Powder Metallurgy

The European Union funded thematic networks PM Modnet and PM Dienet have encouraged development of modelling technology to improve the production of complex ferrous parts and other materials. The networks sought to demonstrate via case studies the potential of the technology to meet industry requirements. The technical results obtained in three case studies (on iron, ceramics, hardmetals and soft magnets) are summarised and requirements for effective dissemination of simulation into industry are proposed. Further progress will involve complementary research on die filling and crack simulation and characterisation.

Finite element modelling is widely used in technological applications. The benefits of using simulation are clear: reduced time and cost when introducing new products to market, better knowledge of part dynamic and static properties, and the opportunity to replace life cycle machine tests, among others. Unfortunately powder metallurgy does not yet belong to this select ‘club’. Much effort has been put into simulating powder filling, compaction and sintering (the hard materials and ceramics industries are more interested in modelling of sintering than ferrous part makers whose main concerns are compaction and, in a further step, filling processes). However, to date, none of the models for powder compaction can effectively meet part makers’ requirements with respect to tool life and crack prediction. The progress towards these goals in Dienet are briefly reviewed.

The European Dienet project addressed modelling of die compaction for a range of materials. This was undertaken through three generic case studies. In the course of the work, potential areas for future research were identified. The views of the members of the Dienet group were canvassed and there was broad agreement that two areas are of the highest priority: modelling of fill, flow, transfer and low pressure compaction, including validation techniques; and modelling of cracking, including validation techniques.

Magnetic micro-actuators and systems require tiny permanent magnets with dimensions of hundreds of micrometres. Such magnets need to have the highest possible energy density, which means Nd–Fe–B is the most appropriate material. Most bottom-up fabrication techniques are either too slow or too expensive; top-down techniques involving machining tend to result in surface damage and a loss of magnetic properties. Thus, PM is an attractive route. It is shown that use of very neodymium-rich Nd–Fe–B powders makes it possible to sinter 100 μm thick specimens to full density at temperatures as low as 800°C. These very thin magnets have coercivities of up to 1000 kA m⁻¹ and are suitable for micro-actuator type applications.

Nanoscale tungsten powders promise access to very hard, strong and wear resistant materials via the press–sinter route. A small particle size changes the response during sintering, requiring lower temperatures and shorter times to attain dense but small grain size structures. On the other hand, oxide reduction and impurity evaporation favour high sintering temperatures and long hold times. Accordingly, press–sinter processing encounters conflicting constraints when applied to small particles. Presented here is an analysis of press–sinter tungsten particle processing to isolate conditions that balance the temperature and size dependent effects. The calculations are pinned by existing data. Opportunities are identified for new consolidation approaches to deliver a small grain size in a full density structure.

The addition of Cu₃P for developing the high strength 465 maraging stainless steel from elemental powders was studied. The sintering parameters investigated were sintering temperature, sintering time and wt-%Cu₃P. In vacuum sintering, effective sintering took place between 1300 and 1350°C. The maximum sintered density of 7·44 g cm⁻³ was achieved at 1350°C for 60 min with 4–6 wt-%Cu₃P. More than 6 wt-%Cu₃P content and temperature >1350°C caused slumping of the specimens. The sintered specimens were heat treated and a maximum ultimate tensile strength (UTS) of 767 MPa was achieved with 4 wt-%Cu₃P content. The maximum hardness of 45·5 HRC was achieved in heat treated condition with 4 wt-%Cu₃P content. Above 4 wt-%Cu₃P content increase in density was observed whereas the response to heat treatment decreased. Fracture morphologies of the sintered specimens were also reported. A comparison of sintering behaviour and mechanical properties of elemental powders with prealloyed powders was also given in the present study.

In the present paper, the effects of the mixed powders prepared from solid state reaction method and glycine–nitrate combustion technique on properties of electrolyte, including sintering behaviour, mechanical property and electrical conductivity, were investigated. The results denoted that the relative density, the flexural strength and the electrical conductivity decreased with increasing powder content prepared by the glycine–nitrate combustion technique. The fracture mode also changed from intergranular to transgranular. The thermal shock resistance was also studied. The results denoted that Sr- and Mg doped lanthanum gallate had a weak thermal shock resistance. The phase constitution was conducted by X-ray diffractometry (XRD). The microstructure was observed by means of scanning electron microscopy (SEM) and the electrical conductivity was measured using impedance analyser.

Charpy V notch (CVN) impact testing was conducted on full size and subsize specimens of sintered and wrought 17–4 PH stainless steel (17–4 PH SS) in the as sintered and H900 heat treated conditions. Test geometries correspond to the American Society for Testing and Materials (ASTM) and Metal Powder Industries Federation (MPIF) impact testing standards. Merits of a notched specimen compared with an unnotched specimen were analysed for both the wrought and sintered materials. The notched ASTM standard bars had a lower coefficient of variance for impact energy than the unnotched MPIF standard bars and displayed greater toughness. Porosity and grain size have a detrimental synergistic effect on impact toughness for the sintered material. Following a discussion about the differences in the wrought and sintered microstructures, it is recommended that impact testing of the injection moulded and sintered specimens should be evaluated according to the ASTM test specifications.

In the present study the authors have used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to assess how surface oxides limit the gas nitriding depth of gas atomised M4 high speed steel powder and compacts. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used for phase identification. In model experiments XPS and AES analyses of vacuum annealed powder were performed in an interconnected furnace, limiting reoxidation. Sintering cycles with and without vacuum annealing treatment were also evaluated. Generally, the authors found that an increased vacuum annealing treatment time decreased the amount of residual oxygen, which improved densification. AES and XPS analyses of the model experiments showed that the vacuum annealing time increased the absorption of nitrogen. In the sintered compacts, SEM, AES and XRD analysis as well as Thermo–Calc simulations showed that similar amounts of nitrogen were tied to vanadium carbonitrides. An AES comparison between the model and sintering experiments showed that the nitrogen absorption had the Sieverts’ law dependence once the surface oxide had been removed.

The authors manufactured the metal hydrides (Ti_0·88Mg_0·12)H₂ and (Ti_0·88Mg_0·12)H_0·7 by a very easy and cheap way using the mechanical milling of Ti–12Mg blending powder in liquid milling media such as isopropyl alcohol, hexane and heptane as hydrogen sources. The powder was analysed by X-ray diffraction (XRD) and convergent beam electron diffraction (CBED) patterns parallel with the chemical analysis by EDX and ion coupled plasma (ICP) measurements. The hydriding reaction began when the mechanically milled particle achieved nanometre size. (Ti_0·88Mg_0·12)H₂ was manufactured in isopropyl alcohol and hexane media, and (Ti_0·88Mg_0·12)H_0·7 in heptane, while the Ti–12Mg blending powder milled in argon atmosphere formed the solid solution of hexagonal close packed (hcp) structure.

Hydroxyapatite (HA)–316L stainless steel composite biomaterials with different 316L stainless steel fibre volume fraction in the composite were fabricated by hot pressing and sintering at elevated temperature. 316L stainless steel fibres were enwrapped in the HA matrix with integration being very tight. Metallographic microscope, SEM and EDAX analysis were carried out in order to investigate the microstructure in the materials and the combination interface between HA and 316L stainless steel fibre was observed in detail. While the composite contained 20 vol.-%316L stainless steel fibre, the bending strength and the compressive strength are equal to ∼200 MPa and 400 MPa, respectively. The research displayed that mechanical properties increase with the increase in volume of the 316L stainless steel fibres, but decrease with increasing diameter and mean length of the fibres. Toughness of the composite increases with the rise of 316L stainless steel contents. It has been shown that some Fe atom diffusion takes place through the interface into ceramic matrix during hot pressing and sintering.

Porous bioceramics hydroxyapatite–tricalcium phosphate (HA–TCP), aimed to be applied in clinic, was fabricated by powder metallurgy and evaluated using both in vitro and in vivo models. Porous HA–TCP was supposed as a partially biodegradable material and designed as a scaffold for bone reconstruction or regeneration. The material processing was proposed and the physical properties as well as the microstructure feature were characterised here. Biological postulation of the relationship between seeding density, proliferation and viability of human osteoblasts cultured on the porous HA–TCP was quantitatively measured. Bone reconstruction was investigated both in vitro and in vivo by use of these biodegradable scaffolds with pore sizes ranged from 200 to 400 μm in diameter. The degradable bioceramic supported cellular proliferation seeded on the scaffold and showed normal differentiated function in vitro. Suitable pore size of the porous bioceramic was required if promotion of bone reconstruction was desired. Clinical trials showed that the bioceramics were successfully applied for bone reconstruction and regeneration and could be partially degraded in human body in 18 months. This approach suggests the feasibility of using porous HA–TCP bioceramics for the transplantation of autogenous osteoblasts to regenerate bone tissue.

The objective of the present study was to investigate high velocity compaction of titanium powder and to prepare a dense composite biomaterial of titanium and hydroxyapatite with the purpose of forming dental components with improved early healing properties. A high purity titanium powder was compacted using high velocity compaction to study the density distribution. Then, a titanium–hydroxyapatite composite was prepared by mixing titanium powders and hydroxyapatite grains. Dental implant components were formed from the high velocity compacted specimens, exposing the hydroxyapatite grains at the component surface. The green density reached more than 98·5% after more than one impact. The composite was heated to 500°C, enough to bind the titanium grains, but to avoid observable reactions. Compacted pure titanium could be sintered to full density. The heated composite material reached 99% density, no reaction was observed between titanium and hydroxyapatite, and the composite material could be formed into dental implants.

A great demand for fine grained metal powders provided an incentive to develop a new method of the electrolytic production of metal powders, which would make it possible to control the electrocrystallisation process. The method consists in the use of a cathode of a special construction, i.e. in the form of an endless moving belt. The belt moves by means of a gear on two drums, one of which is in the electrolyser and the other in the receiver. By setting the appropriate speed of the belt cathode's feed through the electrolyser, it is possible to regulate the time of nucleation and the growth of powder grains. The use of cathode in the form of a moving belt also made it possible to study cathodic process mechanism using polarisation measurements under the conditions of constant area renewal of the electrode's surface. The precise knowledge of the occurrence range of the concentration polarisation is especially important during the process of obtaining a metal in the powder form. It was found that the possibility of the constant area renewal of the surface has a positive effect on the homogeneity of the metal powder.

Physical and mechanical properties for prealloyed 6061 Al powder processed with and without additions of solid and/or liquid lubricants and sintering aids (Pb, Sn, Ag) are presented. For comparison, both vacuum and nitrogen sintering were carried out on as received (gas atomised) and degassed powder compacts pressed at 340 and 510 MPa. Vacuum degassing of the prealloyed powder provided better compressibility and thus higher green densities than those for the as received powder. Highest sintered densities of ∼98–99% of theoretical were obtained for the prealloyed (and degassed) Al compacts by sintering under pure nitrogen with an addition of 0·6 wt-% paraffin wax as solid lubricant or 1·33 vol.-% liquid paraffin, or with a 0·12 wt-%Pb addition as sintering aid and no lubricant. It was found that additions of solid lubricants such as lithium stearate and acrawax to both the premixed (elemental) and prealloyed powders provided reasonable green densities of ∼94·5–95·5% TD, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Other lubricants such as zinc stearate, stearic acid and liquid paraffin provided similar green densities, but higher sintered densities and less porous microstructures, particularly by sintering under pure nitrogen. The prealloyed compacts sintered under pure nitrogen consistently provided much higher sintered densities than elementally premixed compacts sintered under pure nitrogen or vacuum. It is therefore concluded that both lubricant type and sintering atmosphere will have a major effect on the sintered properties of the 6061 Al powder. Sintering under pure nitrogen resulted in higher sintered densities as compared with vacuum sintering for this grade of Al alloy. Tensile properties of the degassed and vacuum sintered (and T6 tempered) prealloyed powder compacts were higher than those of the equivalent alloy prepared by elemental mixing and comparable with those of the commercial (wrought) 6061 Al alloys.