Powder Metallurgy

JPMA, the Japan Powder Metallurgy Association, has announced the winners of its development prizes for 2007, in four categories ranging over design, technology and innovation.

Little research has to date been done to evaluate the impact of nanoparticles on the environment and humans, despite their use in products such as cosmetics, paint and tyres. Experience with other emerging technologies has shown that they are only accepted by society if possible health effects are analysed and published at an early stage of development. INOS, the Identification and Assessment of the Effects of Engineered Nanoparticles on Human and Environmental Health research project funded by the German Federal Ministry of Education and Research (BMBF), has been set up to address these issues. Some initial results of the project are briefly described.

Industrial production of critical parts in aero engine by PM is now more than 30 years old. During this period, evolution of design principles and better evaluation of the impact of cleanliness has led to an evolution of metallurgy (grades and microstructure) and processes. The basis of this evolution will be presented and illustrated by examples taken from companies worldwide.

The development of a floating die shear plate apparatus and its use to explore lubrication mechanisms (both admixed and die wall) and to assess the effectiveness of lubrication under a wide range of both compaction and normal loading conditions are described. The equipment allows the independent exploration of the effect of density and normal stress on the friction between compact and tool set surface. Tests were carried out using DistaloyAE, a zirconia target surface and Kenolube as lubricant (added or applied). A friction correlation that includes both density and normal stress effects and that reduces in response to both of these parameters was found. The results also suggest that the friction coefficient during ejection will be lower than that during compaction; at very high contact stresses, there is a likelihood that the lubrication regime will break down leading to galling. With regard to the friction mechanism, surface analysis using EDX did not detect the presence of lubricant on the compact surface; however, it did reveal two distinct pore types, a ‘dark’ one having a higher than average level of carbon and a ‘light’ one having a higher than average level of nickel. Image and topographical measurement of compacts subjected to sliding against the target surface revealed significant differences. Both analyses suggested significant closure of open pores due to sliding with topographical analysis revealing remnants of pores under the deformed particles that make up the exterior of the compact surface. Results confirmed there is no noticeable benefit in increasing admixed lubricant content above 0˙5%. Die wall lubricant was demonstrated to be very effective, achieving a reduction in friction of up to 58% for unlubricated powder and up to 27% for powder containing 0˙5% admixed lubricant. This was achieved for a lubricant film of typical thickness 5–20 μm.

Materials with high resistance against abrasive wear are of high interest for many tool applications, e.g. in mining industry. These materials usually contain a large amount of hard ceramic phases embedded in a metallic matrix and can only be manufactured by powder metallurgy, most often via hot isostatic pressing. Here a new manufacturing route via powder presintering and direct hot extrusion is presented, which enabled manufacturing of Fe based metal matrix composites with up to 30% of fused tungsten carbide (FTC, WC/W₂C) or titanium carbide (TiC). The matrixes chosen are hot and cold work tool steels. The manufacturing process is elucidated and microstructures formed are presented. The microstructure–property relation of the new Fe based MMC is discussed.

An Fe–35 wt-%Mn alloy, aimed to be used as a metallic degradable biomaterial for stent applications, was prepared via a powder metallurgy route. The effects of processing conditions on the microstructure, mechanical properties, magnetic susceptibility and corrosion behaviour were investigated and the results were compared to those of the SS316L alloy, a gold standard for stent applications. The Fe35Mn alloy was found to be essentially austenitic with fine MnO particles aligned along the rolling direction. The alloy is ductile with a strength approaching that of wrought SS316L. It exhibits antiferromagnetic behaviour and its magnetic susceptibility is not altered by plastic deformation, providing an excellent MRI compatibility. Its corrosion rate was evaluated in a modified Hank's solution, and found superior to that of pure iron (slow in vivo degradation rate). In conclusion, the mechanical, magnetic and corrosion characteristics of the Fe35Mn alloy are considered suitable for further development of a new class of degradable metallic biomaterials.

Experimental determination of Young's moduli of sintered metal–powder mixtures was carried out with results comparison through the use of a resonant frequency technique (dynamic excitation methodology) and an evaluation of the elastic stress–strain ratio in monotonic tensile loading regime. The materials used in this study consist of sintered bodies obtained from several metallic powders employed in the production of powder metallurgy metal composites used in diamond impregnated tools for cutting hard materials (such as stone cutting). The values of Young's modulus determined experimentally are also analysed and compared with a theoretical estimation carried out considering the chemical composition of each material and applying a simple rule of mixtures. The dynamic excitation is assumed to be the simplest experimental procedure, enabling accurate and reproducible measurements. On the other hand, the microstructure of the sintered bodies seems to influence the values of Young's modulus measured experimentally.

SiC_p/Cu composites with high reinforcement content (56–65 vol.-%) were fabricated by spontaneously infiltrating copper alloy into porous SiC preform obtained by powder injection moulding. The main influencing factors of the preparation of the preform and the infiltration behaviour of various preforms were studied. The results indicate that the viscosity increases with an increase in powder loading or decreases with an increase in particle size. The feedstocks with bimodal mixture exhibit low viscosity due to the improvement in packing efficiency. When small particles or bimodal mixture with large size ratio were used, the preforms were difficult to be fully infiltrated. The preforms debound in air exhibit lower infiltration rate than the ones debound in vacuum due to the reduction of porosity and the poor wettability between the oxidised layer and copper alloy. Kinetic analysis on the infiltration curves yielded activation energies of 253 kJ mol^–1.

Titanium alloys are of great interest for several applications, but their processing and application is often difficult and expensive. Metal powder processing permits the reduction of material costs because of limited wasting, even if, sintering of titanium powders has to be carried out under controlled atmospheres, with high processing costs. Spark plasma sintering (SPS) is a new technology which permits processing of hard to sinter materials at lower temperatures and in shorter time compared to conventional technologies; furthermore, SPS can be seen as a potential near net shaping production route for non-complex components. In the present work, the use of SPS on commercial purity titanium grade 1 and grade 3 was investigated. Sintering experiments were conducted in the temperature range 700–1150°C for 5 min, under a uniaxial pressure of 60 MPa, in vacuum. Samples obtained at different temperatures were characterised in term of density, microstructure (grain size), interstitial content (C,O,N), hardness, tensile properties; SEM analysis on fracture surfaces of tensile specimen was also carried out. Results were critically discussed and correlated to sintering temperature, in respect to the typical α→β transformation temperature for each grade. Experimental data confirm that very good microstructural, chemical and mechanical properties can be attained after spark plasma sintering at temperature as low as 900°C.

Mixtures of 94 wt-% tungsten carbide (WC) and 6 wt-% cobalt (Co) powders with different particle sizes have been formed with binders composed of polyethyleneglycol (PEG) and polymethylmethacrylate (PMMA), in some cases with the addition of stearic acid (SA) as a lubricant. The presence of voids has had to be invoked to explain why, when the same moulding conditions were used, the as moulded and as leached densities varied with binder composition and in some cases, why the former were lower than expected from the proportions of binder components and solids used to prepare the feedstocks. For a given nominal binder content (vol.-%), calculated on the assumption of zero voidage, and using constant moulding conditions, it appears that the actual voidage tended to decrease as PMMA progressively replaced PEG, and to increase as SA was introduced to replace PEG. The former change to the binder composition should, with constant binder content and moulding conditions, increase the apparent viscosity of the feedstock, while the latter should reduce it. A slip band model is postulated which allows flow behaviour and void formation to be explained qualitatively. This is further developed in Part 2 of this paper. Data from previous studies using PEG/PMMA binders with stainless steel powders are included as these can now be given a qualitative explanation with the slip band model.

Flow and deformation of powder injection moulding (PIM) feedstocks are considered to occur in similar ways to those observed in materials containing substantial amounts of clay and water, i.e. by a slip band mechanism. A slip band model for PIM feedstocks has been postulated, which allows flow behaviour and void formation to be explained qualitatively. For feedstocks, the slip bands are assumed to be layers of mobile liquid, which, for polyethyleneglycol/polymethylmethacrylate (PEG/PMMA) binders, are considered to be mainly PEG. It is proposed that some of the slip band liquid is drawn from between the randomly densely packed particles between slip bands to form voids. It is considered for given moulding conditions that increasing the apparent viscosity of a feedstock, by decreasing the volume fraction of binder in slip bands, reduces voidage. This can be achieved by increasing the nominal powder loading of a feedstock and/or increasing the effectiveness of the PMMA in holding the particles together more strongly.

A metal injection moulding technique for the production of aluminium alloy components is described. A part is formed by injection moulding a mixture of alloy 6061 and 2 wt-%Sn with a resin consisting of stearic acid, palm oil wax and high density polyethylene. The resin is removed by a combination of solvent and thermal processing. The parts are then sintered in a nitrogen atmosphere to a density of 97%. Sacrificial magnesium blocks which act as an oxygen and moisture getter are placed in the vicinity of the parts during sintering. This ensures surface integrity. Aluminium nitride forms throughout the part, which provides structural rigidity and dimensional stability and limits grain growth. After artificial aging, the tensile strength is 300 MPa. The technique allows the production of small complicated shapes and provides an additional means to manufacture aluminium components for a wide variety of applications.

In metal injection moulding, the quality of the products depends highly on an effective and suitable debinding process. The improvement of the debinding process thus becomes one of the most important topics in metal injection moulding research. To increase the binder debinding rate and decrease the defects of the products, it is essential to understand the influence of thermal control on the debinding process. The present paper aims to investigate the effect of multisteps thermal control on the debinding rate through experiments. Different from previous researches in which single binder is usually adopted, multicomponents binder is used to reflect the real manufacturing situations in industry. The relationship among viscous force, capillary force and pressure force for different working conditions is discussed in details. Experimental results show that the debinding rate and the final debinding fraction are dominated by particle size in the compact rather than that in the wick. For thick compact with low porosity, the shape of green compact is well maintained and the defect of neck shrinkage is reduced by multisteps thermal control. The neck shrinkage situation can further be improved by using the two component binder.

A three-dimensional finite element analysis of a powder compaction process was undertaken to determine the optimum manufacturing conditions for the complex cylinder block found in the hydraulic pump of an excavator. A porous material model was used to ascertain the material behaviour. The finite element predictions for both the density distribution and compaction load were in good agreement with experimental results.