Powder Metallurgy

The EU REACH regulation shifts the responsibility (burden of proof) for collection, evaluation and assessment of ‘chemical’ materials data from the authorities to the industry. As a result, all members of the PM supply chain must contribute to the protection of human health and the environment with regard to all substances they produce, distribute or use. The basic principle is: ‘No data, no market’. In view of this increased responsibility of individual companies, it is important for those with hazardous substances in their portfolio to consider which should be retained in the long term and which ones can be phased out. For sintered parts manufacturers, finding viable solutions to the implementation of REACH that are acceptable to the market requires effective communication with partners along the supply chain. Nickel is a substance of particular concern to PM parts manufactures and the implications of controlling dust exposure are considered.

Recent trends in the technology, production and applications of diamond tooling are reviewed. The continuing fall in the price of synthetic diamond as production volumes increase provides a strong incentive for tooling companies to reduce the cost of matrix materials and of their manufacturing processes, with an accompanying need for research in these areas. Diamond wires for the cutting of granite and other rock continues to grow in application and provides another focus for materials and technology advances.

This work proposes a hypothesis for the interpretation of shrinkage anisotropy during sintering of an Fe–Cu–C alloy based on the effect of the structural modifications of the powder, due to the prior compaction, on the mass transport phenomena. Dislocations are introduced by cold compaction in the contact regions between particles, with different densities along the compaction direction and the transversal one. Therefore, the mass transport by volume diffusion is strongly activated in both directions, and a prevailing effect in the compaction direction is shown. The volume diffusion coefficients derived from the kinetic model correspond to the dislocation pipe diffusion mechanism.

Fe_78·4Si_9·5B₉Cu_0·6Nb_2·5 alloy powder cores were made from powders prepared by the pulverisation of crystallised ribbons, atomisation and high energy ball milling after atomisation. The results showed that the saturation magnetisation of the three powders showed negligible variation. The coercivity was found to be strongly affected by the microstructure of the powder. Cores made from pulverised powder exhibited the largest μ′ and core loss because of the small demagnetising field and large contact area caused by the flat shape of particles. In contrast, cores made from atomised powder exhibited the smallest μ′ and core loss because of the large demagnetising field and small contact area. Cores made from milled powder exhibited an intermediate μ' and core loss because of the change in powder shape caused by severe plastic deformation during ball milling.

Absorption of interstitial elements during the processing of Ti–13Nb–13Zr alloy using hydride powders was investigated. Oxygen, nitrogen and carbon contents were quantified in the steps of hydriding and milling of powders and after sintering of green compacts. The influence of the hydriding treatment was analysed regarding the interstitial contents in the raw materials. Milling and sintering were evaluated according to a 3³ factorial design considering the effects of milling time of titanium hydride from 1 to 11 h, sintering temperature between 1000 and 1400°C and holding time from 1 to 5 h. The results showed that the oxygen contents in sintered samples are strongly affected by the hydriding and milling of powders. The effects of the sintering temperature and holding time were found to be significant mainly for the increase in nitrogen and carbon contents. The results of the factorial design, along with the study of microstructural evolution allowed the optimisation of the process parameters in order to obtain the alloy with the lowest possible contents of interstitial elements and homogeneous microstructure.

We experimentally obtain and analyse green density distribution in stainless steel compact samples and investigate the effect of compaction pressure on sample green density and density distribution. Experimental measurements of local density of stainless steel samples are conducted using scanning electron microscopy. For design purposes, the measured local densities, depth and planar location and compaction pressure are used to train an artificial neural network model to estimate the compaction density as a function of input parameters. Material parameters obtained experimentally are used to calibrate a finite element model. The results show that the artificial neural network and finite element modelling approaches are feasible and could be used in predicting the overall compaction density variations in powder metallurgy components. It is observed that the overall compact green density increases almost linearly with compaction pressure. Phenomena of particle interlocking and cold welding are observed and discussed.

In this study, nanocrystalline Ni_0·64Zn_0·36Fe₂O₄ powders were prepared using a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling were studied by X-ray diffraction technique, scanning electron microscopy, transmission electron microscopy and vibrating sample magnetometre. It is revealed from the results of the phase analysis that nanocrystalline Ni_0·64Zn_0·36Fe₂O₄ ferrite with average crystallite size of 6·18 nm and non-uniform lattice strain of 0·33% has been formed after 60 h of milling time. A progressive increase of saturation magnetisation and a dramatic decrease in coercivity were also observed with increasing milling time.

The present research is focused on porous, grooved and tubular wick development. The basic purpose of a wick is to generate capillary pumping pressure for circulation of a working fluid within the loop heat pipe, which is used in spacecrafts and satellites. The present work establishes metal injection moulding as a potential route for wick development in the near net shape form. Nickel powder, polypropylene powder and thermoplastic binder have been used to produce green compacts of the wicks before sintering. Experiments have been performed at three levels of sintering temperature (900, 930 and 950°C) and sintering dwell time (30, 60 and 90 min). Optimum sintering condition has been found in this work as 900°C temperature and 60 min time, which gives optimal values of porosity (55 vol.-%), average pore diameter (2·6 μm), permeability (1·94×10⁻¹² m²), effective thermal conductivity (9·37 W m⁻¹ K⁻¹) and average capillary pumping pressure (54 kPa) in the wick.

An improved process is proposed for sintering of Fe–3Cr–0·5Mo–0·6C steel in nitrogen, in a box, additionally containing sources of manganese vapour (ferromanganese lumps) and nascent carbon (naphthalene). The use of a semiclosed container with a labyrinth seal minimises compact interaction with the flowing furnace atmosphere and thus ensures that a dry local ‘microclimate’ exists within and around the sintered specimens. Nascent carbon, from ∼1000°C, is significantly chemically more active than graphite (previously used) and the microclimate thus included, from ∼700°C, manganese vapour. The latter additive ensures that manganese is oxidised in preference to chromium, whereby oxygen uptake to the compact is inhibited and reduction rather than oxidation of chromium occurs. A favourable comparison is made of resultant mechanical properties for the same material sintered also in hydrogen, with published properties of similar Cr–Mo–C steels and with MPIF standard for Cu and/or Ni containing steels.

Ultrafine Ti powder was prepared by hydrogenation of titanium sponge, mechanical pulverisation, inhibitor coating, vacuum dehydrogenation and inhibitor removal. The effects of technological parameters on Ti powder morphology, particle sizes and oxygen content were investigated. The results showed that after hydrogenation at 700°C for 2 h and mechanical pulverisation for 5 h, the titanium hydride powder with D₅₀ of 2·61 μm was obtained. Then the ultrafine titanium powder with oxygen content of 0·89 wt-%, D₅₀ of 6·16 μm and irregular morphology was obtained by procedures of coating titanium hydride powder with NaCl, dehydrogenising at 630°C for 2 h and removing NaCl. The addition of NaCl resulted in a negligibly increase in the oxygen content in Ti powder. The Ti particles growth during dehydrogenation was effectively inhibited by the coated NaCl on Ti particles with thickness of 5–10 nm as an isolation layer to block atomic diffusion.

Owing to high performance/cost ratio, sinter hardening process is gaining more popularity towards liquid quench hardening of PM sintered steels. Effect of cooling rate during sinter hardening on the microstructure and properties of sintered steel grade Distaloy HP with heterogeneous and Astaloy CrM with homogeneous microstructure has been studied. The samples were sinter hardened at different cooling rates of 0·5–3°C s⁻¹ in order to investigate the influence of homogeneity towards sinter hardening technology. The microstructure and hardness of the samples were measured carefully. In addition, the chemical analysis of the samples after sinter hardening was measured. It has been shown that sintering process and cooling rate change the microstructure and hence the hardness of the materials. The most differences between samples were detected at a cooling rate of 3°C s⁻¹. At this cooling rate, the homogeneous microstructure was fully martensitic, while the heterogeneous microstructure had lesser amount of martensite. Effect of martensite formation on mechanical properties was also investigated.