Powder Metallurgy

A one-day workshop focusing on energy applications, organised by the Particulate Engineering Committee of the Institute of Materials, Minerals and Mining (IOM3) and co-sponsored by the Materials Knowledge Transfer Network (KTN), was held at the Riverside Centre, Derby on 15 May 2013.

The conventional sintering of titanium requires high temperatures to obtain high densities and low porosity, giving rise to microstructures with high grain size, and high interstitial contents, both of which adversely affect the mechanical properties obtained. A novel approach is reported which uses fine (10 µm) spherical Ti powder to improve the sintering behaviour, together with a small percentage of alumina particles (0·5 µm) to restrict grain growth. Colloidal techniques were used to form spherical agglomerates, 50–300 µm in size, of Ti powder with alumina particles dispersed homogenously on the Ti surface. These agglomerates present good compressibility and make it possible to sinter effectively at low temperatures, increasing the relative density and decreasing grain size.

Titanium alloys containing β stabilising elements such as Nb, Zr and Ta are particularly promising as implant materials because of their excellent combination of low modulus, high strength, corrosion resistance and biocompatibility. A low elastic modulus is important for implants to avoid stress shielding and associated bone resorption. The difficulty of producing complex shapes of these alloys by conventional methods makes metal injection moulding (MIM) attractive. Ti–17Nb alloy parts with densities 94% of theoretical have been produced by MIM of a feedstock based on blended elemental powders. Scanning electron microscopy reveals a typical α−β Widmanstätten microstructure with a precipitated α phase layer along the grain boundaries. The parts exhibit an ultimate tensile strength of 768 MPa and a plastic elongation of over 5%. The modulus of elasticity, about 84 GPa, is more than 20% lower than that of cp Ti and Ti–6Al–4V.

Bone ingrowth into and through porous coatings on orthopaedic implants can substantially improve fixation. However, the introduction of pores increases surface roughness and also the risk of bacterial adherence, which can lead to infection (in extreme cases, to death) and complicate implant surgery due to the high risk of revision being required. Improving osseointegration without increasing infection risk is therefore a major challenge in implantology. Staphylococcal adhesion and biofilm formation on Ti surfaces of varying roughness and porosity have been investigated in vitro. Porous pure titanium coatings, obtained by a PM processing route based on the electrophoretic deposition of TiH₂ followed by thermal treatment in vacuum, significantly reduced bacterial colonisation of the surface compared with a state-of-the-art vacuum plasma sprayed coating. Further reduction of biofilm formation could be obtained by additional surface modification.

The properties of porous Ti disc aerators for water treatment were investigated. It is shown that the addition of 5–15 wt-% of a finer (100–160 µm) sponge titanium powder to the coarser (630–1000 µm) base powder can increase the tensile strength of the discs obtained by a factor of two. This allows weight savings of 44% to be obtained at comparable strength levels for discs 188 mm in diameter. The discs produced from mixed powders also produce finer bubbles and have lower permeability, which can lead to energy savings through reduced pump power consumption.

Aluminium alloy AA 5083 [Al–4·4Mg–0·7Mn–0·15Cr (wt-%)], powder was ball milled in liquid nitrogen via the cryomilling method to obtain a nanocrystalline (NC) structure. Samples of the powder were hot vacuum degassed to remove interstitial contaminants, then consolidated by hot isostatic pressing (HIPing) at six temperatures (from 0·46T_m to 0·89T_m), before being high strain rate forged (HSRF) to produce plate material. The microstructure was analysed at the different processing stages. The compressive properties of the as HIPed material, plus tensile properties of the final product were studied. Despite grain growth during HIPing, an ultrafine grain (UFG) structure was retained in the consolidated material, which consequently had increased strength over conventionally processed AA 5083. As the HIP temperature was increased, the density increased. Strength changes were minimal in compression and tension with varying HIP temperature, once near full density was attained at 275°C (∼0·64T_M). Yield strength data indicate negligible variation in the grain size of the materials.

Centrifugal atomisation of liquid or melts using a rotating cup is an efficient process that is widely used in many industrial processes, which range from chemical reactors to powder production in metallurgy. The theoretical prediction of liquid drop or particle size is desirable for the design of atomisers and operating parameters. In this paper, film disintegration by centrifugal atomisation using a rotating cup was analysed based on wave theory. Two mathematical models were proposed to predict the maximum unstable growth rate, wave length, film length, break-up time and drop size for the rotating cup at high and low Weber number respectively. The film break-up length and drop size were calculated numerically and agreed with the experimental results very well in film disintegration. In addition, the effects of the atomisation parameters in the film disintegration were discussed. The mathematical models are able to accurately predict the atomisation parameters.

The preparation of glass–ceramic foams from slag and other components including glass cullet and foaming agent (SiC) is a challenge in the development of marketable and valuable materials. Box–Behnken experimental design was used to investigate the impact of operating conditions (i.e. temperature, reaction time and foaming agent percentage) on density and water absorption capacity of glass–ceramic foams. The optimal process parameter settings to achieve a maximum water absorption capacity (71·34%) and minimum density (0·61 g cm⁻³) were determined. The failure mode of glass–ceramic foams occurs via layer crushing mechanism. The correlation between the process controlling parameters and the responses (density and water absorption) of the produced foams were represented by two polynomial quadratic models. The determination of main process conditions through Box–Behnken experimental design offers a technological and economic competiveness in the mass production of glass–ceramic foams with specified physical properties for a wide range of applications.

An investigation to identify the chlorides in hydrogenated–dehydrogenated Kroll processed titanium powder was carried out in this study. Jigsaw-like agglomerates containing submicroscopic particles were observed on titanium particles, and the microcompositional analysis suggests the presence of magnesium and chlorine. Further detailed surface chemical analysis carried out by X-ray photoelectron spectroscopy and corresponding curve fitting work revealed that the magnesium and chlorine mainly exist as Mg(OH)Cl and titanium chloride respectively. A mechanism is provided to explain the presence of these chlorides in the Ti powder. The chlorides are considered to arise from the decomposition of hydrated magnesium chloride during the hydrogenation–dehydrogenation process and are supported by thermal analysis of a pure hydrated magnesium chloride. This analysis suggests that the Mg(OH)Cl can further decompose into hydrogen chloride gas and magnesium oxide at high temperature. The implications of the formation of these gaseous species on the sintering of Ti powders are discussed.

Oxidation kinetics based on Kissinger–Akahira–Sunose analysis of high Cr ferritic steel synthesised by mechanical alloying of elemental powder blend of 84Fe–13·5Cr–2·0Al with 0·5 nano-Y₂O₃ (all in wt-%) dispersion followed by cold compaction with 250 MPa pressure and sintering in vacuum (10⁻⁶ mbar) at 1000°C for 1 h have been investigated under isothermal (600–900°C for up to 50 h) and non-isothermal conditions (50–1200°C with heating rates of 10, 20, 30 or 40°C min⁻¹). Mechanism of oxidation was taken place by counterionic transport of oxygen from surface to interior and cations (Cr³⁺/Fe³⁺) from the interior to the surface through grain boundary at low oxidation temperature (∼600°C) and through grains at high oxidation temperature (700–900°C). Early formation of Cr rich spinel layers on the surface improves the oxidation resistance by acting as the diffusion barrier against counterionic transport of ions during oxidation.

Gas atomisation phenomenology of liquid metals and alloys is both intriguing and instructive. Predating atomistic studies of melt disintegration by almost three decades, it provides a functional means of correlation between drop sizes and processing parameters, as is the case in liquid metal spray forming. In this article a concise but thorough compilation of melt disintegration phenomenology is provided, with emphasis on high pressure gas atomisation, inclusive of notation on mean particle diameters, the role of viscosity, density and other dimensionless ratios that enter as variables in high pressure gas atomization and spray forming, as well as break up of a liquid column and theories of drop disintegration in flight. The article concludes by introducing the very efficient Surface Wave Formation principle which facilitates the treatment of complex atomisation geometries and turbulent regimes.