Powder Metallurgy

QinetiQ Nanomaterials Ltd (QNL) has commissioned its first two nanopowder production rigs at what is believed to be the UK's first facility dedicated to the volume production of specialist nanomaterials. A wholly owned subsidiary within the QinetiQ Group, QNL was established in January 2002 with major investment from the QinetiQ Venture Fund to develop bulk nanometric materials capabilities.

The most recent of the biennial series of technology fairs organised by the Particulate Engineering Committee of the Institute of Materials, Minerals and Mining took place on 26 March.

The EPMA held its 14th General Assembly and Management Seminar at the Villa Quaranta Park Hotel, Verona, Italy, on 15–16 May; over 50 members from industry and academia met, and took advantage of the warm Italian sunshine.

MPIF's 2003 International Conference on Powder Metallurgy and Particulate Materials in Las Vegas attracted 1100 delegates from more than 30 countries. JOSEPH M. CAPUS reviews the event, while JOHN DUNKLEY assesses developments pointing the way to higher density, higher performance PM components.

This conference, which incorporated the 5th International Conference on Spray Forming, was once again (after the successful 2000 event) held at Bremen University, under the auspices of Professor Bauckhage's group at IWT (Institut fu¨r Werkstofftechnik), reports JOHN DUNKLEY.

To eliminate the contamination of activator elements, such as Fe and Ni, W-15Cu compacts were prepared from mechanically alloyed powders using an attritor with a zirconia tank, balls and agitator arms. Coarse tungsten and copper powders, 9·9 μm and 13·3 μm, respectively, were milled to 1·26 μm composite powders after 145h of milling. The milled powder contained little free copper and was highly combustible in air. After sintering, the 50 vol.-% dense green compacts attained a density of 15·8g cm^-3 or 96·2%. The microstructure consists of uniformly interdispersed tungsten and copper. When stainless steel grinding balls were used, the powder was heavily contaminated with Fe and Ni. The contamination improved the density slightly, but the grain size and the electrical resistivity increased significantly as well. The sintering behaviours of the two composite powders were similar. Most densification occurred during heating before reaching the melting point of copper.

Porous tungsten is employed extensively for components such as dispenser cathodes in electronic valves and light technology. Normally parts are produced by die pressing or cipping and sintered in a reducing atmosphere at temperatures • 2000°C. In this study, the aim was to look critically at the behaviour of tungsten powder at compaction and sintering so that the distribution of porosity could be better controlled and optimised. Attention was focused on the basic powder properties, such as particle size and shape, surface area and apparent density. The effects of these parameters on the relative density of the parts are described. A small quantity of aluminium was added to the tungsten powder to serve as a binder and a potential sintering aid.

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.

The process of consolidation of nanocrystalline W and clad W-(15-50%)Cu composition powders, as well as jacketing of conventional heavy alloys by steel in hot conditions under shock wave loading, was studied. The temperature of loading was in the range 20-1200°C and intensity of loading was up to 10 GPa. As investigation showed, the application of high temperatures and consolidation of W and W-Cu composition powders in two stages (first at room temperatures and second, in hot conditions) enables the powders to be compacted to near theoretical densities with high value of hardness. The value of densityand hardness depends on treatment temperature and the content of Cu in composition. The investigation of joining processes and jacketing of W-Ni-Feheavy alloys (WHA) by steel showed that, depending on the loading temperature, it is possible to achieve the jacketing by steel with the formation of an intermediate layer upon the whole length of the WHA. The structure of the intermediate layer, as well its properties, depend on the value of the loading temperature.

A novel experimental technique was used to study pressure effects on shear stress and failure behaviour of sintered and sintered plus swaged tungsten heavy alloys under high strain rates at room temperature. Depending on the type of material and amount of superimposed pressure, different shear stress deformation behaviours were measured. Strength and failure properties are compared with values for depth of penetration and crater volumes measured in ballistic tests against semi-infinite targets. A good correlation between stress state dependent dynamic mechanical properties and ballistic results was found. Dr Krüger (lutz.krueger@wsk.tu-chemnitz.de) is with Nordmetall GbR, Eibenberg, Einsiedler Str. 18h, D-09235 Burkhardtsdorf, Germany. Professor Dr Meyer is at the University of Technology Chemnitz, D-09107 Chemnitz, Germany. Mr Hofmann is at the Bundeswehr Research Institute for Materials, Explosives, Fuels & Lubricants (WIWEB), D-85435 Erding, Landshuter Str. 70, Germany. Mr Weinberger is with Israel Military Industries Ltd., P.O.B. 1044 Ramat Hasharon 47100, Israel. Manuscript received 17 December 2002; accepted 14 March 2003.

The penetration of long rods (L/D10) into standard RHA targets differs with rod failure and flow, which produces erosion and deceleration. Oriented, columnar grained W and [001] single crystal W-4%Ta rods exhibit dense shear/shear band flow phenomena consisting of overlapping bands ofpredominantly dynamically recrystallised (DRX) grain structures observed by optical metallography and transmission electron microscopy. This flow is contrasted withWHA (93%W, 5%Ni, 2%Fe) sintered rod penetration characterised by penetrator nose failure in blocks which move on narrow shear bands between the blocks and function like microstructural lubricants. From these comparative observations of residual, penetrated rods, strategies to promote rod penetration seem to involve the development of wide, overlapping bands or layers of equiaxed or preferentially oriented, refined microstructures, which facilitate material flow at high strain rates. Microstructural precursors, such as deformation twinning or the enhancement of recrystallisation or related microstructural issues, promoting frequent or overlapping shear bands, either through alloying or processing routes to control the shear instabilities, seem to provide the best strategies to enhance long rod penetration.

Nanodispersed powders of nitrides/carbonitrides of IV-VI group transition metals (for example TiN, TiC_xN_1-x, ZrC_xN_1-x, NbC_xN_1-x, Cr₃C_1·6N_0·4) with an average particle size of 20-200 nm have been prepared by the method of plasmachemical synthesis. Owing to the dynamic conditions of synthesis (starting product evaporation or decomposition, mixing and condensation of the end product lasting for some tenths of second), the product is formed in a form of nanodispersed powder, mainly as a monocrystal with a great concentration of crystal lattice defects. This determines high chemical activity of these powders, including increased sintering rate and decreased sintering temperature (for example, in the case of TiN, TiC_xN_1-x or NbC_xN_1-x this temperature is 400-600°C lower in comparison with the traditional powders). Addition of transition metal nitride/carbonitride to hardmetal can effect formation of its structure and properties. It was shown during the investigation that addition of 10 wt-% of the TiN nanopowder has a positive effect on a decrease of WC grain size in a cermet and improvement of its mechanic characteristics.

The development of high density spherical rhenium and spherical tungsten-rhenium powders has enabled the use of advanced consolidation techniques for the manufacture of refractory metal components. The investigated consolidation techniques are powder metal injection moulding (PIM) and vacuum plasma spraying (VPS); both produce net shape components. The required particle size distributions for these applications vary. VPS uses a large powder particle size (<44 μm) while PIM requires a fine particle size (<20 μm). The major advantages of spherical powders over traditional powders in plasma spraying are the high density of the powder particles and its good flow characteristics. These two factors combine to produce high density sprayed formed parts. PIM requires that the powder particles be dense as well as fine. Powder particles must be small enough to be entrapped in the binder but without being so small (<0·5 μm) that the body forces dominate. Binders for PIM are tailored to the powder's characteristics, including size, distribution, shape and reactivity. Binder systems are developed to provide maximum packing powder, while maintaining the ability to be moulded, and to allow debinding without chemically reacting with the powder. A discussion of the powder production process and the general characteristics of spherical refractory powders, as well as the alloys produced to date, will be discussed. Further, both consolidation techniques will be discussed in depth, focusing on the role the powder's attributes play in each technique. Spherical rhenium and spherical tungsten-rhenium powders are presently being used in non-erosion throats and other propulsion system applications.

The influences of titanium particle size and ignition mode on the products of SHS reactions in the Cu-Ti-C system have been investigated. Ignition by thermal explosion mode results in Cu-TiC composites with higher levels of porosity and coarser TiC particles than ignition by combustion mode, owing to higher reaction temperatures facilitated by preheating of the reactants. The use of coarse titanium results in incomplete reaction and lower reaction temperatures, irrespective of the ignition method, as a result of increased carbon rich and carbon depleted reacting regions brought about by less homogeneous mixing of the reactants. The lower reaction temperatures result in finer TiC particles and reduced porosity. Incomplete reactions result in higher levels of titanium dissolved in copper and TiC particles of lower stoichiometry. This improves wetting between the two phases and results in a more even distribution of the carbides in the Cu matrix.

The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejection force during the powder shaping process. This paper presents the significant advantages and drawbacks of using the warm compaction process for commercial PM aluminium alloys.

Tensile properties of powder metallurgy 3% manganese-0·8% carbon (content of green compact) steels were determined following laboratory sintering in (nearly) full, semiclosed containers with no getter powders in dry, 0-100% hydrogen-nitrogen atmospheres. Manganese was mixed with the NC 100·24 sponge iron powder as low carbon ferromanganese and carbon as a graphite addition. Dogbone compacts were pressed at 660 MPa, the sintering temperatures were 1120 and 1250°C and cooling rates ∼65 K min- 1. In specimens sintered in nitrogen containing atmospheres at 1120°C, final carbon content was ∼0·7% and for those processed at 1250°C ∼0·6%. Sintering in dry hydrogen resulted in lower carbon and oxygen contents. Independent of the H₂/N₂ ratio in the furnace atmosphere, however, all the specimens were ductile and exhibited similar strengths. Yield strengths R _0·2 were in the range: 426-464 MPa, tensile strengths R_m were 724-780 MPa and strains to failure were 1·6-2·0% after sintering at 1250°C. The 1120°C sintering temperature resulted in 10-15% lower strength values. The microstructures, significantly devoid of oxide networks, comprised mainly mixtures of bainite and fine (divorced) pearlite, with very little martensite and retained austenite. Reproducibly successful sintering of manganese containing compacts requires that reduction conditions exist at the sintering temperature. Ellingham Richardson diagrams dictate that the dewpoints of hydrogen required are-55 and-40°C at 1120 and 1250°C, respectively. A semiclosed container, how ever, ensures a different microclimate. It is suggested that then the initial relevant reactions there are: Mn[vapour]+H₂O=MnO+H₂, 3Fe₂O3 +H₂= 2Fe₃O₄+H₂O, Fe₃O₄+H2=3FeO+H₂O, FeO+H₂= Fe+H2O and C+O2=CO2, which provide hydrogen andwater vapour,also within the pores. The manganese vapour further acts as a ‘shield’ by generating further hydrogen from the water vapour. The following reactions involving carbon monoxide are postulated above 927°C, when CO is a more effective reducing agent than hydrogen: C+H₂O=H₂+CO, 3Fe₂O₃+ CO=2Fe₃O₄+CO₂, Fe₃O₄+CO=3FeO+CO₂, FeO+CO=Fe+CO₂ and C+CO₂=2CO. Accordingly, irrespective of whether it is hydrogen or nitrogen in the semiclosed container, if there is a supply of carbon, reducing conditions prevail at the sintering temperature,embrittling oxidenetworks arenot formed and ductile manganese steels are processed.

The effect of addition of Fe-B-C master alloy powders on the sintering behaviour of iron-molybdenum sintered steels has been investigated by dilatometric experiments. The addition ofthe Fe-B-C master alloy powders to the Fe-Mo-B-C sintered steels can activate the sintering process by liquid phase sintering. Liquid phases form owing to the reaction γ-Fe+ (Fe,Mo)₂B+Fe₃(B,C)• L at 1097°C and the reaction γ-Fe+Mo₂C+Fe₃C• L at 1085°C. The temperature of the liquid phase formation depends on the compositions and the amount of the master alloy powders. SEMQ analysis for the Fe-1·2Mo-0·4B-0·5C (wt-%) steelshows thatgrainboundary phasecontains approximately 5·0 wt-%Mo, much higher than that in the grains (approximately 0·95 wt-%Mo) and higher than that of the base powder Astaloy Mo (1·44 wt-%Mo). This means that molybdenum diffuses from particle to particle boundary to form borides and carbide during sintering.

The electrochemical behaviour of 434L ferritic stainless steels produced by the powder metallurgy (PM) route has been studied. The effects of sintering temperature (1250 and 1400°C) and dispersoid addition (10%Y₂O₃) have been investigated by potent iodynamic polarisation and electrochemical impedance studies in 0·05 M H₂SO₄ solution. All the alloys exhibited active-passive behaviour in the electrolyte. The critical current density for passivation was lower in the case of the PM samples when compared with 430, which has been attributed to the presence of Mo in the samples. The passive current density of the PM samples correlated with the sintered porosities. All the other passivation parameters were similar for the materials. The nature of passive film was investigated using electrochemical impedance spectroscopy. A lower capacitance (i.e. more corrosion resistant) surface film was obtained after sintering at the higher temperature. Yttria dispersoids in ferritic stainless steel decreased the corrosion resistance of the surface film after sintering at the higher temperature, whereas they did not significantly affect the surface film behaviour after lower sintering temperature. The results have been correlated to microstructural parameters of the PM materials.

In free fall gas atomisation of liquid metals, the atomised droplets mainly move downward, while some of them fly in an upward direction. Under certain conditions the upward moving droplets will deposit on the surface of the gas nozzle and metal delivery tube of an atomiser, which may cause a hindrance to the flow of liquid metal and gas. The effect of atomisation parameters, such as gas pressure, focal length, apex angle and diameter of gas nozzle, on the metal buildup on the atomiser have been studied during the free fall gas atomisation of lead, zinc and aluminium. The plenum pressure of gas at which deposition of atomised droplets on the surface of the gas nozzle and metal delivery tube takes place has been termed as limiting plenum pressure, and the corresponding gas velocity at the impingement point as limiting gas velocity. It has been shown that the limiting plenum pressure is different for different metals in the same atomiser. The limiting plenum pressure has been found to increase with free fall distance or specific gravity of liquid metals, and with a decrease in apex angle of atomiser. A correlation is proposed to determine the limiting gas velocity for free fall gas atomisation of metals.