in a hydrogen and nitrogen current, a peak of carbon (amorphous) content is observed at 970 K as a result of catalytic decomposition of its dioxide. Addition of oxidizing components into a gas-carrier leads to decreasing carbon content. For instance, the powder obtained in nitrogen with 2.2% of oxygen at 670 K contains 0.27 wt% carbon. To remove impurities, it is effective to heat treat powders at 1170 K and above. The carbon amount goes down by a digit factor, while carbonyl decomposition in ultrahigh frequency discharges.
Properties of Molybdenum and Molybdenum Alloy Powders The high boiling and melting points of molybdenum (4912 K and 2890 K, respectively) are second only to those of tungsten, rhenium, and osmium. Molybdenum is not used as widely as tungsten because it oxidizes more readily and erodes faster on arcing than tungsten. Nevertheless, due to the density of molybdenum (10.2 g/cm3), which is about half that of tungsten (19.3 g/cm3), use of the former is advantageous where mass limitation is important. The cost of molybdenum is also lower. In addition to its use in make-(before-) break contacts, molybdenum is widely used for mercury switches because it is not attacked, but only wetted, by mercury. Like tungsten, molybdenum strips and sheets are made by swaging or rolling sintered powder compacts. Table 23.13 shows the physical properties of pure molybdenum and tungsten metals. Temperature effects on the physical and mechanical properties of molybdenum and tungsten rods are illustrated in Fig. 23.35. The mechanical properties of molybdenum and molybdenum alloys greatly depend on the amount of working treatment below their recrystallization temperature and on the ductile-to-brittle transition temperature. The minimum recrystallization temperature is 1173 K. The metal maintains superior strength and hardness at elevated temperatures. However, when hot strength is required, a molybdenum alloy (rather than elemental material) is the material of choice. Titanium and zirconium additions to molybdenum create alloys with hot strength and recrystallization temperatures above those of unalloyed molybdenum. TABLE 23.12 Dependence of Carbonyl Molybdenum Powder Properties on Decomposition Temperature Decomposition Temperature in Current of Argon (K)
Color of Powder
Apparent Density (g/cm3)
Carbon Content (wt%)
Lattice Spacing (nm)
670
Gray
0.5
2.36
…
770
Gray
…
3.28
0.42019
870
Dark-gray
…
3.38
0.42030
970
Black
…
3.53
0.42100
1070
Black
2.50
3.48
0.42220
2970–3970
Black
…
0.10–0.20
…
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TABLE 23.13 Typical Properties of Molybdenum and Tungsten Property Density
Unit
Molybdenum
Tungsten
g/cm
10.22
19.3
GPa
1500
70
bcc
bcc
3
Crystal structure Hardness HB Modulus of elasticity At 293 K
GPa
325
405
At 1273 K
GPa
220
325
Melting point
K
2890
3410
Boiling point
K
4912
5660
J/kg K
251
134
Thermal conductivity at 293 K
W/mK
138
154
Coefficient of linear thermal expansion at 293 K
mm/m K
5.53
4.43
Electrical resistivity at 293 K
nO m
53.4
55
Electrical conductivity at 293 K
%IACS
33
31
Specific heat At 293 K
Some of the physical properties of molybdenum and tungsten vary considerably with cross-sectional and grain structure.
Table 23.14 gives basic mechanical property data for 0.38 mm wire fabricated from pure molybdenum and several of the advanced molybdenum alloys. The TZM molybdenum alloy is a high-strength, high-temperature material. This material is manufactured either by the PM or arc-melting processes. The composition of TZM consists of: (0.01–0.04) wt% carbon, (0.40–0.55) wt% titanium, (0.06–0.12) wt% zirconium, <0.0005 wt% hydrogen, <0.010 wt% iron, <0.002 wt% nickel, <0.002 wt% nitrogen, <0.0025 wt% oxygen, <0.008 wt% silicon, and the balance of molybdenum. At 1273 K, its strength is about 720 MPa, which is approximately twice that of unalloyed molybdenum. The alloy is excellent for structural applications under conditions where unalloyed molybdenum is normally used [109]. In comparison with unalloyed molybdenum, TZM has a higher recrystallization temperature and higher strength and hardness at room and elevated temperatures. Its advanced mechanical properties are due to the dispersion of composite carbides in the molybdenum matrix. Basic application fields of TZM include: l l l l l
Die inserts for casting metals Die bodies and punches for hot stamping Tools for metalworking Heat shields and linings for furnaces, structural parts and heating elements Rocket nozzles.
To improve the elevated temperature strength of PM TZM alloys, titanium and zirconium carbides have been replaced by hafnium carbide. An alloy containing 1% Hf and 0.06% C has a tensile strength of 580 MPa at 1588 K, in comparison with 480 MPa for TZM. Creep rate at 1478 K at a stress of 330 MPa is 0.038%/h, compared to 0.05%/h for TZM [110]. Table 23.15 presents the compositions and properties of various molybdenum-silver composites for electrical makebreak contacts [109]. To improve the mechanical properties, including ductility and creep resistance of structural molybdenum-base materials, doped molybdenum alloys were developed addition a small amount of aluminum, potassium, silicon, or rare-earth oxides [111]. Recently, high-toughness molybdenum alloys have been developed with fine grains and finely dispersed titanium carbide particles by mechanical alloying (MA) and hot isostatic pressing (HIP) processes, followed by hot forging and hot and warm rolling. The high toughness of the alloy may be due mainly to grain-boundary strengthening by fine carbide particles having semi-coherency with the adjacent matrix, and the particles have the beneficial effects of increasing
Production of Refractory Metal Powders Chapter
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FIG. 23.35 Physical and mechanical properties of molybdenum and tungsten rods depending on the temperature. (Adopted from Shen Y-S, Lattari P, Gardner J, Wiegard H. Electrical contacts. In: ASM Handbook, vol.2. Novetly (OH, USA): ASM International Publishers; 1990. p. 840–68.)
the recrystallization temperature and hindering grain growth. However, in both cases, a large amount of plastic working was required. Such working may limit the products available to thin sheets or thin wires shapes, which are inapplicable as structural materials. Data regarding the mechanical properties for carbide-dispersed molybdenum alloys are given in Table 23.16. Because the properties of materials with the same composition depend on manufacturing methods, the manufacturing methods are also referenced in these tables. These common methods of producing composite electrical contact materials are described in subparagraph “tungsten powder applications” (see above). Molybdenum alloys with 0.8 mol% zirconium carbide (ZrC) or tantalum carbide (TaC) as dispersed particles were fabricated by mechanical alloying and HIP or spark plasma sintering (SPS) [112]. Powders of pure molybdenum (average
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TABLE 23.14 Tensile Properties of 0.38 mm Molybdenum Wire at Room and Elevated Temperatures Material Sign
Composition
Temperature (K)
Ultimate Tensile Strength (MPa)
Elongation (%)
Unalloyed molybdenum
Mo
293
1350
4.1
1273
305
2.4
1373
140
10.3
1473
115
12.5
293
1565
3.1
1273
1020
2.7
1373
795
3.2
1473
675
2.8
293
1980
3.6
1273
1095
2.3
1373
950
2.3
1473
745
2.2
293
1795
2.9
1273
1270
3.4
1373
1185
3.3
1473
1035
3.0
293
1935
3.2
1273
1350
3.3
1373
1250
3.1
1473
1075
4.6
293
2135
3.6
1273
1460
3.3
1373
1295
2.6
1473
1170
2.4
MT-104
Mo + 45W
HCM
HWM-25
HWM-45
Mo-0.5Ti-0.08Zr0.01C
Mo-45W
Mo-1.1Hf-0.07C
Mo-25W-1.0Hf0.035C
Mo-45W-0.9Hf-0.03C
Source: Adopted from Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.
particle size 4.1 mm), ZrC (2 mm) and TaC (1.1 mm) were mixed to obtain the target composition in an argon atmosphere. These blended powders were subjected to mechanical alloying for 108 ks in a planetary type ball mill with lining and balls made of a cemented carbide (WC-Co). HIP and SPS were conducted at 1573 K and 147 MPa for 10.8 ks in argon, and at 2073 K and 74 MPa for 0.3 ks in a vacuum of 10 Pa, respectively. These alloys were annealed in a vacuum of 2.7 104 Pa at 1873–2473 K for 3.6 ks. Yield strength, ultimate tensile strength and elongation at 300 K and high temperature for Mo-0.8 mol% ZrC, Mo-0.8 mol% TaC, and pure Mo conducted with tensile specimens in form of bar 4 mm wide, 25 mm long, and 1 mm thick are shown in Table 23.16. The superplastic behavior was investigated on a gauge section 3 mm wide, 8 mm long, and 2 mm thick. In both MA with HIP and MA with SPS specimens of Mo-08 mol% ZrC as sintered, the tensile strengths are 1.8 times higher than that of pure molybdenum; however, no plastic behavior is found. While both MA with HIP and MA with SPS specimens of annealed Mo-08 mol%ZrC are superior in terms of room temperature strength, their yield and tensile strengths are about 3 and 2.1 times higher, respectively, than that of pure molybdenum.
10.35 10.35
PSR
PSR
INF
PSR
80Ag-20Mo
75Ag-250Mo
50Ag-50Mo
INF
INF
INF
INF
25 Ag-75Mo
20 Ag-80Mo
15 Ag-85Mo
10 Ag-90Mo
10.23
10.24
10.26
10.27
10.29
10.13
10.18
10.23–10.26
10.27
10.00–10.31
10.12
10.10–10.32
10.10–10.32
10.14
10.10–10.24
10.33
10.36
10.38
Typical
27–30
28–31
28–32
31–34
35–45
45
42–49
44–58
50
45–52
58–61
59–62
65–68
Electrical Conductivity (% IACS)
97–102
97–102
96–98
93–97
85–95
50–68
80–90 HRB
75–82 HRB
65
70–80 HRB
44–47 HRB
38–42 HRB
35–40 HRB
Hardness
… …
758 552
… … 676
… …
… …
… … …
965 …
…
…
…
938
407
414
931
…
…
414
Modulus of Rupture (MPa)
Tensile Strength (MPa)
G
G
C, A
C, A
C, A
C, A
C, A
A
C, A
C, A
A
A
A
Data Sourceb
Semiconducting material
Arcing contacts, heavy-duty electrical applications
Air-circuit breakers. Lowerosion arcing tips
Aircraft switches, breaker arcing tips, electric raisers, air- and oil-circuit breakers
Air- and oilcircuit breakers, arcing tips, traffic signal relays, home circuit breakers
Air conditioner control Light- and medium-duty applications, automotive circuit breakers switches, circuit breakers
Application Examples
a INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy, Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. G: Gibson Electric Inc., Delmont, PA.
INF
10.32
PSR
30Ag-70Mo
10.32
INF
40Ag-60Mo
13.33
INF
45Ag-55Mo
10.42
10.44
10.47
PSR
90Ag-10Mo
Calculated
Manufacturing Methoda
Nominal Composition (%)
Density (g/cm3)
TABLE 23.15 Properties of Typical Molybdenum-Silver Composites for Electrical Make-Break Contacts
Production of Refractory Metal Powders Chapter 23 729
0.110
0.001
0.69Zr
1.2Ta
…
Mo-0.8ZrC
M0–0.8TaC
Pure Mo
0.001
0.019
0.032
0.022
N
0.001
0.137
0.138
0.140
O
MH
MS
MS
MH
Manufacturing Method
100
99.9
99.9
99.9
Relative Density (%)
MH: Mechanical alloying and hot isostatic pressing, MS: Mechanical alloying and spark plasma sintering.
0.117
0.100
0.70Zr
Mo-0.8ZrC
C
Metal
Alloy
Analyzed Composition (wt%)
53.8
11.2
Annealed Recrystallized
10.9
2.8
Annealed As sintered
2.7
1.8
Annealed As sintered
0.3
As sintered
Condition
Grain Size (mm)
344
547
559
469
639
707
1077
838
… 1038
1053
827
… 1015
Ultimate Tensile Strength (MPa) Yield Strength (MPa)
54
38
12
9.0
0.0
0.6
0.0
Elongation (%)
SECTION
TABLE 23.16 Mechanical Properties at 300 K for Carbide Dispersed Molybdenum Alloys
730 D Production of Non-Ferrous Metal Powders
Production of Refractory Metal Powders Chapter
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731
FIG. 23.36 Dependence of ultimate tensile strength and elongation of the carbide dispersed molybdenum alloys on temperature: (MH) corresponds to HIP with MA powders, (MS) corresponds to SPS with MA powders. (Source: Takida T, Kurishita H, Mabuchi M, Igarashi T, Doi Y, Nagae T. Mechanical properties of fine-grained sintered molybdenum alloy processed by mechanical alloying. In: Kneringer G. editor. Proceedings of 15th international Plansee seminar, vol. 1. Reutte (Tirol, Austria): Plansee Holding AG; 2001. p. 293–304.)
The carbide dispersed molybdenum alloys exhibit excellent strength up to 1373 K. Their tensile strengths are 3–5 times higher below 1473 K (about 0.5 melting point), and 1.5–2 times higher at 1773 K, than those of pure molybdenum and Mo-0.8 mol%TaC (Fig. 23.36). The elongation of Mo-08 mol%ZrC with grain size of 3.0 mm obtained by means of SPS with MA increased rapidly in the temperature range over 1773 K, and a superior limit elongation of 180% is attained at 1973 K. Variations in the nature of the oxide and consolidation method have been shown to have a significant effect on the mechanical properties of oxide dispersion strengthened (ODS) molybdenum material [113]. Comparison of the effect of doping technique species on tensile properties of ODS molybdenum-lanthana and ODS molybdenum-yttria alloys illustrates that, for both oxide species, the wet doping technique results in a much finer dispersion of oxide particles ranged from 0.08 to 0.12 mm. The wet doping process uses an aqueous tantalum nitrate solution to dope the molybdenum dioxide (MoO2) precursor before its reduction to molybdenum metal powder [114]. At ambient temperature, the alloy’s strength is not strongly sensitive to either doping technique, but, at high temperatures, the wet doping technique produces superior material for a given oxide. Lanthana-doped material is also superior to yttria-doped material at elevated temperature. The ODS molybdenum material contains nominally 2 vol% La2O3 that corresponds to a lanthanum (La) content of 1.09 wt%. The addition of a small amount of group VIII transition metals, such as nickel and palladium, can lower the activation energy of sintering and decrease the sintering temperature from >1970 to <1570 K [115]. However, such activationsintered compacts have no ductility, and thus, have restricted application in industry [116]. Through activated liquid phase sintering, the addition of 1.5 wt% Ni to molybdenum was shown to improve the sintered density significantly to 97% at 1573 K, while the pure molybdenum only reached 82% theoretical density. But, this impairs ductility. This embrittlement appears to be caused by a grain boundary layer of a Ni-Mo compound about 2 nm thick formed during sintering [117]. Due to the brittle nature of this compound, the Ni-containing molybdenum fractures through this thin compound layer. There are several grades of ODS Mo alloys available for commercial use. The most popular doping levels of lanthana (La2O3) to molybdenum are: 0.3% by weight, 0.6% by weight and 1.1% by weight. In work [118], the thin sheets, that is,
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FIG. 23.37 SEM image of molybdenum powder with 1 at.% Na after ball milling for 75 h. (Source: Bergk B, Muhle € U, Kieback B, Koutna´ N, Holec H, Clemens H. Nanocrystalline alloys of molybdenum with sodium and yttrium obtained by mechanical alloying. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2017 euro congress. Milan (Italy); 2017: p. 1–6.)
0.5 mm and 0.2 mm thick, of these alloys were studied. The ingots were made by cold-isostatic pressing of the Mo-lanthana (MoLa) powders with subsequent sintering. The ingots were subsequently thermo-mechanically processed, first into plates during hot rolling, then into sheets. It was concluded [118] that the annealed 0.6 wt% and 0.3 wt% lanthana MoLa sheets exhibited deep drawability with a 900–1020 MPa UTS range, 800–920 MPa YS range, and about 25%–27% elongation, and can be considered the best material for the majority of furnace components applications, while the thin MoLa sheets, that is, 0.2 mm thick, exhibited ideal isotropy of bendability under annealed condition with no regard to lanthana content. There was also reported that hardness is not a reliable differentiating factor among lanthana-content MoLa sheets. Molybdenum alloys with low contents of immiscible sodium and yttrium elements made by mechanical alloying were studied [119]. The alloys were produced by high-energy ball milling utilizing a Retsch SM 200 mill placed inside an argon glove box to guarantee high purity conditions and to avoid uptake of oxygen and nitrogen. Fig. 23.37 shows a SEM image of an alloy with 1.0 at.% Na after milling for 75 h. The particle sizes distribute in 1–20 mm range. As reported, the nominal concentration of 1 at.% Na or Y the solute content remains constant up to temperatures of 1173 K. For higher temperatures, the solute content starts decreasing until at 1473 K almost no sodium like yttrium is left. The alloy’s structure is characterized by the following: the grains have been grown to about 150 nm and are reduced to about one tenth of the value for the as milled samples; it is revealed that sodium (as well as yttrium) adapts itself to the molybdenum lattice. Because the conventional sintering of molybdenum requires high temperatures and long holding times, which leads to large grain growth, much research has been focused on the enhancement of the sinterability of Mo using spark plasma sintering (SPS) technique [120–122]. In the work [123], molybdenum nanopowder was sintered by SPS technique under various temperatures, pressures, heating rates, and holding times in order to decrease the grain size. Nanosized molybdenum powder from US Research Nanomaterials, Inc., (TX, United States) with a purity of 99.9% (metal basis) has been used in this study. The powders correspond to agglomerates of roundish particles with a size range of about 50–200 nm (Fig. 23.38). The SPS device was set to apply the periodic sequence of 12 ms of DC pulses followed by a pause of 2 ms. For 60 MPa or 120 MPa SPS pressure levels, measured initial green (after pre-cold compaction) densities of the Mo powder compacts were 58% and 67%, respectively. Sintering conditions of the conducted SPS tests included heating rates of 100 K/min or 200 K/min from 873 K to the maximum temperature (1473, 1573, 1873, or 2073 K) with dwell times of 0 or 10 min. The grain sizes of Mo pellets showed large dependence on sintering temperature. Fig. 23.39 shows the grain size of SPSed Mo pellets sintered under different temperatures with the same conditions of 60 MPa, 100 K/min, and 0 min holding time. As shown in Fig. 23.39A, 1573 K conditions are related to a small grain size of 2 mm, while grain size increased to 37 mm or 54 mm when the sintering temperature was over 1873 K (Fig. 23.39B and C). At sintering temperatures between 1573 K and 1873 K, the Mo oxide phase melted and moved to the grain boundary regions. In this study [123], the highest achieved microhardness was 435 kgf/mm2 (4.266 GPa). Relative density of 95.77% was achieved with 1473 K, 60 MPa, 100 K/min, and 10 min holding time. Optimal conditions for Mo pellet processing that resulted in small grain size, high density, and high hardness are determined to be 1473 K, 60 MPa, 100 K/min, and 10 min holding time under vacuum.
Production of Refractory Metal Powders Chapter
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FIG. 23.38 SEM images of molybdenum nanopowders at two magnifications. (Source: Lee G, Olevsky E, McKittrick J. Spark plasma sintering of molybdenum nanopowders. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
FIG. 23.39 SEM phase images of spark plasma sintering (SPS) pellets consolidated at different temperatures: (A) 1573 K, (B) 1873 K, and 1973 K; white regions indicate Mo, black regions indicate Mo oxide. (Source: Lee G, Olevsky E, McKittrick J. Spark plasma sintering of molybdenum nanopowders. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
The elaboration possibilities of the refractory Mo3Al intermetallic compound by self-propagating high-temperature synthesis (SHS) starting from a near stoichiometric mixture of powder components were considered [124]. High-purity fine powders of molybdenum and aluminum (MO006021 and AL006031 grades) had been used as staring materials. Appropriate amounts of them, corresponding to 76:24 (Mo:Al) atomic ratio (a near stoichiometric), had been preliminarily homogenized in a Turbulla blender for 30 min. To facilitate SHS reaction, ahead of time, researchers carried out an intimate homogenization and an energetic activation of the component powder mixture by a controlled mechanical alloying-mechanical disordering (MA + MD), by milling for 3 h in a planetary ball mill under argon atmosphere. An SEM image of Mo-Al powder mixture subjected by 3 h (MA + MD) processing is shown in Fig. 23.40. As reported [124], by subjecting compacts realized from the above powder mixture by SHS in thermo-explosion mode under pressure, cylindrical compacts of a microstructure similar to superalloys, consisting of a multitude of submicron Mo3Al crystallites embedded in a Mo-Al alloy matrix, density of 73.26%, and higher Vickers hardness than of Mo (1742 MPa, vs. 1400 MPa) have been obtained.
Workplace Atmospheres Safety Under the Workplace exposure limits in the European PM industry the long- and short-term (15-min reference period) workplace exposure limits (WELs) for soluble molybdenum compounds (as Mo) are 5 mg/m3 and 10 mg/m3, accordingly, and the limits for insoluble compounds (as Mo) in the workplace atmosphere are 10 mg/m3 and 20 mg/m3, respectively [57]. According to US New Jersey Department of Health and Senior Services. Hazardous Substance Sheet, the legal OSHA airborne permissible exposure limit for molybdenum is 15 mg/m3.
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FIG. 23.40 SEM image of Mo-Al powder mixture subjected by 3 h mechanical alloying- mechanical disordering (MA + MD) processing. (Source: Orban RL, Salomie D, Orban M, Neamt¸u B, Sechel N, Lung T. SHS synthesis from mechanical activated powdered components mixture of molybdenum aluminade Mo3Al for use at extremely high temperatures. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
The aerosols of metallic molybdenum, either soluble or insoluble molybdenum compounds and molybdenum silicide, have a predominantly fibrous effect and are related to the third class of danger according to State Standard 12.1.005-88 under the legislation in the CIS [56]. The stated average limit value (ALV) of the metallic molybdenum aerosol in the workplace atmosphere as the shift time-weighted average concentration is 0.5 mg/m3, while the threshold limit value (TLV) is 3 mg/m3. The ALVs of the insoluble and soluble molybdenum compounds and molybdenum silicide in dust form are 1.0, 4.0 and 4.0 mg/m3, respectively, while the ALV of soluble molybdenum compounds in the form of condensed aerosols is 2.0 mg/m3. Molybdenum content in potable water has a toxicological effect and is related to the second class of danger, and the TLV is 0.25 mg/L according to CIS State Standard 4630-88 [58]. Molybdenum powder, even with particle sizes at a fraction 74 mm, is not explosive in the air; the minimum ignition temperature of powder deposits (self-ignition temperature) is 583 K. The products with such characteristics of inflammability and explosion risk are related to the “low explosion hazard” class of danger according to the Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used to prevent pollution and environmental damage during powder manufacture and processing. Detailed information on health and environment protection measures in such conditions can be found in Chapter 27.
NIOBIUM AND TANTALUM Niobium and tantalum are often found in close combination in their ores. The most important niobium-tantalum-bearing minerals are columbite and tantalite, which form isomorphous series FeNb2O6-MnNb2O6 and FeTa2O6-MnTa2O6, and the ores are variations of the compound (Fe, Mn)(Nb, Ta)2O6. Tantalum and niobium are separated from ore and from one another by digestion in aqueous hydrofluoric acid, followed by solvent extraction with methyl isobutyl ketone (MIBK), as illustrated in Fig. 23.41 [125]. In the first contact, only the niobium and tantalum fluorides are soluble in the MIBK. Thus, they are separated from the contaminants iron, manganese, titanium, and zirconium, which remain in aqueous solution. The fact that the solubility of tantalum in MIBK is high over an extensive range of acidity, while niobium is soluble only at high acidity, is used for separation of tantalum fluoride and niobium fluoride. This is accomplished in a series of subsequent steps, where MIBK contacts aqueous solutions of varying acidity. Commercially, deposits of niobium are found also in the pyrochlore [(Na, Ca)2(Nb, Ta, Ti)2O4(OH, F)H2O], which is mined in Canada and Brazil. These ores are mostly tantalum free and are enriched to concentrates containing 55%–60% niobium pentoxide by means of a series of operations including grinding, flotation, and leaching. The extraction of niobium by high-temperature chlorination of the ore is also an appropriate technique for niobium powder production [126].
Niobium and Niobium Alloys Niobium is used in commercial applications as an alloying element in steels. Nearly 75% of all niobium metal is used as a minor alloying addition in low-alloy steel. About 20%–25% is used as an alloy addition in nickel-base superalloys and heat-
Production of Refractory Metal Powders Chapter
23
735
Columbite/tantalite ore
Hydrogen fluoride
Ball mill
Hydrogen fluoride digester
Waste to neutralizer
Liquid-liquid separator
Niobium fluoride solution
Tantalum fluoride solution Ammonia
Potassium fluoride Potassium fluorotantalate crystalizer
Precipitator
Centrifuge-filter
Filter press
Dryer
Dryer
Sodium reduction
Niobium pentoxide calciner
Acid washer
Thermite reactor
Sodium, sodium chloride
Slag
Aluminium, accelerator Vacuum dryer
Ingot conditioning
Bar press Alloy additive Electron beam melter
Hydrider
Crush and degassing
Sodium-reduced tantalum capacitor powder
Electron beam tantalum or tantalum alloy powder
Niobium or niobium alloy powder
FIG. 23.41 Niobium and tantalum production flowchart. (Source: Kirk RE, Othmer DF, editors. Encyclopedia of chemical technology, vol. 22, 3rd ed. John Wiley & Sons; 1983.)
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resisting steel. Only 1%–2% of all niobium used is in the form of niobium base alloys and pure niobium metal including super-conducting niobium-titanium alloy, which accounts for over one half of all niobium alloys produced [127].
Production of Niobium Powders Metallothermic Reduction: Initially, niobium metal was produced by PM methods, which involved high-temperature vacuum sintering and carbon reduction. However, in the early 1960s, aluminothermic reduction and electron beam purification came into general use. At present, the basic process for the recovery of niobium, along with recovery from columbite and tantalite ores shown in Fig. 23.41, is the aluminothermic reduction of pyrochlore concentrates to ferroniobium. Niobium metal is purified by a chlorination process, where volatile niobium pentachlorate (NbCl5) is distilled and then hydrolyzed to the oxide. The metal is next recovered by a second aluminothermic reduction 3Nb2 O5 + 10Al ! 6Nb + 5Al2 O3 : During the exothermic reaction, oxide impurities are slagged and removed from the molten niobium. Calcium and magnesium have also high reactivity for the reduction of niobium oxide to niobium. However, magnesium has an advantage over calcium in that it has a high vapor pressure (0.078 bar), even at a moderate boiling temperature of 1363 K. Therefore, niobium metal powders can be also prepared by using a magnesiothermic reduction process [128–131]. Okabe reported a preforms reduced process (PRP) for niobium metal using a magnesiothermic reduction process [129]. Using a cyclone separator assembly, niobium is prepared by the reaction between the magnesium vapor and niobium oxide [128]. However, preparing niobium metal using the cyclone reduction techniques resulted in high concentration of oxygen (7.68%) [128]. In the work [132], an attempt was made to prepare the niobium metal powder with lower oxygen content using the magnesium vapor reduction process. The influence of various factors in the magnesium vapor reduction of niobium oxide and deoxidation of niobium metal powder in a two-stage reduction process is discussed. Prepared by magnesium vapor reduction at 1123 K for 5 h, niobium powder contained about 8 mass% oxygen. After second stage reduction, the oxygen concentration decreased to 0.65 mass%. An SEM micrograph of the niobium powder after second stage reduction is shown in Fig. 23.42. As it appears, fine particles of niobium powder less 1.0 mm form agglomerates from 5 mm to 10 mm in size. Carbothermic Reduction: Carbothermic reduction is also practiced [133]. Reduction is conducted in two steps involving the formation of niobium carbide: Nb2 O5 + 7C ¼ 2NbC + ð5COÞ Nb2 O5 + 5NbC ¼ 7Nb + 5CO It is not desirable to conduct carbothermic reduction in one stage. Stock of Nb2O5 contains only 57.2% of niobium, and even in briquette form, it has low density ( 1.8 g/cm3). High volumes of CO ( 0.34 m3) are consumed per 1 kg of stock leading to the reduction process’ low capacity. Therefore, the first step is carried out in graphite-tube resistance push-type furnaces in a hydrogen atmosphere at a temperature of 2070 K. The graphite cartridge with the stock (Nb2O5 + 7C) is pushed through the furnace for 1–1.5 h. Lightly sintered niobium carbide is ground to powder and blended with Nb2O5 in a surplus of 3%– 5% against the stoichiometric ratio. Then, this mixture is pressed into briquettes at a pressure of 100 MPa and calcined in a FIG. 23.42 SEM micrograph of the niobium powder made by magnesium vapor reduction. (Source: Kumar TS, Kumar SR, Rao ML, Prakash TL. Preparation of niobium metal powder by twostage magnesium vapor reduction of niobium pentoxide. J Metallurgy 2013; (Article ID 62934): 1–6.)
Production of Refractory Metal Powders Chapter
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vacuum furnace at 2070 K and a residual pressure of 1.3–0.13 Pa for 6 h. The niobium reduction goes by intermediary stages of lower oxide formation (NbO2 and NbO). It is important that at temperatures above 900 K, a part of Nb2O5 is evaporated, while NbO2 and NbO evaporate at higher temperatures. Niobium oxide vapor is adsorbed on the carbon and niobium carbide particles, where niobium is reduced. The second step goes with higher capacity, therefore, the mixture Nb2O5 + 5NbC has 82.4% niobium in briquettes with a density of about 3 g/cm3. Reduction by Sodium: The reduction of potassium fluorine niobate (K2NbF7) by sodium may be represented as follows: K2NbF7 + 5Na ¼ Nb + 2KF + 5NaF This process is realized in a shaft furnace using a layerwise loaded charge in a steel crucible. Sodium surplus (15%–20%) in the form of sodium chloride layer is poured onto the top of the charge; the crucible is placed into the preheated furnace (up to 870 K) and kept for 1.0–1.5 h at 1170–1270 K. Niobium powder is washed, first in cold then in hot water, and is next washed with dilute nitric or hydrochloric acid (to eliminate iron, titanium and other impurities) and 2.5% cold hydrofluoric acid for 5–15 min to decrease the niobium oxide content. Electrolysis of Melt: Electrolysis of melt is also used. Refined salts are melted in a nickel crucible (cathode), and Nb2O5 is introduced into the melt; the anode is a graphite rod, and the electrolyte temperature is 1020 K. Voltage on the cell is 2.5 V; cathode current density ranges from 5 to 6 kA/m2, and current efficiency is not >30%. The niobium deposit is coarsegrained and contaminated with the lowest niobium oxides. Two types of electrolyte are used K2NbF7-KCl-Nb2O5 and K2NbF7-KCl-NaCI (for refining). In liquid melt of the former, before Nb2O5 is introduced, the following reactions take place: K2NbF7 + KF ¼ K3NbF8 and/or K2NbF7 + KCl ¼ K3NbF7Cl Having added niobium pentoxide in surplus of KF, the reaction can be written: K2NbF7 + 13KF + 2Nb2O5 ¼ 5K3NbO2F4 2 3 Complex niobium containing ions hypothetically NbF3 8 , NbF7 and NbO2F4 are present in the electrolyte. At temperatures in the range from 970 K to 1020 K and cathode current density ranging from 1 to 8 kA/m2, it is possible to obtain niobium powder, in which, compared to anode material, the impurities content is less by an order of magnitude: for iron, 1–3, for silicon, 1–1.5, for carbon, 1–1.5, and for oxygen, 1.0. Niobium sponge can be produced also by electrodeposition from halide melts. Electrolysis of bromide and iodide melts (4%–6% Nb) is carried out with addition of alkali metal chlorides at 973 K and cathode current density of 5 kA/m2. Current efficiency is about 40%. The obtained sponge is lightly milled to powder. For niobium reduction from halide melts, the starting material is niobium produced by carbothermic reduction and the electrolyte is a mixture of NaCl and K2NbF7, with 10%–40% of the latter. In an argon atmosphere, niobium is oxidized on the anode and reduced on the cathode using a cathode current density of 32–41 kA/m2 and temperature between 1100 K and 1200 K. The electrolysis cell (Fig. 23.43) made of nickel is put into the furnace with graphite heaters and a heat shield. On the furnace cover, a water-cooled chamber for charging is erected, in which the cathode is vertically moved. Current efficiency,
FIG. 23.43 Elecrolyzer for niobium refining.
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TABLE 23.17 Compositions of Contaminant Elements in Anodes and Electrolytic Niobium Powder Elements
Unit
Anode
Powder
Aluminum
ppm
10
10–20
Calcium
ppm
50
50
Chromium
ppm
20
10
Copper
ppm
10
1
Iron
ppm
30
70–150
Lead
ppm
…
10
Magnesium
ppm
10
10
Manganese
ppm
…
10
Nickel
ppm
10
100–500
Silicon
ppm
30
4–5
Tin
ppm
…
10
Titanium
ppm
1000
100
Vanadium
ppm
…
10
Zirconium
ppm
5000
100
calculated to Nb2+, ranges from 23.1% to 34.6%. Compositions of contaminant elements in electrolytic refined niobium powders and in anodes are given in Table 23.17. The powders (cathode product) consist of modified coarse dendrites. Hydride-Dehydride Process: Powders are produced from ingots by means of hydriding, crushing, and subsequent dehydriding of electron beam (EB) melted ingots. These melt-grade powders have higher purity than the sodium-reduced types and have better dielectric properties. However, unit capacitance is usually lower for EB-powder. Although the dielectric properties of niobium oxide are inferior to those of tantalum oxide, niobium has been evaluated as a means of manufacturing low-cost capacitors. Considerable niobium powder development work is required before niobium capacitors will be offered commercially. Usually, powders are crushed to pass a 180 mm (80-mesh) screen and a mean particle size of 10–15 mm is typical.
Niobium Alloys Typical compositions of niobium and niobium alloy powders are given in Table 23.18. Niobium powder is frequently used as the starting material to blend with other alloying powders. The blend is pressed to bars and melted. Niobium metal scrap, which is reduced to powder by the hydride-dehydride process, can also be incorporated into the alloy blends. Some efforts [127] have been directed to producing complex metastable alloy powders, such as niobium-aluminum and niobium-silicon alloys, by liquid metal atomization and rapid solidification. Most commercial niobium alloys are in wrought form. Primary (bulk) processing is generally done by high temperature extrusion or forging. Secondary working manufactures a variety of mill products such as rods, sheets, wire, foil, and tubing. Powder metallurgy methods have been evaluated for common niobium alloys, such as C-103 (Nb-10Hf-1Ti) and Nb-30Hf9W. It has been found that the high production cost of atomization prevents economical manufacturing of net shape parts. However, hydride-dehydride niobium powders are used to produce commercial quantities of hot working preforms of alloy Nb-30Hf-9W [127]. The NbCu composite is used to produce electric contacts, resistances, welding electrodes, microwaves absorbers, and heat sinks. Some studies of the Nb-Cu composite with varying (5–20 wt%) copper content were carried out [134–136]. In the work [137], the Nb-Cu composites with 12.5 wt% and 25 wt% Cu content where produced by mechanical alloying using planetary ball milling. Niobium powder made by hydride-dehydride technique (HDH) and copper powder made by carbonyl process were used in this work. The characteristic morphology of these powders is shown in Fig. 23.44.
Production of Refractory Metal Powders Chapter
23
TABLE 23.18 Typical Compositions of Niobium and Nb-10Hf-1Ti Alloy Powders Made by the Hydride-Dehydride Process Analysis Niobium
Unit
Source: Fansteel Inc
GOST 26252–84 I-grade
Nb-10Hf-1Ti (C-103 Niobium Alloy) Source: Fansteel Inc
Niobium
wt%
99.7%
balance
87.2%
Hafnium
ppm
<20
…
9.8 wt%
Titanium
ppm
<20
<10
0.91 wt%
Aluminum
ppm
<20
<10
<20
Boron
ppm
<1
…
<10
Carbon
ppm
500
<50
194
Cobalt
ppm
<10
<10
<10
Copper
ppm
<40
<30
<40
Hydrogen
ppm
150
<20
50
Iron
ppm
100
<30
200
Magnesium
ppm
…
<10
…
Manganese
ppm
…
<10
…
Molybdenum
ppm
<20
<30
100
Nickel
ppm
<20
<10
<20
Oxygen
ppm
1820
<2000
1980
Nitrogen
ppm
197
<200
62
Silicon
ppm
30
<30
<20
Tantalum
ppm
800
<600
2800
Tin
ppm
…
<10
…
Tungsten
ppm
<50
<30
1100
ppm
<20
<10
1800
Other elements
ppm
<20
…
<20
Physical properties:
ppm
Apparent density
g/cm3
…
0.7
…
…
1.0; 1.1; 1.3
…
…
0.76
…
Property Elements:
Zirconium a
b,c,d
Average particle sizes,
(mm)
Surface area
m2/g
a
Other elements include cadmium, chromium, magnesium, manganese, lead, tin, vanadium. Method of particle sizes determination: BET particle size calculated. Mercury porosimetry. d FESEM particle size. b c
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FIG. 23.44 Morphology of (A) carbonyl cooper powder and (B) HDH niobium powder. (Source: Dias A, Grossi J, Muterlle PV. Production of NbCu composite by powder metallurgy. In: Completed by European powder metallurgy association. CD/USB proceedings of PM2017 euro congress. Milan (Italy): EPMA; 2017: p. 1–6.)
FIG. 23.45 Morphology of Nb-Cu composite powders with 12.5 wt% of cooper content (A) after two and (B) eight hours milling for the samples. (Source: Dias A, Grossi J, Muterlle PV. Production of NbCu composite by powder metallurgy. In: Completed by European powder metallurgy association. CD/USB proceedings of PM2017 euro congress. Milan (Italy): EPMA; 2017: p. 1–6.)
Fig. 23.45A and B exhibit the morphology of powders with 12.5 wt% of copper after two and eight hours milling for the samples, respectively. Two hours of milling was not effective to obtain the NbCu composite by mechanical alloying, because the powders still preserve their elementary conditions. Milling for eight hours shows that the elementary powders are not yet visible. After milling, the powders were compacted at 400 MPa using a uniaxial mechanical press. The sintering of green samples was carried out in a tubular furnace with an argon atmosphere at a 1373 K temperature. As reported, all samples presented higher than 75% of full density, and in the samples with 25wt%Cu, the densification was higher than 80%. Nb-Si intermetallic composites feature is high-temperature strength at rather low densities. They have a density of around 7 g/cm3, which is significantly lower than the density of nickel-based superalloys with about 9 g/cm3 [138,139]. For high-temperature resistant components in the high-pressure section of aircraft and land-based turbines, nickel-based superalloys are currently applied. The efficiency of these turbine engines can be further improved by increasing the service temperature. Currently, heat-resistant nickel alloys (HRNAs) for casting monocrystalline GTE blades have reached the limiting operating temperatures of 1370–1420 K for the material, which is 80%–85% of their melting point. Therefore, refractory alloys such the niobium-based silicide intermetallic composites (Nb-Si) are a promising material group for these applications. Until now, the utilized processing technologies to develop Nb-Si alloys were mainly casting, directional solidification, arc melting plus subsequent extrusion, physical vapor deposition, and hot extrusion of gas-atomized powders [140,141]. The challenge remains to find a processing technique that achieves a homogeneous microstructure and allows the production of near net shape Nb-Si components for industrial production [139,140]. The paper [142] reports the development of several metal injection molding (MIM) test geometries by utilizing gasatomized powder. The same powder was processed by hot isostatic pressing (HIP) as well. After sintering, some MIM samples were subjected to subsequent HIPing to achieve further densification. Residual porosity, microstructure, phase distribution, and density achieved by these three processing techniques (MIM, HIP and MIM + HIP) are compared. Mechanical properties at ambient and elevated temperatures are assessed for both the sintered MIM samples and HIP
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FIG. 23.46 SEM of the gas atomized Nb-Si powder <25 mm size fraction. (Source: Mulser M, Hartwig T, Seemuller € C, Heilmaier M, Adkins N, Wickins M. Influence of the processing technique on the properties of Nb-Si intermetallic composites for high-temperature applications processed by MIM and HIP. In: Compiled by Russell A, Chernenkoff W, Brian J. Proceedings of 2014 powder metallurgy world congress. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 04-8-16.)
material. Powder of the nominal composition Nb-20Si-23Ti-3Cr-6Al-4Hf (at.%) was produced by gas atomization in an argon atmosphere by electrode induction melting. A scanning electron micrograph of materials gas-atomized in an argon atmosphere by electrode induction melting is shown in Fig. 23.46. The powder particles are spherical with a smooth surface and limited satellites. Compared with an HIP sample of powder <25 mm size fraction (Fig. 23.47A), the MIM sample of the same powder size fraction (Fig. 23.47B) shows a higher porosity with pores up to 10 mm in diameter. As reported [142], the microstructure achieved by MIM reveals a much finer grain structure and more homogeneously distributed silicide particles compared to HIP. However, the MIM samples show larger HfO2 precipitates and a higher residual porosity. By subsequent HIP processing, the residual porosity was reduced, increasing the density to 99.2%. The mechanical properties at RT and elevated temperatures of the as-sintered condition are significantly lower than those achieved by HIP material. In the work [143], the alloying behavior and properties of Nb5Si3 were studied. The study used data for the silicide parameters as valence electron concentration (VEC) and electronegativity (Dw) and for the silicide solubility range, which were determined using the concentration X ¼ Al + B + Ge + Si + Sn in (Nb,TM)5X3). Actual chemical compositions of tetragonal Nb5Si3 in developmental Nb-silicide based alloys were used to calculate VEC, Dw and X. Relationships between solvent and solute additions in Nb5Si3 and its parameters VEC and Dw were found. Changes in the hardness and creep of tetragonal Nb5Si3 were related to the parameters VEC and Dw. The influence of the technological regimes of vacuum induction melting and oriented crystallization melting on the content of alloying elements and the structure of an in-situ composite of the Nb-Si system alloyed with titanium, hafnium, aluminum, chromium, molybdenum, tungsten, and zirconium was studied [144]. The following conclusions were made:
FIG. 23.47 Scanning electron micrographs of (A) HIP and (B) MIM of <25 mm size powder fraction of the Nb-Si intermetallic composite. (Source: € Mulser M, Hartwig T, Seemuller C, Heilmaier M, Adkins N, Wickins M. Influence of the processing technique on the properties of Nb-Si intermetallic composites for high-temperature applications processed by MIM and HIP. In: Compiled by Russell A, Chernenkoff W, Brian J. Proceedings of 2014 powder metallurgy world congress. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 04-8-16.)
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l
l
SECTION
D Production of Non-Ferrous Metal Powders
The microstructure of samples of a composite based on Nb-Si is represented by eutectic cells oriented along the direction of crystallization. The phase composition contains the Nb solid solution (matrix) with the bcc lattice, niobium silicide g-Nb5Si3 with the hexagonal lattice, and hafnium oxide HfO2 with the bcc lattice. Niobium silicide g-Nb5Si3 has an orientation close to I100I along the direction of the growth axis, while the Nb matrix is oriented along the direction I110I. In cast material, an inhomogeneous distribution of alloying elements in the solid solution is observed. The titanium, chromium, and aluminum concentrations at the cell boundaries are higher than in the center, while the molybdenum, niobium, and tungsten concentrations are lower. Exposure at a temperature of 1773 K for 20 h contributes to leveling the concentration of alloying elements in the solid solution, but leads to the intensified formation of hafnium oxides. The tensile strengths of niobium composites at 1473 K are almost two times higher than those for heat-resistant nickel alloy (HRNA) single crystals; with consideration of the lower density of composites (7.386 g/cm3 compared to 8.9–9.1 g/cm3 for HRNAs), the advantage in the specific strength is even more significant.
Tantalum Several methods for reducing tantalum compounds to tantalum metal have been developed, but sodium reduction of the double salt, potassium tantalum fluoride (K2TaF7), to produce tantalum metal powder is the most commonly used. The general equation for the reduction with sodium may be represented as: K2 TaF7 + 5Na ¼ Ta + 5NaF + 2KF The reaction is carried out in a stirred reactor containing molten sodium chloride diluent under an inert atmosphere. The reaction is exothermic and accompanied by heat evolution amounting to 1500 kJ/mol (2 kJ per 1 g of molten sodium). After cooling, the resulting salt cake is removed from the reactor and crushed. The tantalum powder is recovered by thoroughly washing with hot distilled water with an addition of hydrochloric acid and subsequently dried at 380–400 K. Particles are usually rounded in shape, with a tendency to form grape-like clusters during reduction. Individual particles in such clusters range in size from 1 mm to 10 mm, depending on reduction variables, including temperature, agitation, and salt purity. Electronic capacitor applications generally require fine particles with high surface area. In one modification of the process, potassium tantalum fluoride, sodium chloride, and sodium metal are mixed into a paste and reacted without pre-melting [145]. Flake-shaped tantalum particles are produced by this technique. Because flakes have a higher surface-to-volume ratio than spheres, they are suitable for capacitor applications. In another modification of the process, tantalum powder with a highly developed surface can be produced by conducting the process in the presence of small amounts of boron, phosphorus, sulfur, and silicon. An even more appreciable effect is achieved by additions ranging from 0.002% to 0.5% of halides, oxides, or sulfides of Ti, Zr, and Hf. Generally, a wide range of sodium reduced capacitor powders is currently available, with unit capacitances ranging from 5000 mFV/g to >25,000 mFV/g. Capacitor powders are also manufactured by the HDH process. The electron beam (EB) melting step allows improvement of the purification of tantalum metal powders. In this process, sodium-reduced powders are pressed into bars which are subsequently EB melted. The resulting ingot is placed in a furnace with a hydrogen atmosphere. The tantalum is fully hydrided on slow cooling from 1070 K under hydrogen. The brittle hydride is crushed, ground, and classified to yield powder with average particle sizes ranging from 3 mm to 6 mm. The particles have an angular shape. Additions of small amounts (10–100 ppm) of sintering inhibitors, such as phosphorus, are frequently employed. These agents allow finer powders to be used than would be possible in the absence of an inhibitor. Alternatively, higher sintering temperatures can be used without loss of surface area, thereby aiding vaporization of undesirable impurities. Dielectric properties are thus improved. Because capacitance is directly proportional to the tantalum surface area accessible to an electrolyte, a porous structure is desirable. Thus, it is desirable to press the powders to as low a density as possible, while retaining sufficient green strength for handling. In general, sodium-reduced powders can be pressed to lower densities than EB melted, degassed hydride powders. The former material is used in high-capacitance devices at lower voltage ratings (<50 V). However, because of the higher purity of the latter powders, they allow operation at higher voltage. Thus, comparing the two basic types of tantalum powders when used for capacitor manufacture shows that the sodium-reduced powder material has a higher capacitance per gram of powder and better green strength for pressed compacts, while electron beam melted, degassed hydride powder material has higher purity, better flow characteristics and higher voltage capability in capacitor devices. Currently, sodium-reduced tantalum powders are generally used for 80%–90% of capacitor applications. There is an electrolytic process for producing tantalum using an electrolyte containing potassium fluorotantalate (K2TaF7), potassium (sodium) chloride and/or fluoride in which tantalum pentoxide is dissolved. Carbon dioxide and
Production of Refractory Metal Powders Chapter
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743
carbon monoxide are formed at the anode. An anode effect arises during electrolysis of the K2TaF7 melt. Potassium chloride and fluoride provide easy fusibility, fluidity, and conductivity of the electrolyte, while the tantalum pentoxide improves wettability of the graphite anode. The cathode can be made of molybdenum, nickel, nichrome, or steel. 3 Thus, the oxide-fluoride-chloride electrolytes contain ions of K+, Cl–, F, TaF3 8 , TaOF6 (when the content of Ta2O5 in 3 the electrolyte is <5%) or TaO2F4 (when content of Ta2O5 in the electrolyte is 5% and more). The discharge of Ta5+ ions occurs on the cathode: Ta5+ + 5e ! Ta Scheme of the anode process can be written: TaOF3 6 2e ¼ TaF5 + F + 0:502
TaF5 + 3K + + 3F ¼ K3 TaF8 0:5O2 + C ¼ CO ðO2 + C ¼ CO2 Þ Thus, Ta2O5 undergoes an electrolysis treatment after finally being introduced into the melt. Precipitation of tantalum on the cathode and oxygen on the anode can be described: 4Ta5 + + 20e ! 4Taand10O2 20e ! 5O2 Reaction of graphite oxidation on the anode accompanied by evolution of energy leads to its depolarization, so that the amount of energy necessary to decompose of Ta2O5 is decreased. The electrolyte contains 2.5–8.5 wt% Ta2O5, 8.5–32.5 wt% K2TaF7, 27.5–57.5 wt% KF, and 25.5–55 wt% KC1; its melting temperature is about 920–970 K, depending on electrolyte composition. Electrolysis is accomplished at 970–1020 K. An advanced electrolyte containing 3–3.5 wt% Ta2O5, K2TaF7, 66.5–72 wt% (NaCl + KC1) with a reduced melting point of 270 K allows electrolysis to be conducted at 970 K. There are two types of electrolyzers. In the first type, a graphite crucible serves as an anode, and a metal bar located in the center of the crucible is the cathode. The second type includes a metal crucible as the cathode and a graphite rod or a hollow perforated graphite tube as an anode disposed in the center of the crucible (Fig. 23.48). The latter is preferable. In this electrolyzer, Ta2O3 is loaded into the cell through the hollow anode, and the gases are eliminated through the orifice in the side of the electrolyzer. Electrolysis is performed at a current density 5 kA/m2 on the cathode and 1.2–1.6 kA/m2 on the anode. Tantalum dendrites are deposited on the bottom and the sides of the crucible. The process is terminated as soon as the FIG. 23.48 Elecrolyzer for tantalum production.
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SECTION
D Production of Non-Ferrous Metal Powders
cathode deposit fills two-thirds of the crucible capacity. Tantalum powder particles are imbedded in the solidified electrolyte that protects the metal from oxidation on cooling. The powder particle sizes range from 30 mm to 120 mm. The cathode product is milled in a ball mill that operates in a closed cycle with an air separator. The electrolyte phase is separated and returned to the cell for electrolysis. The tantalum powder is washed on shaking tables in water jets to eliminate electrolyte, treated with a hot mixture of hydrochloric acid and nitric acid in porcelain reactors to eliminate molybdenum and iron, washed with water again, and dried. A vacuum heating process is also available in which the cathode crucible is heated in argon to 1270 K to melt the electrolyte. The vessel is then evacuated and the electrolyte sucked out. The metallic tantalum in form of weakly fritted dendrites in 100–120 mm sizes precipitated on the crucible walls is scraped off and ground. Current efficiency is 80%–83%; power consumption is 2300 kWh per ton. Tantalum powder can be produced by electrolysis of melts containing tantalum pentachloride (TaCl5). The liquid or vapor TaCl5 ( 10% of tantalum) is introduced into the melt (NaCl + NaF, NaCl + KCl, or NaCl + KCl + KF) at temperature ranges from 1020 K to 1220 K, by voltage in the cell 6–8 V, cathode current density 5 kA/m2, and anode current density of 10–20 kA/m2; current efficiency amounts to 85%–88%. The method of tantalum refining in chloride-fluoride and fluoride melts is still attractive, as the majority of impurities are eliminated completely; iron content is decreased 10–50 times, oxygen and hydrogen contents are decreased 10–100 times, and molybdenum and tungsten content is decreased 104–105 times, but it is difficult to eliminate niobium. The electrochemical method for producing tantalum powder based on reducing electrochemically dissolved tantalum ions (Tan+) by dysprosium divalent ions (Dy2+) in molten salt was investigated [146]. A tantalum rod (anode) was immersed in the NaCl-36 mol%KCl-4 mol%MgCl2-6 mol%DyCl2 molten salt at 1000 K, and it was anodically dissolved in this salt. The electrochemically dissolved Tan+ ions were reduced in situ by Dy2+ ions in the molten salt to produce tantalum powder. After completion of the electrolysis process, tantalum powder mixed with salt was recovered by leaching the mixture with acids. The salt and some metallic impurities were dissolved in an acetic acid solution. The resulting tantalum powder was rinsed in aqueous HCl at room temperature; it was then rinsed with distilled water, alcohol, and acetone, and subsequently dried under vacuum. Fig. 23.49 shows the SEM images of the tantalum powders obtained at various currents. As shown, the particles are loosely agglomerated to form a porous structure; the average particle size of the powder successively increased by a small amount when the currents were increased to 2.0 A and 4.0 A, As reported, tantalum powder with a purity of around 97 mass% and an average primary particle size of around 0.1 mm was obtained by this electrochemical method in the NaCl-36 mol%KCl-4 mol%MgCl2-6 mol%DyCl2 molten salt at an applied current of 1.0 A. Typical impurity compositions of sodium-reduced tantalum powder, electron beam melted, degassed hydride powder, electrolytic powder, and purified electrolytic powder are given in Table 23.19.
Tantalum Carbide The conventional technique for tantalum carbide powder production is similar to methods by which tungsten carbide powders are produced. Stoichiometric tantalum carbide can be used as an additive to WC-Mo grade powder attritions to enhance the physical properties of sintered material. It can be also used as a grain growth inhibitor preventing the formation of large grains, resulting in increased hardness of the sintered parts. Additions from a tenth of 1%–30% are common
FIG. 23.49 SEM images tantalum powder obtained by the electrochemical method in NaCl–36 mol%KCl–4 mol%MgCl2–6 mol%DyCl2 molten salt at various currents: (A) i ¼ 1A; (B) i ¼ 2A; (c) i ¼ 4A.
Production of Refractory Metal Powders Chapter
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745
TABLE 23.19 Typical Compositions of Tantalum Powders Manufactured by Different Methods Powder Analysis (ppm)
Element
SodiumReduceda
Electron Beam Melted DegassedHydride
Electrolytic from Oxide Chloride or Oxide Fluoride-Chloride Melts
Purified from FluorideChloride or Fluoride Melts With K2TaF7
Aluminum
10
5
..
…
Calcium
10
5
…
…
Carbon
100
45
200–300
20–30
Chromium
25
5
…
…
Cobalt
10
5
…
…
Copper
10
5
…
…
Fluorine
…
…
10
…
Hydrogen
30
30
…
…
Iron
50
30
…
20–30
Iron +Nickel
…
…
300–1000
…
Lead
10
5
…
…
Magnesium
10
5
…
…
Manganese
10
10
…
…
Nickel
50
5
…
…
Niobium
50
20
…
…
Nitrogen
90
40
…
…
Oxygen
2400
1650
1000–2000
100–200
Silicon
25
10
1000
…
Sodium
10
5
…
…
Tin
10
5
…
…
Titanium
10
5
1000
…
Tungsten
25
25
…
…
Vanadium
10
5
…
…
Zirconium
10
5
…
…
a
Source: Fansteel Inc.
and completely dependent on the particular application. Typical composition of commercial tantalum carbide powder, produced by OSRAM Sylvania Company, is given in Table 23.20. It also makes tantalum-niobium carbide (TaNbC) of several compositions, such as 90/10, 80/20, and 60/40. In the work [147], the hydrate ammonium tantalate was selected as precursor of the tantalum carbide. The reactions between precursor ((NH4)TaO(C2O4)3H2O) and mixture of methane and hydrogen were carried out at relatively low temperature of 1270 K in a resistive furnace, consisting of an alumina fixed bed reactor. As shown, the precursor powder obtained in this work has large particles composed of irregular shape agglomerates consisting of small crystallites (Fig. 23.50A), while the tantalum carbide powder particles preserve configuration, but their surface looks somewhat fitted (Fig. 23.50B).
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TABLE 23.20 Typical Properties of Tantalum Carbide Powder Property
Range
Typical
Particle size FSSS
0.5–2.0 mm
0.8–1.5 mm
Chemical analyses
Maximum percent
Typical percent
Total carbon
6.15–6.30
6.20–6.30
Free carbon
0.15
0.07
Calcium
0.02
0.005
Iron
0.05
0.04
Niobium
0.25
0.10
Oxygen
0.15
0.13
Silicon
0.02
0.01
Titanium
0.02
0.01
Source: OSRAM Sylvania.
FIG. 23.50 SEM images of the (A) precursor powder and (B) and tantalum carbide powder.
As concluded [147], this precursor can be used in TaC synthesis at much lower temperatures and reaction times compared to those used in conventional synthesis. For the conventional method, the tantalum carbide is obtained at temperatures ranging from 1570 K to 1770 K; however, for good homogeneity in the final product, high temperatures around 2270 K throughout the reaction are necessary. The method using hydrate ammonium tantalate precursor allowed production of tantalum carbide at 1273 K and an isotherm of two hours. The results presented for TaC revealed the formation of pure phase with characteristics different from those conventionally obtained such as particle size and average crystallite size in the nanometer order, reaching 12.05 nm.
Consolidation and Manufacturing of Semi-Products Powder metallurgy semiproducts of tantalum and its alloys are commonly made by cold isostatic pressing the powders into bars. The bars are then resistance sintered in a vacuum furnace at 1.33 102 Pa. Heating occurs due to the passage of an electric current. For tantalum, temperatures above 2570 K are required to achieve full density and sufficient purification as a result of removal of interstitial impurity elements. Sintered bars are then rolled or drawn at room temperature, with suitable intermediate vacuum anneals. Models of initial stage sintering that consider the starting particle morphology have been applied to tantalum to develop higher energy capacitors [148,149]. An example of the agreement is shown in [150] for nonisothermal sintering of agglomerated tantalum flakes.
Production of Refractory Metal Powders Chapter
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Powder metallurgy products differ from those produced from cast materials primarily because surface oxides are present in the initial powders. These oxides are contained in the microstructure of the consolidated PM material. Recrystallization is inhibited, and finer grained metal is obtained. The material is usually harder and has higher yield and tensile strength, but it is somewhat less ductile. Microalloying additions, such as thoria and yttria, are sometimes added to control grain size and strength after recrystallization. Small quantities of silicon are also added, particularly to bars that are subsequently drawn to capacitor wire. Silicon promotes the retention of ductility of the wire by preventing embrittlement of the tantalum lead wire embedded in the capacitor pellet. During vacuum sintering, oxygen diffuses from the surrounding powder into the embedded wire, which has lower oxygen, and collects at grain boundaries, thereby stimulating embrittlement. Silicon operates as a getter for the excess oxygen. A part of the silicon vaporizes harmlessly in the form of volatile silicon monoxide. Tungsten and hafnium are the alloying additions commonly added to improve the strength properties of tantalum. Typical properties for commercially produced tantalum and tantalum PM alloys and for comparison cast alloys are given in Table 23.21. Ta-2.5%W is used for the fabrication of corrosion-resistant heat exchangers, valves, and other chemical equipment. Less ductile Ta-10%W alloy is a harder, stronger, and more wear-resistant alloy and is especially suited for the manufacture of furnace hardware, missile parts, nozzles, and fasteners. A Ta-7.5%W alloy, made by powder metallurgy, is useful for springs, siphons, and leaf springs in severe corrosion environments, such as hydrochloric acid, bromine, or dry chlorine. Because the yield strength of the PM product is higher than that of ingot material of the same composition, its modulus of resilience is also considerably higher, as illustrated in Table 23.21.
Workplace Atmosphere Safety The aerosols of metallic niobium and tantalum are not toxic and relegated to the fourth, lowest class of danger according to State Standard 12.1.005-88 under the legislation in the Commonwealth of Independents States (CIS) [56]. Inspirable dust of both metals predominantly has a fibrous effect. The stated average limit value (ALV) of the aerosols of tantalum and its alloys in the workplace atmosphere, as the shift time-weighted average concentration, is 10 mg/m3. As for niobium, the State Standard 12.1.005-88 standardizes only niobium nitride ALV, which amounts to 10 mg/m3 (as the shift timeweighted average concentration). Under the workplace exposure limits in the European PM industry the long- and short-term (15-min reference period) workplace exposure limits (WELs) for tantalum are 5 mg/m3 and 10 mg/m3, accordingly [57]. Niobium in potable water has a toxic effect and is placed into the second class of danger and the TLV is 0.01 mg/L (for the inorganic compounds, including transition metals, subject to summation of all form contents) according to CIS State Standard 4630-88 [58]. The low concentration ignition limit (LCIL) of the metallic tantalum powder with particle sizes below 44 mm in the air is 200 g/m3; minimum ignition temperatures of powder aerosols and aerogels (self-ignition temperature) are 903 K and 563 K, respectively. The products with such characteristics of inflammability and explosion risk are placed in the “moderate explosion hazard” class of danger according to the EPMA Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used for pollution prevention and environmental control by powder manufacturing and its processing. Detailed information on health and environment protection measures and also on prevention of inflammability and explosion risk in such conditions can be found in Chapter 27.
RHENIUM Except for tungsten and carbon, among the elements, rhenium has the highest melting point, which is 3453 K. Its density is exceeded only by osmium, iridium, and platinum. Rhenium has the highest modulus of elasticity among the refractory metals. A ductile-to-brittle transition temperature does not exist in pure rhenium. Rhenium is one in refractory metals that does not form carbides.
Production of Rhenium Powder The main method for rhenium powder production is reduction of ammonium perrhenate (NH4ReO4) by hydrogen. The reduction can be written:
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TABLE 23.21 Typical Properties for Commercially Tantalum and Tantalum Alloys Standard Grades Commercial Pure Tantalum
Property
Unit 3
Ta-7.5W, Sheetb
Ta-7.5W, PM
EB Melteda
PM
Ta-10W, EB Melteda
Wire
Sheet
Ingot
PM
16.9
16.6
16.8
16.8
16.8
…
…
Density, at RT
g/cm
Melting point
°C
3000
3000
3030
3025
3025
…
...
Hardness, HV
HV
110
120
245
325
400
…
…
Hardness, DPH
DPH
…
…
…
…
…
150
291
At 293 K
MPa
205
310
550
1035
1165
…
…
473 K
MPa
190
…
515
…
…
…
…
1023 K
MPa
140
…
380
…
…
…
…
1273 K
MPa
90
…
305
…
…
…
…
At 293 K
MPa
165
220
460
1005
875
483
676
473 K
MPa
69
…
400
…
…
…
…
1023 K
MPa
41
…
275
…
…
…
…
1273 K
MPa
34
…
205
…
…
…
…
%
40
30
25
6
7
…
…
473 K
30
…
…
…
…
…
…
1023 K
45
…
…
…
…
…
…
1273 K
33
…
…
…
…
…
…
c
c
Tensile strength:
Yield strength
Elongation At 293 K
Modulus of elasticity At 293 K
GPa
185
185
205
200
200
196
196
1023 K
GPa
160
…
150
…
…
…
…
MPa
…
…
…
…
…
593
1172
Modulus of resilience (UR)d At RT a
EB, electronic beam. Measured for 2.1 mm thick, annealed and recrystallized sheet. Estimated. d UR ¼ s20/2E; s20 is the yield strength; E is Young’s modulus. b c
Source: Adapted from Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.
NH4 ReO4 + 2H2 ¼ Re + 1=2 N2 + 4H2 O Typical impurity composition of hydrogen reduced powders is: 5 ppm (Аl + Fе + Сu + Мо), 10 ppm (Са + Si + Р + Na), 20 ppm (Mg + Ni + S), 50 ppm K, 1 ppm Mn, and Cu 0.5 ppm. Raw material is milled in rubber-lined drums, then reduced in pusher type furnaces. The ammonium perrhenate is loaded into flat molybdenum boats that are pushed through the furnace. Reduction takes place in two stages. At
Production of Refractory Metal Powders Chapter
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TABLE 23.22 Typical Chemical Composition of ≤74 mm and ≤44 mm Rhenium Powders Analysis (ppm) Elements
≤74 mm (2200 mesh)
≤44 mm (2325 mesh)
Aluminum
1.4
1.4
Boron
0.1
0.1
Calcium
0.2
0.2
Carbon
<10
<10
Chromium
1.5
1.5
Copper
0.5
0.5
Hydrogen
56
56
Iron
2.3
2.3
Manganese
0.1
0.1
Molybdenum
0.4
0.4
Nickel
1.7
1.7
Nitrogen
22
22
Oxygen
898
898
Silicon
0.2
0.2
Tantalum
<5
<5
Tungsten
0.33
0.33
Vanadium
<0.005
<0.005
Zinc
0.1
0.1
616–636 K, RO2 is formed, and a final reduction to metal is done at 1220–1240 K. The boats’ time in the hot zone of furnace is 1–2 h. Average size of powder particles ranges from 1 mm to 3 mm. Apparent density ranges from 1.5 g/cm3 to 1.9 g/cm3. Typical chemical compositions of 74 mm and 44 mm rhenium powders are shown in Table 23.22. Among the refractory metals, rhenium has the highest tensile strength (Fig. 23.51). Rhenium has high electrical resistivity. It typically exhibits higher resistivity than tungsten. This feature, associated with a low vapor pressure, makes it FIG. 23.51 Dependence of ultimate tensile strength for pure refractory metals on temperature. (Source: Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.)
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perfectly suited for filament applications; parallel with this, it preserves ductility and is not affected by the oxidation/ reduction cycle experienced in these applications, as is tungsten. Rhenium does not react with molten copper, silver, tin, or zinc. Rhenium is resistant to hydrochloric acid and sea-water corrosion and to the mechanical effects of electrical erosion. At elevated temperatures, rhenium resists reaction in hydrogen and inert atmospheres. Rhenium bars are produced by uniaxial pressing, while tubes are produced by cold isostatic pressing. Compaction pressures are typically 200 MPa to 250 MPa [151]. Green parts are presintered at 1470 K to 1670 K for 30 min in hydrogen to increase their handling strength. Final sintering at about 2970 K can be performed either in hydrogen or high vacuum. Typical densities are 80%–90% of theoretical density, and mill powders are cold worked with frequent annealing because of rhenium’s rate of work hardening. Net-shape rhenium components can be sintered at temperatures of about 2570 K to near-full density [152].
Rhenium Alloys Rhenium is used in combination with platinum in catalysts, especially for selective hydrogenation and crude oil reforming. Rhenium catalysts show high resistance to poisons, such as nitrogen oxides, sulfur, and phosphorus. Rhenium is an advantageous alloying addition with other refractory metals. It greatly enhances the ductility and tensile strength of refractory metals and their alloys. This tendency is retained on heating above the recrystallization temperature. For example, alloys of molybdenum and rhenium are more ductile than pure molybdenum. An alloy with 35% Re can be rolled at room temperature to >95% reduction in thickness before cracking. For economic reasons, molybdenum-rhenium alloys are not widely used commercially. Alloys of molybdenum with 5% and 41% Re are used for thermocouple wires.
Applications Rhenium alloys are used in nuclear reactors, semiconductors, thermocouples, electronic tube components, gyroscopes, parts for rockets, electrical contacts, thermionic converters, and various parts for high temperature aerospace applications. Recent developments in the production of net shapes by powder metallurgy, rhenium manufacturing techniques have generated significant saving in both time and budget. For instance, the thrusters of different and unique types of aerospace components (rhenium combustion chambers) were manufactured by means of near-shape technique [153]. Near-net shapes are produced using elastometric molds with and without hard tooling inserts. The mold cavity is dimensioned so that, after CIP and sintering, final dimensions require only a minimum amount of finish machining [154]. The use of this technique allows the making of parts with a density of 95%–98% after sintering and 98.5%–99.9% on subsequent hot isostatic pressing [155]. The typical microstructure of the initial rhenium metal powders is shown in Fig. 23.52. The production of near-shape PM rhenium combustion chambers has reduced the manufacturing cost to 35%, and shortened the manufacturing time by 30%–40%. The quantity of rhenium metal powder used to produce a chamber is reduced by FIG. 23.52 Scanning electron micrograph of the rhenium metal powder. (Source: Leonhard T, Downs J. Near net shape of powder metallurgy rhenium parts In: Infringer G, editor. Proceedings of 15th International Plan see Seminar, vol. 1. Reutter (Tirol, Austria): Plansee holding AG; 2001. p. 647–57.)
Production of Refractory Metal Powders Chapter
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approximately 70%, and subsequent reduction in machining costs is nearly 50%. Thus, cold isostatic pressing to near-net shape has provided significant cost savings and increased the quality and density of rhenium parts. Tungsten-rhenium alloys, applied by vapor deposition, are used to coat the surface of molybdenum targets in X-ray tube manufacture. Other rhenium alloys, with tungsten and molybdenum, are used for filaments, grid heaters, cathode cups, and ignitor wires in photoflash bulbs. One of the largest applications for rhenium is for mass spectrometer filaments. These are available in commercial (99.99%) and zone-refined (99.95%) purities.
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International Carbide Data: East Barnet (England); 1996. [62] Bock A, Zeiler B. Production and characterization of ultrafine WC powders. In: Kneringer G, editor. Proceedings of 15th International Plansee Seminar. vol. 4. Reutte (Tirol, Austria): Plansee Holding AG; 2001. p. 229–44. [63] US Patent 3,379,503. Apr. 1968.
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[64] Schubert WD, Bock A, Lux B. General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production. In: Bildstein H, Eck R, editors. Proceedings of 13th International Plansee Seminar. vol. 4. Reutte (Tirol, Austria): Plansee Metall AG; 1993. p. 283–305. [65] Yamamoto Y, Matsumoto A, Doi Y. Properties of ultrafine tungsten carbide and cemented carbide by direct carburization. In: Kneringer G, editor. Proceedings of 14th International Plansee Seminar. vol. 2. Reutte (Tirol, Austria): RWF Werbegesellschaft; 1997. p. 596–608. [66] US Patent 5,380,688. Jan 1995. [67] Carroll DF. Processing and properties of ultrafine WC/Co hard materials. In: Kneringer G, editor. Proceedings of 14th International Plansee Seminar. vol. 2. Reutte (Tirol, Austria): RWF Werbegesellschaft; 1997. p. 168–82. [68] US Patent 5,230,729. Jul 1993. [69] EU Patent EP 0 568861 B1, 1993. [70] Wolfe TA, Johnson JL, Mehrotra PK. Production of carbide powders. In: ASM Handbook. vol. 7. ASM International Publishers; 2015. p. 711–4. [71] Fang Z. Wear resistance of powder metallurgy alloys. In: ASM Handbook. vol. 7. ASM International Publishers; 1998. p. 965–77. [72] Gilardi R, Alzati L. The role of carbon source in the production of WC powders. In: Compiled by Bose A, Dowding RJ, Johnson JL. Proceedings of the 9th Tungsten, Refractory & Hardmaterials Conference. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 37–44. [73] Song X, Wang H, Yang T, Liu X, Wang X. In situ synthesis of nanoscale WC-Z composite powder and its application in cermet coating. In: Compiled by European Powder Metallurgy Association. Proceedings of PM 2016 World Congress. Hamburg (Germany); 2016: p. 1–6. [74] Schubert WD, K€ubel E. Mechanical testing of specimens and impact of trace elements on properties of hard. In: Bildstein H, Ortner HM, editors. Proceedings of 12th International Plansee Seminar. vol. 2. Reutte (Tirol, Austria): Metallwerk Plansee GmbH; 1989. p. 869–909. [75] Uhrenius B, Brandrup-Wognsen H, Gustavson U, Lethinen B, Manninen H. On the formation of impurity-containing phases in cemented carbides. In: Bildstein H, Ortner HM, editors. Proceedings of 12th International Plansee Seminar. vol. 2. Reutte (Tirol, Austria): Metallwerk Plansee GmbH; 1989. p. 71–96. [76] Schedler W. Hardmetall f€ur den Praktiker. VDI-Verlag GmbH; 1988. [77] Hu B. Metal and alloy powders for welding, hardfacing, brazing, and soldering. In: ASM Handbook. vol. 7. ASM International Publishers; 2015. p. 769–82. [78] US Patent 5,089,182. Feb 1992. [79] Rinco´n C, Zambrano G, Carvajal A, Esteve J. Tungsten carbide/diamond-like carbon multilayer coating on steel for tribological applications. Surf Coat Technol 2001;148(2–3):277–83. [80] Santos A, Gonzalez C, Ramirez ZY. Characterization of tungsten carbide coatings deposited on AISI 1020 steel. J Phys 2017;786(012011):1–7. https://doi.org/10.1088/1742-6596/786/1/012011. Conference Series. [81] Oyana ST, editor. The Chemistry of Transition Metals Carbides and Nitrides. Blacky Academic and Professional; 1996. [82] Brookes KJA. World Directory and Handbook of Hardmetals and Hard Materials. 6th ed. Hertfordshire (UK): International carbide data; 1996. [83] Schubert WD, Neumaster H, Kinger G, Lux B. Hardness to toughness relationship of fine-grained WC-Co hard metals. In: Kneringer G, editor. Proceedings of 14th International Plansee Seminar. vol. 4. Reutte (Tirol, Austria): RWF Werbegesellschaft; 1997. p. 249–64. [84] Zhang K, et al. Microstructure and composition of the grain/binder interface in WC–Ni3Al composites. Int J Refract Met Hard Mater 2014;44:88–93. [85] Almond E, Roebuck B. Identification of optimum binder phase compositions for improved WC hard metals. Mater Sci Eng A 1988;105:237–48. [86] Sato J, Omori T, Oikawa K, Ohnuma I, Kainuma R, Ishida K. Cobalt-base high-temperature alloys. Science 2006;312:90–1. [87] Long J, et al. A new type of WC–Co–Ni–Al cemented carbide: Grain size and morphology of g0 -strengthened composite binder phase. Scr Mater 2017;126:33–6. [88] Mannesson K, Borgh I, Borgenstam A, Agren J. Abnormal grain growth in cemented carbides — Experiments and simulations. Int J Refract Met Hard Mater 2011;29:488–94. [89] Yang DY, Kang SJL. Suppression of abnormal grain growth in WC–Co via pre-sintering treatment. Int J Refract Met Hard Mater 2009; 2009(27):90–4. [90] Park Y, Hwang N, Yoon D. Abnormal growth of faceted (WC) grains in a (Co) liquid matrix. Metall Mater Trans A 1996;27:2809–19. [91] Long J, et al. Microstructure evolution of WC grains in WC–Co–Ni–Al alloys: effect of binder phase composition. J Alloys Compd 2017;710:338–48. [92] Long J, Chen F, Du Y, Yi M. Growth behavior of WC-Co-Ni-Al alloys. In: Completed by European Powder Metallurgy Association. Proceedings of PM2017 Euro Congress. Milan (Italy); 2017: p. 1–6. [93] Kelley A. Tungsten-carbide by three dimensional printing, Releigh (NC, USA): NC State University; 1998, https://pdfs.semanticscholar.org/ 01e8/33b179d50f58158d5a0f9407062b8fee13bd.pdf. Accessed April 2018. [94] P€ otschke J, Berger C, Richter H-J. Additive manufacturing of hardmetals. In: Compiled by European Powder Metallurgy Association, (Shrewsbury, UK). CD/USB Proceedings of PM 2017 Euro Congress. Milan (Italy); 2017: p. 1–6. [95] Uhlmann E, Bergmann A, Gridin W. Investigation on additive manufacturing of tungsten carbide-cobalt by selective laser melting. Proc CIRP 2015;35:8–15. [96] Cermets and cemented carbides. In: ASM Handbook. vol. 7. ASM International Publishers; 1998. p. 922–40. [97] German RM. Powder metallurgy & prticxulate materials processing. Metal Powder Industries Federation; 2005. [98] Bose A. A earliest perspective on the commercial PM metal-ceramic composite: cemented tungsten carbide. Int J Powder Metall 2011;47(2):31–50. 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The effect of thermo-mechanical processing on the mechanical properties of molybdenum–2 vol% lanthana. In: Kneringer G, editor. Proceedings of 15th International Plansee Seminar. vol. 1. Reutte (Tirol, Austria): Plansee Holding AG; 2001. p. 485–97. US Patent 5,868,876. Feb 1999. German RM, Labombard CA. Sintering molybdenum treated with Ni, Pd and Pt. Int J Powder Metall Powder Tech 1982;18(2):147–56. Hwang KS, Huang HS. The liquid phase sintering of molybdenum with Ni und Cu addition. Mater Chem Phys 2001;67:92–100. Huang HS, Hwang KS. Embrittlement of activation sintered molybdenum. In: Proceedings of 2002 Powder Metallurgy World Congress, compiled by Arnold V, Chu C-L, Jandeska WF, Sanderow HI, Princeton (New Jersey, USA): MPIF Publishers; 2002. p. 6-77-88. Winnicka MB. The effect of lanthana content on the mechanical properties and formability of ODS Mo alloys. In: Compiled by Bose A, Dowding RJ, Johnson JL. CD/USB Proceedings of the 9th Tungsten, Refractory & Hardmaterials Conference. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 501–12. Bergk B, M€ uhle U, Kieback B, Koutna´ N, Holec H, Clemens H. Nanocrystalline alloys of molybdenum with sodium and yttrium obtained by mechanical alloying. In: Compiled by European Powder Metallurgy Association. CD/USB Proceedings of PM 2017 Euro Congress. Milan (Italy); 2017: p. 1–6. Ohser-Wiedemann R, Martin U, Seifert HJ, M€uller A. Densification behavior of pure molybdenum powder by spark plasma sintering. Int J Refract Met Hard Mater 2010;28:550–7. Park HK, Ryu JH, Youn HJ, Yang JM, Oh IH. Fabrication and property evaluation of Mo compacts for sputtering target application by spark plasma sintering process. Mater Trans 2012;53:1056–61. Mouawad B, et al. Full densification of molybdenum powders using spark plasma sintering. Metall Mat Trans A 2012;43:3402–9. Lee G, Olevsky E, McKittrick J. 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Production of Refractory Metal Powders Oleg D. Neikov*, Stanislav S. Naboychenko†, Irina B. Murashova† and Nicholas A. Yefimov* *Frantsevich Institute for Problems of Materials Science (IPMS), Kiev, Ukraine,
†
Ural State Technical University (UPI), Yekaterinburg, Russia
Chapter Outline Introduction Production of Tungsten and Tungsten Carbide Powders Tungsten Metal Powders Tungsten Carbide Powders Tungsten Carbide Electrical Contacts Diamond Tools Hardfacing Cemented Carbides Production of Cemented Carbide Powders Cemented Carbide Compaction Sintering and Secondary Operations Recycling Molybdenum Metal Powders Production of Molybdenum Powders
685 686 686 707 712 714 714 717 717 719 721 721 723 723
Properties of Molybdenum and Molybdenum Alloy Powders Workplace Atmospheres Safety Niobium and Tantalum Niobium and Niobium Alloys Tantalum Tantalum Carbide Workplace Atmosphere Safety Rhenium Production of Rhenium Powder Rhenium Alloys Applications References
725 733 734 734 742 744 747 747 747 750 750 751
INTRODUCTION Tungsten, molybdenum, niobium, tantalum, and rhenium, classified as the refractory metals, are distinguished by several general characteristics, including high melting point, high density, and superior resistance to wear and corrosion. With the exception of two of the platinum-group metals, osmium and iridium, they have the highest melting temperatures and lowest vapor pressures of all metals. Tungsten has a melting point of 3683 K, the highest of all metals, and has a density more than twice that of iron. However, they are also subject to heavy oxidation above 773 K, and they must be protected for service at elevated temperatures using coatings or non-oxidizing atmospheres. These metals have body-centered cubic crystal structures (except for rhenium, whose crystal form is hexagonal). Powder metallurgy is the only production route for commercial tungsten, rhenium, and their alloys, because melting technology is costly and results in a coarse-grained microstructure, which negatively affects both further processing and the properties of the final product. Selection of a specific refractory metal alloy is often based on fabricability rather than strength or corrosion resistance. Special attention must be taken during the processing and use of refractory metals because of their ductile-to-brittle transition temperatures (DBTTs) and their reactions with gases [1]. Niobium, tantalum, and their alloys are the most easily fabricated refractory metals. They are ductile in the pure state and have high interstitial solubility for carbon, nitrogen, oxygen, and hydrogen. Both niobium and tantalum adsorb hydrogen and form hydrides at moderate temperatures. Niobium and tantalum begin oxidizing at 473 K and 673 K, respectively. Below 1643 K, the oxides are non-volatile, but the ductility of the parent material is reduced because of oxygen absorption. Molybdenum, molybdenum alloys, tungsten, and tungsten alloys have limited solubility for carbon, nitrogen, oxygen, and hydrogen. Molybdenum begins to oxidize at approximately 773 K. At temperatures above 1053 K, the eutectic temperature of MoO2 and MoO, oxides become partly volatile, and the oxidation rate accelerates rapidly with increasing temperature. Extrusion (or hot forging) is used for transformation the arc-cast refractory metal ingots into rounds or rectangular bars. These bars, as well as PM-sintered products, are processed into sheets, plates, foil, tubing, and rods. When the refractory Handbook of Non-Ferrous Metal Powders. https://doi.org/10.1016/B978-0-08-100543-9.00023-3 © 2019 Elsevier Ltd. All rights reserved.
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products are involved and cannot be fabricated from standard mill products, they require machining or pressing and sintering to near-net shape. Many refractory metals and alloys are available as wire. Tungsten wire, which is produced in diameters as small as 0.01 mm, is used extensively for lighting, electronic devices, and thermocouples. Tantalum wire is used widely in capacitor manufacturing and in surgical applications. Molybdenum mill product forms, such as sheets and foil, can be stamped into simple shapes. Molybdenum is also machined with ordinary tools and can be gas tungsten arc and electron beam welded or brazed. The refractory metals are extracted from ore concentrates, transformed by processing into intermediate products, and then reduced to the metal. Except for niobium, they are produced exclusively as metal powders, which are consolidated by sintering and/or melting. The process for niobium differs only in that the metal is generally reduced by aluminothermic reduction of oxide.
PRODUCTION OF TUNGSTEN AND TUNGSTEN CARBIDE POWDERS Tungsten was adopted in industry at the beginning of the 20th century due to its application as an alloying element in highspeed steels and as filament wire in incandescent lamps. In both applications, pure tungsten was manufactured in powder form. At the end of the 19th century, very hard tungsten carbides, WC and W2C, were studied by Henri Moissan [2,3]. However, for a long time, all attempts to compact tungsten carbide powders resulted in porous and brittle products. In 1922, researchers of the OSRAM study group combined the brittle tungsten carbide with a ductile metal binder. After sintering such powder mixtures, a material of both high strength and toughness was obtained. Sintered hardmetals, based on tungsten carbide bonded with cobalt, were first commercially produced by WIDIA in Germany in 1926. The global hardmetal industry is estimated to have totaled 32,250 mt in 2002 with a sales value of over $8.5 billion followed by further rises to 51,900 mt in 2006, with sales at an estimated $43 billion. Of these production quantities, around 60% is for sintered cutting tool materials used for machining all types of metallic components, for woodworking, mining tools, drilling tools used in oil exploration, etc., and the remainder is used for components such as rolls used in steel mills, metal forming dies, and structural components for severe wear resistance applications [4]. Western Europe once led the market in this sector, but its share diminished to 39% in 2006, while China’s market share increased to 33%, the United States’ share is at around 13%, and Japan accounts for 12%, with others at 3%. Since 1993, the global production of tungsten, which is the main raw material used in sintered hardmetal products, has fluctuated in a narrow range between 30,000 mt and 35,000 mt. China is the predominant tungsten producer accounting for almost 85% of global output in 1999. The usages of tungsten scrap as a raw material is estimated to be as high as 30% in some countries, with the incentive to use scrap increasing as tungsten prices increase. As a result, annual global consumption of tungsten has been in the range of 40,000–50,000 t [5], and over half of the tungsten consumption is for tungsten carbide powder for hardmetal production. Along with this, important products are based on tungsten alloy powders, such as: l l l
Heavy metal alloys (W-Fe-Cu-Ni-Co) typically comprising 90–98 wt% tungsten Electrical contacts (W-Cu, W-Ag) Ductile tungsten, including lamp filaments.
Tungsten, as an alloying element for steels and superalloys, is predominantly used as ferro-tungsten, or as melting base alloys, which are prepared from either ore concentrates or tungsten-bearing scrap. An insignificant part of the tungsten consumption is used in the form of various chemical products, such as catalysts, pigments, solid lubricants, and heavy liquids.
Tungsten Metal Powders Production of Tungsten Powder by Hydrogen Reduction Feed to Process: Industrial raw materials to process tungsten-bearing minerals are wolframite ([Mn, Fe]WO4), scheelite ubnerite (MnWO4). The lower limit of workable tungsten concentration ranges from 0.1 (CaWO4), ferberite (FeWO4), and h€ to 0.3 wt% WO3. Concentrations of >2 wt% are infrequent. The average WO3 content is usually about 0.5 wt% [6,7]. Recently, Wolfram Bergbau and H€ utten GmbH (WBH) in Austria, a world-leading producer of tungsten oxide, tungsten metal, and tungsten carbide, mined about 400,000 t of scheelite ore, with a 536,306 t peak in 1986, with an average WO3 content of 0.55 wt% [8]. Depending on the mineral type, different techniques, such as gravity methods and flotation, are
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used to concentrate the ore to reduced WO3 contents of about 40–60 wt%. Tungsten-bearing scrap, which usually has a high tungsten content, is a considerable alternative tungsten source to the natural mineral resources. Process: The main process steps from concentrates or scrap through the formation of high purity ammonium paratungstate (APT: (NH4)10 (H2W12O42)2H2O) intermediate to tungsten metal powder [6] are shown schematically in Fig. 23.1. In the dissolution step, the tungsten-bearing feed materials (wolframite concentrate and oxidized tungsten scrap) are treated with NaOH, or scheelite concentrate is treated with Na2CO3 to form water-soluble sodium tungstate NaWO4. The purification steps include: l l
l
Removal of aluminum, arsenic, fluorine, phosphorus, and silicon by precipitation. Substitution of sodium by NH4 forms an ammonium solution (NH4)2WO4. This is ordinarily done by solvent extraction, which is a liquid ion exchange process, in which tungsten is extracted by means of an aliphatic amine dissolved in isodecanol and kerosene. This process is followed by re-extraction into an aqueous ammonia solution. Crystallization of APT by evaporation of ammonia and water. APT is the intermediate product of highest purity for metallic tungsten and tungsten carbide powder production.
The calcination step includes heating the APT to 673–1173 K, predominantly in rotary furnaces, for ammonia and water elimination. If accomplished in an excess of air, the resulting oxide is WO3 yellow tungsten oxide. Under exclusion of oxygen, a slightly reduced, blue-colored tungsten oxide is formed (WO3-x tungsten blue oxide). WO3 is the most stable a-oxide with a homogeneity range of WO3-WO2.96; it has three crystalline modifications: monoclinic modification stable at temperatures lower than 1000 K, tetragonal modification in the range from 1000 K to 1370 K, and cubic modification at temperatures higher than 1370 K. It sublimes noticeably at 1070–1120 K and exists in the form of polymer molecules in the vapor phase. Reduction step with hydrogen reducer: The reduction of the oxide to metal proceeds through several intermediate oxide phases. Generally, two furnace designs for the reduction process are used industrially: a multitube pusher type and rotary furnaces. During the reduction process, the sequence in which oxides occur depends on the applied temperature and on the
Scheelite concentrate
Wolframite concentrate Dissolution
Purification Precipitation: Si, P, F, Al, Solvent extruction Crystalization
Amonium paratungstate
Calcination
Tungsten oxide WO3–x
Reduction
Metallic tungsten powder
Tangsten-bearing scrap
FIG. 23.1 Tungsten metal powder production flowsheet.
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moisture within the powder bed, accordingly, which are static in pusher-type popular furnaces and dynamic in rotary furnaces. Tungsten reduction by hydrogen includes four steps: WO3 + 0:1H2 $ WO2:9 + 0:1H2 O, Keq ¼ 3792:0=T + ð4:8286Þ WO2:9 + 0:18H2 $ WO2:72 + 0:18H2 O, Keq ¼ 142:5=T + 1:684 WO2:72 + 0:72H2 $ WO2 + 0:72H2 O, Keq ¼ 801:7=T + 0:8615 WO2 + 2H2 $ W + 2H2 O, Keq ¼ 2219=T WO3 + 3H2 $ W + 3H2 O The numerical value of summary equilibrium constant Keq,sum may be calculated as a composition of four previous equilibrium constants for preceding stages. It measures for temperature T, K Keq:sum ¼
9:61 1011 1:26 1010 2:67 107 1:55 104 + T4 T3 T2 T
In the pusher furnace, tungsten oxide is loaded into flat metal “boats,” which are pushed through the horizontal furnace tubes (Fig. 23.2). Increased demand for tungsten products made it possible for WHB Company to modernize the reaction stage by replacing the manually operated pusher furnaces by automated furnaces (Fig. 23.3). The powder layer preserves its static state throughout the whole reduction process. Typical reduction temperatures range from 873 K to 1373 K. The boats are pushed at a rate usually in the range of 5 to 30 mm/min. The hydrogen flow is commonly in the countercurrent direction. Water vapor formed during reduction is carried away by the hydrogen flow. The cooler is located on the discharge end of the tube. Electrolytic hydrogen (collected in gas holders and freed from oxygen) and steam are used. As the hydrogen supply is greatly in excess of that required for the reduction (the efficiency is not >20%), the outgoing gas passes through a regeneration plant and returns to the process after drying. The boats are shifted in the direction of higher temperatures and less-damp water concentrations. Reduction rate is controlled by water vapor diffusion through the layer of WO3 powder, and then, transformation throughout the all layer section WO3 ! WO2.9 ! WO2.72 ! WO2 ! W is accomplished. The pusher-type furnace is very adaptable to the manufacture of different particle size powders and can quickly be adapted to produce the desired powder properties. By variation of reduction parameters such as temperature, powder layer depth in the boats, and boat speed, the particle size of the metallic tungsten powder can be controlled. Grain size of the WO2 particles depends significantly on the tungsten
FIG. 23.2 Multitube push type furnace.
Production of Refractory Metal Powders Chapter
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FIG. 23.3 Automatic push type furnace. (Courtesy: Wolfram Bergbau- und Hutten-GmbH.) €
particle size. Increase of the tungsten powder particle size is encouraged by high temperature and quick temperature increase along the furnace tube, high push rate, large depth of the powder layer, low hydrogen flow rate, and its high humidity. By using rotary furnaces, the tungsten oxide is directly introduced into the rotating furnace tube. An overall view of a rotary furnace used at the refining plant of WBH Company is shown in Fig. 23.4. The rotating furnace tube has, as a consequence, not a static but a dynamic powder layer. The depth of the layer is influenced by the feed rate, rotational speed, incline, and lifters inside the tube. The temperature range is comparable to that of pusher-type furnaces, and the hydrogen flow direction is also usually countercurrent. With respect to changes in particle size, the rotary furnace is not as flexible as the pusher furnace. A characteristic of application is the continuous production of powders, with medium size ranging from 1 to 3 mm.
FIG. 23.4 Rotary furnace. (Courtesy: Wolfram Bergbau€ und Hutten-GmbH.)
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FIG. 23.5 Morphological transformations of tungsten oxides during reduction at 1273 K. (Source: Spross M. Berg- und Huettenmaennische Montashefte. J Min Metall Mater Geotech Plant Eng 1996;141 (8):359–362.)
The intermediate oxides differ as regards both color and morphology. The latter is illustrated in Fig. 23.5. The cause of these morphological changes is the transport of tungsten via the gas phase, that is, chemical vapor transport (CVT) during the reduction sequence. Tungstic acid, WO2 (OH)2, has been identified as the most volatile compound in the system W-O-H and is responsible for the CVT. If the partial pressure ratio pH2O/pH2 is low enough, the reduction from WO2 to tungsten starts. From a thermodynamic point of view, WO2(OH)2 may form from both WO2 and tungsten, but the equilibrium pressure is lower above tungsten, which causes the CVT of WO2(OH)2 from WO2 to tungsten, where this vapor deposits as metallic tungsten, thus leading to growth of the tungsten crystal. The CVT mechanism permits the control of the metallic tungsten particle size through the interaction of temperature and humidity (partial pressure ratio pH2O/pH2), which is shown schematically in Fig. 23.6 for the pusher furnace [6,9,10].
Reduction by Solid Carbon Reduction follows the relationship shown in Fig. 23.7 and can be written: WO3 + 3CO ¼ W + 3CO2 3ðCO2 + C ¼ 2COÞ ________________________ WO3 + 3C ¼ W + 3CO, Keq ¼ P3 CO where Keq is equilibrium constant.
Relation temperature/humidity
Process parameters in push furnace
Low temperature
• Low temperature
Low humudity
High temperature High humidity
Tungsten transport via gas phase
• High H2 flow rate • Low oxide boatload
WO2(OH)2
Tungsten deposition W powder formation WO2(OH)2 + 3H2 = W + 4 H2O
Fine tungsten metal powder
• High temperature • Low H2 flow rate • High oxide boatload
FIG. 23.6 Control of the metallic tungsten particle size in the push furnace.
Coarse tungsten metal powder
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FIG. 23.7 Equilibrium compositions of gaseous mix of CO and CO2 depending on temperatures for CO–CO2 and CO–CO2–WO2–W systems. 1: C + CO2 $ 2CO; 2: WO2 + 2CO $ W + 2CO2.
FIG. 23.8 Push type furnace for tungsten anhydride reduction by carbon.
At temperatures lower than 1000 K, the oxide is not reduced because of insufficient partial pressure of CO. The temperature necessary to accelerate the process is above 1470 K. Reduction is accomplished in pusher type furnaces with carbon tubes (Fig. 23.8). Maximum temperature in the furnace is 2100 K; consumption power is 25 kW, daily yield is 350–380 kg [11]. To produce fine powders, the temperature is maintained in the range from 1670 K to 1770 K, the boats are moved through the high temperature zone rather faster, or some excess of black is injected. The powder produced contains inclusions of carbides and is friable. These powders are used for the production of tungsten carbide powders.
Precipitation from Gaseous Phase of Tungsten Hexafluoride and Tungsten Hexachloride The powder is produced from gaseous substances: l l l
at low temperature by dissociation of tungsten hexafluoride or tungsten hexachloride in hydrogen; by decomposition of tungsten hexachloride in inert gas (argon or nitrogen); by high temperature decomposition of tungsten hexafluoride in hydrogen fluoride.
The melting point of tungsten hexafluoride is 276 K, and its boiling point is 290.1 K. The melting and boiling points of tungsten hexachloride are 554.5 K and 623 K, respectively. Reduction of tungsten hexafloride takes place according to the reaction (HHal) formation: WHalx + 0:5 xH2 ¼ W + xHHal,
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FIG. 23.9 Flow diagram of the reduction process by precipitation from gaseous phase of tungsten hexafluoride and tungsten hexachloride in fluidized bed.
Keq ¼ pxHHal = p0:5x p MHal x H2 A flow diagram of the reduction process by precipitation from the gaseous phase of tungsten hexafluoride and tungsten hexachloride is shown in Fig. 23.9. The plant consists of a halide evaporator, reactor of fluidized bed and devices for regeneration and gaseous product collection, and a porous bottom made of sintered coarse tungsten powder, replaced sometimes by a layer of tungsten small fragments (1–5 mm) or a metal grate placed in the lower part of the reactor. Tungsten halide vapor from the evaporator, together with hydrogen and argon, is supplied by pump to the lower part of the preheated reactor where, at the bottom, tungsten fine powder (20–60 mm) as seed is found. During reduction, the seed is converted into granules which are discharged periodically or continuously. The hydrogen chloride or fluoride is collected in a scrubber. After discharge of the hydrogen and argon mixture from the desiccant, the hydrogen is absorbed by heated titanium sponge, and argon is returned to the cycle. The sponge is dehydrogenated periodically, and purified hydrogen is also returned to the cycle. The temperature in the reactor influences the degree of metal precipitation. However, increasing the temperature above 1170 K leads to eventual sintering of seed particles and, as a result, to the failure of the fluidized bed. Tungsten reduction from tungsten hexachloride is carried out at 1020–1070 K and from tungsten hexafluoride at 820–870 K. In the first case, the molar ratio of hydrogen to tungsten hexachloride in the gas-vapor mixture is maintained at approximately 30–40 to 1; while the molar ratio of hydrogen to argon is approximately 1.75 to 1; tungsten recovery amounts to 90%–99% and degree of precipitation on powder is 96%–97%. In the second case, the molar ratio of hydrogen to tungsten hexafluoride, maintained at 15–18 to 1, provides full recovery of tungsten. The gaseous flow rate must be in the range of 5–10 cm/s for fluidized bed formation. For the production of tungsten powders with a spherical particle shape, the metal is reduced from concentrated solutions of salts, oxides, or hydroxides. Spheroidizing takes place in the high temperature zone where powder is transferred by carrier gas. High purity tungsten nanopowders are produced from fluorine-containing compounds according to the following scheme: tungsten metal ion extraction from aqueous solutions (fluorine organic acids, ethers, amines, carbonyl acids, ketones and others), re-extraction by solutions of fluorine compounds (hydrogen fluoride, ammonium fluoride, ammonium hydrogen fluoride), and reduction of tungsten fluoride and tungsten oxyfluoride in hydrogen current. Metal powder particle size is approximately 30 nm. Similar powders are also produced by evaporation of volatile chlorides and hydrides in a reaction gas atmosphere (hydrogen, ethane, ammonia), heating being done by high-frequency induction or laser beam.
Production of Tungsten Powder by a Plasma Process The plasma process is realized at temperatures much higher than the boiling point of the basic materials. Tungsten is produced in a vaporous state. Powder is formed by condensation, coagulation, or crystallization accompanied by polymorphous transformations. Complete reduction to tungsten is achieved by processing tungsten trioxide in an argonhydrogen plasma (by the molar ratio of argon to hydrogen at 1 to 1) at temperatures ranging from 4270 K to 5270 K. A two-stage scheme is used for the production of powder with a narrow range of granulometric composition (0.6–2 mm) preserving its properties by storing in air. In the first step, the tungsten trioxide is preliminarily reduced at 2270 K to a
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FIG. 23.10 Electric arc device for tungsten and molybdenum oxides reduction by hydrogen.
residual oxygen content in the powder of 8%; in the second step, annealing at 1020–1070 K in reduction furnaces is carried out. Plasma plant (Fig. 23.10) contains a consumable tungsten electrode inside a chamber, in which is a direct current arc (200 A, 30 V). The powder particles formed with average size about 0.05 mm are carried to a bag filter by a stream of hydrogen-argon mixture. The capacity up to 60 kg/h of tungsten powder with particle size 0.03–0.05 mm is provided for by control of jet temperature and reaction area geometry. Tungsten powder produced in compound current of arc and radio-frequency plasmas has average particle size about 10 nm and impurities content not >33 ppm Fe, 23 ppm Mo, 17 ppm Si and 280 ppm carbon.
Amalgam Method Tungsten and molybdenum powders can be produced by reducing their pentachlorides by 2–5 wt% zinc amalgam (Fig. 23.11). The reduction reactions may be represented as follows: 2MoCI5 + 5ZnðHgÞ ¼ 2MoðHgÞ + 5ZnC12 + ðHgÞ 2WCl5 + 5ZnðHgÞ ¼ 2WðHgÞ + 5ZnCl2 + ðHgÞ Amalgam consumption is 110%–200% of stoichiometry reactions. The process temperature is a little bit lower than the boiling point of the zinc chloride formed (591 K). After metal reduction, the temperature is increased to 623 K and maintained for 30 min for growth of the crystals. Molten zinc chloride is floated on the amalgam surface, cooled, and then a
FIG. 23.11 Flow sheet of molybdenum and tungsten powder production by amalgam method.
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solidified cake of zinc chloride is separated from the amalgam surface. Excess zinc and mercury are distilled off in a vacuum by increasing the temperature to 1073 K.
Carbonyl Technique Tungsten and molybdenum carbonyl powders are produced by carbon monoxide action on their compounds (salts) at high pressures and elevated temperatures with metal carbonyl formation, and subsequently, its thermal decomposition with emission of dispersed metal. This process has been developed on enlarged laboratory and semi-industrial scales. Synthesis of carbonyls is done in liquid media. Fig. 23.12 shows a flow diagram of this process. The basic materials for this process are the following: higher metal chlorides (MoCI5 and WCl6), produced from ore concentrates, zinc powder as reducer, and dichlorethane or ether as solvent. After solvent cleaning, basic materials are fed into an autoclave in the following sequence: dichlorethane, metal chloride, zinc, and ether. For tungsten carbonyl production, the reaction components are charged in a weight ratio of tungsten hexachloride to zinc powder of 1 to 0.667 and volumetrical ratio of tungsten hexachloride to ether of 1 to 25. The process is carried out at 320–350 K and under 4–6 MPa of carbon monoxide pressure. On completion of the synthesis, carbon monoxide is removed from the autoclave, and the reaction mass is taken to the still where first, the solvents are distilled off, and then, carbonyl is evaporated off by live steam. Solvents then go to a regeneration unit and, after purifying, are returned to the process. Carbonyl is supplied to a water spraying capacitor, and the suspension is delivered to a nutsche filter and the filtrate exhausted to the collector by vacuum pump. Squeezed carbonyl is fed to the topping still, cleaned from iron pentacarbonyl (Fe(CO)5) by 1% aqueous solution of sodium hydroxide, and is distilled once more using live steam. Cleaned carbonyl is filtered in the nutsche filter, washed with distilled water and dried in a vacuum oven. The plant capacity is 200–500 kg per day; autoclave volume is 1–15 L, with carbonyl efficiency amounts to 80%–90%. There is also a method of dry synthesis from chlorides. The process uses iron swarf as the reducing agent and is conducted under carbon monoxide at a pressure up to 28 MPa and a temperature of 550 K. The advantage of this process is the absence of inflammable solvents. Tungsten and molybdenum powders are also produced by carbonyl decomposition under atmospheric pressure and at temperature ranges from 620 K to 1470 K in a gas current (hydrogen, nitrogen, and others). Sometimes, it is decomposed at reduced pressure without using a carrier gas. Powder efficiency is about 90%. The flow sheet of powder production in hydrogen is shown in Fig. 23.13. Crystalline carbonyl is charged into a horizontal sublimator of the “pipe-in-pipe” type with exterior heating. After sealing, the system is cleaned out using pure nitrogen. In the sublimator, carbonyl is heated up to temperatures ranging from 330 K to 400 K, and its vapor is carried by preheated and purified hydrogen to a decomposition unit provided with electroheating. Carbonyls are decomposed at temperatures ranging from 620 K to 670 K. The powder is collected in a container. Tungsten powder efficiency is 94%; molybdenum powder efficiency is 86%. More detailed information on the carbonyl process is contained in Chapter 6. FIG. 23.12 Diagram of plant for tungsten and molybdenum carbonyl synthesis in solvent media.
Production of Refractory Metal Powders Chapter
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FIG. 23.13 Flow sheet of powder production by carbonyl decomposition in hydrogen current.
Properties of Tungsten Powders Tungsten has two outstanding properties: the highest melting point of all metals at 3683 K and a very high density of 19.25 g/cm3 at 293 K. It cannot be shaped using traditional casting techniques. Consolidation of tungsten powders is based almost exclusively on powder metallurgical methods [12,13]. Therefore, definite and detailed powder specifications are very important in order to obtain the required final properties of a consolidated tungsten product. These specifications include physical and chemical properties. The basic physical properties are particle size, flow rate, and different powder density values. All the powder sizes, from ultrafine to hundreds of microns, are produced on an industrial scale, but the bulk of the powder used at present is in the size range 2–5 mm. Particle sizes in the range of 1–10 mm are measured by Fisher subsieve sizer (FSSS) (ASTM B 330 and MPIF 32 standards) [14] and correspond to particle sizes observed directly by optical microscopy (OM) (MPIF E 20 and GOST 23402-78 standards) and by scanning electron microscopy (SEM). SEM and OM are also the best ways to estimate the overall particle morphologies. Particle size distributions can be measured by light scattering techniques (ASTM B 822 standard) or by sedimentation based on X-ray monitoring of gravity sedimentation scattering (sedigraph) (ASTM B 761 standard). Controlling the reduction parameters make it possible to produce either narrow or wide size distribution and to provide any specific requirement on tungsten powders for various applications. Apparent density (ISO 3923 standard, and in accordance with it ASTM B 212, MPIF O4, and GOST 19440 standards), tap density (ISO 3953 standard, and in accordance with it, is accepted in ASTM B 527, MPIF 46, and GOST 25279 standards) as well as green density and green strength have great importance for determining the behavior of a powder during subsequent consolidation steps, including pressing and sintering. A feature of tungsten powder is a relatively high purity, in comparison with most other metal powders used for powder metallurgical products. Table 23.1 contains analytical data of typical tungsten powders produced by hydrogen reduction. Trace elements, including molybdenum, aluminum, arsenic, silicon, phosphorus, sodium, potassium, calcium, magnesium, and sulfur, are removed during ammonium paratungstate manufacture. The contaminants, such as nickel, chromium, iron, and cobalt, characteristic alloying elements of reduction boats or furnace tubes, have to be avoided. Physical analyses of typical medium sized tungsten metal powders are presented in Table 23.1. The characteristic morphology of the tungsten powders produced by hydrogen reduction is shown in Fig. 23.14. The particles have angular prismatic shape in either coarse or submicron sizes. In the powders reduced by solid carbon with stoichiometric carbon black consumption, the carbon content is decreased to 0.5 wt% and oxygen to 0.10–0.15 wt. Apparent density of these powders ranges between 3 and 7 g/cm3 by 1–10 mm powder particles size. Such powders may be used in hard alloy manufacture to produce tungsten carbide. Tungsten powders obtained by precipitation from the gaseous phase of tungsten hexafluoride and tungsten hexachloride (reduction of highest halides) with particle sizes between 0.05 and 0.80 mm and roundish shape are of high purity (Table 23.2) and differ significantly from tungsten powder reduced from oxides. Tungsten powders produced from WF6 in hydrofluoric medium and by WOx decomposition in hydrogen current are finer than particles produced by high-temperature WF6 dissociation or by high-temperature treatment of preliminary chlorinated wastes. High oxygen content in some “gas-phased” powders is caused by their high absorption properties in wet air.
<10
<20
<5
…
800
<10
<5
<50
<5
<5
<5
<20
<5
<5
<5
…
<5
<20
<10
<20
<5
…
800
<15
<10
<100
<5
<5
<5
<25
<5
<20
<5
…
<10
<25
Guar. Virgin
4.4 0.3
2.5 0.3
0.8 0.1
1.0 0.1
W 10
<20
<30
<5
…
800
<5
<10
<150
<5
<5
<5
<20
<10
<100
<5
…
<10
<20
Typ. reclaimedb
99.95
4.8 0.3
2.8 0.3
1.4 0.2
1.5 0.2
W 15
<20
<30
<10
…
800
<15
<15
<200
<5
<5
<5
<30
<15
<100
<5
…
<15
<20
Guar. Reclaimed
5.2 0.4
3.3 0.3
2.4 0.3
2.5 0.3
W 25
b
Tungsten powder made from tungsten ore. € Tungsten powder made by recycling sintered carbide scrap; Companies: WBH: Wolfram Bergbau- und HuttenGmbH; OSRAM: OSRAM Brunta´l spol. s r. o.
a
<5
<5
ppm
Sulfur
<10
ppm
Silicon
<10
ppm
Potassium
…
ppm
Phosphorus …
ppm
Oxygen …
<5
<5
ppm
Nickel
…
…
…
ppm
Sodium
250
<20
<20
ppm
Molybdenum
2650
…
…
ppm
Manganese
…
…
ppm
Magnesium
…
…
ppm
Lead
<5
<5
ppm
Iron
…
…
ppm
Copper
<5
<5
ppm
Cobalt
…
…
ppm
Chromium
…
…
ppm
Carbon
<2
<2
ppm
Calcium
<5
<5
ppm
Aluminum
Typ. Virgina
99.98
3.8 0.3
2.0 0.2
0.5 0.1
0.6 0.1
W 06
OSRAM
6.0 0.4
4.0 0.3
3.3 0.3
3.5 0.3
W 35
6.3 0.5
4.5 0.4
4.2 0.3
4.5 0.3
W 45
6.5 0.8
4.7 0.5
3.5 1.0
6.5 1.0
W 65
SECTION
Trace impurities:
wt%
Tungsten min. Excluded oxygen
…
…
g/cm
Tap density
…
…
3
g/cm
Scott density
…
… 3
mm
Lab milled
48
1 mm Coarse
1
mm
Unit
As supplied
Particle sizes FSSS
Property
WBH
Source: Standard Grades
TABLE 23.1 Properties of Typical Tungsten Powders Produced by Conventional Calcination/Reduction Process 696 D Production of Non-Ferrous Metal Powders
Production of Refractory Metal Powders Chapter
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FIG. 23.14 Scanning electron micrographs of coarse and submicron tungsten powders produced by hydrogen reduction.
TABLE 23.2 Composition of Typical Tungsten Powders Produced by Reduction of Highest Halides Technique [7] Property
Unit
LTF
LTHL
HTF
Average particle size
mm
0.8
0.1
0.2
Aluminum
ppm
…
…
14
Arsenic
ppm
…
…
80
Calcium
ppm
30
…
12
Chromium
ppm
…
7
…
Fluorine
ppm
130
…
900
Iron
ppm
1500
600
130
Magnesium
ppm
10
20
…
Manganese
ppm
30
5
…
Molybdenum
ppm
…
320
1300
Oxygen
ppm
200
6500
…
Silicon
ppm
10
60
300
Titanium
ppm
100
…
…
Trace impurities:
LTF: Low temperature dissociation by reduction of tungsten hexafluoride in hydrogen; LTHL: Low temperature dissociation by reduction of tungsten hexachloride in hydrogen; HTF: high temperature decomposition of tungsten hexafluoride in hydrogen fluoride medium.
Properties of carbonyl powders are shown in Table 23.3. Tungsten and molybdenum powders produced at decomposition temperature 620–670 K in hydrogen are represented by 60%–70% of particles with round shape with rough and sharp spurs on the surface with mean particle sizes of 2–4 mm (Fig. 23.15). Tungsten powders contain 1.4–2.2 wt% carbon and up to 2.0 wt% of oxygen, while molybdenum powders contain 3.4–3.9 wt% carbon and up to 5.8 wt% of oxygen. Particles of tungsten and molybdenum powders reduced at temperatures in the range from 670 K to 1170 K in an argon current are spherical and non-uniform in size, especially at low decomposition temperatures. The surface of molybdenum particles is comparatively smooth, while the surface of tungsten particles is uneven and includes some separate spheroids <1 mm in size. The particles become significantly smaller in size, and specific surface increases on increase of the
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TABLE 23.3 Variation of Properties With Decomposition Temperature for Carbonyl Tungsten Powders Decomposition Temperature in Current of Argon (K)
Color of Powder
Average Particle Size (mm)
Specific Surface Area (m2/g)
Carbon Content (wt%)
Lattice Spacing (nm)
673
Gray
1.2
0.18
1.08
0.4178
773
Gray
1.0
0.21
1.00
0.4172
873
Dark-gray
0.74
0.30
1.50
0.4167
700
Black
0.12
1.56
1.83
0.4163
973
Black
0.07
3.00
1.82
0.4159
1073
Black
0.03
8.20
…
…
2273–4243
Black
0.01–0.03
…
0.50–1.00
…
(a) SHF: discharge in nitrogen current.
FIG. 23.15 Scanning electron micrographs of carbonyl tungsten (A) and molybdenum (B) powders.
decomposition temperature from 670 K to 1170 K; for example, for tungsten powders, the average size decreases from 1.2 mm to 0.03 mm, and the specific surface increases from 0.18 m2/g to 8.2 m2/g, respectively. Carbon content in tungsten powders obtained in argon is 1.9 wt%, while in molybdenum powders, it amounts to 3.5 wt%. The addition of an oxidizing component to the carrier gas decreases the carbon content in the powder. Sintering temperatures of carbonyl tungsten and molybdenum powders are some hundreds of degrees lower than that of powder produced by other methods. They are used for the production of porous and compact parts by powder metallurgy. Metal carbides, for example, ZrC and other abrasive materials such as boron nitride, can be coated with tungsten or molybdenum by thermal decomposition of their carbonyls resulting in improved of service performance. For example, the hardness of cutting wheels of cubic boron nitride metallized by tungsten is increased twofold. Diamond grains metallized by tungsten increase their cohesion in instrument assembly, preventing diamond chipping. Other methods of abrasives metallization are not effective enough because they do not provide good coating cohesion. The quantity of carbon and oxygen in the powder depends, first of all, on the process conditions in its production. Peak of carbon content is observed at about 970 K connected with free amorphous carbon formation because of catalytic decomposition of the carbon oxide. Carbon content can be decreased by a process of carbonyl W(CO)6 decomposition in superhigh-frequency discharge at temperatures ranging between 3043 K and 4000 K in a nitrogen current. The particles have a
Production of Refractory Metal Powders Chapter
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699
spherical shape and their size ranges from 12 nm to 30 nm; nitrogen content is 0.3 wt%, and oxygen content is not >1 wt%. The content of other impurities does not exceed the value 1.0 104 wt%. Carbonyl tungsten powders are metastable carbides with a face-cantered cubic lattice. Their properties change with increasing temperature; thus, lattice spacing decreases (see Table 23.3). Powders obtained by plasma-chemical reduction are characterized by high dispersivity; particles of an average size of about 0.07 mm form weak conglomerates with a developed specific surface and a very low apparent density of 0.25 g/cm3. When first produced, a large quantity of hydrogen chloride is absorbed on the powder surface, which volatilizes during storage in air, while the oxygen concentration is increased to 4%–8%. Nanopowders of approximately 20 nm average particle size are produced by melt atomization in arc argon plasma. Tungsten particles of 510 nm sizes have face-cantered or hexagonal close-packed lattice in comparison with typical body-centered for solid state. Tungsten powders obtained through high-temperature reduction in argon-hydrogen plasma represent double component systems consisting of a-W (body-centered lattice) and b-W (A-15) with main peaks (210) of b-phase and (110) of a-phase.
Tungsten Powder Applications Ultrahigh purity powders are used for manufacturing semiconductors and high-performance electrodes of high-intensity discharge (HID) lamps to avoid outgassing of impurities and guarantee a consistent lamp quality with increased lifetime. In particular, the incorporation of alpha emitters such as uranium and thorium is extremely crucial and must be held below 1 ppb [15]. The mobile alkali ions (for instance, sodium and potassium) have to be held below 0.1 ppm. Purification of APT by means of multiple extraction and crystallization steps ensures the required purity, so all production steps must be carried out in clean room conditions [6,16]. In the work [17], a method based on ammonium paratungstate (APT) row material, used for the production of ultrahighpurity tungsten powder (6N W), was developed. The APT was reduced by hydrogen to high-purity tungsten metal powder (Fig. 23.16). Two types of high purity tungsten material for electrodes in HID were prepared. One was a tungsten rod with a diameter in the range of 12–40 mm, which was processed by powder compaction with cold isostatic pressing (CIP), sintering at a temperature higher than 2273 K in an induction heating furnace under a hydrogen protection atmosphere, and hot forging. The other was a tungsten wire with a diameter in the range of 0.02–12 mm, which was fabricated by powder compaction by CIP, self-resistance sintering at a temperature approximately 2970–3070 K, swaging, and wire drawing. The Fisher subsieve sizer (FSSS) particle size of the produced 6N W powder is in the range of 2.2–3.2 mm, and the apparent density is approximately 2.5 g/cm3. The SEM image of the powder morphology is shown in Fig. 23.17. The d10, d50, and d90 of the tungsten powder (analyzed using a laser particle size distribution analyzer) are 2.027 mm, 5.161 mm, and 15.816 mm, respectively. The flowablility of the 6 N W powder is suitable for producing tungsten rods, wires, and targets. The purity (excluding the gaseous ingredients) of the tungsten rods and wires is 5 N or higher (Table 23.4). Tungsten is currently the most promising material for the first wall in demonstration fusion reactors (DEMO) and as a plasma facing material (PFM) in future fusion reactors [18]. Feasibly utilizing tungsten in the next generation of fusion reactors will require components with a thin tungsten layer. Coating technologies such as vacuum plasma spray (VPS), physical vapor deposition (PVD), and chemical vapor deposition (CVD) have been studied in recent years [19–21]. High purity (>99.9999%), full density (19.23 g/cm3) tungsten parts and coatings were manufactured with chemical vapor deposition (CVD) method with a deposition rate higher than 0.6 mm/h [22]. The thermal conductivity of the CVD-W material with a columnar grain structure is higher than that of forged tungsten, but the coefficient of thermal expansion (CTE) of both types of tungsten is very similar. Thermal shock tests show that the threshold energy of CVD-W for crack initiation is between 1.5 and 2.25 MJ/m2, while that of forged tungsten is lower than 1.1 MJ/m2, because the pyramidal shapes of the surface of CVD-W coating can withstand a greater level of distortion, preventing the appearance of cracks. The high melting point (3683 K) of all metals, low tritium retention, high thermal conductivity, and the high low erosion rate under plasma loading makes tungsten a suitable PFM. Pure tungsten powders: Pure tungsten powders are made into rods by pressing and sintering. In order to achieve high densification, a high temperature in the range of 2273–3373 K is required [2,6]. The sintered tungsten bars thus obtained are forged or rolled at elevated temperatures in order to produce rods (which are subsequently drawn to manufacture wires) or sheets. Such pure tungsten materials are widely used for high temperature applications, for example, in high-temperature furnace parts [2,6]. For these materials, powders produced by the highest halide reduction can be used. Non-sag tungsten wires: To prevent sagging of tungsten filaments in incandescent lamps during high temperature operation, doping elements—aluminum, silicon, and potassium—are included in the powder [3,23]. The present technique consists of adding an aqueous solution of compounds containing aluminum, silicon, and potassium into the tungsten oxide
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FIG. 23.16 Flow sheet of the process of ultrahigh purity tungsten products.
FIG. 23.17 SEM micrograph of the ultrahigh purity tungsten (6N W) powder.
Production of Refractory Metal Powders Chapter
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TABLE 23.4 Purity of the High Purity Tungsten products Product
Tungsten Powder
Tungsten Rod
Tungsten Wire
Purity (tungsten element content, wt%)
99.999931
99.99989
99.999922
powder before reduction to tungsten metal. During this reduction, a portion of the above trace elements is incorporated into the tungsten metal powder particles. During the sintering of the pressed tungsten powder, aluminum and silicon evaporate, while the potassium atom is too big to diffuse through the tungsten lattice. This process results in the incorporation of potassium droplets as inclusions. During wire drawing, the small potassium droplets are elongated and finally break into a multitude of little tiny potassium bubbles in lines along the wire axis, which results in an elongated interlocked grain structure. During recrystallization at elevated temperatures, the tiny potassium bubbles effectively stabilize through an effect similar to dispersion hardening, the movement of dislocations, and thus, the migration of the grain boundaries perpendicular to the wire axis. Large inclusions in the metal particles generate large potassium pores in the sintered ingot from which long bubble rows are formed in the wire. This characteristic structure explains the non-sag properties of the tungsten filaments in incandescent lamps [2,6,23–26]. Spheroidization of tungsten powders: The spherical tungsten powders exhibit outstanding properties such as excellent flowability, high bulk density, and low sintering shrinkage, making them suitable for abroad range of applications such as thermal spraying, additive manufacturing, etc. The thermal plasma technique has been proven effective for preparing spherical tungsten powders because of its high temperature and high enthalpy [27–29]. Plasma spheroidization of tungsten powders consists of two main processes: first, the raw tungsten powder, injected into the high temperature region of radio frequency (RF) thermal plasma, is heated and melted to form spherical droplets (due to surface tension), and then, the melted droplets solidify into spherical particles due to rapid quenching when they spurt out of the plasma. However, not all powder particles can be melted completely, despite the plasma temperature being much higher than the melting point of tungsten. Possible reasons for this incomplete melting could be that the powder particles cannot absorb sufficient plasma energy due to the non-uniform temperature inside the plasma torch, the local cooling effect of plasma by the cold carrier gas and flow of powder particles [28], or insufficient optimization of experimental parameters. The paper [30] describes a study of the spheroidization of tungsten powders by radio frequency thermal plasma, including the melting, solidification, and growth behavior of the tungsten powder particles during the spheroidization process. The flight time and melting time of tungsten powder particles in the plasma were estimated, and the growth behavior of the particles, specifically, the change in the average tungsten powder particle size before and after plasma spheroidization, was analyzed. The flight time and melting time for tungsten powder particles with radius of 7.8 mm were calculated to be 9.3 ms and 2.9 ms, respectively. The particle size distribution before and after spheroidization were determined (Table 23.5). It can be seen (Fig. 23.18A) that the raw powder particles are irregular, and the particles after RF processing (Fig. 23.18B) possess a spheroidal shape with few satellites present on the rough particles’ surfaces. The authors reported [30] that the change in powder particle size during the process showed that growth of tungsten powder particles was mainly caused by the coalescence of droplets in the thermal plasma system. The experimental results demonstrated that the spheroidization rate could reach up to 95% under the operating conditions used. Tungsten composite materials: Tungsten metal powder blended with other powders, such as copper, iron, cobalt, and nickel, form so-called “tungsten heavy alloys” (WHAs), which typically contain 90–98 wt% W and can be liquid phase sintered at about 1773 K. Their properties include high density (17–19 g/cm3) and ductility, and energy penetration and
TABLE 23.5 Particle Size Distribution of the Tungsten Powders Before and After Spheroidization Undersize (mm) of Cumulative Distribution Powder Condition
d5
d10
d50
d90
Raw powder
5.78
7.09
15.60
29.02
In-flight particles
…
…
20
…
Spheroidized powder
4.52
7.11
28.79
68.02
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D Production of Non-Ferrous Metal Powders
FIG. 23.18 Field emission SEM images of tungsten powders: (A) raw powder, (B) spheroidized powder.
radiation shields are the application fields of these tungsten materials [6]. WHAs are typically formulated from elemental powders with mean particle sizes in the range of 1 mm to 7 mm. Because of exclusive high melting temperature of tungsten, PM manufacturing methods are predominantly used for such materials. Infiltration (INF) is used exclusively for making tungsten-based composite contact materials. Refractory metal powder is first blended to the desired composition with or without a small amount of binder to impart green strength, then it is pressed and sintered into a skeleton of the required shape. Silver or copper is next infiltrated into the pores of the skeleton. This technique produces the highest density composites, generally 97% or more of theoretical density. Because of the presence of some closed pores in the sintered skeleton, complete densification is not possible. The press-sinter method can be used for small refractory metal parts (not exceeding about 25 mm in diameter). A high density material is obtained by pressing a blended powder of the exact final composition into shape and then sintering it at the melting point of the low-melting-point component (liquid-phase sintering). In some cases, an activating agent such as nickel, cobalt, or iron is added to improve the sintering effect on the refractory metal particles. For this process, powders of much finer particle size are preferable to enable more bonding surface. However, material formed by this process is weaker compared that formed via the infiltration process. The press-sinter-re-press (PSR) process is used for all categories of contact materials, especially silver-based materials. Blended powders are compacted to the desired shape and then sintered. The materials are further densified by a second pressing (re-pressing). Sometimes, the properties can be modified by a second sintering or annealing. The flexibility of this process makes it applicable to parts of any configuration and of any material. Material thus produced may also have weaker bonding between particles. The first heavy alloy developed was a W-Ni-Cu alloy. Alloys of this ternary system are still used occasionally, mainly for applications in which ferromagnetic character and resistance must be minimized. However, the present industry standard W-Ni-Fe exceeds the above alloy in corrosion resistance and mechanical properties. Most of the current purposes for WHAs are best satisfied by the W-Ni-Fe system. Alloys such as 93W-4.9Ni-2.1Fe and 95W-4Ni-1Fe represent common compositions. Mechanical properties depend on the nickel-to-iron ratio. They are sensitive to hydrogen embrittlement due to the body-centered cubic (bcc) structure of the principal tungsten phase [31]. By means of the nickel-to-iron ratio selection and post-sinter heat treatment, optimal mechanical properties are obtained. There are two common post-sinter heat treatments for WHAs. Vacuum heat treatment is useful for the reduction of hydrogen embrittlement, as is resolution and quench for reduction of segregation-induced embrittlement. If alloys are subjected to vacuum heat treatment, nickel-to-iron ratios in the range of 2.3–4 should be chosen, as better mechanical properties are generated [32,33]. For example, 91W-6.3Ni-2.7 Fe alloy ensures an ultimate tensile strength of 940 MPa, elongation of 35%, and hardness of 29 HRC. The addition of cobalt to a W-Ni-Fe alloy is a common approach for a slight increase of both strength and ductility. The presence of cobalt provides solid solution strengthening of the binder and slightly enhanced tungsten-matrix interfacial strength. Even higher mechanical properties are attainable from the W-Ni-Co system with nickel-to-cobalt ratios ranging from 2 to 9 [34]. These alloys provide high static tensile strength, ductility and hardness, for example, for 91W-6Ni-3Co alloy, ultimate tensile strength is 960 MPa, elongation is 40%, and hardness is 31 HRC. The use of such alloys is generally limited to defense applications requiring advanced mechanical properties. A number of other special WHAs are known, too. An example is an addition of molybdenum to the W-Co-Ni-Fe quaternary alloy to restrict tungsten dissolution and spheroid growth, resulting in higher strength, but reduced ductility, in the sintered state [35]. The addition of boron in the form of NiB and FeB to W-Ni-Fe system alloys leads to a small increase of
Production of Refractory Metal Powders Chapter
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hardness in some cases, in particular, for W-7NiB-3Fe and W-7NiB-3FeB compositions [36]. The sintering temperature selected for these alloys was 1773 K. The structural and mechanical properties of 90W-7Ni-3Fe alloy compacts produced using a high-voltage electric pulse sintering technique were studied [37]. High-voltage electric pulse consolidation (HVEPC) involves passing a high-voltage discharge passage (30 kV) with a high current density ( 100 kAcm2) and short pulse duration (<300 ms). This current pulse is passed through a powder filling during the application of external pressure. The rapid increase in temperature at the areas of contact of the powder particles leads to their instantaneous welding. In this study, the average particle size of selected heavy alloy was equal to 6.03 mm; powder density was equal to 17.13 g/cm3. The value of the applied voltage varied from 4.5 kV to 5.8 kV. The pressure applied to the filling was equal to 45 MPa. It was reported [37] that the relative density of the sintered compacts is in the range of 94.9%–95.9% of theoretical density. The highest density increase values and increased microhardness of compacts are achieved at the maximum value of discharge 5.8 kV, although stabilized density values were obtained at 5.6 kV. Further increase in the microhardness by increasing discharge voltage leads to embrittlement of the material. The microstructure of the cylindrical shape samples is shown in Fig. 23.19. The average grain size is approximately 10 mm; that is, high-voltage sintering contributes to maintaining a fine-grained structure, more uniform distribution of iron-nickel binder, and almost total absence of porosity. Porous skeletons of pressed tungsten metal powder infiltrated with copper alloys or silver forming W-Cu as well as W-Ag composites are widely used as electrical contacts [38–40] due to their high electrical conductivity, low thermal coefficient of expansion, and their resistance to damage caused by electrical arc formation. Many researchers have tried to obtain W-Cu alloys of full density without sintering additives by use of fine tungsten powder and increasing the homogeneity of the W-Cu powder in order to facilitate rearrangement of tungsten particles in liquid copper during liquid-phase sintering [41,42]. Through mechanical alloying of W/Cu or tungsten oxide and copper oxide, homogeneous W-Cu composite powders with nanosized tungsten particles can be produced, and fully densified specimens can be fabricated. W-(25/30)Cu composite powders were fabricated by the sol-spray drying and subsequent hydrogen reduction, then pressed and directly sintered [43]. The authors reported that when sintering time (ranging from 15 min to 120 min) and temperature (between 1573 and 1693 K) are increased, relative densities of W-(25/30)Cu increase. When sintered at 1693 K for 120 min, relative densities of W-25Cu and W-30Cu composite materials are 98.09% and 99.13%, respectively. According to kinetic characteristics during sintering, the first stage of W-Cu liquid sintering finishes at some time from 0 to 30 min into the process. During the sintering process, spherical tungsten grains grow and uniformly distribute into the matrix Cu phase. According to liner fitting of W grain sizes, grain growth of W-25/30Cu composite materials conforms to the dissolve-precipitation mechanism during liquid phase sintering. BE-SEM images of W-30Cu sintered at 1693 K for 30–120 min show (Fig. 23.20) that nearly spherical tungsten particles are uniformly distributed by copper phase, and with sintering going on, tungsten particles grow. In the work [44], W-30 wt% Cu alloys were fabricated using cold pressing and infiltration sintering methods from two types of powders, that is, mixed copper-tungsten (abbreviated name: M-Cu-W) powders and newly developed coppercoated tungsten composite (abbreviated name: Cu@W) powders. The latter were prepared using the thermo-electroless method [45]. Tungsten-copper mixed powders with an average particle size of tungsten 6–8 mm and copper-coated tungsten powders with an average particle size 4–10 mm were used to fabricate bulk W-30Cu alloys through infiltration sintering at FIG. 23.19 Microstructure of the cylindrical shape sample of 90W7Ni-3Fe alloy compacted by high-voltage electric pulse consolidation.
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D Production of Non-Ferrous Metal Powders
FIG. 23.20 SEM images of W-30Cu composite sintered at 1653 K for 30–120 min: (A) 30 min, (B) 60 min, (C) 120 min.
1623 K for 90 min under a hydrogen atmosphere. The polyhedral structures of tungsten particles are observed (Fig. 23.21A), while the surface of roundish copper particles appears rough (Fig. 23.21B). The cross-section image of the coated powder (Fig. 23.21C) shows that the copper phase is coated onto the surface of tungsten particle with coating thickness in the 2–6 mm range. Microstructures of W-30Cu alloys fabricated by the infiltration sintering method are shown in Fig. 23.22, in which the gray-white particles are tungsten phase, and the black ones are copper phase. It can be seen in Fig. 23.22A that the copper
FIG. 23.21 The morphology of copper coated tungsten composite powders: (A) pure tungsten powders, (B) copper coated tungsten powders, (C) cross section of the coated powder. (Modified from Chen W, et al. Infiltration sintering of WCu alloys from copper-coated tungsten composite powders for superior mechanical properties and arc-ablation resistance. J Alloys Compd 2017; 728(25 Dec): 196–205.)
FIG. 23.22 Microstructures of W-30Cu alloys fabricated by the infiltration sintering method: (A) mixed copper-tungsten powder; (B) copper-coated tungsten composite. (Source: Chen W, et al. Infiltration sintering of WCu alloys from copper-coated tungsten composite powders for superior mechanical properties and arc-ablation resistance. J Alloys Compd 2017; 728(25 Dec): 196–205.)
Production of Refractory Metal Powders Chapter
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phase in the (M-Cu-W) alloy has a certain segregation (denoted by arrow). The copper particles are distributed among the tungsten particles with size distribution in the 40–100 mm range. In contrast, in the (CuW) alloy (Fig. 23.22B), the tungsten particles are distributed more homogeneously, and no obvious agglomerates can be observed. Chen et al. [44] reported that the arc-erosion resistance of the CuW alloy is better than that of the (M-Cu-W) alloy. The ablation area of the former was larger, and its ablation crater was shallower, and the arc erosion is mainly attributed to the evaporation of the low melting point of copper. For the (M-Cu-W) alloy, arc erosion was mainly attributable to the sputtering of copper. A mechanothermochemical process using the liquid solution as starting material and a grinding process using salt-free oxide powder enables the production of ultrafine, homogeneous W-Cu composite powders [46]. The spheroidal composite W-Cu oxide powder is produced by spray drying an aqueous solution of tungsten and copper salts, and subjecting it to subsequent oxidation. The spheroidal structure of the oxide powder is destroyed during ball milling, and W-10 wt% Cu composite with nanosized particles is produced through reduction of the ground oxide powder at 974 K in hydrogen. But as the copper content in W-Cu composite powder decreases, densification of W-Cu powder compacts is difficult due to pore formation by local densification in agglomerated powders. W-10 wt% Cu-based oxide powders with tungsten trioxide addition before reduction sintered at 1673 K in hydrogen can achieve a sintered density of 17.05 g/cm3, which amounts to 98.5% of theoretical density. The thermal and electrical conductivities of such a composite are 203 W/ (mK) and 36.5%IACS, respectively [46]. To fabricate tungsten-copper nanocomposite powder, the tungsten-copper oxide mixture ball-milled in argon was reduced at a low temperature of 573 K [47]. The reduced W-Cu nanocomposite powder was consolidated at 1473 K for 1 h with a heating rate of 10 K/min. It has been shown recently that porous tungsten can be obtained at approximately 1473 K, a much lower temperature than the 2273 K used in the conventional process. This has been achieved by employing reactive sintering. Reactive sintering method makes use of the addition of aluminum, the low melting phase of which decreases the process temperature while acting as a potential sintering aid. As a result, higher homogeneity and uniform porosity distribution have been obtained [48]. High-energy ball milling is a well-known technique for producing oxide-dispersion strengthened materials. Due to activation of a material during grinding without oxide and metal powder admixtures, this method has essential advantages [49–51]. It makes the sintering stage easier, resulting in both higher rate and lower temperature. The reduced size of mechanically ground particles and their high density in lattice defects promote the sintering transport phenomena. It also leads to finer and more homogeneous oxide dispersion. This latter feature, together with the nanocrystalline structure of the ground powders, results in a small grain size after sintering. As reported [52], long milling times on the order of 30 h of W-1 vol% Y make it possible to reach a relative density of 96.5% after sintering for 4 h in vacuum at 2073 K, while within a milling time of 80 min, the relative density amounts only to 76.5%. Recent developments [53,54] define the production of W/Cu composite powders which can be directly pressed to shape and sintered, being produced without infiltration or machining, which is of particular importance with respect to their application as heat sinks in the semiconductor industry. Tungsten with oxide dispersions is manufactured with the addition of thorium-containing compounds, such as thorium oxide or thorium nitrate. The mixtures consisting of tungsten metal powder and the above additives are pressed, sintered, and further processed into wires or sheets. The main application of this material is welding electrodes, which, due to the high electron emission ability of thorium, ensure easier arc formation, and offer stabilization and improved physical properties arising from dispersion hardening in sheets and wires, particularly for vibration-resistant automotive lamps [2,6]. Because of the radioactive danger of thorium, alternatives have been developed. Cerium and lanthanum have been tried as potential candidates to replace thorium in welding electrodes. Properties of typical tungsten based composites are shown in Table 23.6.
Recovery of Metallic Tungsten Virtually almost all types of tungsten products, if available in large enough amounts and workable, are presently recycled [4,6]. Even unselected, low-quality scrap containing enough tungsten can be used as a feed material for the chemical ammonium paratungstate production process, electrochemical recovery, or for alloying in steel. But any possible, practicable use for selected tungsten scrap is, of course, desirable for economic reasons. Typical kinds of tungsten scrap include: l
Selected manufacturing scrap of pure tungsten can easily be crushed in high energy mills and directly re-used for applications with lower quality demands.
INF
INF
44Cu-56W
30Cu-70W
PSR
91W-6Ni-3Co
… …
…
13.85–14.18
12.76
11.90–11.96
17.25
14.20–14.77
12.10–12.60
…
14.45
12.87
11.39
17.81
12.84
…
…
36–51
55
45–63
29–35
49
47–50
45–53
60–65
72–80
85–90
Electrical Conductivity (% IACS)
31 HRC
29 HRC
86–96 HRB
79 HRB
60–81 HRB
95–105 HRB
90 HRB
55–65 HRB
80–93 HRB
50–60 HRB
40–47 HRB
25–38 HRB
Hardness
… 827 572
… … …
960d
[34] [34]
…
C, A
C
A
C
C
C
C
A
A
A
Data Source
…
1000
…
940c
827
…
758
434
…
379
896
…
…
483
…
Modulus of Rupture (MPa)
…
Tensile Strength (MPa) b
Defense applications
Arcing tips, vacuum interrupter, oil circuit breakers Circuit breaker runners, arcing tips, tap change arcing tips
Motor governor, semiconducting material
Motor starters, aircraft equipment, circuit breakers, contactors, computers, arcing tips
Automotive starting switches, circuit breakers
Current-carrying contacts in circuit breakers’ lightduty contactors
Application Examples
INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. c Elongation 35%. d Elongation 40%.
a
PSR
91W-6.3Ni2.7Fe
Ternary alloys
INF
50Cu-50W
Tungsten-copper
PSR
10Ag-90W
15.56
…
PSR
INF
14.92
INF
35Ag-65W
27.5Ag-72.5W
14.65–14.74
14.92
PSR
12.00
60Ag-40W
12.16
PSR
10.60–11.30
Typical
70Ag-30W
11.27
Calculated
PSR
Manufacturing Methoda
85Ag-15W
Tungsten-silver
Nominal Composition (%)
Density (g/cm3)
TABLE 23.6 Properties of Typical Tungsten Base Composites
Production of Refractory Metal Powders Chapter
l
l
l
l
l
23
707
Chips of heavy metal, obtained during the shaping operations, can be transformed into re-usable powder by means of oxidation and subsequent reduction. Large solid pieces can be machined into smaller pieces to enable subsequent recycling. The oxidation step leads to a total disintegration of the chips into oxide powder containing tungsten trioxide and tungstates of iron, nickel, or cobalt. Hydrogen reduction results in a metal powder, in which all the components (W/Fe/ Co/Ni) of the initial heavy metal are presented. Composite scrap mixtures, such as W/Cu, are difficult to oxidize, and are, therefore, not suitable for chemical recycling. Also, re-using in the steel industry cannot be realized due to the deleterious effect of copper in steel. A possible option is salt melt digestion with sodium nitrate, sodium nitrite, and sodium carbonate mixtures. However, this process presents environmental problems due to the nitrogen oxide formation. An alternative would be the electrochemical recovery of the tungsten. The slimes recovered from grinding high-speed tool steel, such as those containing up to 6 wt% tungsten, are de-watered by hot air and oxidized in a tube furnace in oxygen (0.05 m/s) with self-heating up to 1220 K. The oxides are vaporized at a temperature higher than 1670 K and condensed on cold screens. Tungsten extraction yield reaches up to 86% after 6 h at 1620 K. The condensate from tungsten and molybdenum oxides is reduced in hydrogen for 2 h at 1173 K. This results in powders with an average particle size of 15 mm and the following element content: 50 wt% tungsten, 50 wt% molybdenum, 0.02 wt% vanadium, 0.02 wt% chromium, and 0.1 wt% iron. The recovery process for tungsten bearing wastes includes the following steps: oxidation in hot air; milling of formed mixture of tungsten oxide and cobalt-tungsten oxide; and reduction at T < 947 °C in endogas or dissociated ammonia with an addition of natural gas. The recycling of composites of tungsten with thorium-containing compounds has problems due to radioactivity. Therefore, direct recycling in the steel industry is impossible. Chemical recovery in the conventional process leads to inadequate separation of thorium, and consequently is also inapplicable. At present, the only possible way to recycle this material is by electrolysis, such as through the method based on the electrolytic dissolution of tungsten in ammonia [55]. The difficulty is carrying out the process in such a way as to prevent dissolution of any of the radioactive products.
Accident Prevention When Working with the Tungsten Powder Metallic tungsten dust is not toxic and is ranked in the fourth category according to State Standard 12.1.005-88 under the legislation in the Commonwealth of Independent States (CIS) [56]. Tungsten inspirable dust takes a predominantly fibrous effect. The limit for dust in the atmosphere of the work place unhealthy is 6 mg/m3. Under the Workplace exposure limits in the European PM industry, the long- and short-term (15-min reference period) workplace exposure limits (WELs) for tungsten and insoluble compounds (as W) in the workplace atmosphere are 5 mg/m3and 10 mg/m3, respectively [57]. Tungsten in drinking water is also unhealthy and falls into the second category. A limit of 0.05 mg/L is specified in CIS State Standard 4630-88 [58]. Fine oxidizable powder mixed with air can constitute an explosion hazard, but the risk with tungsten powder is minimal even with powder fraction <74 mm; a minimal ignition temperature of powder deposits (self-ignition temperature) is 583 K. The products with such characteristics of inflammability and explosion risk are relegated to the “low explosion hazard” class of danger according to the Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used for pollution prevention and environmental control during powder manufacture and its processing. Detailed information on health and environment protection measures and on the prevention of inflammability risk in such conditions can be found in the Chapter 27.
Tungsten Carbide Powders Tungsten carbide powder is the basis for the production of cemented carbides, the vast class of hard, wear-resistant, refractory materials, in which the hard carbide particles are bound together, or cemented, by a soft and ductile metal binder. Conventional manufacturing of tungsten carbide powders accounts worldwide for the bulk of the tungsten carbide powder produced. This technique permits the production of tungsten carbide powders with particle sizes ranging from 0.15 mm to 50 mm [60]. This field offers all industrial applications and is described below, along with other methods.
Production of Tungsten Carbide Powders Conventional technique: There are two methods [15,61] by which tungsten carbide powders are produced from pure tungsten powder. In the first method, fine tungsten powders are blended with high purity carbon black, filled into graphite boats, and pushed through a high temperature furnace under hydrogen at temperatures between 1673 K and
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D Production of Non-Ferrous Metal Powders
FIG. 23.23 Single tube push type furnace for tungsten carbide powder production. (Courtesy: Wolfram Bergbau€ und Hutten-GmbH.)
€ FIG. 23.24 Scanning electron micrographs of coarse and ultrafine tungsten carbide powders. (Courtesy: Wolfram Bergbau- und Hutten-GmbH.)
1773 K. Compared to the reduction step for tungsten, temperatures are much higher, and single tube pusher type furnaces (Fig. 23.23) are used. Each particle is composed of numerous tungsten carbide crystals. The particle size of the tungsten carbide powder (Fig. 23.24), for the most part, preserves the particle size of the submicron tungsten metal powder (see Fig. 23.14). Further, cooled caked tungsten carbide powder is de-agglomerated. This step just separates particles from sticking to each other and is not used to change the particle size. This process, including the ore-processing steps, is also called the conventional calcination-reduction-carburization (CRC) process and offers the potential to manufacture commercial WC ultrafine powders with median grain sizes below 0.5 mm. Strict process control and a high degree of automation permits greater uniformity and batch-to-batch reliability [62]. This process is unsuitable for the production of coarse-grained powder. Techniques of the former conventional tungsten carbide production are usually limited during carburization to temperatures around 2270 K. At these temperatures, carburization with tungsten powder with particles <100 mm can be achieved, but the resulting product is polycrystalline. By the latter, the increase of tungsten carbide single crystals is realized by the use of a liquid phase, which is able to dissolve both tungsten and carbon. The formation of tungsten carbide within the melt of solvent metal, such as iron, is the
Production of Refractory Metal Powders Chapter
23
709
basis of the Menstruum process, also called “macro process.” [63] The powder mixtures of tungsten ore concentrates, iron oxide, aluminum, calcium carbide, and/or carbon react exothermally (after ignition by a detonator) to generate tungsten carbide, iron, calcium oxide, and aluminum oxide. A high temperature exothermic reaction (2Al + 3FeO $ Al2O3 + 3Fe) at about 2770 K produces a molten mass that, when solidified, consists of tungsten carbide crystals dispersed in iron and a slag containing impurities. This technique permits complete reduction and carburization of a batch of about 22 t of tungsten carbide, which is achieved in 60 min. To get the tungsten carbide in powder form, the solidified solvent metal is dissolved, which is ordinarily done with hot hydrochloric acid. The obtained tungsten carbide powder is separated according to desired particle size by sieving. Menstruum tungsten carbide powders have a high yield of well-faceted single crystal particles with sizes of 400 mm and below. Production of fine grained powder: Recently, the demands for tungsten carbide powders finer than 1 mm are increasing, as material for precision tools such as drills and end mills for metal cutting, printed circuit boards, cutters and precision pins, and so on. This increased demand, together with conventional techniques’ efficiency limitations in producing fine-grained metal powders [64], creates a strong motivation to develop alternative techniques. Tokyo Tungsten Co., Ltd. has developed [65] the direct carburizing process, in which tungsten trioxide and carbon black react in the following steps: WO3 ! ðWO2:9 Þ ! ðWO2:72 Þ ! WO2 ! W ! W2 C ! WC At the (WO2.72) ! WO2 step, ultrafine particles are generated and then transformed into tungsten carbide without grain growth. The starting material is prepared in the form of pellets from a mixture of tungsten oxide with carbon black. Subsequent reaction is accomplished in rotary furnaces in two steps: the direct reduction of the WO3 to tungsten by carbon under nitrogen, followed by carburization under hydrogen at temperatures up to 1873 K to tungsten carbide powder. This method permits the production of ultrafine powder with particle sizes ranging from 0.1 mm to 0.7 mm. Vickers hardness, HV (50 kg), decreases from 2150 to 1900 as particle size increases from 0.1 mm to 0.3 mm. Another direct carburization process has been developed by Dow Chemical Company [66,67]. A mixture of tungsten trioxide and carbon black falls through a vertical reactor at temperatures of approximately 2270 K. Accordingly, the reaction time is a few seconds and results in the formation of an intermediate mixture of W, W2C, WC, and C. This product must be subjected to a second, more conventional carburization step. Particle sizes were reduced by controlling the milling time, carbon content, inhibitor addition, and sintering temperature. The characteristic particle size range for this process is between 0.1 mm and 0.8 mm, which is identical to the lowest limits of the conventional process [64]. The spray conversion process has been developed for Nanodyne Inc. [68] to produce nanopowders. Aqueous solutions of tungsten and cobalt salts are spray dried, then subsequently reduced and carburized with a gas mixture such as hydrogen and ammonia or carbon dioxide and carbon monoxide in a fluidized bed reactor. The product obtained is relatively large (about 75 mm) hollow WC-Co composite powder particles that consist of tungsten carbide crystallites with a grain size of about 30 nm. This product is milled in order to obtain a powder suitable for consolidation. Another technique to produce tungsten carbide nanopowders is a chemical vapor reaction process [69]. This process is based on the gas phase reaction of metal halides with different gas mixtures to produce nanosized powder ranging from 5 nm to 50 nm. Production of tungsten carbides is possible in principle; however, the process seems better adapted to the production of nitrides or carbides of other group IV and V refractory metals.
Properties of Tungsten Carbide Powders High hardness is one of the most important properties of pure tungsten carbide and is the basis for the application in hardmetals, also called “cemented carbides.” As of 2014, >50% of the 30,000 tons of tungsten produced annually was used in the production of cemented carbides [70]. A number of physical and technological characteristic values are quite similar to those for tungsten metal powders. Tungsten carbide powders may have powder particle structures which are either single or polycrystalline. SEM and bright field TEM images (Fig. 23.25) show the typical morphology and physical constitution of ultrafine tungsten carbide [62]. The grain boundaries within a single particle are revealed by dotted lines. Recent developments in the field of standard ultrafine grades were driven, above all, by the need to control the sinter activity and grain growth tendency of ultrafine carbides. Table 23.7 shows the basic characteristics of standard ultrafine powder grades. Abbreviation CRC symbolizes the milestones of conventional process from tungsten concentrate to tungsten carbide through calcination, reduction, and carburization. The values obtained using a Fisher subsieve sizer (FSSS), based on the air permeability method of measuring the average particle size of powders, are not absolute and irreproducible in the range of ultrafine powders. It could be
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D Production of Non-Ferrous Metal Powders
FIG. 23.25 Morphology and physical constitution of ultrafine tungsten carbide. (A) Scanning electron micrograph of powder particles; (B) transmission electron microscopy image of single particle. (Courtesy: Wolfram Bergbau - und Huttengesellschaft mbH NfG KG, Austria.)
TABLE 23.7 Properties of Ultrafine Tungsten Carbide Powder Grades Standard Grades Unit
CRC025
WC O6
CRC020
WC 05
CRC015
mm
0.63
0.64
0.55
0.56
0.49
2
m /g
1.71
1.52
2.03
1.95
2.43–2.83
mm
0.22
0.25
0.19
0.20
0.16–0.14
mm
…
0.30
…
0.25
0.18
Total carbon
wt%
6.12
6.17
6.09
6.19
6.11–6.15
Free carbon
wt%
0.03
0.03
0.03
0.02
0.01–0.05
VC
wt%
Undoped
0.1–0.5
Undoped
0.3–1.0
0.3–1.0
Cr3 C2
wt%
Undoped
0.1–0.5
Undoped
0.3–1.0
0.3–1.0
Oxygen
ppm
900
1030
1080
1450
1810–2800
Property a
FSSS grain size BET specific surface area (SBET) b
BET calculated particle size FESEM particle size dSEM
c
a
Fisher Sub-Sieve Size. 6 BET particle size calculated, based on measurement 300 particles, according equation: dBET ¼ 15:6S mm. Pd ofPminimum Pd Pd BET d FESEM particle size according equation: dSEM ¼ dji d= dji N mm, where dji d is sum of diameter of all particles in range from di to dj; dji N is total number of particles in range from di to dj. VC, vanadium carbide.
b c
demonstrated using scanning electron microscopy that obtained FSSS particle sizes are overstated by two to three times in comparison with reality [64,71]. With field emission secondary electron microscopy (FESEM), particle sizes are slightly higher than calculated by BET (Table 23.7) because the surface roughness may give a high surface area measurement, leading to the wrong conclusion that they are finer. In turn, an arithmetic mean FESEM particle size, calculated according to Eq. (c) in Table 23.7 used for particle size distribution: dj X
jðdÞ ¼
d
di dN X d1
d
Production of Refractory Metal Powders Chapter
23
711
FIG. 23.26 Scanning electron micrographs of standard tungsten carbide powder grades CRC025 (A) and CRC015 (B).
P where ddN1 d is sum of particle diameter of all measured particles N (mm) does not represent reliable weight particle distribution. In consequence of cubic dependence of particle volume on its size, the weight distribution will be leveled by FESEM particle size distribution calculated by the sum of particle diameters. In reality, for example, if powder particle size boundaries ranged from 0.05 to 1.0, the linear particle sizes would differ by 20 times, while their volumes would differ by 8000 times. Thus, the average FESEM particle size may be understated. A higher fines yield can be advantageous in order to increase the sinter activity/densification rate (at low sintering temperatures during solid state sintering) or to enable a slight rise in hardness due to maximum grain growth inhibition. Special applications (in particular, in the area of wear parts) set the demand for carbide grades with BET specific surface areas higher than 2.4 m2/g. Developed CRC 015 grade satisfies these requirements (see Table 23.7). These carbides with calculated BET particle sizes of 0.15 mm are produced in full-scale units by the conventional CRC process. Powder grades CRC 025 and CRC 015 differ appreciably in their particle sizes (Fig. 23.26). Added chromium plays an important role in grain growth inhibition during sintering. Energy dispersion analysis provides evidence [61] that chromium added during powder processing is preferentially present at WC/WC grain boundaries, even in the interior of polycrystalline particles or in necks between carbide particles. 10 wt% Co alloys produced by conventional liquid phase sintering at 1693 K and doped with chromium carbide (0.3–1.0 wt% Cr3 C2) and vanadium carbide (0.3–0.6 wt% VC) resulted in coercitivity values ranging from 440 Oe to 560 Oe with a corresponding increase in BET specific surface area from 1.5 to 2.8 m2/g. Additionally, the increase in maximum coercitivity is directly proportional to the BET surface area. The most important chemical component of tungsten carbide powder is its carbon content. Stoichiometric pure tungsten carbide has a total carbon content (Ct) of 6.135 wt%. From the point of view of thermodynamic equilibrium, WC has a very narrow stability range [2]. Thus, not even a small carbon shortage leads to W2C to the formation of free carbon (Cf). In commercial WC powders, there is always a small portion of free carbon present, about 0.03% at a stoichiometric Ct level. Even when the total carbon is below the stoichiometric level, the coexistence of traces of W2C and Cf can occur. The carbon balance is important for hardmetal manufacturing, because a shortage of carbon leads to the formation of the brittle phases, for instance, Co2W3C, and any excess in carbon leads to graphite precipitation. Thus, both cases result in a detrimental effect on the mechanical properties of the final product [2,61]. The role of carbon source in the production of WC powders starting from metal tungsten (W) and tungsten oxide (WO3) has been studied [72]. Those conclusions follow. In the W + C ! WC process conditions, inert atmospheres are recommendable for the synthesis of WC when metallic tungsten powders are used as precursors. In these conditions, fine WC powders can be obtained at 1373 K using either graphite or carbon black powders. The resulting WC powders consist of agglomerates of submicron particles with irregular platelet morphology. In the WO3 + 4C ! WC + 3CO process conditions, is possible to synthesize WC directly from WO3 powders. In this case, atmospheres containing hydrogen are needed to activate reduction of oxides at lower temperatures, whereas, at higher temperatures, reduction is promoted by the presence of carbon. Carburization in Ar-50%H2 of mixes containing WO3 + carbon black is complete at 1373 K, whereas, for WO3 + graphite powders, complete transformation to WC is
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D Production of Non-Ferrous Metal Powders
achieved at higher temperatures (1573 K). High energetic milling of WO3 + graphite allows the complete carburization at 1373 K. The resulting WC powders have spherical morphology, submicron particle size, and crystalline grain sizes below 30 nm. Nanoscale WC-Z composite powder was synthesized by in situ reaction of tungsten oxide, cobalt oxide, and carbon black mixtures for fabricating a nanostructured WC-based coating in which the conventional Co matrix was substituted for Z phases (Co6W6C, Co3W3C, Co2W4C, etc.) [73]. WC-Z nanocomposite powder with a particle size of 50–200 nm was prepared by in situ reaction of metal oxides and carbon, based on which, the nanostructured, WC-based coating was then fabricated by high-velocity, oxy-fuel thermal spraying. Song, et al. [73], based on fulfilled studies, reported the following: (a) Above 1170 K, the synthesized composite powder contains only WC and Z phases. At lower temperatures, the CoWO4 phase preferentially forms in the synthesized powder and then transforms into W2C, W, Co6W6C, and Co3W3C phases as the temperature increases. (b) The amount of Z phases of the obtained WC-Z composite powder depends largely on the carbon addition of the raw materials. The appropriate carbon content was estimated as 3.9%–4.2% for the WC-Co thermal spray feedstock powder resulting in no metallic Co phase. (c) The thermal spraying feedstock prepared using the WC-Z nanocomposite powder has a homogeneous WC particle size distribution with a mean size of about 260 nm. In contrast, the conventional powder consists of WC particles of 3 5 mm (Fig. 23.27). Elements such as vanadium, chromium, tantalum, and niobium, due to their grain-growth inhibiting effect during hardmetal sintering, are added to tungsten carbide powder in amounts around 0.1–1.5 wt%. Such elements can be introduced as carbide to the tungsten carbide powder, or as oxide or carbide prior to the carburization step. Trace elements similar to nickel, iron, and cobalt can be admitted in amounts around 100 ppm. At the same time, there are other impurities that should be limited in the very low ppm range because of their detrimental action on hardmetal properties. Calcium, sulfur, silicon, and phosphorus are typical of such elements [74,75]. Compositions and properties of several standard tungsten carbide powders are given in Table 23.8.
Tungsten Carbide Electrical Contacts Tungsten carbide analogous to other refractory metals and their carbides is distinguished by high melting and boiling points and very high hardness at both room and elevated temperatures, but also by poor electrical and thermal conductivities and poor oxidation resistance. Forming a composite can compensate for these drawbacks. The development of composite contact materials involving silver with tungsten or molybdenum or their carbides has resulted in materials that can withstand higher currents and more arcing than other contact materials, without experiencing sticking or rapid erosion. Refractory metals, the content of which can vary from 10% to 90%, offer good mechanical wear resistance and resistance to arcing. Silver provides good electrical and thermal conductivities.
FIG. 23.27 Cross-sectional microstructures of the feedstock particles prepared with the WC-Z composite powder (A) and (B) the conventional powder. (Source: Song X, Wang H, Yang T, Liu X, Wang X. In situ synthesis of nanoscale WC- composite powder and its application in cermet coating. In: Compiled by European Powder Metallurgy Association. Proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
TABLE 23.8 Properties of Typical Tungsten Carbide Powders Source: Standard Grades WBH
OSRAM
Property
Unit
WC 05D [3]
WC 06
BC0 4 U
BC1 0 U
BC45S
BC65S
BC75H
WC-TiC 70/30a
WC-Ti 50/50a
FSSSb
mm
0.53
0.64
0.5–0.7
0.91–1.20
4.31–5.00
11.01–15.00
> 30
2.0 0.3
2.0 0.3
2
m /g
1.90
1.52
…
…
…
…
…
…
…
Total carbon
wt%
6.23
6.17
6.13 0.05
6.13 0.05
6.13 0.05
6.13 0.05
6.13 0.05
9.7–10.2
12.6 0.2
Free carbon
wt%
0.04
0.03
max 0.08
max 0.08
max 0.06
max 0.06
max 0.06
max 0.25
max 0.4
VC
wt%
0.32
0.1–0.5
…
…
…
…
…
…
…
Cr3C2
wt%
0.64
0.1–0.5
…
…
…
…
…
…
…
Aluminum
ppm
10
<5
…
…
…
…
…
…
..
Calcium
ppm
3
<2
…
…
…
…
…
max 0.01
max 0.01
Chromium
ppm
…
…
100
100
50
50
75
…
…
Cobalt
ppm
<5
<5
…
…
…
…
…
…
…
Copper
ppm
<2
…
…
…
…
…
…
…
…
Iron
ppm
46
20
350
350
500
500
500
max 0.05
max 0.05
Magnesium
ppm
<1
…
…
…
…
…
…
…
…
Molybdenum
ppm
<20
<20
…
…
…
…
…
…
…
Nickel
ppm
<5
<5
150
150
100
100
100
…
…
Oxygen
ppm
1440
1030
3000
2500
500
500
500
…
…
Phosphorus
ppm
<20
…
…
…
…
…
…
…
…
Potassium
ppm
<4
…
…
…
…
…
…
max 0.01
max 0.01
Sulfur
ppm
6
<5
…
…
…
…
…
max 0.01
max 0.01
Silicon
ppm
<10
<10
…
…
…
…
…
max 0.005
max 0.005
BET specific surface area Nominal composition:
Trace elements:
Density
g/cm
14.72
14.91
14.94
14.94
14.94
…
…
Hardness
HV30
2000
1650
1380
1220
<1050
…
…
Coercivity
Oe
500
260
122
80
<60
…
…
3
€ Fine-grained Grade; WBH: Wolfram Bergbau- und Hutten Company (Sankt Martin im Sulmtal, Austria); OSRAM: OSRAM Sulvanias, North America (Wilmington, MA, United States). Fisher Sub-Sieve Size.
a
b
714
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D Production of Non-Ferrous Metal Powders
Silver does not alloy with tungsten, molybdenum, or their carbides. Therefore, PM processes are used in their manufacture. Depending on the composition, contact materials are accomplished either by pressing and sintering or by the pressinterinfiltrate method. At temperatures rise above the melting point of the infiltrant, the liquid metal penetrates and fills the interconnecting voids of the pressed and sintered compact. Densities that are 96%–99% of the theoretical density can be achieved by this technique. Infiltrated contact materials find use as current-carrying contacts in air- and oil-immersed circuit breakers, heavy-duty relays, automotive starters, and switches. Copper infiltrant, which costs less but has very poor corrosion resistance, is used for parts that operate in non-corrosive environments such as oil, vacuum, or inert atmospheres. Table 23.9 contains data on typical tungsten and carbide electrical contact grades published by manufacturers, which usually include density, hardness, and electrical conductivity.
Diamond Tools Tungsten carbide powder, together with certain metal powders, can be used to adjust the characteristics of alloys in which diamonds are embedded [76].
HARDFACING Hardfacing is the application of hard, wear-resistant material to the surface of a component by welding, thermal spraying, or a similar process for the main purpose of reducing wear. Carbide/metal mixtures very like hardmetal compounds are applied to machine parts to improve their resistance. This can be realized by flame or plasma spraying of powders or by welding techniques. The detonation gun spray process is also used. The extremely high particle velocities achieved in this process result in coating with higher density, greater internal strength, and superior bond strength than can be achieved with conventional plasma spraying or single-step flame spraying. Another category of hardfacing techniques is cladding. Hardfacing materials can be clad onto substrates by furnace fusing prearranged layers of loosely bonded hardmetal onto the substrate. Thus, the Conforma Clad Process (Conforma Clad, Inc.) uses a flexible cloth made of polytetrafluoroethylene and a hardfacing powder of the desired composition. The main benefit from this technique is that the cloth can be cut into any required size and placed on the surface to be coated, regardless of the profile of the workpiece. During subsequent furnace sintering, polytetrafluoroethylene is vaporized, and the hardfacing powder forms the deposit layer [77]. Sintered tungsten carbide powders are widely used for thermal spray coatings and torch welding hardfacing. Powder can be a product of crushing bulk WC-Co alloys or specifically made spherical shaped pellets. Tungsten carbide powders for thermal spray coatings usually contain from 6% to 18% cobalt. Particle sizes of WC-Co for thermal spray coatings range from 5 mm to 150 mm. Powder particle sizes for torch welding hardfacing range from 20 mm to 100 mm. Macrocrystalline tungsten carbide powders are a special kind of tungsten carbide powder manufactured by a hightemperature thermit process during which ore concentrate is converted directly to tungsten carbide. This product maintains the correct stoichiometric carbon content of 6.13 wt%. Macrocrystalline tungsten carbide is grown in crystals ranging from 1 mm to 5 mm. Coarse mesh sizes of macrocrystalline carbide are extensively used in abrasion erosion protection applications. The finer size powder is employed as wear rate modifier. Coarse tungsten carbide powder can also be obtained by conventional carburization method. Commercial grade powders are manufactured by H. C. Starck GmbH (Munich, Germany), Kennametal India Ltd. (Bengaluru, India), etc. Cast tungsten carbide is another category of carbide that also finds wide applications in hardfacing. Cast carbide refers to eutectic of WC and W2C that can range in carbon content from 3.5 to 4.5 wt%. It is manufactured by melting compounds of metallic tungsten, tungsten carbide, and carbon at a temperature above 3270 K. The melt is cast into billets, which are then crushed to the size range of powder. The cast is crushed to powder <850 mm and as fine as below 45 mm. Cast carbide, mostly used in torch welding hardfacing, is often used in combination with sintered tungsten carbide and other carbides to enhance wear resistance. At the same time, the addition of cast carbide tends to produce coatings with lower toughness. A relatively new spherical cast tungsten carbide product has all the basic characteristics of crushed cast carbide. However, this material has extremely high wear resistance and higher resistance to chipping and cracking in comparison with crushed cast carbide due to its spherical shape. Spherical cast carbide is manufactured by a technique ensuring rapid solidification of molten eutectic carbide droplets [78]. Diamond-like carbon (DLC) coatings technology due to their surface smoothness, hardness, and chemical inertness in combination with a low friction coefficient against most metals make DLC films interesting for wear-protective purposes. However, DLC films present high compressive internal stress and low chemical interaction between films and their metal
TABLE 23.9 Properties of Typical Tungsten Carbide Electrical Contact Grades Density (g/cm3) Nominal Composition (%)
Manufacturing Methoda
Calculated
Typical
Electrical Conductivity (% IACS)
Hardness
Tensile Strength (MPa)
Modulus of Rupture (MPa)
Data Sourceb
Application Examples
Tungsten carbide-silver INF
11.86
11.53–11.85
55–60
50–65 HRB
272
483
C, A
60Ag-40WC
PSR
12.09
11.40–11.92
46–55
60–70 HRB
…
…
A
50Ag-50WC
INF
12.56
12.12–12.50
43–52
75–83 HRB
276
793
C, A
Aircraft contactors, lighting relays, low-voltage switches, Circuit breakers
40Ag-60WC
INF
13.07
12.0–12.92
40–47
90–100 HRB
379
827
C, A
Heavy-duty
38Ag
INF
13.18
12.92–13.29
35–38
90–100 HRB
552
…
C
Circuit breakers
20Ag-80WC
PSR
14.23
13.2
19
400 HV (c)
…
…
M
Semiconducting material
Arcing contacts in oil, wiping shoes in power transformers
Tungsten carbide-copper 50Cu
INF
11.39
11.0–11.27
42–47
90–100 HRB
…
1103
C, A
44Cu
INF
11.77
11.64
43
99 HRF
…
1241
C
30Cu
INF
12.78
12.65
30
38 HRC
…
…
…
a INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy, Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. M: Metz Degussa, South Plainville, NJ.
Production of Refractory Metal Powders Chapter
65Ag-35WC
23 715
716
SECTION
D Production of Non-Ferrous Metal Powders
FIG. 23.28 Schematic of the magnetron sputtering system.
substrates. Now, the most general approach to solve the adhesion problem is to use metal or silicon interlayers between steel and DLC. In work [79], a physical vapor deposition (PVD) method for the deposition of tungsten carbide and DLC multilayer coatings by using a single sputtering target comprised of two equal halves (one half carbon, the other tungsten) was realized. A set of coatings were deposited by means of a RF (radio frequency) (13.56 MHz) magnetron sputtering system (Fig. 23.28). The different layers are obtained by the variation of the argon and methane/argon sputtering gas composition. The pressure of both argon an argon/methane mixture was 4 Pa. Using this deposition method, researchers prepared multilayer coatings onto steel substrates. The coatings are formed by a multilayer stack of different WC layers and an upper DLC layer. The DLC thickness was approximately 0.3 mm, and the total thickness was approximately 1.0 mm. Fig. 23.29 shows the SEM cross-section morphology of a WC/DLC multilayer. As reported [79], the structure of this multilayer stack consists of cubic WC1x polycrystalline layers separated by carbon rich narrow interlayers. The upper DLC layer has been obtained in the same process by increasing the methane concentration in the gas mixture. The experiments have demonstrated that multilayer coatings improve the adhesion and wear resistance of the DLC coatings on steel substrates while keeping their low friction properties. FIG. 23.29 SEM micrograph of cross-section of a WC/DLC multilayer coating. (Source: Rinco´n C, Zambrano G, Carvajal A, Esteve J. Tungsten carbide/diamond-like carbon multilayer coating on steel for tribological applications. Surf Coat Technol 2001; 148(2–3): 277–83.)
Production of Refractory Metal Powders Chapter
23
717
The paper [80] presents the results of abrasive wear resistance, microhardness, and adhesive wear resistance tests conducted on AISI 1020 commercial steel when it is standardized, heat treated through a process of quenching-tempering, and when a coating of tungsten carbide in powder form of nanometric size is deposited on it. This had been reported as follows: Knoop microhardness tests allowed researchers to conclude that thermally treated specimens with a quenchingtempering process improve this property by 5.5%, while the WC coatings have a more significant increase, reaching a value of 124.2% compared to standard steel. The average loss of material per unit of covered length during abrasive wear resistance tests on AISI 1020 standard steel, thermally treated with a quenching-tempering process and coated with nanometricsized WC is of 0.0382 mm3/m, 0.0291 mm3/m, and 0.0151 mm3/m, respectively. The average loss of material per unit of covered length during adhesive wear resistance tests on AISI 1020 standard steel, thermally treated with a quenchingtempering process and coated with nanometric-sized WC is of 0.0827 mm3/m, 0.0736 mm3/m, and 0.0271 mm3/m, respectively.
CEMENTED CARBIDES Production of Cemented Carbide Powders Optimal interfacial bonding in hardmetals is achieved by means of optimization of balance between the high hardness of the tungsten carbide and the high toughness of a ductile binder metal, such as cobalt, nickel, or iron [76,81]. The steps of hardmetal manufacture include: blending of grade powders, milling, powder consolidation, liquid phase sintering, postsintering operations, and post-sinter forming. The excellent wetting of tungsten carbide by cobalt is one of its key properties. By variation of the tungsten carbide particle size and the percentage of cobalt, desired properties of the hardmetals can be obtained [76,81,82]. Fig. 23.30 illustrates, in diagram form, that tungsten carbide particles with equal particle size may differ concerning the size of the individual crystals within the particles, which, depending on the powder manufacturing process, can either be single or polycrystalline. This characteristic has a strong influence on the behavior of the powder during the manufacturing of the cemented carbide. Generally, the higher the number of subgrains per WC particle, the finer the microstructure of a hardmetal can be [3]. A general rule, moreover, is the finer the carbide and the lower the cobalt content, the harder, but less tough, the composite is. But, higher binder content at a certain hardness (combined with a lower average WC grain size) does not necessarily mean a higher toughness. In particular, in the lower hardness range, this relationship does not hold. At the higher hardness range, however, a higher binder can reduce the carbide contiguity, and thus, improve the hardness to toughness relationship. However, the improvements which can be obtained at a high hardness are obviously limited for hardness above 2000 HV30 [83], while the basic WC-Co material has been modified to produce a variety of hardmetals, which are used in a wide range of applications, including metal cutting, mining, construction, rock drilling, metal forming, structural components, and wear parts. FIG. 23.30 Influence of crystalline size of tungsten carbide particles on the microstructure forming of hard metal.
718
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D Production of Non-Ferrous Metal Powders
Additions of other carbides such as titanium, tantalum and niobium, or titanium carbonitrides are used in steel cutting grades and are produced from metal oxides of titanium, tantalum and niobium. These oxides in particulate form are mixed with metallic tungsten powder and carbon black. The oxides are reduced under a hydrogen atmosphere or vacuum at elevated temperatures and form solid solution carbides WC-TiC, WC-TiC-TaC, or WC-TiC-(Ta, Nb)C. The Menstruum process can be used to produce a WC-TiC solid solution. In this method, both carbides are dissolved in liquid nickel, which functions as a contributory metal. Solid solution carbides are then precipitated during cooling. The cemented carbide powders may consist of tungsten carbide mixed with a finely dispersed metallic binder (cobalt, nickel, or iron) or with additions of other above-named cubic carbides, depending on the desired properties and application of the tool. Intensive milling is necessary to disintegrate the initial carbide crystallites and to blend all components so that every carbide particle is coated with binder material. This process is accomplished in ball mills, vibratory mills, or attritors using carbide balls. The mills are usually lined with carbide sleeves; however, mills lined with low-carbon steel or stainless steel are also used. Grinding is carried out under an organic liquid such as heptane or acetone to prevent oxidation of the powder. After milling, the liquid is distilled off, and the powder is dried. A solid lubricant, such as paraffin wax, is added to the powder mix in the final stage of the milling or subsequently in a blender. The lubricant protective coating prevents or greatly reduces the oxidation of the powder and imparts strength to the powder mix during its consolidation. In the spray drying process commonly used in the hardmetals industry, a hot inert gas such as nitrogen impinges on a stream of carbide particles. Thus, a product in the form of spheroidal powder aggregates is produced. The favorable mechanical properties of WC-Co-based cemented carbides are significantly deteriorated at elevated temperatures or in corrosive environment [84]. In order to overcome these shortcomings, the introduction of ordered fcc_L12g0 precipitates [85] into a Co-binder is a way to strengthen the binder phase and improve its high temperature properties or corrosion resistance, similar to that in cobalt-base superalloys [86]. Thus, it offers new possibilities for development of novel WC-Co-Ni-Al cemented carbide that possesses both high-temperature strength and environmental resistance. Long et al. [87] recently developed a type of WC-Co-Ni-Al cemented carbides with novel g + g0 Co-based binder. The abnormal WC grain is much larger than average, and this greatly worsens the mechanical properties of the alloys because these abnormal grains act as initiation points for cracking and breakage [88]. Thus, suppression of abnormal grain growth (AGG) becomes an important issue for the preparation of ultrafine or nanocrystalline WC-Co cemented carbide [89]. However, the mechanism for AGG is not fully understood. Park et al. [90] considers the AGG behavior to be related to two-dimensional (2-D) nucleation. Recently, Long et al. [91] considered the 2-D nucleation of WC grains in ultracoarse WC-Co-Ni-Al alloys in connection with solid-liquid interfacial energy and solubility of tungsten in the binder phase. In the work [92], the growth behavior of WC grains in WC-Co-Ni-Al alloys with different Co-Ni-Al binder phase compositions was studied. Powder mixtures of WC-18(Co-Ni-Al) samples were milled in ethyl alcohol in a ball mill at the ballto-powder ratio of 6:1 for 48 h. Powder mixtures of WC-8Co like WC-8Ni3Al were milled in ethyl alcohol in a ball mill at the ball-to-powder ratio of 15:1 for 96 h. These powder mixtures were then dried under vacuum at 358 K. These samples were uniaxially cold pressed into bars of 25 8 6.7 mm at a pressure of 150 MPa and then sintered at 1723 K under the 6 MPa pressure for 1 h and cooled with a furnace. Then, WC grains in the as-sintered samples were extracted by removing the binder matrix with saturation hydrochloric and Fe3Cl solution in supersonic cleaner. Fig. 23.31 shows the morphology of cemented carbide grains extracted from the WC-18(Co-Ni-Al) alloys sintered at 1723 K for 1 h. It can be seen that, as the Ni3Al content increases, WC grain size is significantly reduced, especially the size of the coarse WC grains.
FIG. 23.31 Morphology of cemented carbide grains extracted from the WC–18 (Co-Ni-Al) alloys obtained by various Ni3Al content: (A) without Ni3Al, (B) 5.3 wt% Ni3Al, (C) 11.3 wt% Ni3Al.
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FIG. 23.32 Microstructure of binder jet printed and sintered parts with 12 wt%Co (A) and 17 wt%Co (B).
It was concluded [92] that with the increase of Ni3Al content in Co-Ni-Al binder, the increasing interface energy slows the growth rate of WC; therefore, with the increase of Ni3Al content in Co-Ni-Al binder, the AGG of the alloy is appreciably inhibited. Additive manufacturing (AM) of polymer, metal, and ceramic materials has become more and more common. For hardmetals however, very little work has been reported so far. Yet, the implementation of additive manufacturing in the field of hardmetals would enhance the possibility of producing complex shaped parts, which cannot be produced by conventional means, and allowing the production of parts within hours instead of days because of the tool-free production technology. Binder jetting of cemented carbide green parts was first published by Kelley in 1998 [93]. Like select selective laser sintering (SLS), it is a powder-bed-based AM technique. In contrast to SLS, the particles are not selectively “sintered” but “glued” together by organic binder, which is applied layer-by-layer using a special print head. Thus, the printed parts are green parts, and their green density is mainly based on the powder density of the used starting material. During a postprinting debinding step, the organic materials used during printing must be removed, and the samples must be sintered. Using this technique, in the work [94], the samples with cobalt contents between 12 wt% and 20 wt% were produced by sintering temperatures between 1673 K and 1743 K. Densities achieved are above 99.8% of theoretical density and hardness as well as fracture toughness are comparable to standard hardmetal grades with the same composition. The 17wt%Co samples showed homogeneous microstructure, while samples with 12wt%Co showed some larger WC grains, which can be avoided by using a composition with grain growth inhibitors such as Cr3C2 or vanadium carbide (Fig. 23.32). A suspension-based method of thermoplastic 3D printing (T3DP) has been developed as an AM technology within the last 10 years. As reported [94], using T3DP showed promising results for cemented carbides. WC-10wt%Co samples were produced with nanosize WC powders of very fine cobalt powder. The hardmetal content in the organic suspension was 67 vol% which results in a quite high green density compared to conventional hardmetal fabrication. Samples were debindered under standard conditions with hydrogen and sinter-HIPed at 1623 K with 6 MPa argon HIP pressure. The results showed that by T3DP, total density of parts was 14.23 0.01 g/cm3 above full density, and the properties were comparable to conventional (via uniaxial pressing) produced samples. Uhlmann et al. [95] showed that cemented carbide with a content of 17 wt% Co with a density of 96% theoretical density and different geometries could be produced by additive manufacturing selective laser melting process.
Cemented Carbide Compaction A variety of techniques is used to compact hardmetal powders to the desired green density and shape before they can be sintered to full density. Powder consolidation methods include uniaxial pressing, cold isostatic pressing (CIP), extrusion, and injection molding. Processes such as uniaxial pressing and CIP are performed using powders directly after their creation, whereas extrusion and injection molding require extra processing by binders. Consolidation and shaping of cemented carbide powders are described in several publications [96–100].
Pressing Pressing involves compaction of powder under pressure by constraining it between die walls and rams to increase green density to a required level. In uniaxial pressing, powder in a die is pressed between top and bottom rams. Pressing is a onestep process, combining consolidation of the powder to required green density and achieving required shape. Compaction force ranges from 165 MPa to 331 MPa. Pressing is often used to produce near-net shape carbide parts, which requires
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optimization of various processes and powder properties, like controlling the relationship between pressing pressure and shrinkage factor (green dimension/sintered dimension or green-sintered dimension/green dimension). Simple metal-cutting insert shapes are typically round or polygonal with straight walls, with or without a central hole. Rods are also produced in straight-walled dies with curved upper and lower punches by length pressing, with the length perpendicular to the pressing axis. These are used to make blanks for drills and end mills. Parts with clearance angles are also pressed in single-action tooling with an angled die wall. Multiplaten tooling is used to create shapes with significant steps in height or changes in cross-section. For geometries that contain undercuts, cross holes, and other geometries that prevent removing the part from the die, moving side die elements are used in side pressing.
Extrusion Rods, end mills, boring bars, and wires are formed by extrusion. Components having internal structures, such as internal holes for delivering coolant to the cutting edge, are also made using extrusion. The extrusion process is accomplished in an extrusion press, using either a piston-type or screw-type mechanism to push a powder-binder mix through a nozzle. Batch and continuous extrusion presses are used. The most common nozzle shape is round, but a variety of cross-sections can be extruded. Optimal powder-binder mix content and good mixing are required. Generally, powder properties, binder type and amount, as well as extrusion temperature, determine extrusion pressure and speed. Powder compaction is required to remove porosity and achieve sufficiently high green density so that full density is achieved on sintering. Poor mixing of powder and binder results in binder pools, which create imperfections and microstructural defects. Binder content and temperatures that are too high can cause a high rate of extrusion without sufficient powder compaction. Too low a binder content results in excessively high pressing pressures, which can cause surface cracks and damage to tooling and press.
Cold Isostatic Pressing Cold isostatic pressing (CIP), in which the powder is subjected to equal pressure from all directions, followed by machining, is also in general use for wear and metal forming tools. Cold isostatic pressing is commonly used for parts that are too large to be pressed in uniaxial presses and that do not require high precision in the sintered state. Dry-bag and wet-bag cold isostatic presses are used. In this process, an impervious moldable bag placed in pressure chamber is filled with powder and subjected to an isostatic pressure of 69 MPa to 207 MPa using a water-based liquid medium at ambient temperature. Shaped bags with cores are used to obtain near-net shapes. Powders with no or low wax are used in cold isostatic pressing. Dimensional tolerances for parts produced by CIP are relatively large, so size and shape control are not as critical as in uniaxial pressing. Parts are machined in the green, or presintered, state, and after sintering.
Injection Molding Injection molding of carbide powders is similar to injection molding metal powders and is used for high-volume production of relatively complex-shaped parts. Fig. 23.33 shows a schematic representation of the injection molding process. Carbide injection molding is achieved by mixing additional waxes, polymers, and surfactants with a dried powder to attain the
FIG. 23.33 Schematic of injection molding process.
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desired solids loading, injecting the thermoplastic feedstock into a die, removing from the die, and debinding in one or more stages prior to sintering. Large parts with wall thicknesses less than about 15 mm are produced by low-pressure systems, while smaller geometries with higher dimensional control and surface finish requirements are generally produced by highpressure systems. The differences in high- and low-pressure systems are usually the polymer versus wax content and solids loading. Due to the angular shape of carbide grains, the attainable solid loading in high-pressure systems is usually limited to 53%–58% by volume. Binder systems must minimize shrinkage on cooling in the die, and avoid carbon and oxygen pickup. Polymers must be selected for their clean burning during thermal debinding to avoid addition of impurities into the system. Surfactants are required to reduce molding pressure and to reduce tool wear. It is important that binder components are compatible with each other and bonded to powder to avoid separation. Mixing of additional lubricants into the powder to form a feedstock is performed in sigma and continuous mixers. Carbides are more sensitive to porosity than metallic systems; therefore, avoidance of air traps in the molding process is very important. The additional steps of mixing and debinding, as well a longer cycle time in molding, add to the cost as compared with traditional die pressing.
Sintering and Secondary Operations Hardmetals (cemented tungsten carbide, cemented carbide) are two-phase materials consisting of hard carbide phases and binder materials. The binder phase is primarily cobalt, but can also contain other binders including nickel, iron, vanadium, chromium, and molybdenum, or a combination of these. Successful application of these materials in metal cutting, oil drilling, mining, construction, metal forming, and shaping dies depends on achieving low levels of residual porosity with uniform, homogenous microstructure and desired chemical composition. Generally, there are two major sintering methods: pressureless sintering and pressure-assisted sintering. Pressureless sintering techniques include vacuum and partialpressure, hydrogen, and microwave sintering. Pressure-assisted techniques include overpressure sintering (sinter-HIP), sintering followed by postsinter HIP, hot pressing, and several rapid hot consolidation techniques. The sintering of carbide materials is comprehensively considered in several publications [98,101–103]. The first step in the sintering process is the removal of the lubricant from the powder compact. Then, by the pressureless sintering method, the compacts are usually set on graphite trays coated with graphite paint. Using a semicontinuous or batch type graphite furnace, the compacts are first heated to about 770 K in a hydrogen atmosphere or vacuum. Subsequent to lubricant removal, the compacts are sintered in a vacuum of 0.1 Pa (or 103 Torr) at a final sintered temperature ranging from 1623 K to 1873 K, depending on the content of the cobalt binder and the desired microstructure. This leads the cobalt to melt and binds the carbide particles together. Shrinkage of the compact ranges from 17% to 25% on a linear scale and provides a practically pore-free, fully dense material. The sinter-HIP process was developed in the early 1980s [104]. In this process, low-pressure hot isostatic pressing (up to about 7 MPa) is combined with vacuum sintering, and pressure is maintained at the sintering temperature while the metallic binder is still molten. This technique facilitates the production of pore-free products at costs only slightly higher than those of vacuum sintering green compacts. In the 1970s, the hardmetal industry took advantage of hot isostatic pressing (HIP), in which vacuum-sintered material is re-heated under a gas (argon or helium) pressure of 100–150 MPa. The temperatures of this process are 278–323 K below the sintering temperature. Sintering plus HIP is used to further reduce porosity and improve part durability. Acceptable levels, distribution, and sizes of porosity depend on the applications, and their measurement method is described in the article [105]. The high temperatures and pressures employed in the HIP autoclave improve the nearly perfect properties of hardmetal parts by removing any residual internal porosity and pits. A large number of hardmetal parts are machined after sintering because of surface finish, geometry and tolerance requirements. This operation is both time-consuming and expensive. The sintered parts can undergo many secondary operation including processing by means of metal-bonded diamond or silicon carbide wheels, turned with a single-point tool, or lapped with diamond-containing slurries, electrical discharge machining, and brazing to achieve desired condition, such as shape, specified size, edge state, and surface finish.
RECYCLING Hardmetal scrap containing tungsten carbide can be divided into two groups: l
Soft scrap, such as hardmetal grinding sludge, filter dusts, broken green parts, and floor sweepings
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SECTION
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Hard scrap, which represents sintered parts consisting of either used parts or fragments.
The contamination of the scrap with impurities is another criterion for classification. While particulate scrap contains impurity levels that cannot be admitted for direct reuse, hard scrap, even if heavily contaminated, can be classified, thus making direct reuse easier. Hardmetal recycling processes can be subdivided into the following kinds: l l
l
l
l
Direct conversion of sorted hard scrap into graded powder ready for pressing and resintering. “Zinc process” is the most significant process used [106]. It includes the following main processing steps: sorted, cleaned, and crushed hardmetal scrap; reaction in molten zinc at 1173–1273 K in argon or hydrogen atmosphere; vacuum distillation at 1273–1323 K; obtained cake is consecutively crushed, and ball milled, and the resultant powder is screened; then, oversize product (>75 mm) is returned to melting, undersize product (<75 mm) is ball milled, and blend carbon adjusted, resulting in graded hardmetal powder. Also used on a commercial scale is the so-called “coldstream process,” a high-velocity process. The cleaned, sorted, and previously crushed hardmetal scrap is entrained in a gas stream (7 MPa) and projected against a stationary baffle plate. After the material has struck the target and shattered, it is removed from the impact chamber by suction. Material is then subjected to screening. The oversize product is returned into a feed vessel for subsequent impact against the baffle plate. Further classification of undersize material and mixing of different size fractions result in graded hardmetal powder ready for use. Removing the binder by means of leaching is another method of hardmetal recycling. The composition of the hardmetal scrap used for the leaching process determines the quality of the resulting carbide residue. The chemical conversion process can be used for all contaminated scraps, either soft or hard, to recover primary powders of the hardmetal components. Fig. 23.34 schematically shows the process from the contaminated and unsorted scrap via the conventional WC powder manufacturing route, which involves calcination, reduction, and carburization intermediate to tungsten carbide powder.
Due to the intrinsic value of the hardmetal components, and due to the increasingly more severe legislation concerning waste disposal and the preservation of natural resources, today, recycling has considerable significance, which will increase in the future.
FIG. 23.34 Flowsheet for the chemical conversion process. (Source: Lux B, Zeiler B. Production of tungsten and tungsten carbide powders. In: ASM handbook, vol. 7. ASM International Publishers; 1998 p. 188–201.)
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MOLYBDENUM METAL POWDERS Molybdenum powders find application in different fields of technique. The major use for molybdenum is as an alloying element for alloy and tool steels, stainless, and nickel- or cobalt-based superalloys to increase hot strength, toughness, and corrosion resistance. Molybdenum ingots, produced by melting PM electrodes, are extruded, rolled into sheets or rods, and subsequently worked to other mill product shapes, such as wire and tubing. Molybdenum has outstanding electrical and heat conducting capabilities. The latter is approximately 50% higher than that of steel, iron, and nickel alloys. It finds wide usage as a heat sink. The coefficient of thermal expansion of molybdenum plots almost linearly with temperature over a wide range. This characteristic, in combination with its heat conducting capabilities, accounts for its use in bimetal thermocouples. Methods of doping molybdenum powder with potassium aluminosilicate to obtain a non-sag microstructure comparable to that of tungsten have also been developed [107]. In the electrical and electronic industries, molybdenum is used in cathodes and cathode supports, for radar devices, as current leads for thoria cathodes, and cores for winding tungsten filaments. Molybdenum plays a significant role in the rocket industry, where it is used for high-temperature structural parts. Molybdenum alloys satisfy the requirements for use in airframes because of their retention of mechanical properties after thermal cycling due to high recrystallization temperatures and good creep strength. Molybdenum has been also effective in the nuclear, chemical, glass, and metallizing industries.
Production of Molybdenum Powders Molybdenum powder is produced by a technique similar to that used for tungsten powder. Molybdenum disulfide (MoS2) is the major source of raw material. Molybdenum disulfide is concentrated by flotation and transformed to an impure technical molybdenum trioxide (MoO3) by means of roasting, which oxidizes the sulfur and removes it as gaseous sulfur dioxide [108]. Subsequent purification takes advantage of the sublimation characteristics of molybdenum trioxide. Above 820 K, it sublimes easily and can be distilled from its impurities and condensed again as pure molybdenum trioxide. Solvent extraction processes are also acceptable. Additional purification can also be achieved by dissolving molybdenum trioxide in ammonia to form ammonium molybdate, (NH4)2MoO4. The pure molybdenum trioxide or ammonium molybdate is reduced in tube push type or rotary furnaces similar to those used for tungsten reduction. Typical purification reduction reactions can be written: MoS2 + 7=2O2 ¼ MoO3 + 2SO2 endothermic ðreactionÞ MoO3 + 3H2 ¼ Mo + 3H2 O MoO3 + 2NH4 OH ¼ ðNH4 Þ2 MoO4 + H2 O ðNH4 Þ2 MoO4 + 3H2 ¼ Mo + 2NH4 OH + 2H2 O Reduction of molybdenum trioxide by dry hydrogen involves two steps: MoO3 ! MoO2 ! Mo [11]. The bond strength metal-oxygen in MoO3 is less than in WO3, and in MoO2, is larger than in WO2. The magnitudes of the equilibrium constants for the two steps of Mo reduction (Keq ¼ pH2O/pH2) are given in Table 23.10. The first step is fulfilled at temperatures ranging from 670 K to 920 K to realize slow increase of temperature in order to accomplish the formation of MoO2 at 770–820 K and not to form molten material at 820–870 K (low-melting eutectic Mo4O11-MoO2). The second reduction step is carried out at 920–1220 K. The reduction parameters like temperature, reduction time, and powder layer thickness (oxide load of the boats in push type furnace) are chosen so that the residual oxygen content does not exceed 1000 ppm after the second reduction step.
TABLE 23.10 Magnitudes of the Equilibrium Constants for the Two Steps of Molybdenum Reduction Temperature (K)
670
700
870
915
1070
1200
Keq, 1st step (n-d)
5.0 107
…
1.7 106
…
1.58 105
…
Keq, 2nd step (n-d)
…
0.076
…
0.234
0.389
0.55
n-d, Non-dimensional.
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TABLE 23.11 Properties of Typical Molybdenum Metal Powders Produced by Hydrogen Reduction Sources [3]
SIC Standards
Property
Unit
1–6 mm
TU48-19-316
TU48-19-313a
Particle sizes
mm
1–6b
<5
40–150
wt%
99.9c
99.5
99.9
Aluminum
ppm
5–25
50
…
Calcium
ppm
3–15
70
70
Carbon
ppm
10–50
…
…
Chromium
ppm
5–25
…
…
Cobalt
ppm
…
…
…
Copper
ppm
5–10
…
…
Iron
ppm
10–100
140
…
Lead
ppm
5–10
…
…
Magnesium
ppm
1–10
30
30
Nickel
ppm
5–50
50
200
Oxygen + water
ppm
500–1000
3000
…
Potassium
ppm
…
500
…
Silicon
ppm
…
50
…
Sodium
ppm
…
150
150
Tin
ppm
15–50
…
…
Tungsten
ppm
100–300
400
…
NVM (summation of Ca, Na, K, S)
ppm
500–1000
…
…
Nominal composition Molybdenum Trace elements:
a
Coarse powder for plasma spraying, CIS: Commonwealth of Independent States’ standards. Particle size FSSS. c Metallic molybdenum content, exclusive of gases. b
Properties of typical molybdenum metal powders produced by reduction of molybdenum trioxide by hydrogen are shown in Table 23.11. To produce high-purity molybdenum powder, an aqueous solution of ammonium molybdate is mixed with a solution of iron chloride (III); formed precipitate of iron molybdate is separated from the solution containing some of the impurities. A solution is acidified to make molybdic acid (H2MoO4); that is separated and then dissolved in an aqueous solution of ammonia (NH4OH). At the second stage of purification, the obtained solution of refined ammonium molybdate is mixed with a solution of iron chloride (III), and the cycle is repeated; formed precipitate of iron molybdate is separated, washed, and, after acidifying the molybdic acid, is dissolved in an aqueous solution of ammonia. The solution is separated by filtration, and finally, pure ammonium molybdate is cooled. The crystals of pure ammonium molybdate are separated and reduced in a hydrogen atmosphere at 1170–1220 K. Reduction of low boiling point molybdenum chlorides and fluorides also allows production of high-purity molybdenum powder. The melting point of molybdenum pentachloride (MoCI5) is 467 K, and its boiling point is 541 K. A fluidized bed unit for reduction is described in Chapter 4, where molybdenum pentachloride is being supplied at temperatures ranging from 470 K to 570 K (MoCI3 is decomposed at higher temperature). Optimal conditions providing metal precipitation of
Production of Refractory Metal Powders Chapter
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88% and reduction degree of 98% are as follows: temperature range from 870 K to 1070 K, MoCI5 concentration in gas vapor phase of 4–5 vol%, and a molar ratio of hydrogen to molybdenum pentachloride NLE 30 to 1. Obtained fine powder contains: 40–60 wt% fraction minus 0.6 mm, 20–23 wt% fraction 0.6–1.2 mm, 10–20 wt% fraction 1.6–1.8 mm, and 1–3 wt% fraction 1.8–2.4 mm. If a rotating reaction chamber is used, where ammonia paramolybdate is reduced by hydrogen, porous conglomerates of dispersed powders are obtained which are narrow in size and shape. Powders are characterized by high bulk mass and good flow, and their dispersivity depends on temperature condition. Highly dispersed (3–5 mm) powders of rhenium and molybdenum alloy (1:1) are produced by treatment of ReOMoO(OCH3)7 in two steps: calcinations in oxygen (523–573 K, 1–2 h) and reduction by hydrogen (1073–1173 K, 2–3 h). The carbonyl process for molybdenum powder production is based on the decomposition of carbonyl molybdenum (Mo (CO)6). The powder particle sizes are around 1 mm, and the surface of the particles is rather smooth. Having increased the temperature of carbonyl decomposition, there have been changes in powder color, carbon content, apparent density, and lattice spacing (Table 23.12). Carbonyl molybdenum powders enriched by carbon represent the metastable carbides with face-centered lattice. The increasing decomposition temperature leads to the extension of lattice spacing. Impurities of carbon and oxygen are typical for molybdenum powders, and the content of sulfur, phosphorus, and arsenic does not exceed 1 ppm. By execution of the carbonyl process in a hydrogen and nitrogen current, a peak of carbon (amorphous) content is observed at 970 K as a result of catalytic decomposition of its dioxide. Addition of oxidizing components into a gas-carrier leads to decreasing carbon content. For instance, the powder obtained in nitrogen with 2.2% of oxygen at 670 K contains 0.27 wt% carbon. To remove impurities, it is effective to heat treat powders at 1170 K and above. The carbon amount goes down by a digit factor, while carbonyl decomposition in ultrahigh frequency discharges.
Properties of Molybdenum and Molybdenum Alloy Powders The high boiling and melting points of molybdenum (4912 K and 2890 K, respectively) are second only to those of tungsten, rhenium, and osmium. Molybdenum is not used as widely as tungsten because it oxidizes more readily and erodes faster on arcing than tungsten. Nevertheless, due to the density of molybdenum (10.2 g/cm3), which is about half that of tungsten (19.3 g/cm3), use of the former is advantageous where mass limitation is important. The cost of molybdenum is also lower. In addition to its use in make-(before-) break contacts, molybdenum is widely used for mercury switches because it is not attacked, but only wetted, by mercury. Like tungsten, molybdenum strips and sheets are made by swaging or rolling sintered powder compacts. Table 23.13 shows the physical properties of pure molybdenum and tungsten metals. Temperature effects on the physical and mechanical properties of molybdenum and tungsten rods are illustrated in Fig. 23.35. The mechanical properties of molybdenum and molybdenum alloys greatly depend on the amount of working treatment below their recrystallization temperature and on the ductile-to-brittle transition temperature. The minimum recrystallization temperature is 1173 K. The metal maintains superior strength and hardness at elevated temperatures. However, when hot strength is required, a molybdenum alloy (rather than elemental material) is the material of choice. Titanium and zirconium additions to molybdenum create alloys with hot strength and recrystallization temperatures above those of unalloyed molybdenum. TABLE 23.12 Dependence of Carbonyl Molybdenum Powder Properties on Decomposition Temperature Decomposition Temperature in Current of Argon (K)
Color of Powder
Apparent Density (g/cm3)
Carbon Content (wt%)
Lattice Spacing (nm)
670
Gray
0.5
2.36
…
770
Gray
…
3.28
0.42019
870
Dark-gray
…
3.38
0.42030
970
Black
…
3.53
0.42100
1070
Black
2.50
3.48
0.42220
2970–3970
Black
…
0.10–0.20
…
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TABLE 23.13 Typical Properties of Molybdenum and Tungsten Property Density
Unit
Molybdenum
Tungsten
g/cm
10.22
19.3
GPa
1500
70
bcc
bcc
3
Crystal structure Hardness HB Modulus of elasticity At 293 K
GPa
325
405
At 1273 K
GPa
220
325
Melting point
K
2890
3410
Boiling point
K
4912
5660
J/kg K
251
134
Thermal conductivity at 293 K
W/mK
138
154
Coefficient of linear thermal expansion at 293 K
mm/m K
5.53
4.43
Electrical resistivity at 293 K
nO m
53.4
55
Electrical conductivity at 293 K
%IACS
33
31
Specific heat At 293 K
Some of the physical properties of molybdenum and tungsten vary considerably with cross-sectional and grain structure.
Table 23.14 gives basic mechanical property data for 0.38 mm wire fabricated from pure molybdenum and several of the advanced molybdenum alloys. The TZM molybdenum alloy is a high-strength, high-temperature material. This material is manufactured either by the PM or arc-melting processes. The composition of TZM consists of: (0.01–0.04) wt% carbon, (0.40–0.55) wt% titanium, (0.06–0.12) wt% zirconium, <0.0005 wt% hydrogen, <0.010 wt% iron, <0.002 wt% nickel, <0.002 wt% nitrogen, <0.0025 wt% oxygen, <0.008 wt% silicon, and the balance of molybdenum. At 1273 K, its strength is about 720 MPa, which is approximately twice that of unalloyed molybdenum. The alloy is excellent for structural applications under conditions where unalloyed molybdenum is normally used [109]. In comparison with unalloyed molybdenum, TZM has a higher recrystallization temperature and higher strength and hardness at room and elevated temperatures. Its advanced mechanical properties are due to the dispersion of composite carbides in the molybdenum matrix. Basic application fields of TZM include: l l l l l
Die inserts for casting metals Die bodies and punches for hot stamping Tools for metalworking Heat shields and linings for furnaces, structural parts and heating elements Rocket nozzles.
To improve the elevated temperature strength of PM TZM alloys, titanium and zirconium carbides have been replaced by hafnium carbide. An alloy containing 1% Hf and 0.06% C has a tensile strength of 580 MPa at 1588 K, in comparison with 480 MPa for TZM. Creep rate at 1478 K at a stress of 330 MPa is 0.038%/h, compared to 0.05%/h for TZM [110]. Table 23.15 presents the compositions and properties of various molybdenum-silver composites for electrical makebreak contacts [109]. To improve the mechanical properties, including ductility and creep resistance of structural molybdenum-base materials, doped molybdenum alloys were developed addition a small amount of aluminum, potassium, silicon, or rare-earth oxides [111]. Recently, high-toughness molybdenum alloys have been developed with fine grains and finely dispersed titanium carbide particles by mechanical alloying (MA) and hot isostatic pressing (HIP) processes, followed by hot forging and hot and warm rolling. The high toughness of the alloy may be due mainly to grain-boundary strengthening by fine carbide particles having semi-coherency with the adjacent matrix, and the particles have the beneficial effects of increasing
Production of Refractory Metal Powders Chapter
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FIG. 23.35 Physical and mechanical properties of molybdenum and tungsten rods depending on the temperature. (Adopted from Shen Y-S, Lattari P, Gardner J, Wiegard H. Electrical contacts. In: ASM Handbook, vol.2. Novetly (OH, USA): ASM International Publishers; 1990. p. 840–68.)
the recrystallization temperature and hindering grain growth. However, in both cases, a large amount of plastic working was required. Such working may limit the products available to thin sheets or thin wires shapes, which are inapplicable as structural materials. Data regarding the mechanical properties for carbide-dispersed molybdenum alloys are given in Table 23.16. Because the properties of materials with the same composition depend on manufacturing methods, the manufacturing methods are also referenced in these tables. These common methods of producing composite electrical contact materials are described in subparagraph “tungsten powder applications” (see above). Molybdenum alloys with 0.8 mol% zirconium carbide (ZrC) or tantalum carbide (TaC) as dispersed particles were fabricated by mechanical alloying and HIP or spark plasma sintering (SPS) [112]. Powders of pure molybdenum (average
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TABLE 23.14 Tensile Properties of 0.38 mm Molybdenum Wire at Room and Elevated Temperatures Material Sign
Composition
Temperature (K)
Ultimate Tensile Strength (MPa)
Elongation (%)
Unalloyed molybdenum
Mo
293
1350
4.1
1273
305
2.4
1373
140
10.3
1473
115
12.5
293
1565
3.1
1273
1020
2.7
1373
795
3.2
1473
675
2.8
293
1980
3.6
1273
1095
2.3
1373
950
2.3
1473
745
2.2
293
1795
2.9
1273
1270
3.4
1373
1185
3.3
1473
1035
3.0
293
1935
3.2
1273
1350
3.3
1373
1250
3.1
1473
1075
4.6
293
2135
3.6
1273
1460
3.3
1373
1295
2.6
1473
1170
2.4
MT-104
Mo + 45W
HCM
HWM-25
HWM-45
Mo-0.5Ti-0.08Zr0.01C
Mo-45W
Mo-1.1Hf-0.07C
Mo-25W-1.0Hf0.035C
Mo-45W-0.9Hf-0.03C
Source: Adopted from Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.
particle size 4.1 mm), ZrC (2 mm) and TaC (1.1 mm) were mixed to obtain the target composition in an argon atmosphere. These blended powders were subjected to mechanical alloying for 108 ks in a planetary type ball mill with lining and balls made of a cemented carbide (WC-Co). HIP and SPS were conducted at 1573 K and 147 MPa for 10.8 ks in argon, and at 2073 K and 74 MPa for 0.3 ks in a vacuum of 10 Pa, respectively. These alloys were annealed in a vacuum of 2.7 104 Pa at 1873–2473 K for 3.6 ks. Yield strength, ultimate tensile strength and elongation at 300 K and high temperature for Mo-0.8 mol% ZrC, Mo-0.8 mol% TaC, and pure Mo conducted with tensile specimens in form of bar 4 mm wide, 25 mm long, and 1 mm thick are shown in Table 23.16. The superplastic behavior was investigated on a gauge section 3 mm wide, 8 mm long, and 2 mm thick. In both MA with HIP and MA with SPS specimens of Mo-08 mol% ZrC as sintered, the tensile strengths are 1.8 times higher than that of pure molybdenum; however, no plastic behavior is found. While both MA with HIP and MA with SPS specimens of annealed Mo-08 mol%ZrC are superior in terms of room temperature strength, their yield and tensile strengths are about 3 and 2.1 times higher, respectively, than that of pure molybdenum.
10.35 10.35
PSR
PSR
INF
PSR
80Ag-20Mo
75Ag-250Mo
50Ag-50Mo
INF
INF
INF
INF
25 Ag-75Mo
20 Ag-80Mo
15 Ag-85Mo
10 Ag-90Mo
10.23
10.24
10.26
10.27
10.29
10.13
10.18
10.23–10.26
10.27
10.00–10.31
10.12
10.10–10.32
10.10–10.32
10.14
10.10–10.24
10.33
10.36
10.38
Typical
27–30
28–31
28–32
31–34
35–45
45
42–49
44–58
50
45–52
58–61
59–62
65–68
Electrical Conductivity (% IACS)
97–102
97–102
96–98
93–97
85–95
50–68
80–90 HRB
75–82 HRB
65
70–80 HRB
44–47 HRB
38–42 HRB
35–40 HRB
Hardness
… …
758 552
… … 676
… …
… …
… … …
965 …
…
…
…
938
407
414
931
…
…
414
Modulus of Rupture (MPa)
Tensile Strength (MPa)
G
G
C, A
C, A
C, A
C, A
C, A
A
C, A
C, A
A
A
A
Data Sourceb
Semiconducting material
Arcing contacts, heavy-duty electrical applications
Air-circuit breakers. Lowerosion arcing tips
Aircraft switches, breaker arcing tips, electric raisers, air- and oil-circuit breakers
Air- and oilcircuit breakers, arcing tips, traffic signal relays, home circuit breakers
Air conditioner control Light- and medium-duty applications, automotive circuit breakers switches, circuit breakers
Application Examples
a INF, press-sinter-infiltrate; PSR, press-sinter-repress (blended powder of the desired composition are compacted to the required shape and then sintered. Afterwards, the material is further densified by a second pressing. Sometimes the properties can be modified by a second sintering or annealing). b A: Advance Metallurgy, Inc., McKeesport, PA. C: Contacts, Materials, Welds, Inc., Indianapolis, IN. G: Gibson Electric Inc., Delmont, PA.
INF
10.32
PSR
30Ag-70Mo
10.32
INF
40Ag-60Mo
13.33
INF
45Ag-55Mo
10.42
10.44
10.47
PSR
90Ag-10Mo
Calculated
Manufacturing Methoda
Nominal Composition (%)
Density (g/cm3)
TABLE 23.15 Properties of Typical Molybdenum-Silver Composites for Electrical Make-Break Contacts
Production of Refractory Metal Powders Chapter 23 729
0.110
0.001
0.69Zr
1.2Ta
…
Mo-0.8ZrC
M0–0.8TaC
Pure Mo
0.001
0.019
0.032
0.022
N
0.001
0.137
0.138
0.140
O
MH
MS
MS
MH
Manufacturing Method
100
99.9
99.9
99.9
Relative Density (%)
MH: Mechanical alloying and hot isostatic pressing, MS: Mechanical alloying and spark plasma sintering.
0.117
0.100
0.70Zr
Mo-0.8ZrC
C
Metal
Alloy
Analyzed Composition (wt%)
53.8
11.2
Annealed Recrystallized
10.9
2.8
Annealed As sintered
2.7
1.8
Annealed As sintered
0.3
As sintered
Condition
Grain Size (mm)
344
547
559
469
639
707
1077
838
… 1038
1053
827
… 1015
Ultimate Tensile Strength (MPa) Yield Strength (MPa)
54
38
12
9.0
0.0
0.6
0.0
Elongation (%)
SECTION
TABLE 23.16 Mechanical Properties at 300 K for Carbide Dispersed Molybdenum Alloys
730 D Production of Non-Ferrous Metal Powders
Production of Refractory Metal Powders Chapter
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FIG. 23.36 Dependence of ultimate tensile strength and elongation of the carbide dispersed molybdenum alloys on temperature: (MH) corresponds to HIP with MA powders, (MS) corresponds to SPS with MA powders. (Source: Takida T, Kurishita H, Mabuchi M, Igarashi T, Doi Y, Nagae T. Mechanical properties of fine-grained sintered molybdenum alloy processed by mechanical alloying. In: Kneringer G. editor. Proceedings of 15th international Plansee seminar, vol. 1. Reutte (Tirol, Austria): Plansee Holding AG; 2001. p. 293–304.)
The carbide dispersed molybdenum alloys exhibit excellent strength up to 1373 K. Their tensile strengths are 3–5 times higher below 1473 K (about 0.5 melting point), and 1.5–2 times higher at 1773 K, than those of pure molybdenum and Mo-0.8 mol%TaC (Fig. 23.36). The elongation of Mo-08 mol%ZrC with grain size of 3.0 mm obtained by means of SPS with MA increased rapidly in the temperature range over 1773 K, and a superior limit elongation of 180% is attained at 1973 K. Variations in the nature of the oxide and consolidation method have been shown to have a significant effect on the mechanical properties of oxide dispersion strengthened (ODS) molybdenum material [113]. Comparison of the effect of doping technique species on tensile properties of ODS molybdenum-lanthana and ODS molybdenum-yttria alloys illustrates that, for both oxide species, the wet doping technique results in a much finer dispersion of oxide particles ranged from 0.08 to 0.12 mm. The wet doping process uses an aqueous tantalum nitrate solution to dope the molybdenum dioxide (MoO2) precursor before its reduction to molybdenum metal powder [114]. At ambient temperature, the alloy’s strength is not strongly sensitive to either doping technique, but, at high temperatures, the wet doping technique produces superior material for a given oxide. Lanthana-doped material is also superior to yttria-doped material at elevated temperature. The ODS molybdenum material contains nominally 2 vol% La2O3 that corresponds to a lanthanum (La) content of 1.09 wt%. The addition of a small amount of group VIII transition metals, such as nickel and palladium, can lower the activation energy of sintering and decrease the sintering temperature from >1970 to <1570 K [115]. However, such activationsintered compacts have no ductility, and thus, have restricted application in industry [116]. Through activated liquid phase sintering, the addition of 1.5 wt% Ni to molybdenum was shown to improve the sintered density significantly to 97% at 1573 K, while the pure molybdenum only reached 82% theoretical density. But, this impairs ductility. This embrittlement appears to be caused by a grain boundary layer of a Ni-Mo compound about 2 nm thick formed during sintering [117]. Due to the brittle nature of this compound, the Ni-containing molybdenum fractures through this thin compound layer. There are several grades of ODS Mo alloys available for commercial use. The most popular doping levels of lanthana (La2O3) to molybdenum are: 0.3% by weight, 0.6% by weight and 1.1% by weight. In work [118], the thin sheets, that is,
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FIG. 23.37 SEM image of molybdenum powder with 1 at.% Na after ball milling for 75 h. (Source: Bergk B, Muhle € U, Kieback B, Koutna´ N, Holec H, Clemens H. Nanocrystalline alloys of molybdenum with sodium and yttrium obtained by mechanical alloying. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2017 euro congress. Milan (Italy); 2017: p. 1–6.)
0.5 mm and 0.2 mm thick, of these alloys were studied. The ingots were made by cold-isostatic pressing of the Mo-lanthana (MoLa) powders with subsequent sintering. The ingots were subsequently thermo-mechanically processed, first into plates during hot rolling, then into sheets. It was concluded [118] that the annealed 0.6 wt% and 0.3 wt% lanthana MoLa sheets exhibited deep drawability with a 900–1020 MPa UTS range, 800–920 MPa YS range, and about 25%–27% elongation, and can be considered the best material for the majority of furnace components applications, while the thin MoLa sheets, that is, 0.2 mm thick, exhibited ideal isotropy of bendability under annealed condition with no regard to lanthana content. There was also reported that hardness is not a reliable differentiating factor among lanthana-content MoLa sheets. Molybdenum alloys with low contents of immiscible sodium and yttrium elements made by mechanical alloying were studied [119]. The alloys were produced by high-energy ball milling utilizing a Retsch SM 200 mill placed inside an argon glove box to guarantee high purity conditions and to avoid uptake of oxygen and nitrogen. Fig. 23.37 shows a SEM image of an alloy with 1.0 at.% Na after milling for 75 h. The particle sizes distribute in 1–20 mm range. As reported, the nominal concentration of 1 at.% Na or Y the solute content remains constant up to temperatures of 1173 K. For higher temperatures, the solute content starts decreasing until at 1473 K almost no sodium like yttrium is left. The alloy’s structure is characterized by the following: the grains have been grown to about 150 nm and are reduced to about one tenth of the value for the as milled samples; it is revealed that sodium (as well as yttrium) adapts itself to the molybdenum lattice. Because the conventional sintering of molybdenum requires high temperatures and long holding times, which leads to large grain growth, much research has been focused on the enhancement of the sinterability of Mo using spark plasma sintering (SPS) technique [120–122]. In the work [123], molybdenum nanopowder was sintered by SPS technique under various temperatures, pressures, heating rates, and holding times in order to decrease the grain size. Nanosized molybdenum powder from US Research Nanomaterials, Inc., (TX, United States) with a purity of 99.9% (metal basis) has been used in this study. The powders correspond to agglomerates of roundish particles with a size range of about 50–200 nm (Fig. 23.38). The SPS device was set to apply the periodic sequence of 12 ms of DC pulses followed by a pause of 2 ms. For 60 MPa or 120 MPa SPS pressure levels, measured initial green (after pre-cold compaction) densities of the Mo powder compacts were 58% and 67%, respectively. Sintering conditions of the conducted SPS tests included heating rates of 100 K/min or 200 K/min from 873 K to the maximum temperature (1473, 1573, 1873, or 2073 K) with dwell times of 0 or 10 min. The grain sizes of Mo pellets showed large dependence on sintering temperature. Fig. 23.39 shows the grain size of SPSed Mo pellets sintered under different temperatures with the same conditions of 60 MPa, 100 K/min, and 0 min holding time. As shown in Fig. 23.39A, 1573 K conditions are related to a small grain size of 2 mm, while grain size increased to 37 mm or 54 mm when the sintering temperature was over 1873 K (Fig. 23.39B and C). At sintering temperatures between 1573 K and 1873 K, the Mo oxide phase melted and moved to the grain boundary regions. In this study [123], the highest achieved microhardness was 435 kgf/mm2 (4.266 GPa). Relative density of 95.77% was achieved with 1473 K, 60 MPa, 100 K/min, and 10 min holding time. Optimal conditions for Mo pellet processing that resulted in small grain size, high density, and high hardness are determined to be 1473 K, 60 MPa, 100 K/min, and 10 min holding time under vacuum.
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FIG. 23.38 SEM images of molybdenum nanopowders at two magnifications. (Source: Lee G, Olevsky E, McKittrick J. Spark plasma sintering of molybdenum nanopowders. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
FIG. 23.39 SEM phase images of spark plasma sintering (SPS) pellets consolidated at different temperatures: (A) 1573 K, (B) 1873 K, and 1973 K; white regions indicate Mo, black regions indicate Mo oxide. (Source: Lee G, Olevsky E, McKittrick J. Spark plasma sintering of molybdenum nanopowders. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
The elaboration possibilities of the refractory Mo3Al intermetallic compound by self-propagating high-temperature synthesis (SHS) starting from a near stoichiometric mixture of powder components were considered [124]. High-purity fine powders of molybdenum and aluminum (MO006021 and AL006031 grades) had been used as staring materials. Appropriate amounts of them, corresponding to 76:24 (Mo:Al) atomic ratio (a near stoichiometric), had been preliminarily homogenized in a Turbulla blender for 30 min. To facilitate SHS reaction, ahead of time, researchers carried out an intimate homogenization and an energetic activation of the component powder mixture by a controlled mechanical alloying-mechanical disordering (MA + MD), by milling for 3 h in a planetary ball mill under argon atmosphere. An SEM image of Mo-Al powder mixture subjected by 3 h (MA + MD) processing is shown in Fig. 23.40. As reported [124], by subjecting compacts realized from the above powder mixture by SHS in thermo-explosion mode under pressure, cylindrical compacts of a microstructure similar to superalloys, consisting of a multitude of submicron Mo3Al crystallites embedded in a Mo-Al alloy matrix, density of 73.26%, and higher Vickers hardness than of Mo (1742 MPa, vs. 1400 MPa) have been obtained.
Workplace Atmospheres Safety Under the Workplace exposure limits in the European PM industry the long- and short-term (15-min reference period) workplace exposure limits (WELs) for soluble molybdenum compounds (as Mo) are 5 mg/m3 and 10 mg/m3, accordingly, and the limits for insoluble compounds (as Mo) in the workplace atmosphere are 10 mg/m3 and 20 mg/m3, respectively [57]. According to US New Jersey Department of Health and Senior Services. Hazardous Substance Sheet, the legal OSHA airborne permissible exposure limit for molybdenum is 15 mg/m3.
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FIG. 23.40 SEM image of Mo-Al powder mixture subjected by 3 h mechanical alloying- mechanical disordering (MA + MD) processing. (Source: Orban RL, Salomie D, Orban M, Neamt¸u B, Sechel N, Lung T. SHS synthesis from mechanical activated powdered components mixture of molybdenum aluminade Mo3Al for use at extremely high temperatures. In: Compiled by European Powder Metallurgy Association. CD/USB proceedings of PM 2016 world congress. Hamburg (Germany); 2016: p. 1–6.)
The aerosols of metallic molybdenum, either soluble or insoluble molybdenum compounds and molybdenum silicide, have a predominantly fibrous effect and are related to the third class of danger according to State Standard 12.1.005-88 under the legislation in the CIS [56]. The stated average limit value (ALV) of the metallic molybdenum aerosol in the workplace atmosphere as the shift time-weighted average concentration is 0.5 mg/m3, while the threshold limit value (TLV) is 3 mg/m3. The ALVs of the insoluble and soluble molybdenum compounds and molybdenum silicide in dust form are 1.0, 4.0 and 4.0 mg/m3, respectively, while the ALV of soluble molybdenum compounds in the form of condensed aerosols is 2.0 mg/m3. Molybdenum content in potable water has a toxicological effect and is related to the second class of danger, and the TLV is 0.25 mg/L according to CIS State Standard 4630-88 [58]. Molybdenum powder, even with particle sizes at a fraction 74 mm, is not explosive in the air; the minimum ignition temperature of powder deposits (self-ignition temperature) is 583 K. The products with such characteristics of inflammability and explosion risk are related to the “low explosion hazard” class of danger according to the Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used to prevent pollution and environmental damage during powder manufacture and processing. Detailed information on health and environment protection measures in such conditions can be found in Chapter 27.
NIOBIUM AND TANTALUM Niobium and tantalum are often found in close combination in their ores. The most important niobium-tantalum-bearing minerals are columbite and tantalite, which form isomorphous series FeNb2O6-MnNb2O6 and FeTa2O6-MnTa2O6, and the ores are variations of the compound (Fe, Mn)(Nb, Ta)2O6. Tantalum and niobium are separated from ore and from one another by digestion in aqueous hydrofluoric acid, followed by solvent extraction with methyl isobutyl ketone (MIBK), as illustrated in Fig. 23.41 [125]. In the first contact, only the niobium and tantalum fluorides are soluble in the MIBK. Thus, they are separated from the contaminants iron, manganese, titanium, and zirconium, which remain in aqueous solution. The fact that the solubility of tantalum in MIBK is high over an extensive range of acidity, while niobium is soluble only at high acidity, is used for separation of tantalum fluoride and niobium fluoride. This is accomplished in a series of subsequent steps, where MIBK contacts aqueous solutions of varying acidity. Commercially, deposits of niobium are found also in the pyrochlore [(Na, Ca)2(Nb, Ta, Ti)2O4(OH, F)H2O], which is mined in Canada and Brazil. These ores are mostly tantalum free and are enriched to concentrates containing 55%–60% niobium pentoxide by means of a series of operations including grinding, flotation, and leaching. The extraction of niobium by high-temperature chlorination of the ore is also an appropriate technique for niobium powder production [126].
Niobium and Niobium Alloys Niobium is used in commercial applications as an alloying element in steels. Nearly 75% of all niobium metal is used as a minor alloying addition in low-alloy steel. About 20%–25% is used as an alloy addition in nickel-base superalloys and heat-
Production of Refractory Metal Powders Chapter
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735
Columbite/tantalite ore
Hydrogen fluoride
Ball mill
Hydrogen fluoride digester
Waste to neutralizer
Liquid-liquid separator
Niobium fluoride solution
Tantalum fluoride solution Ammonia
Potassium fluoride Potassium fluorotantalate crystalizer
Precipitator
Centrifuge-filter
Filter press
Dryer
Dryer
Sodium reduction
Niobium pentoxide calciner
Acid washer
Thermite reactor
Sodium, sodium chloride
Slag
Aluminium, accelerator Vacuum dryer
Ingot conditioning
Bar press Alloy additive Electron beam melter
Hydrider
Crush and degassing
Sodium-reduced tantalum capacitor powder
Electron beam tantalum or tantalum alloy powder
Niobium or niobium alloy powder
FIG. 23.41 Niobium and tantalum production flowchart. (Source: Kirk RE, Othmer DF, editors. Encyclopedia of chemical technology, vol. 22, 3rd ed. John Wiley & Sons; 1983.)
736
SECTION
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resisting steel. Only 1%–2% of all niobium used is in the form of niobium base alloys and pure niobium metal including super-conducting niobium-titanium alloy, which accounts for over one half of all niobium alloys produced [127].
Production of Niobium Powders Metallothermic Reduction: Initially, niobium metal was produced by PM methods, which involved high-temperature vacuum sintering and carbon reduction. However, in the early 1960s, aluminothermic reduction and electron beam purification came into general use. At present, the basic process for the recovery of niobium, along with recovery from columbite and tantalite ores shown in Fig. 23.41, is the aluminothermic reduction of pyrochlore concentrates to ferroniobium. Niobium metal is purified by a chlorination process, where volatile niobium pentachlorate (NbCl5) is distilled and then hydrolyzed to the oxide. The metal is next recovered by a second aluminothermic reduction 3Nb2 O5 + 10Al ! 6Nb + 5Al2 O3 : During the exothermic reaction, oxide impurities are slagged and removed from the molten niobium. Calcium and magnesium have also high reactivity for the reduction of niobium oxide to niobium. However, magnesium has an advantage over calcium in that it has a high vapor pressure (0.078 bar), even at a moderate boiling temperature of 1363 K. Therefore, niobium metal powders can be also prepared by using a magnesiothermic reduction process [128–131]. Okabe reported a preforms reduced process (PRP) for niobium metal using a magnesiothermic reduction process [129]. Using a cyclone separator assembly, niobium is prepared by the reaction between the magnesium vapor and niobium oxide [128]. However, preparing niobium metal using the cyclone reduction techniques resulted in high concentration of oxygen (7.68%) [128]. In the work [132], an attempt was made to prepare the niobium metal powder with lower oxygen content using the magnesium vapor reduction process. The influence of various factors in the magnesium vapor reduction of niobium oxide and deoxidation of niobium metal powder in a two-stage reduction process is discussed. Prepared by magnesium vapor reduction at 1123 K for 5 h, niobium powder contained about 8 mass% oxygen. After second stage reduction, the oxygen concentration decreased to 0.65 mass%. An SEM micrograph of the niobium powder after second stage reduction is shown in Fig. 23.42. As it appears, fine particles of niobium powder less 1.0 mm form agglomerates from 5 mm to 10 mm in size. Carbothermic Reduction: Carbothermic reduction is also practiced [133]. Reduction is conducted in two steps involving the formation of niobium carbide: Nb2 O5 + 7C ¼ 2NbC + ð5COÞ Nb2 O5 + 5NbC ¼ 7Nb + 5CO It is not desirable to conduct carbothermic reduction in one stage. Stock of Nb2O5 contains only 57.2% of niobium, and even in briquette form, it has low density ( 1.8 g/cm3). High volumes of CO ( 0.34 m3) are consumed per 1 kg of stock leading to the reduction process’ low capacity. Therefore, the first step is carried out in graphite-tube resistance push-type furnaces in a hydrogen atmosphere at a temperature of 2070 K. The graphite cartridge with the stock (Nb2O5 + 7C) is pushed through the furnace for 1–1.5 h. Lightly sintered niobium carbide is ground to powder and blended with Nb2O5 in a surplus of 3%– 5% against the stoichiometric ratio. Then, this mixture is pressed into briquettes at a pressure of 100 MPa and calcined in a FIG. 23.42 SEM micrograph of the niobium powder made by magnesium vapor reduction. (Source: Kumar TS, Kumar SR, Rao ML, Prakash TL. Preparation of niobium metal powder by twostage magnesium vapor reduction of niobium pentoxide. J Metallurgy 2013; (Article ID 62934): 1–6.)
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vacuum furnace at 2070 K and a residual pressure of 1.3–0.13 Pa for 6 h. The niobium reduction goes by intermediary stages of lower oxide formation (NbO2 and NbO). It is important that at temperatures above 900 K, a part of Nb2O5 is evaporated, while NbO2 and NbO evaporate at higher temperatures. Niobium oxide vapor is adsorbed on the carbon and niobium carbide particles, where niobium is reduced. The second step goes with higher capacity, therefore, the mixture Nb2O5 + 5NbC has 82.4% niobium in briquettes with a density of about 3 g/cm3. Reduction by Sodium: The reduction of potassium fluorine niobate (K2NbF7) by sodium may be represented as follows: K2NbF7 + 5Na ¼ Nb + 2KF + 5NaF This process is realized in a shaft furnace using a layerwise loaded charge in a steel crucible. Sodium surplus (15%–20%) in the form of sodium chloride layer is poured onto the top of the charge; the crucible is placed into the preheated furnace (up to 870 K) and kept for 1.0–1.5 h at 1170–1270 K. Niobium powder is washed, first in cold then in hot water, and is next washed with dilute nitric or hydrochloric acid (to eliminate iron, titanium and other impurities) and 2.5% cold hydrofluoric acid for 5–15 min to decrease the niobium oxide content. Electrolysis of Melt: Electrolysis of melt is also used. Refined salts are melted in a nickel crucible (cathode), and Nb2O5 is introduced into the melt; the anode is a graphite rod, and the electrolyte temperature is 1020 K. Voltage on the cell is 2.5 V; cathode current density ranges from 5 to 6 kA/m2, and current efficiency is not >30%. The niobium deposit is coarsegrained and contaminated with the lowest niobium oxides. Two types of electrolyte are used K2NbF7-KCl-Nb2O5 and K2NbF7-KCl-NaCI (for refining). In liquid melt of the former, before Nb2O5 is introduced, the following reactions take place: K2NbF7 + KF ¼ K3NbF8 and/or K2NbF7 + KCl ¼ K3NbF7Cl Having added niobium pentoxide in surplus of KF, the reaction can be written: K2NbF7 + 13KF + 2Nb2O5 ¼ 5K3NbO2F4 2 3 Complex niobium containing ions hypothetically NbF3 8 , NbF7 and NbO2F4 are present in the electrolyte. At temperatures in the range from 970 K to 1020 K and cathode current density ranging from 1 to 8 kA/m2, it is possible to obtain niobium powder, in which, compared to anode material, the impurities content is less by an order of magnitude: for iron, 1–3, for silicon, 1–1.5, for carbon, 1–1.5, and for oxygen, 1.0. Niobium sponge can be produced also by electrodeposition from halide melts. Electrolysis of bromide and iodide melts (4%–6% Nb) is carried out with addition of alkali metal chlorides at 973 K and cathode current density of 5 kA/m2. Current efficiency is about 40%. The obtained sponge is lightly milled to powder. For niobium reduction from halide melts, the starting material is niobium produced by carbothermic reduction and the electrolyte is a mixture of NaCl and K2NbF7, with 10%–40% of the latter. In an argon atmosphere, niobium is oxidized on the anode and reduced on the cathode using a cathode current density of 32–41 kA/m2 and temperature between 1100 K and 1200 K. The electrolysis cell (Fig. 23.43) made of nickel is put into the furnace with graphite heaters and a heat shield. On the furnace cover, a water-cooled chamber for charging is erected, in which the cathode is vertically moved. Current efficiency,
FIG. 23.43 Elecrolyzer for niobium refining.
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TABLE 23.17 Compositions of Contaminant Elements in Anodes and Electrolytic Niobium Powder Elements
Unit
Anode
Powder
Aluminum
ppm
10
10–20
Calcium
ppm
50
50
Chromium
ppm
20
10
Copper
ppm
10
1
Iron
ppm
30
70–150
Lead
ppm
…
10
Magnesium
ppm
10
10
Manganese
ppm
…
10
Nickel
ppm
10
100–500
Silicon
ppm
30
4–5
Tin
ppm
…
10
Titanium
ppm
1000
100
Vanadium
ppm
…
10
Zirconium
ppm
5000
100
calculated to Nb2+, ranges from 23.1% to 34.6%. Compositions of contaminant elements in electrolytic refined niobium powders and in anodes are given in Table 23.17. The powders (cathode product) consist of modified coarse dendrites. Hydride-Dehydride Process: Powders are produced from ingots by means of hydriding, crushing, and subsequent dehydriding of electron beam (EB) melted ingots. These melt-grade powders have higher purity than the sodium-reduced types and have better dielectric properties. However, unit capacitance is usually lower for EB-powder. Although the dielectric properties of niobium oxide are inferior to those of tantalum oxide, niobium has been evaluated as a means of manufacturing low-cost capacitors. Considerable niobium powder development work is required before niobium capacitors will be offered commercially. Usually, powders are crushed to pass a 180 mm (80-mesh) screen and a mean particle size of 10–15 mm is typical.
Niobium Alloys Typical compositions of niobium and niobium alloy powders are given in Table 23.18. Niobium powder is frequently used as the starting material to blend with other alloying powders. The blend is pressed to bars and melted. Niobium metal scrap, which is reduced to powder by the hydride-dehydride process, can also be incorporated into the alloy blends. Some efforts [127] have been directed to producing complex metastable alloy powders, such as niobium-aluminum and niobium-silicon alloys, by liquid metal atomization and rapid solidification. Most commercial niobium alloys are in wrought form. Primary (bulk) processing is generally done by high temperature extrusion or forging. Secondary working manufactures a variety of mill products such as rods, sheets, wire, foil, and tubing. Powder metallurgy methods have been evaluated for common niobium alloys, such as C-103 (Nb-10Hf-1Ti) and Nb-30Hf9W. It has been found that the high production cost of atomization prevents economical manufacturing of net shape parts. However, hydride-dehydride niobium powders are used to produce commercial quantities of hot working preforms of alloy Nb-30Hf-9W [127]. The NbCu composite is used to produce electric contacts, resistances, welding electrodes, microwaves absorbers, and heat sinks. Some studies of the Nb-Cu composite with varying (5–20 wt%) copper content were carried out [134–136]. In the work [137], the Nb-Cu composites with 12.5 wt% and 25 wt% Cu content where produced by mechanical alloying using planetary ball milling. Niobium powder made by hydride-dehydride technique (HDH) and copper powder made by carbonyl process were used in this work. The characteristic morphology of these powders is shown in Fig. 23.44.
Production of Refractory Metal Powders Chapter
23
TABLE 23.18 Typical Compositions of Niobium and Nb-10Hf-1Ti Alloy Powders Made by the Hydride-Dehydride Process Analysis Niobium
Unit
Source: Fansteel Inc
GOST 26252–84 I-grade
Nb-10Hf-1Ti (C-103 Niobium Alloy) Source: Fansteel Inc
Niobium
wt%
99.7%
balance
87.2%
Hafnium
ppm
<20
…
9.8 wt%
Titanium
ppm
<20
<10
0.91 wt%
Aluminum
ppm
<20
<10
<20
Boron
ppm
<1
…
<10
Carbon
ppm
500
<50
194
Cobalt
ppm
<10
<10
<10
Copper
ppm
<40
<30
<40
Hydrogen
ppm
150
<20
50
Iron
ppm
100
<30
200
Magnesium
ppm
…
<10
…
Manganese
ppm
…
<10
…
Molybdenum
ppm
<20
<30
100
Nickel
ppm
<20
<10
<20
Oxygen
ppm
1820
<2000
1980
Nitrogen
ppm
197
<200
62
Silicon
ppm
30
<30
<20
Tantalum
ppm
800
<600
2800
Tin
ppm
…
<10
…
Tungsten
ppm
<50
<30
1100
ppm
<20
<10
1800
Other elements
ppm
<20
…
<20
Physical properties:
ppm
Apparent density
g/cm3
…
0.7
…
…
1.0; 1.1; 1.3
…
…
0.76
…
Property Elements:
Zirconium a
b,c,d
Average particle sizes,
(mm)
Surface area
m2/g
a
Other elements include cadmium, chromium, magnesium, manganese, lead, tin, vanadium. Method of particle sizes determination: BET particle size calculated. Mercury porosimetry. d FESEM particle size. b c
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D Production of Non-Ferrous Metal Powders
FIG. 23.44 Morphology of (A) carbonyl cooper powder and (B) HDH niobium powder. (Source: Dias A, Grossi J, Muterlle PV. Production of NbCu composite by powder metallurgy. In: Completed by European powder metallurgy association. CD/USB proceedings of PM2017 euro congress. Milan (Italy): EPMA; 2017: p. 1–6.)
FIG. 23.45 Morphology of Nb-Cu composite powders with 12.5 wt% of cooper content (A) after two and (B) eight hours milling for the samples. (Source: Dias A, Grossi J, Muterlle PV. Production of NbCu composite by powder metallurgy. In: Completed by European powder metallurgy association. CD/USB proceedings of PM2017 euro congress. Milan (Italy): EPMA; 2017: p. 1–6.)
Fig. 23.45A and B exhibit the morphology of powders with 12.5 wt% of copper after two and eight hours milling for the samples, respectively. Two hours of milling was not effective to obtain the NbCu composite by mechanical alloying, because the powders still preserve their elementary conditions. Milling for eight hours shows that the elementary powders are not yet visible. After milling, the powders were compacted at 400 MPa using a uniaxial mechanical press. The sintering of green samples was carried out in a tubular furnace with an argon atmosphere at a 1373 K temperature. As reported, all samples presented higher than 75% of full density, and in the samples with 25wt%Cu, the densification was higher than 80%. Nb-Si intermetallic composites feature is high-temperature strength at rather low densities. They have a density of around 7 g/cm3, which is significantly lower than the density of nickel-based superalloys with about 9 g/cm3 [138,139]. For high-temperature resistant components in the high-pressure section of aircraft and land-based turbines, nickel-based superalloys are currently applied. The efficiency of these turbine engines can be further improved by increasing the service temperature. Currently, heat-resistant nickel alloys (HRNAs) for casting monocrystalline GTE blades have reached the limiting operating temperatures of 1370–1420 K for the material, which is 80%–85% of their melting point. Therefore, refractory alloys such the niobium-based silicide intermetallic composites (Nb-Si) are a promising material group for these applications. Until now, the utilized processing technologies to develop Nb-Si alloys were mainly casting, directional solidification, arc melting plus subsequent extrusion, physical vapor deposition, and hot extrusion of gas-atomized powders [140,141]. The challenge remains to find a processing technique that achieves a homogeneous microstructure and allows the production of near net shape Nb-Si components for industrial production [139,140]. The paper [142] reports the development of several metal injection molding (MIM) test geometries by utilizing gasatomized powder. The same powder was processed by hot isostatic pressing (HIP) as well. After sintering, some MIM samples were subjected to subsequent HIPing to achieve further densification. Residual porosity, microstructure, phase distribution, and density achieved by these three processing techniques (MIM, HIP and MIM + HIP) are compared. Mechanical properties at ambient and elevated temperatures are assessed for both the sintered MIM samples and HIP
Production of Refractory Metal Powders Chapter
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FIG. 23.46 SEM of the gas atomized Nb-Si powder <25 mm size fraction. (Source: Mulser M, Hartwig T, Seemuller € C, Heilmaier M, Adkins N, Wickins M. Influence of the processing technique on the properties of Nb-Si intermetallic composites for high-temperature applications processed by MIM and HIP. In: Compiled by Russell A, Chernenkoff W, Brian J. Proceedings of 2014 powder metallurgy world congress. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 04-8-16.)
material. Powder of the nominal composition Nb-20Si-23Ti-3Cr-6Al-4Hf (at.%) was produced by gas atomization in an argon atmosphere by electrode induction melting. A scanning electron micrograph of materials gas-atomized in an argon atmosphere by electrode induction melting is shown in Fig. 23.46. The powder particles are spherical with a smooth surface and limited satellites. Compared with an HIP sample of powder <25 mm size fraction (Fig. 23.47A), the MIM sample of the same powder size fraction (Fig. 23.47B) shows a higher porosity with pores up to 10 mm in diameter. As reported [142], the microstructure achieved by MIM reveals a much finer grain structure and more homogeneously distributed silicide particles compared to HIP. However, the MIM samples show larger HfO2 precipitates and a higher residual porosity. By subsequent HIP processing, the residual porosity was reduced, increasing the density to 99.2%. The mechanical properties at RT and elevated temperatures of the as-sintered condition are significantly lower than those achieved by HIP material. In the work [143], the alloying behavior and properties of Nb5Si3 were studied. The study used data for the silicide parameters as valence electron concentration (VEC) and electronegativity (Dw) and for the silicide solubility range, which were determined using the concentration X ¼ Al + B + Ge + Si + Sn in (Nb,TM)5X3). Actual chemical compositions of tetragonal Nb5Si3 in developmental Nb-silicide based alloys were used to calculate VEC, Dw and X. Relationships between solvent and solute additions in Nb5Si3 and its parameters VEC and Dw were found. Changes in the hardness and creep of tetragonal Nb5Si3 were related to the parameters VEC and Dw. The influence of the technological regimes of vacuum induction melting and oriented crystallization melting on the content of alloying elements and the structure of an in-situ composite of the Nb-Si system alloyed with titanium, hafnium, aluminum, chromium, molybdenum, tungsten, and zirconium was studied [144]. The following conclusions were made:
FIG. 23.47 Scanning electron micrographs of (A) HIP and (B) MIM of <25 mm size powder fraction of the Nb-Si intermetallic composite. (Source: € Mulser M, Hartwig T, Seemuller C, Heilmaier M, Adkins N, Wickins M. Influence of the processing technique on the properties of Nb-Si intermetallic composites for high-temperature applications processed by MIM and HIP. In: Compiled by Russell A, Chernenkoff W, Brian J. Proceedings of 2014 powder metallurgy world congress. Orlando (FL, USA): Metal Powder Industries Federation; 2014: p. 04-8-16.)
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l
l
SECTION
D Production of Non-Ferrous Metal Powders
The microstructure of samples of a composite based on Nb-Si is represented by eutectic cells oriented along the direction of crystallization. The phase composition contains the Nb solid solution (matrix) with the bcc lattice, niobium silicide g-Nb5Si3 with the hexagonal lattice, and hafnium oxide HfO2 with the bcc lattice. Niobium silicide g-Nb5Si3 has an orientation close to I100I along the direction of the growth axis, while the Nb matrix is oriented along the direction I110I. In cast material, an inhomogeneous distribution of alloying elements in the solid solution is observed. The titanium, chromium, and aluminum concentrations at the cell boundaries are higher than in the center, while the molybdenum, niobium, and tungsten concentrations are lower. Exposure at a temperature of 1773 K for 20 h contributes to leveling the concentration of alloying elements in the solid solution, but leads to the intensified formation of hafnium oxides. The tensile strengths of niobium composites at 1473 K are almost two times higher than those for heat-resistant nickel alloy (HRNA) single crystals; with consideration of the lower density of composites (7.386 g/cm3 compared to 8.9–9.1 g/cm3 for HRNAs), the advantage in the specific strength is even more significant.
Tantalum Several methods for reducing tantalum compounds to tantalum metal have been developed, but sodium reduction of the double salt, potassium tantalum fluoride (K2TaF7), to produce tantalum metal powder is the most commonly used. The general equation for the reduction with sodium may be represented as: K2 TaF7 + 5Na ¼ Ta + 5NaF + 2KF The reaction is carried out in a stirred reactor containing molten sodium chloride diluent under an inert atmosphere. The reaction is exothermic and accompanied by heat evolution amounting to 1500 kJ/mol (2 kJ per 1 g of molten sodium). After cooling, the resulting salt cake is removed from the reactor and crushed. The tantalum powder is recovered by thoroughly washing with hot distilled water with an addition of hydrochloric acid and subsequently dried at 380–400 K. Particles are usually rounded in shape, with a tendency to form grape-like clusters during reduction. Individual particles in such clusters range in size from 1 mm to 10 mm, depending on reduction variables, including temperature, agitation, and salt purity. Electronic capacitor applications generally require fine particles with high surface area. In one modification of the process, potassium tantalum fluoride, sodium chloride, and sodium metal are mixed into a paste and reacted without pre-melting [145]. Flake-shaped tantalum particles are produced by this technique. Because flakes have a higher surface-to-volume ratio than spheres, they are suitable for capacitor applications. In another modification of the process, tantalum powder with a highly developed surface can be produced by conducting the process in the presence of small amounts of boron, phosphorus, sulfur, and silicon. An even more appreciable effect is achieved by additions ranging from 0.002% to 0.5% of halides, oxides, or sulfides of Ti, Zr, and Hf. Generally, a wide range of sodium reduced capacitor powders is currently available, with unit capacitances ranging from 5000 mFV/g to >25,000 mFV/g. Capacitor powders are also manufactured by the HDH process. The electron beam (EB) melting step allows improvement of the purification of tantalum metal powders. In this process, sodium-reduced powders are pressed into bars which are subsequently EB melted. The resulting ingot is placed in a furnace with a hydrogen atmosphere. The tantalum is fully hydrided on slow cooling from 1070 K under hydrogen. The brittle hydride is crushed, ground, and classified to yield powder with average particle sizes ranging from 3 mm to 6 mm. The particles have an angular shape. Additions of small amounts (10–100 ppm) of sintering inhibitors, such as phosphorus, are frequently employed. These agents allow finer powders to be used than would be possible in the absence of an inhibitor. Alternatively, higher sintering temperatures can be used without loss of surface area, thereby aiding vaporization of undesirable impurities. Dielectric properties are thus improved. Because capacitance is directly proportional to the tantalum surface area accessible to an electrolyte, a porous structure is desirable. Thus, it is desirable to press the powders to as low a density as possible, while retaining sufficient green strength for handling. In general, sodium-reduced powders can be pressed to lower densities than EB melted, degassed hydride powders. The former material is used in high-capacitance devices at lower voltage ratings (<50 V). However, because of the higher purity of the latter powders, they allow operation at higher voltage. Thus, comparing the two basic types of tantalum powders when used for capacitor manufacture shows that the sodium-reduced powder material has a higher capacitance per gram of powder and better green strength for pressed compacts, while electron beam melted, degassed hydride powder material has higher purity, better flow characteristics and higher voltage capability in capacitor devices. Currently, sodium-reduced tantalum powders are generally used for 80%–90% of capacitor applications. There is an electrolytic process for producing tantalum using an electrolyte containing potassium fluorotantalate (K2TaF7), potassium (sodium) chloride and/or fluoride in which tantalum pentoxide is dissolved. Carbon dioxide and
Production of Refractory Metal Powders Chapter
23
743
carbon monoxide are formed at the anode. An anode effect arises during electrolysis of the K2TaF7 melt. Potassium chloride and fluoride provide easy fusibility, fluidity, and conductivity of the electrolyte, while the tantalum pentoxide improves wettability of the graphite anode. The cathode can be made of molybdenum, nickel, nichrome, or steel. 3 Thus, the oxide-fluoride-chloride electrolytes contain ions of K+, Cl–, F, TaF3 8 , TaOF6 (when the content of Ta2O5 in 3 the electrolyte is <5%) or TaO2F4 (when content of Ta2O5 in the electrolyte is 5% and more). The discharge of Ta5+ ions occurs on the cathode: Ta5+ + 5e ! Ta Scheme of the anode process can be written: TaOF3 6 2e ¼ TaF5 + F + 0:502
TaF5 + 3K + + 3F ¼ K3 TaF8 0:5O2 + C ¼ CO ðO2 + C ¼ CO2 Þ Thus, Ta2O5 undergoes an electrolysis treatment after finally being introduced into the melt. Precipitation of tantalum on the cathode and oxygen on the anode can be described: 4Ta5 + + 20e ! 4Taand10O2 20e ! 5O2 Reaction of graphite oxidation on the anode accompanied by evolution of energy leads to its depolarization, so that the amount of energy necessary to decompose of Ta2O5 is decreased. The electrolyte contains 2.5–8.5 wt% Ta2O5, 8.5–32.5 wt% K2TaF7, 27.5–57.5 wt% KF, and 25.5–55 wt% KC1; its melting temperature is about 920–970 K, depending on electrolyte composition. Electrolysis is accomplished at 970–1020 K. An advanced electrolyte containing 3–3.5 wt% Ta2O5, K2TaF7, 66.5–72 wt% (NaCl + KC1) with a reduced melting point of 270 K allows electrolysis to be conducted at 970 K. There are two types of electrolyzers. In the first type, a graphite crucible serves as an anode, and a metal bar located in the center of the crucible is the cathode. The second type includes a metal crucible as the cathode and a graphite rod or a hollow perforated graphite tube as an anode disposed in the center of the crucible (Fig. 23.48). The latter is preferable. In this electrolyzer, Ta2O3 is loaded into the cell through the hollow anode, and the gases are eliminated through the orifice in the side of the electrolyzer. Electrolysis is performed at a current density 5 kA/m2 on the cathode and 1.2–1.6 kA/m2 on the anode. Tantalum dendrites are deposited on the bottom and the sides of the crucible. The process is terminated as soon as the FIG. 23.48 Elecrolyzer for tantalum production.
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SECTION
D Production of Non-Ferrous Metal Powders
cathode deposit fills two-thirds of the crucible capacity. Tantalum powder particles are imbedded in the solidified electrolyte that protects the metal from oxidation on cooling. The powder particle sizes range from 30 mm to 120 mm. The cathode product is milled in a ball mill that operates in a closed cycle with an air separator. The electrolyte phase is separated and returned to the cell for electrolysis. The tantalum powder is washed on shaking tables in water jets to eliminate electrolyte, treated with a hot mixture of hydrochloric acid and nitric acid in porcelain reactors to eliminate molybdenum and iron, washed with water again, and dried. A vacuum heating process is also available in which the cathode crucible is heated in argon to 1270 K to melt the electrolyte. The vessel is then evacuated and the electrolyte sucked out. The metallic tantalum in form of weakly fritted dendrites in 100–120 mm sizes precipitated on the crucible walls is scraped off and ground. Current efficiency is 80%–83%; power consumption is 2300 kWh per ton. Tantalum powder can be produced by electrolysis of melts containing tantalum pentachloride (TaCl5). The liquid or vapor TaCl5 ( 10% of tantalum) is introduced into the melt (NaCl + NaF, NaCl + KCl, or NaCl + KCl + KF) at temperature ranges from 1020 K to 1220 K, by voltage in the cell 6–8 V, cathode current density 5 kA/m2, and anode current density of 10–20 kA/m2; current efficiency amounts to 85%–88%. The method of tantalum refining in chloride-fluoride and fluoride melts is still attractive, as the majority of impurities are eliminated completely; iron content is decreased 10–50 times, oxygen and hydrogen contents are decreased 10–100 times, and molybdenum and tungsten content is decreased 104–105 times, but it is difficult to eliminate niobium. The electrochemical method for producing tantalum powder based on reducing electrochemically dissolved tantalum ions (Tan+) by dysprosium divalent ions (Dy2+) in molten salt was investigated [146]. A tantalum rod (anode) was immersed in the NaCl-36 mol%KCl-4 mol%MgCl2-6 mol%DyCl2 molten salt at 1000 K, and it was anodically dissolved in this salt. The electrochemically dissolved Tan+ ions were reduced in situ by Dy2+ ions in the molten salt to produce tantalum powder. After completion of the electrolysis process, tantalum powder mixed with salt was recovered by leaching the mixture with acids. The salt and some metallic impurities were dissolved in an acetic acid solution. The resulting tantalum powder was rinsed in aqueous HCl at room temperature; it was then rinsed with distilled water, alcohol, and acetone, and subsequently dried under vacuum. Fig. 23.49 shows the SEM images of the tantalum powders obtained at various currents. As shown, the particles are loosely agglomerated to form a porous structure; the average particle size of the powder successively increased by a small amount when the currents were increased to 2.0 A and 4.0 A, As reported, tantalum powder with a purity of around 97 mass% and an average primary particle size of around 0.1 mm was obtained by this electrochemical method in the NaCl-36 mol%KCl-4 mol%MgCl2-6 mol%DyCl2 molten salt at an applied current of 1.0 A. Typical impurity compositions of sodium-reduced tantalum powder, electron beam melted, degassed hydride powder, electrolytic powder, and purified electrolytic powder are given in Table 23.19.
Tantalum Carbide The conventional technique for tantalum carbide powder production is similar to methods by which tungsten carbide powders are produced. Stoichiometric tantalum carbide can be used as an additive to WC-Mo grade powder attritions to enhance the physical properties of sintered material. It can be also used as a grain growth inhibitor preventing the formation of large grains, resulting in increased hardness of the sintered parts. Additions from a tenth of 1%–30% are common
FIG. 23.49 SEM images tantalum powder obtained by the electrochemical method in NaCl–36 mol%KCl–4 mol%MgCl2–6 mol%DyCl2 molten salt at various currents: (A) i ¼ 1A; (B) i ¼ 2A; (c) i ¼ 4A.
Production of Refractory Metal Powders Chapter
23
745
TABLE 23.19 Typical Compositions of Tantalum Powders Manufactured by Different Methods Powder Analysis (ppm)
Element
SodiumReduceda
Electron Beam Melted DegassedHydride
Electrolytic from Oxide Chloride or Oxide Fluoride-Chloride Melts
Purified from FluorideChloride or Fluoride Melts With K2TaF7
Aluminum
10
5
..
…
Calcium
10
5
…
…
Carbon
100
45
200–300
20–30
Chromium
25
5
…
…
Cobalt
10
5
…
…
Copper
10
5
…
…
Fluorine
…
…
10
…
Hydrogen
30
30
…
…
Iron
50
30
…
20–30
Iron +Nickel
…
…
300–1000
…
Lead
10
5
…
…
Magnesium
10
5
…
…
Manganese
10
10
…
…
Nickel
50
5
…
…
Niobium
50
20
…
…
Nitrogen
90
40
…
…
Oxygen
2400
1650
1000–2000
100–200
Silicon
25
10
1000
…
Sodium
10
5
…
…
Tin
10
5
…
…
Titanium
10
5
1000
…
Tungsten
25
25
…
…
Vanadium
10
5
…
…
Zirconium
10
5
…
…
a
Source: Fansteel Inc.
and completely dependent on the particular application. Typical composition of commercial tantalum carbide powder, produced by OSRAM Sylvania Company, is given in Table 23.20. It also makes tantalum-niobium carbide (TaNbC) of several compositions, such as 90/10, 80/20, and 60/40. In the work [147], the hydrate ammonium tantalate was selected as precursor of the tantalum carbide. The reactions between precursor ((NH4)TaO(C2O4)3H2O) and mixture of methane and hydrogen were carried out at relatively low temperature of 1270 K in a resistive furnace, consisting of an alumina fixed bed reactor. As shown, the precursor powder obtained in this work has large particles composed of irregular shape agglomerates consisting of small crystallites (Fig. 23.50A), while the tantalum carbide powder particles preserve configuration, but their surface looks somewhat fitted (Fig. 23.50B).
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D Production of Non-Ferrous Metal Powders
TABLE 23.20 Typical Properties of Tantalum Carbide Powder Property
Range
Typical
Particle size FSSS
0.5–2.0 mm
0.8–1.5 mm
Chemical analyses
Maximum percent
Typical percent
Total carbon
6.15–6.30
6.20–6.30
Free carbon
0.15
0.07
Calcium
0.02
0.005
Iron
0.05
0.04
Niobium
0.25
0.10
Oxygen
0.15
0.13
Silicon
0.02
0.01
Titanium
0.02
0.01
Source: OSRAM Sylvania.
FIG. 23.50 SEM images of the (A) precursor powder and (B) and tantalum carbide powder.
As concluded [147], this precursor can be used in TaC synthesis at much lower temperatures and reaction times compared to those used in conventional synthesis. For the conventional method, the tantalum carbide is obtained at temperatures ranging from 1570 K to 1770 K; however, for good homogeneity in the final product, high temperatures around 2270 K throughout the reaction are necessary. The method using hydrate ammonium tantalate precursor allowed production of tantalum carbide at 1273 K and an isotherm of two hours. The results presented for TaC revealed the formation of pure phase with characteristics different from those conventionally obtained such as particle size and average crystallite size in the nanometer order, reaching 12.05 nm.
Consolidation and Manufacturing of Semi-Products Powder metallurgy semiproducts of tantalum and its alloys are commonly made by cold isostatic pressing the powders into bars. The bars are then resistance sintered in a vacuum furnace at 1.33 102 Pa. Heating occurs due to the passage of an electric current. For tantalum, temperatures above 2570 K are required to achieve full density and sufficient purification as a result of removal of interstitial impurity elements. Sintered bars are then rolled or drawn at room temperature, with suitable intermediate vacuum anneals. Models of initial stage sintering that consider the starting particle morphology have been applied to tantalum to develop higher energy capacitors [148,149]. An example of the agreement is shown in [150] for nonisothermal sintering of agglomerated tantalum flakes.
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Powder metallurgy products differ from those produced from cast materials primarily because surface oxides are present in the initial powders. These oxides are contained in the microstructure of the consolidated PM material. Recrystallization is inhibited, and finer grained metal is obtained. The material is usually harder and has higher yield and tensile strength, but it is somewhat less ductile. Microalloying additions, such as thoria and yttria, are sometimes added to control grain size and strength after recrystallization. Small quantities of silicon are also added, particularly to bars that are subsequently drawn to capacitor wire. Silicon promotes the retention of ductility of the wire by preventing embrittlement of the tantalum lead wire embedded in the capacitor pellet. During vacuum sintering, oxygen diffuses from the surrounding powder into the embedded wire, which has lower oxygen, and collects at grain boundaries, thereby stimulating embrittlement. Silicon operates as a getter for the excess oxygen. A part of the silicon vaporizes harmlessly in the form of volatile silicon monoxide. Tungsten and hafnium are the alloying additions commonly added to improve the strength properties of tantalum. Typical properties for commercially produced tantalum and tantalum PM alloys and for comparison cast alloys are given in Table 23.21. Ta-2.5%W is used for the fabrication of corrosion-resistant heat exchangers, valves, and other chemical equipment. Less ductile Ta-10%W alloy is a harder, stronger, and more wear-resistant alloy and is especially suited for the manufacture of furnace hardware, missile parts, nozzles, and fasteners. A Ta-7.5%W alloy, made by powder metallurgy, is useful for springs, siphons, and leaf springs in severe corrosion environments, such as hydrochloric acid, bromine, or dry chlorine. Because the yield strength of the PM product is higher than that of ingot material of the same composition, its modulus of resilience is also considerably higher, as illustrated in Table 23.21.
Workplace Atmosphere Safety The aerosols of metallic niobium and tantalum are not toxic and relegated to the fourth, lowest class of danger according to State Standard 12.1.005-88 under the legislation in the Commonwealth of Independents States (CIS) [56]. Inspirable dust of both metals predominantly has a fibrous effect. The stated average limit value (ALV) of the aerosols of tantalum and its alloys in the workplace atmosphere, as the shift time-weighted average concentration, is 10 mg/m3. As for niobium, the State Standard 12.1.005-88 standardizes only niobium nitride ALV, which amounts to 10 mg/m3 (as the shift timeweighted average concentration). Under the workplace exposure limits in the European PM industry the long- and short-term (15-min reference period) workplace exposure limits (WELs) for tantalum are 5 mg/m3 and 10 mg/m3, accordingly [57]. Niobium in potable water has a toxic effect and is placed into the second class of danger and the TLV is 0.01 mg/L (for the inorganic compounds, including transition metals, subject to summation of all form contents) according to CIS State Standard 4630-88 [58]. The low concentration ignition limit (LCIL) of the metallic tantalum powder with particle sizes below 44 mm in the air is 200 g/m3; minimum ignition temperatures of powder aerosols and aerogels (self-ignition temperature) are 903 K and 563 K, respectively. The products with such characteristics of inflammability and explosion risk are placed in the “moderate explosion hazard” class of danger according to the EPMA Guide to Legislation and “Health and Safety” in the European PM Industry [59]. Common techniques are mainly used for pollution prevention and environmental control by powder manufacturing and its processing. Detailed information on health and environment protection measures and also on prevention of inflammability and explosion risk in such conditions can be found in Chapter 27.
RHENIUM Except for tungsten and carbon, among the elements, rhenium has the highest melting point, which is 3453 K. Its density is exceeded only by osmium, iridium, and platinum. Rhenium has the highest modulus of elasticity among the refractory metals. A ductile-to-brittle transition temperature does not exist in pure rhenium. Rhenium is one in refractory metals that does not form carbides.
Production of Rhenium Powder The main method for rhenium powder production is reduction of ammonium perrhenate (NH4ReO4) by hydrogen. The reduction can be written:
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TABLE 23.21 Typical Properties for Commercially Tantalum and Tantalum Alloys Standard Grades Commercial Pure Tantalum
Property
Unit 3
Ta-7.5W, Sheetb
Ta-7.5W, PM
EB Melteda
PM
Ta-10W, EB Melteda
Wire
Sheet
Ingot
PM
16.9
16.6
16.8
16.8
16.8
…
…
Density, at RT
g/cm
Melting point
°C
3000
3000
3030
3025
3025
…
...
Hardness, HV
HV
110
120
245
325
400
…
…
Hardness, DPH
DPH
…
…
…
…
…
150
291
At 293 K
MPa
205
310
550
1035
1165
…
…
473 K
MPa
190
…
515
…
…
…
…
1023 K
MPa
140
…
380
…
…
…
…
1273 K
MPa
90
…
305
…
…
…
…
At 293 K
MPa
165
220
460
1005
875
483
676
473 K
MPa
69
…
400
…
…
…
…
1023 K
MPa
41
…
275
…
…
…
…
1273 K
MPa
34
…
205
…
…
…
…
%
40
30
25
6
7
…
…
473 K
30
…
…
…
…
…
…
1023 K
45
…
…
…
…
…
…
1273 K
33
…
…
…
…
…
…
c
c
Tensile strength:
Yield strength
Elongation At 293 K
Modulus of elasticity At 293 K
GPa
185
185
205
200
200
196
196
1023 K
GPa
160
…
150
…
…
…
…
MPa
…
…
…
…
…
593
1172
Modulus of resilience (UR)d At RT a
EB, electronic beam. Measured for 2.1 mm thick, annealed and recrystallized sheet. Estimated. d UR ¼ s20/2E; s20 is the yield strength; E is Young’s modulus. b c
Source: Adapted from Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.
NH4 ReO4 + 2H2 ¼ Re + 1=2 N2 + 4H2 O Typical impurity composition of hydrogen reduced powders is: 5 ppm (Аl + Fе + Сu + Мо), 10 ppm (Са + Si + Р + Na), 20 ppm (Mg + Ni + S), 50 ppm K, 1 ppm Mn, and Cu 0.5 ppm. Raw material is milled in rubber-lined drums, then reduced in pusher type furnaces. The ammonium perrhenate is loaded into flat molybdenum boats that are pushed through the furnace. Reduction takes place in two stages. At
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TABLE 23.22 Typical Chemical Composition of ≤74 mm and ≤44 mm Rhenium Powders Analysis (ppm) Elements
≤74 mm (2200 mesh)
≤44 mm (2325 mesh)
Aluminum
1.4
1.4
Boron
0.1
0.1
Calcium
0.2
0.2
Carbon
<10
<10
Chromium
1.5
1.5
Copper
0.5
0.5
Hydrogen
56
56
Iron
2.3
2.3
Manganese
0.1
0.1
Molybdenum
0.4
0.4
Nickel
1.7
1.7
Nitrogen
22
22
Oxygen
898
898
Silicon
0.2
0.2
Tantalum
<5
<5
Tungsten
0.33
0.33
Vanadium
<0.005
<0.005
Zinc
0.1
0.1
616–636 K, RO2 is formed, and a final reduction to metal is done at 1220–1240 K. The boats’ time in the hot zone of furnace is 1–2 h. Average size of powder particles ranges from 1 mm to 3 mm. Apparent density ranges from 1.5 g/cm3 to 1.9 g/cm3. Typical chemical compositions of 74 mm and 44 mm rhenium powders are shown in Table 23.22. Among the refractory metals, rhenium has the highest tensile strength (Fig. 23.51). Rhenium has high electrical resistivity. It typically exhibits higher resistivity than tungsten. This feature, associated with a low vapor pressure, makes it FIG. 23.51 Dependence of ultimate tensile strength for pure refractory metals on temperature. (Source: Lambert JB et al. Refractory metals and alloys. In: Properties and selection: nonferrous alloys and special-purpose materials. ASM Handbook, vol. 2. Novetly (OH, USA): ASM International Publishers; 1990. p. 558–85.)
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perfectly suited for filament applications; parallel with this, it preserves ductility and is not affected by the oxidation/ reduction cycle experienced in these applications, as is tungsten. Rhenium does not react with molten copper, silver, tin, or zinc. Rhenium is resistant to hydrochloric acid and sea-water corrosion and to the mechanical effects of electrical erosion. At elevated temperatures, rhenium resists reaction in hydrogen and inert atmospheres. Rhenium bars are produced by uniaxial pressing, while tubes are produced by cold isostatic pressing. Compaction pressures are typically 200 MPa to 250 MPa [151]. Green parts are presintered at 1470 K to 1670 K for 30 min in hydrogen to increase their handling strength. Final sintering at about 2970 K can be performed either in hydrogen or high vacuum. Typical densities are 80%–90% of theoretical density, and mill powders are cold worked with frequent annealing because of rhenium’s rate of work hardening. Net-shape rhenium components can be sintered at temperatures of about 2570 K to near-full density [152].
Rhenium Alloys Rhenium is used in combination with platinum in catalysts, especially for selective hydrogenation and crude oil reforming. Rhenium catalysts show high resistance to poisons, such as nitrogen oxides, sulfur, and phosphorus. Rhenium is an advantageous alloying addition with other refractory metals. It greatly enhances the ductility and tensile strength of refractory metals and their alloys. This tendency is retained on heating above the recrystallization temperature. For example, alloys of molybdenum and rhenium are more ductile than pure molybdenum. An alloy with 35% Re can be rolled at room temperature to >95% reduction in thickness before cracking. For economic reasons, molybdenum-rhenium alloys are not widely used commercially. Alloys of molybdenum with 5% and 41% Re are used for thermocouple wires.
Applications Rhenium alloys are used in nuclear reactors, semiconductors, thermocouples, electronic tube components, gyroscopes, parts for rockets, electrical contacts, thermionic converters, and various parts for high temperature aerospace applications. Recent developments in the production of net shapes by powder metallurgy, rhenium manufacturing techniques have generated significant saving in both time and budget. For instance, the thrusters of different and unique types of aerospace components (rhenium combustion chambers) were manufactured by means of near-shape technique [153]. Near-net shapes are produced using elastometric molds with and without hard tooling inserts. The mold cavity is dimensioned so that, after CIP and sintering, final dimensions require only a minimum amount of finish machining [154]. The use of this technique allows the making of parts with a density of 95%–98% after sintering and 98.5%–99.9% on subsequent hot isostatic pressing [155]. The typical microstructure of the initial rhenium metal powders is shown in Fig. 23.52. The production of near-shape PM rhenium combustion chambers has reduced the manufacturing cost to 35%, and shortened the manufacturing time by 30%–40%. The quantity of rhenium metal powder used to produce a chamber is reduced by FIG. 23.52 Scanning electron micrograph of the rhenium metal powder. (Source: Leonhard T, Downs J. Near net shape of powder metallurgy rhenium parts In: Infringer G, editor. Proceedings of 15th International Plan see Seminar, vol. 1. Reutter (Tirol, Austria): Plansee holding AG; 2001. p. 647–57.)
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approximately 70%, and subsequent reduction in machining costs is nearly 50%. Thus, cold isostatic pressing to near-net shape has provided significant cost savings and increased the quality and density of rhenium parts. Tungsten-rhenium alloys, applied by vapor deposition, are used to coat the surface of molybdenum targets in X-ray tube manufacture. Other rhenium alloys, with tungsten and molybdenum, are used for filaments, grid heaters, cathode cups, and ignitor wires in photoflash bulbs. One of the largest applications for rhenium is for mass spectrometer filaments. These are available in commercial (99.99%) and zone-refined (99.95%) purities.
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