Powder Metallurgy

Current orthopaedic implant technology focuses on fixation by osseointegration to maximise the implant longevity and reduce the need for burdensome and expensive revision surgery. In this respect, porous Ti coated implants, which enable bone ingrowth into the porous structure and the establishment of biological anchoring of the implant, are of interest. In previous work a new powder metallurgical processing route was reported for the application of porous Ti coatings on Ti alloy substrates by electrophoretic deposition (EPD) of TiH₂ powder suspensions. To validate the function of these coatings for potential clinical applications, the early peri-implant bone response was evaluated in vivo in a rabbit model. The results clearly demonstrate bone ingrowth in porous coatings with pore channels down to 10 μm, as opposed to the minimum pore size of 50–100 μm commonly claimed in the literature. Moreover, the observed inter-connectivity with surrounding cortical bone confirmed the envisaged mechanical interlocking of the implant.

Recent developments are presented on powder injection moulding of titanium from metal hydride powders and binders composed of polyethylene, paraffin wax and stearic acid. The feasibility of using this route to process fit for purpose, complex parts is assessed. Titanium hydride offers a low cost solution compared with pure titanium powders. Feedstocks for powder injection moulding were prepared in a sigma mixer. Tensile test specimens and demonstration parts were injection moulded. Solvent debinding in heptane was followed by thermal debinding and dehydrogenation under argon. Titanium parts were sintered at 1200°C under argon. Sintered parts exhibit a linear shrinkage of about 20%, good shape preservation and reproducibility. The yield strength (519 MPa), ultimate tensile strength (666 MPa), elongation to fracture (15%) and interstitial content measured by quantitative analysis meet the requirements for titanium grade 4.

Biocompatibility, bone-like mechanical properties, and good bone-to-implant anchorage are current requirements for permanent implants. Porous titanium can satisfy these requirements provided that sufficient porosity, large enough pores and interconnections allowing bone ingrowth can reliably be obtained with controlled processes. In the present work, porous parts are processed from titanium hydride based feedstocks containing space holders. Two formulations have been developed: a feedstock with a polyethyleneglycol based binder and NaCl space holders, and a feedstock with a paraffin based binder and PMMA space holders. Depending on the sintering conditions, porosity levels between 30 and 60% and open porosity between 10 and 40% are obtained, with pore sizes in the range 50–500 μm. The microstructure, porosity and mechanical properties of porous titanium sintered at various temperatures have been characterised by scanning electron microscopy and compression tests.

In this study, Ti powder (average size: 45 μm) was plated/coated by electroless Ni with hydrazine hydrate as reductant. The Ni plating was carried out at 85°C and pH 9–10. The influence of process parameters such as plating period as well as reductant concentration was investigated. The Ni plated Ti powder was characterised by scanning electron microscopy, energy dispersive spectrometer analysis and X-ray fluorescence. It is found that a pure/uniform Ni layer may be deposited on the Ti powder particles. The deposited mass increases as plating period/reductant concentration increases.

The production process of a reduced activation oxide dispersion strengthened ferritic steel with nominal composition of Fe–12Cr–2·5W–0·2Ti–0·25Y₂O₃ (designated 12Cr-ODS) was investigated in this study. Master alloy with designed composition was prepared first, and the powder with two sizes obtained by atomising was prealloyed by mechanical alloying with added Y₂O₃ powder, followed by consolidated spark plasma sintering. Characteristics of the samples were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and relative density, and the mechanical properties such as hardness (HV), bending strength and tensile strength were also tested. Sample with 50% coarse particles and 50% fine particles was found to be of better comprehensive properties. Mechanical properties of the sample changed with the improved microstructure of refining grains and optimised oxide particles that were acquired through annealing. However, further research on the dispersion and properties of oxide particles is needed in the future.

Powder metallurgical (PM) steels with elemental Ni additions exhibit non-homogenous microstructures with soft Ni rich areas, lean in C, after conventional sintering. Though, the exact correlation between the distribution of Ni and mechanical properties is not well known and depends on the conditions, e.g. the load state, it is desirable to be able to control the distribution of Ni since it plays a major role in the properties of Ni PM components. By introducing other alloying elements, the microstructure homogeneity of Ni containing PM steels, can be influenced. Thus, the effect of common alloying elements on the homogeneity of sintered microstructures has been investigated in the present work. It is found that additions of either C or Mo have minor effect on Ni distribution in the Fe–Ni system. However, addition of both C and Mo to Fe–Ni improves the Ni distribution. In addition, a strong interaction between Ni and Cu is observed and it enhances the Ni homogeneity. Furthermore, the influence of Cu is more pronounced in presence of C.

The structural evolution of Cu–12 wt·%Ge (∼Cu–11 at·%Ge) alloy processed by means of mechanical alloying (MA) with subsequent heat treatment was studied using X-ray diffraction profiles, scanning electron microscopy, transmission electron microscopy (TEM) and high resolution TEM observations as well as differential thermal analysis(DTA). The fcc Cu(Ge) solid solution (α) was produced at early stages of MA and amorphised upon further milling. This was followed by the formation of ζ−Cu₅Ge intermetallic nanocrystals after 40 h of milling. The subsequent annealing process led to the nanocrystallisation of the amorphous powder. The X-ray crystallite size data showed the exceptional thermal stability of the alloy prepared. The mechanism of the stability was also investigated. In addition, a thermodynamic analysis based on Miedema's semiempirical model was carried out for the evaluation of the experimental results and a good agreement was found between them.

Micrometal injection moulding (μ-MIM) is a promising alternative for fabricating micro parts. Low coefficient of thermal expansion Fe_x–Ni_1−x alloys are suitable to meet high dimensional stability with temperature keeping acceptable mechanical properties. However, these alloys are sensible of elements contamination during the debinding and sintering stages, and their dimensional stability and mechanical properties could be affected. The present work studies the effect of using combinations of debinding and sintering atmospheres in the micropowder injection moulding of Invar 36 feedstocks based on cellulose acetate butyrate binders. Microtensile specimens were successfully injected. Densification and microstructure at different conditions were linked with mechanical and physical properties of the parts. The processing conditions, and thus, the residual C, O and especially H influence severely these properties.

The oxygen content of a Cu powder has a significant influence on the transient liquid phase sintering (TLPS) behaviour of a Cu–Ni powder mixture. For Cu with low oxygen content, interdiffusion occurs between the Cu and Ni powders during heating and melting of the Cu phase. This interdiffusion leads to isothermal solidification of the Cu rich liquid and full TLPS behaviour. For Cu with a high oxygen content, a displacement reaction at the Cu/Ni interface occurs. Under certain conditions, this reaction leads to the formation of a continuous NiO layer over the Ni powder surface. This prevents Cu/Ni interdiffusion and wetting of the Ni particles by the molten Cu, thus eliminating the possibility for a TLPS process. Analysis of the results, indicate that the extent of NiO layer formation depends on oxygen content of the Cu, the composition of the powder mixture and the Ni particle size.

Studies are performed to enhance low temperature sintering of Ti–6Al–4V. High energy ball milling is found to be effective in lowering the sintering temperature through the mechanisms of particle size reduction and nanograin formation. The former reduces the diffusion distance for densification, whereas the latter introduces an additional densification mechanism allowing mass transport from the interior of the particle to the neck zone. Together, these two effects can reduce the onset temperature for densification by about 300°C. Spark plasma sintering can further improve low temperature sintering when compared with radiant heat sintering and microwave sintering. The enhanced densification is discussed on the basis of the applied pressure (50 MPa) and the intrinsic joule effect that leads to increase in the local temperature at the contact point between particles.

Magnetic properties of Fe based composite materials with different particle sizes under a cryogenic condition have been investigated. Realisation of this venture has been carried out at the liquid nitrogen temperature. Results of energy loss density were obtained from measurements of the static (dc) hysteresis cycles ranging from 0·1 to 1·0 T. In turn, results of power loss density were obtained from measurements of the dynamic (ac) hysteresis cycles ranging from 50 to 1000 Hz and at the maximum flux density of 0·5, 0·8 and 1·0 T. The study confirmed the influence of temperature on magnetic parameters. It has been shown that total power loss density has increased with decreasing temperature. We report changes in a nature of energy loss after immersing specimens made of soft magnetic composites in liquid nitrogen. Measurements of the maximum relative permeability were also conducted.