NANOVAL process offers fine powder benefits -
NANOVAL GmbH of Berlin, Germany, has developed a unique gas atomizing technology that enables the economic production of fine and ultrafine metal powders. The small company supplies both atomizing equipment and speciality metal powders. Dr Gunther Schulz, application and sales engineer, outlines the characteristics of the process, as well as the powders and their applications.
he founder of Nanoval, Dr Lfider Gerking, well remembers the words of his former teacher Professor Alfred Walz - “Fine metal powders are the future”. Professor Walz had analyzed existing gas atomizing processes from a fluid dynamic approach and had found that they were all characterized by turbulent gas jets attacking a melt stream at a certain angle. “This is as precise as tooling with a sledge hammer”, Prof. Walz said. The problem is that a gas stream exiting a nozzle decelerates on its way to the melt stream, while the density also drops with distance. Additionally, in a turbulent flow gas density and velocity fluctuate with time and location, with the net result that the eventual transfer of momentum to the melt is small
FIGURE pilot
30
MPR
I: NANOVAL’s plant atomizer.
November
1996
0026-0657/96/US$15.00 All rights reserved.
and non-uniform. “A laminar gas flow is the key towards fine metal powders with narrow particle size distributions”, concluded Prof. Walz, to whom the basic patents on laminar atomization of metal powders had been granted. Applications for fine powders were scarcely developed in those days, however, and none of the powder producers or atomization equipment suppliers that Walz approached were interested in developing the basic ideas into a working industrial process. But Dr Gerking was, with the eventual result being the founding of NANOVAL in December 1987.
Current
status
Having developed its laminar gas flow atomization technique to a commercially viable stage (see box), NANOVAL has been marketing its metal powders for the last two years. During this time the market for fine and ultrafine powders with mean particle sizes of 20 resp. 10 pm has developed rapidly. Powders of such a fine size were previously only available in a restricted range of alloy compositions. No process has been available allowing a direct production of such fine and ultrafine gas atomized powders, with most materials being the byproducts of the manufacture of coarser powders. Consequently, the availability of fine powders has been limited, both in quantity and quality, and the prices have been high. NANOVAI. is actively promoting the development of applications for fine metal powders. Customers initial enquiries are often for powders of around 100 pm mean particle size, but, provided its feasible for the application intended, NANOVAL. will also offer a finer powder. Usually, the customers’ tests give better results with the finer powders. Today, NANOVAL employs six people and supplies small lots of specialty powders, mainly to European clients for research and development (R&D) purposes. The company uses a small pilot plant atomizer (Figure I), which has a typical batch size of about 5 kg. In the first nine month of 1996, NANOVAL has shipped more than 1000 kg of powder produced on this machine. The response from customers has been very good and several R&D Copyright
c
1996, Elsevier
Science
Ltd.
ANOVAL he basic: idea of
process
cross section, the gas transfers kinetic energy to the melt. The radial symmetric laminar, self st2abilizing gas flow, gas flow stabilizes the melt stream preaccompanying r,he melt stream in parallel. vent,ing it from undesired disintegration Figure 2 shows the nozzle configuraor wave crest stripping. Thus, a very thin tion for the NA?iOVAL process. The gas melt filament of constant thickness is (1) under pressure accompanies the obtained at the narrowest cross section metal melt (2): both passing together mluch thinner than would be possible through a LA\:.& nozzle (4). Within the under the free flow of a melt. Together short distance between the entrance of with the constant gas impulse, this gives the LAVAL nozzie to the narrowest cross a very constant and high specific gas section (3) t,he gas accelerates from a few momentum and, therefore, a uniform, m/s ta sonic velocity (306 m/s for Ar, fine powder. :177 m/s for N2. and 971 m/s for We, at Beneath the narrowest cross section 273.15K and 0.10li3 MPa). The gas the gas decompresses very quickly and decompresses, starting at p1 and reaching accelerates into supersonic velocity. The a pressure 13sp2 in the narrowest cross melt stream monofil deforms at an ever sect,ion, which is given by the critical increasing speed by virtue of shear pressure ratio pl/p2 (provided the counstresses between the gas and liquid ter pressure at the end of the LAVAL interface until, as the external gas presIrozzie is smaller than pa). This critical sure drops, it becomes unstable and pressure ra7io is a material constant for a bursts into many much thinner filaments. given atomkhg gas, so both factors These are also hydrodynamically unstable determining the gas momentum (presand disintegrate into small pieces. Under sure resp. density and velocity) are fixed the influence of surface tension, spherical \tith a given starting pressure p,. melt droplets are formed which cool down Because of the very high acceleration and solidity to form powder particles. eithin the NAVAL nozzle the gas flow A moderate gas pressure of about stays laminar as it stabilizes itself. The 2 NPa is sufficient to give powders with metal melt is drawn by the gas flow, in mean particle diameters dSO of about parallel, via shear stresses to a thin 10 pm, with argon or nitrogen as the fiiament! i.e. it is accelerated steadily. atomizing gas, or 5 pm, when helium is For its entircb journey to the narrowest used. The powders in their as atomized state have standard deviations in their particle size distributions that are typically between 1.6 to 1.8, which is quite narrow compared to a > 3 with conventional nozzle systems. Particle shape is spherical. The specific gas consumption for a given mean particle diameter is only about l/3 of that which a close coupled nozzle would require, and about l/7 of the gas consumption a free fall nozzle system would take, as Figure 5 demonstrates. The specific gas consumption is a cost critical figure in the production of fine and ultrafine powders, as the atomization process requires large amounts FIGURE 2, Ike ~A~~~A~ nozzle configuration ensures a of gas. laminated, accelerated gas flow. pr()c&s
the NANOVAL [.’ is the use of a strictly
Metal
FIGURE
3: SEM
photograph
of a 14 ct gold
alloy M/M powder.
applications are approaching production stage, bringing the demand for larger quantities of powder. For such large scale applications, NANOVAL usually sells the customer atomizing equipment to allow them to produce their own powders. Based on standard components, atomizing equipment is constructed to meet customers requirements as close as possible, starting with small lab scale atomizers to full industrial scale. A large number of demanding applications, however, will remain too small to make the investment in an atomizer worthwhile for a customer. Additionally, some customers are not interested in diversiting into powder production. So, NANOVAL has decided to invest in a second atomizer, which is currently being commissioned. This machine will have a batch size of about 100 kg for steel powders, allowing NANOVAL to supply tonne lots in 1997. It can be batch or continuously operated as required, and is equipped with all the safety features required for the safe atomization of light metals like aluminium or magnesium.
40 particle FIGURE
32
4. The
MPR
particle
November
size
1996
distribution
size [pm]
of an as-atomized
14 ct gold alloy a M/M application.
for
injection
moulding
Metal injection moulding (MIM) is a rapidly growing application for fine powders. Until now, the only prealloyed, gas atomized powders that have been available are a small number of steel powders. NANOVAL’s technology, however, allows the direct production of a wide variety of MIM powders, typically with a dsO of about 10 pm and a yield of the -20 pm fraction of about 90 %. No screening is necessary with these materials, the powders can be used ‘as atomized’. This opens the way to a full choice of metallic materials for MIM part producers. Among the new materials that have been supplied for MIM are several precious metal alloys, special grades of stainless and high speed steels, copper-based alloys and superalloys. A scanning electron microscope photograph of an as atomized powder is shown in Figure 3, while a typical particle size distribution curve is shown in Figure 4.
Composite
materials
Another promising application for fine and ultrafine powders is in the production of composite materials. The properties of composites made by powder metallurgical techniques are always improved if the metal powder component has a reduced size. Nonmetallic powders, such as oxides, carbides, nitrides and graphite, are available in very small particle sizes of a few microns and the best distribution of these particles is obtained with admixing very fine metal powders. Product examples are tungsten carbide, diamond tools and functionally graded materials, as well as silver-based contact materials. The atomization of fine powders also opens up access to rapid solidification materials. The cooling velocity of melt droplets is inversely proportional to the particle diameter. In argon gas, a 10 pm particle cools at about lo6 K/s and in helium gas at about lo7 K/s. This speed is fast enough to minimize segregation and, with some alloys, to suppress crystallization. Fine, nanocrystalline or even amorphous structures are obtained, which often have superior properties. Powders for solder pastes and other electronic applications are another major field of application. Today, typical particle sizes used for surface mounted devices (SMD) solder pastes range from 25 to 75 pm. The yield within that size fraction rises from about 30% with conventional turbulent gas atomizing to about 70% with the NANOVAL process. Powder sphericity and oxygen content are excellent, and the NANOVAL route also offers continuous atomization, recharging kilogram bars of solder alloy through a sluice system, giving
a very good ecornxny to the overall process. As ~iniat~lr~~atior~ proceeds in electronics, there is a strong trend to use even finer paste powders. NAKOV~s sales for ultrafine pitch pastes have been flourishing in the last few months. Besides conventional tin-lead solders, NANOVAL’s specialities comprise lead-free solders, active solders and brazes. Conductive inks, glues and metallization pxxes made from ultrafine silver and copper powders are other de~~i~n~i~% appiications served.
In thermal sprati applications, the more advanced technologies like high velocity oxygen fuel (HVOF) or Low Pressure Plasma Spaying (LPI%) require fine powders, typicahy below 50 pm, and a narrow size fracti,on, as undersize material evaporates during spraying. Good spheric&y is a must, as othwvise the fine powders cause feeding problems in the spraying gun because of poor ~~o~*ab~~i~. Rapid prrti;otyping techniques, like selective laser sintering, also require fine powder feed material to guarantee precision. Several customers have already sue cessfully tested ~~~~~‘s fine sprayi powders for these applications.
I
10
gas/metal ratio [m3/kg] FIGURE
5: Specific
gas
consumption
(argon) diameter
as a function for different
eferences [l] A. Walz, Patent DE 3,311,343; Patent US 4534,917 and other equivalents in all major countries. [Z] L. Gerking, Powder Metallurgy internationrci, Vol. 25, No. 2> 1993, pp. 59-65.
HE -cut
warm compccf~oc era in powder
II
density
*
strength
-% imrxoved uction
in green
acti n
a
meiaiiurgy
The process offers: z& an fxrease in green an increase
t
powder flow it2 green scrap
of between af between
caused
-# increcsed potential for machining The Sio? heate: preheats the powde:
O-l- 0.3g/cm3
1.7 IT
50 -100%.
by damage in the green state and can be fitted to all press types.
Linde Metaliteknik AB, R&g&ngsg. 4, S-252 27 Helsin gborg, Sweden, Tel +46 42 18 00 90. Fax +46 42 18 00 94
of mean particle nozzle systems.