A faster FAST: Electro-Sinter-Forging

Metal Powder Report  Volume 00, Number 00  July 2017

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A faster FAST: Electro-Sinter-Forging Alessandro Fais EPoS srl, Strada delle Cacce 73, 10132 Torino, Italy

A novel sintering technology based on high density electric currents and sintering cycles shorter than a second is presented. Data will be shown that compare it to Hot Pressing and Spark Plasma Sintering and to conventional Press and Sinter. Considerations will be made on the materials, the physical and mechanical characteristics of the objects produced, on the range of present and future products, on the tooling and on the productivity. Introduction Using an electric current to heat and sinter or aid a sintering process is not really a novelty. The first pioneers of such methods began at the start and continued throughout the last century [1–4] when the manipulation of electric currents became increasingly popular for processing. Research in this field has been dense in the last sixty years and there have been a number of reviews that clearly illustrate the main differences of each procedure and method [5–8]. The electric current assisted technologies that have found direct industrial application, however, are few: Hot Pressing (HP) and, in a more limited way, Spark Plasma Sintering (SPS). Both these very similar methods employ graphite dies and plungers placed inside a controlled atmosphere and heated through a current flow, retrofitted in temperature either through a thermocouple or a pyrometer. Typical control parameters include maximum temperature, heating ramp (8C/s), holding time (in minutes) and pressure/force to apply on the plungers. The main difference between the two methods are in that HP uses alternate currents directly converted from the electrical distribution lines with transformers whereas SPS has a conversion of the current pulses with elaborate power electronics to direct currents and trains of waves of 350 Hz [9]. SPS machines usually also have the possibility to choose between the on and off time of the current pulses and the time of direct current versus alternate current. Besides these sophistications though, the results, as far as sintering time and densification rates or values of maximum density, are quite E-mail address: [email protected].

similar. A typical cycle made of heating and holding the temperature in pressure lasts between 10 and 25 min in both methods and different studies have reported no differences in terms of mechanical properties between them [10,11]. Instead of heating a graphite die, an alternative method, uses direct heating of the powders to consolidate them directly and to sensibly reduce the processing time. These other ways of using currents constricts the electron flow exclusively through the powders and deliver a high amount of energy in fractions of a second. They are either called Electro-Discharge-Sintering (EDS) procedures or Electric Pulse Consolidation [7] (and beware of the names, Inoue, the inventor of SPS, names in his patent, his technology Electro Discharge Sintering. but later in time EDS was employed for single pulse, single discharge techniques). Here the first EDS-type consolidation method to be ever industrialized will be presented in its most significant details in order to understand, with the data presently available, how it compares to other, more renown, powder metallurgical technologies. The newly developed methodology has been equipped with specifically designed long lasting, cost effective molds and plungers that allow it to be practical and usable in an industrial environment. The process, named ElectroSinter-Forging (ESF) [12], is executed by a machine as the one shown in Fig. 1, which works with cycles very similar to a conventional powder press but with a twist. As the powders are compressed to shape them, a high intensity electric pulse with current density from 0.1 to over 1 kA/mm2 and duration of 30–100 milliseconds (ms) is injected in the tooling through the plungers. Visually you see a press with too many copper components than

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Metal Powder Report  Volume 00, Number 00  July 2017

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FIGURE 1

Electro-Sinter-Forging machine.

normal, that, with a low roughly 60 Hz stroke of electric current and pressure, can produce a fully sintered mechanically precise component. The densities can be adjusted from very low (60–70%) to very high (99.9+%). The tolerances on inner and outer electro sinter forged steel rings reach a standard deviation (sigma) of 0.006 mm (6 mm) on both the inner diameter (of 13 mm) and outer diameter (of 16 mm). A display of such rings is shown in Fig. 2.

The method ESF is simple: two pulses, one mechanical and one electrical, are superimposed in a die previously loaded with the powders. The electrical pulse is synchronized as to begin energy transfer when a predetermined level of pressure has been reached (Pstart) so that a homogeneous flow of current is guaranteed. While the energy source is transferring electro-magnetic energy, the mechanical system acts to compensate, at high speed, the powder shrinkage and to increase pressure (Pend) in the time frame of the electromagnetic pulse that lasts from 30 to 100 ms (t*). Pressure is held from a few milliseconds to, usually 1–3 s, before the upper plunger is automatically drawn out of the die and the lower plunger is moved to the upper part of the die in order to extract the sintered piece from the die assembly. The typical cycle is shown in Fig. 3. The electro-magnetic discharge is usually generated with a high

FIGURE 3

ESF cycle: (a) pre pressing, (b) superimposed pulses, (c) holding in pressure/ cooling and (d) release of pressure and extraction from die. Mechanical pressure in light gray, p(t), and electric power pJ(t) in thick red. Mechanical pressure during (b) can oscillate or have different from monotonous behavior due to how the consolidation occurs.

voltage capacitor bank connected to step down transformers for the conversion of high voltage low current on the primary circuit to low voltage high current on the secondary sintering circuit. The voltages on the sintering die are below 50 V so the procedure is quite safe. The process seems visually identical to a conventional powder pressing system with a double action, but the result, instead of a compacted green is a sintered part. The values of pressures employed usually range between 10 and 50 MPa for Pstart and 40 to 400 MPa for Pend. Currents are intense: typical current densities range between 0.1 to over 1 kA/mm2. The parameter, used to have a good feeling of how much energy has been discharged on the part during the process, is called specific energy input, or SEI: Z Z  1 1 þ1 1 t SEI ¼ EJ ¼ vðtÞiðtÞdt  vðtÞiðtÞdt w w 0 w 0 where w is the weight of powders inserted in the die (which is usually nearly coincident with the weight of the extracted part), EJ is the energy dissipated on the system by Joule effect, v(t) and i(t)

FIGURE 2

Array of ESFed iron cylinders (left) and close up on single as sintered ESFed cylinder (right). 2 Please cite this article in press as: A. Fais, Met. Powder Rep. (2017), http://dx.doi.org/10.1016/j.mprp.2017.06.001

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and respectively the curve of voltage and current, measured during the process, while t* is the characteristic time of the process. The usual parameters that are set on the machine are: the axes positions (handled by a numerical control) before and after the discharge, the amount of energy [%] loaded on the capacitor banks, the amount of time that the pressure is held after t*. The powders can be of any size, in function of the final application: from submicron (0.7 mm Co powders used for cutting tools) to coarse 0.5 mm. Powders must be used completely dry. Any trace of lubricant or polymeric agent used in conventional PM, is detrimental to the characteristics of ESFed objects. This is a consequence of the process being so fast: there’s no time for evacuation of the binder.

Materials and applications A wide range of materials can be consolidated with ESF. Details on the ones sintered and a focus on specific application-oriented materials will be presented.

Ceramics Timid trials on electrically conductive ceramics (such as TiB2) have been made but sintering them has not been completely successful. Electrically insulating ceramics such as oxides do not seem to be sinterable since the dies are made in insulating materials to force the current flow through the powders. Combining insulating dies and electrically conductive ceramic powders, although the electric potential increases to 50 V on the compacted powders, barely any current will flow and the material will not sinter (at least at ambient temperature).

Metals, metal matrix composites and intermetallic materials Metals [13,14], metal matrix composites (MMCs) [15,16] and intermetallic [17] materials instead have been sintered up to theoretical density. All of the following systems have been at least partially explored: iron and alloys [13] of pre-alloyed powders of AISI M2, T2, H13, 316L and 410; pure nickel and pre-alloyed Inconel; pure cobalt and pre-alloyed stellite; copper and various types of pre-alloyed bronzes; titanium and pre-alloyed grade 5 powders; molybdenum; tantalum; gold and pre alloyed powders and composites [14]; silver and pre-alloyed powders; cemented carbides [15], carbonitrides and nitrides; diamond reinforced metals for stone cutting [16]; pre-alloyed nitinol [17]; Y-titanium;

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Nd-Fe-B flakes; bismuth tellurides, magnesium silicides and skutterudites for thermoelectric applications. All these systems have been sintered in air without problems. Trials with vacuum up to 105 bar have been executed with commercial iron powders and with cemented carbides but no statistically relevant differences between the two conditions (with and without vacuum) where noticed. Nitinol, for example, a system conventionally cast under vacuum to prevent oxidation, was consolidated to theoretical density without protective atmosphere. No measurable differences in the amount of oxygen between pre and post ESF were noticed [17].

Metal machining tools/hard metals and cermets Preliminary experiments on cemented carbides of different composition and their wear behavior when used as machining tools have been the object of a MANUNET EU funded research project named EDS-Cut. Carbides of tungsten, mixtures of tungsten and titanium carbides and straight titanium carbide grades with cobalt or iron-nickel [18] alloys were used for turning, grooving and milling. Given the speed of the ESF consolidation, carbides do not have enough time to recrystallize in the canonical faceted shapes that characterize most hard metals and appear roundish and small, Fig. 4, since grain growth cannot occur. The high speed also prevents the formation of fragile h complex carbides (M6C), reason for which an iron-nickel alloy could be used with ESF, in alternative to cobalt. All the X-ray diffraction spectra of the ESFed cemented carbides, an example of which is reported Fig. 5, show no other phase besides WC (and or TiC when it was used) and the iron-nickel binder. Wear, during machining of steels, aluminum and titanium alloys, was monitored in milling, grooving and turning. The most interesting result regarded a full titanium carbide iron nickel cermet that was able to vastly outperform a P20 type commercial hard metal in dry turning of 18NiCrMo4 steel at speeds of 140 and 210 m/s [15], as can be seen in Fig. 6. The trials in other geometries and machining conditions where not as outstanding but still demonstrate a potential of the technology in this field.

Diamond abrasive cutting Electro-Sinter-Forged diamond metal matrix composites for circular blades (called bits) used in stone cutting have been extensively developed and produced [16]. Previous researchers have also

FIGURE 4

SEM, bright field 2000 and 15,000 enlargements images of pure tungsten carbide grade with and iron-nickel binder. 3 Please cite this article in press as: A. Fais, Met. Powder Rep. (2017), http://dx.doi.org/10.1016/j.mprp.2017.06.001

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Metal Powder Report  Volume 00, Number 00  July 2017

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Metal Powder Report  Volume 00, Number 00  July 2017

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XRD spectrum of a straight tungsten carbide with pure an iron-nickel binder.

explored this application with an analogous although not quite similar machine [19,20] but the results have been disappointing or not interesting enough if compared to ESF. Continuous automatic productions of electro-sinter-forged bits are ongoing and can be considered the pioneer field in which this technique is being used. Due to the harsh wear involved, the application is also a great benchmark for the behavior and duration of the tools. ESFed bits have been tested on a number of granites and marbles with professional mills on the field of industrial cutting. A summary of some of these results gathered for abrasives rocks (granites and quartzites) is summarized in Table 1. The high cutting speeds reached are considered to be given by a combination of fine metal matrix grain sizes and an optimal balance between diamond retention and diamond degradation, all features available in ESFed bits, thanks to extremely low sintering time. An idea of the fracture surfaces of diamonds bit prepared with Hot Pressing (HP), in (a), and with increasing levels of energy, from (b) do (d), is given in Fig. 7.

Iron and steels Previous work on an analogous, less effective method named Capacitor Discharge Sintering [21–23] had already given clues on how iron and alloys can behave with ESF. More recently a ¨ ganas Astaloy CrL) has been used to commercial iron powder (Ho investigate the peculiar behavior of powders during ESF [13] and the results were compared, with three point bend tests, to press and sinter (P&S) (uniaxially pressed at 650 MPa, sintered at 1250 8C for 120 min). The best ESFed samples had transverse rupture strengths (TRS) of around 1500 MPa whereas the P&Sed samples reached barely 700 MPa. The fracture surfaces of the two groups, shown in Fig. 8, are a demonstration of how different an ESFed material can be compared to a traditional press and sinter method. In this work we also observed a strong, yet to be confirmed in other systems, influence of the initial pressure (Pstart) over the density and over the mechanical properties of ESFed iron alloys: the lower the initial pressure, the higher the TRS. Ongoing experiments on simply mixed iron alloys and carbon are focused on obtaining alloys during ESF. Preliminary tests confirm the possibility to directly sinter to full density (99+%) high carbon alloys. Particularly interesting is the fact that these high carbon alloys are extracted, from ESF, in a hardened state, at 59–63 HRC, and that no contamination from the tooling could be measured.

Tools and productivity

FIGURE 6

Flank wear of a naked commercial P20 cutting tool versus an ESFed straight titanium carbide iron nickel composite during the dry turning of 18NiCrMo4 steel.

Tooling consisting of dies, compression rings, plungers and plunger tips as well as the complete secondary circuit are original in their design for the assembly, the features and the choice of materials. Presently the parts that can be produced have limitation on the geometries: they must be relatively simple (rounds and polygons, no gears yet) and flat, on a single level or with surface features on the plungers. The key points from which the technology benefits are the following: (a) no contamination from the die has ever been measured by any university or company that has analyzed ESFed parts, SPS and HP on the contrary have an issue with

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TABLE 1

Blade

Diameter [mm]

Segments [mm]

Material

Cutting speed/rate [cm2/min]

G-D54M001 G-D54M001 G-D54M001 G-D54M001 G-D54M002 G-D54M002 G-D54M002 G-D54M001 G-D54M001 G-D54M001

390 390 390 390 490 490 490 490 490 490

20  10 20  10 20  10 20  10 20  10 20  10 20  10 20  10 20  10 20  10

Red Indian Quartzite (class V), h = 2.2 cm Class IV Chinese rose granite, h = 4 cm Class IV Sardinian rose, h = 3 cm Class IV, gray granite, h = 4 Black Africa Class V granite, h = 4 Black Africa Class V granite, h = 3 Black Africa Class V granite, h = 4, multiple passes (0.6 + 0.5) Green Fire (Australian granite class V), h = 2, multiple passes (0.6 + 0.5) White tarn, granite class II, h = 3 Indian Jouparana, granite class III, h = 2 in multiple passes (0.6 + 0.5)

550 400 980 800 440 570 550 400 900 500

contamination from carbon form the graphite dies; (b) minor contamination from the plunger tips has been observed in rare cases when the tips were not optimized for the specific application; (c) the dies have long life, the ones used to ESF diamond abrasives last beyond 7000 parts (compared to 60 rounds with graphite dies in HP), the expected tool life for the production of steel parts is >500,000 parts per die; (d) the tolerances of ESFed parts are tight: 6 mm on the inner and outer radii and 0.04 mm on the thickness, repressing and mechanical finishing is necessary to have the same tolerances with standard press and sinter procedures; (e) no control atmosphere is required even for pyrophoric materials. The machine cycles time can be

reasonably accelerated to at least one piece every 5 s and a single operator can handle up to 4–6 machines at the same time.

Advantages and disadvantages Electro-Sinter-Forging presents the following advantages compared state of the art sintering and casting, forging and machining techniques:  Lean: Products with tight mechanical tolerances and high mechanical properties (hardness and TRS) are producible which reduce traditional processing steps sensibly. Example: Powder mix ! ESF ! deburring vs Casting ! Forming ! Cutting ! Machining ! Heat Treatment ! Finishing.

FIGURE 7

Fracture surfaces of cobalt-diamond abrasive bits. (a) Hot pressed (HPed) bit in optimized conditions; (b) ESFed bit in optimized conditions, max current density of 0.8 kA/mm2; (c) ESFed bit with maximum current density of 1 kA/mm2 and light degradation of the diamonds; (d) ESFed bit with maximum current density of 1.1 kA/mm2 and diamond degradation/graphitization. 5 Please cite this article in press as: A. Fais, Met. Powder Rep. (2017), http://dx.doi.org/10.1016/j.mprp.2017.06.001

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Cutting speeds of circular blades produced with ESFed cobalt-diamond bits.

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Metal Powder Report  Volume 00, Number 00  July 2017

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Fracture surfaces of the electro-sinter-forged Astaloy CrL samples (upper figures) and the press and sintered samples (lower figures) at 500 (left) and 3000 (right).  Automatic: The manpower required is low (1 person every 4–6

machines).  Clean-tech: A comparison between the energy consumption of ESF and direct hot pressing (DHP) has been made [16,24] and the energy needed for ESF is 6% of the energy required for DHP, 

    

the calculations are reported in Fig. 9. Due to its speed, ESF, also has no need of a vacuum or a gas protected atmosphere. The powder used in ESF have to be dried, which complicates powder handling but allows to eliminate the de-binding phase and the need to treat combustion fumes. It can be used to produce objects with densities that are nearly theoretical. And the disadvantages: No ceramics can be sintered with cold dies. Tooling and machine are proprietary. New tools have a lead time of 8–12 weeks. Shapes are relatively simple. Objects are relatively small: <200 g.

Conclusions

FIGURE 9

A comparison between the use of electric energy in Direct Hot Pressing (DHP) and Electro-Sinter-Forging (ESF).

A novel sintering technology with the potential to substitute press and sinter, sinter-forging, direct hot pressing and spark plasma sintering for relatively small objects in metals, intermetallic materials and metal matrix composites (cutting tools) is here presented. The technique is gaining traction for the production of diamond abrasive tools where the reduced production costs and the performance of the tools on the field have combined to produce appealing products. Many fields of applications are still under investigation: steels, precious metals and

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composites, hard metal carbides and thermos-electrics, to mention a few. ESF machines seem particularly apt for the production of medium to large scale objects (100,000–1,500,000 pieces per year). The lack of flexibility and complexity and the size limitations hinders its use for small series and for intricate or large objects. References [1] A.G. Bloxam, GBP 27, 002, 1906. [2] GF Taylor, U.S. Patent No. 1,896,854, 1933. [3] K. Inoue, Apparatus for Electrically Sintering Discrete Bodies, U.S. Patent No. 3,250,892, 1966. [4] K. Inoue, Electric-Discharge Sintering, U.S. Patent No. 3,241,956, 1966. [5] R. Orru`, R. Lichieri, A.M. Locci, A. Cincotti, G. Cao, Mater. Sci. Eng. R 63 (2009) 127. [6] S. Grasso, Y. Sakka, G. Maizza, Sci. Technol. Adv. Mater. (2009) 10. [7] M.S. Yurlova, V.D. Demenyuk, L.Y. Lebedeva, D.V. Dudina, E.G. Grigoryev, E.A. Olevsky, J. Mater. Sci. (2013) 1. [8] E.A. Olevsky, E.V. Aleksandrova, A.M. Ilyina, D.V. Dudina, A.N. Novoselov, K.Y. Pelve, E.G. Grigoryev, Materials (Basel) 6 (2013) 4375. [9] U. Anselmi-Tamburini, S. Gennari, J.E. Garay, Z.A. Munir, Mater. Sci. Eng. A 394 (2005) 139.

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[10] Y.F. Yang, M. Qian, Spark Plasma Sintering and Hot Pressing of Titanium and Titanium Alloys, Elsevier Inc., 2015. [11] J.J.M. Jua´rez, D.V. Jaramillo, R.A. Cuenca, F.L. Jua´rez, Powder Technol. 221 (2012) 264. [12] A Fais, EP 2 198 993 B1, 2008. [13] A. Fais, M. Actis Grande, I. Forno, Mater. Des. 93 (2016) 458. [14] I. Forno, M. Actis Grande, A. Fais, Gold Bull. 48 (2015) 127. [15] A. Fais, Int. Powder Metall. Congr. Exhib. Euro PM 2013. European Powder Metallurgy Association (EPMA), 2013. [16] A. Fais, Diam. Appl. Technol. (2015) 21. ˇ elko, S. Spriano, [17] C. Balagna, A. Fais, K. Brunelli, L. Peruzzo, M. Horynova´, L. C Intermetallics 68 (2016) 31. [18] A Fais, S Bonetti, A Method for Sintering Metal-Matrix Composite Materials – EP2606996, 2011. [19] D. Egan, S. Melody, Met. Powder Rep. 64 (2009) 10. ¨ tte, J. [20] W. Tillmann, C. Kronholz, M. Ferreira, A. Knote, W. Theisen, P. Schu Schmidt, PM2010 World Congr. Diam. Tools, 2010. [21] A. Fais, G. Maizza, J. Mater. Process Technol. 202 (2008) 70. [22] P. Scardi, M. D’Incau, M. Leoni, A. Fais, Metall. Mater. Trans. A: Phys. Metall. Mater. Sci. 41 (2010) 1196. [23] A. Fais, J. Mater. Process Technol. 210 (2010) 2223. [24] A. Fais, Powder Metall. Congr. Exhib. Euro PM 2013. European Powder Metallurgy Association (EPMA), 2013.

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Metal Powder Report  Volume 00, Number 00  July 2017