Powder Metallurgy

Metal injection moulding was applied to fabricate Ti–22Nb alloy as a low modulus material for biomedical applications. Tensile test specimens were injection moulded, followed by debinding and sintering. Sintering was at 1500°C for 4 h under vacuum (10^–3 Pa). Selected as-sintered Ti–22Nb samples were hot isostatically pressed at 915°C/100 MPa for 2 h. The nature of the titanium carbide precipitates in the as-sintered Ti–22Nb alloy was investigated. Selected area electron diffraction patterns revealed that the carbides are Ti₂C with a fcc structure. The calculation of the phase diagram showed a significant decrease of carbon solubility in Ti–22Nb compared with that in Ti from 500 to 1500 °C, contributing to the carbide precipitation in Ti–22Nb. Due to the carbide precipitation, the as-hipped Ti–22Nb alloy exhibited higher tensile strength but lower elongation than conventionally processed Ti–22Nb.

Metal injection moulding (MIM) is an established technique for serial production of complex, mostly stainless steel parts. However, for other materials, especially for titanium parts, there is still the need for superior purity and enhanced surface quality. This paper describes recent advances at Fraunhofer IFAM to address the challenge of producing fit-for-purpose titanium MIM medical parts. Investigations have shown the effect of using very fine powders and mould surface finish: with polished moulds a very low surface roughness, <0·8 µm, was obtainable on the sintered part. Chemical and mechanical properties of the sintered parts are determined by complex interactions between sintering conditions, purity, particle size and binder components. Tests on the feasibility of moulding high aspect ratios, wall thicknesses <200 µm and special features such as a fine internal threads and hexagon headset structures are also reported.

Scaffolds for bone tissue engineering are highly porous materials having interconnected and homogeneously distributed pores to facilitate the formation of new bone tissue. At the same time, appropriate mechanical strength is required in the scaffolds to withstand stresses in the in vivo environment. The space holder method has been used to fulfil these contradictory requirements in the fabrication of titanium scaffolds. Space holding particles are mixed with titanium particles then removed before or during sintering, to leave pores in the scaffolds. Despite its importance, the removal of space holders has rarely been studied. In the present study, removal by water leaching was investigated. Leaching was characterised using a novel real-time measurement technique adopted from ASTM B963-08 that achieved precise scaffold weight loss data reflecting the removal of space holding particles. The acquired data fit existing solvent debinding models for powder injection moulded parts, allowing the mechanism involved during water leaching to be determined.

The effect of additions of transitional refractory metals on the structure and properties of Al–Zn–Mg alloys, made by ingot and PM routes, was investigated. The strength of the ingot alloys especially is increased by scandium and zirconium. The modifying action of scandium inhibits recrystallisation and precipitation of the fine-grained coherent Al₃(Sc_1–xZr_x) phase. The effect is weaker in PM alloys where the ultra-high cooling rate during high pressure water atomisation produces the fine-grained structure. PM semi-products of the base composition Al–5Zn–3Mg and alloys without scandium are not recrystallised during heating to 500°C, whereas cast alloys of similar composition recrystallised on the hot extrusion stage at 400–450°C. Of the Sc alloys, Al–5Zn–3Mg–0·5Mn–0·7Zr–0·3Sc showed the highest strength (UTS = 651 MPa, YS = 596 MPa), whereas of the PM alloys without scandium Al–5Zn–3Mg–0·85Zr–0·22Cr–0·17Ni–0·15Ti alloy showed UTS = 618 MPa and YS = 553 MPa. At melt cooling rates of 10⁵–10⁶ K s^–1 the total content of transitional refractory metals must not exceed 1·5–1·7 wt-% and total content (Zn+Mg) should be <8 wt-% at a Zn/Mg ratio of 5:3.

Digital radiography is a promising non-destructive testing tool for powder metallurgy (PM) parts, in which transmitted X-rays are recorded to generate data for an advanced defect detection system. An important part of this system is the data processing platform for pattern recognition in X-ray images. Combinations of advanced techniques for noise reduction, contrast enhancement and image segmentation are employed. Algorithms of registration for images in regions of interest are discussed, e.g. the scale invariant feature transform (SIFT). Modern pattern recognition methodologies such as smoothing, moment representation, image alignment and optical flow towards feature classification are evaluated. The proposed defect detection and classification capability for automatic analysis of digital radiographic images from PM parts potentially allows integration into multiple-view inspection systems, which should enhance quality control in the PM manufacturing and production environment. Defect detection systems able to work at the speed of current production lines are of great interest to both PM manufacturers and users.

Electrochemical machining (ECM) is a complex technology used to shape conductive materials, for example hard metals. The workpiece material is removed by anodic dissolution in aqueous electrolytes at extremely large current densities. Hard metals are non-homogeneous and consist of hard particles, e.g. carbides, embedded in a softer matrix formed by metals such as cobalt, nickel, iron or alloys. The final shape is determined by a custom made tool, the cathode, or by a new technique with a confined electrolyte jet (Jet-ECM). An optimisation of this process intents to ascertain detailed information about the influence of current and potential distribution, composition of electrolyte, electrolyte flow geometry, electrochemical behaviour and dissolution mechanism of the components and structure of the interface material/electrolyte. Experiments on WC6Co, a common hard metal consisting of WC particles with Co as a binder, are presented as an example.

This paper aims to give an insight into the reliability of fatigue life assessment results for powder metallurgy (PM) components subjected to variable amplitude loading. In this study, notched specimens of two PM steel and two PM aluminium alloys are examined. As the experimental basis, constant amplitude tests with different stress amplitudes and stress ratios (to determine the S–N curves and mean stress sensitivities) as well as service load tests (variable amplitude loading) were carried out. The fatigue life was calculated, applying different modifications of the linear damage accumulation. The results of the experiments and calculations were compared and statistically evaluated. Regarding the resulting scatter, the fatigue life assessment with the modified damage accumulation by Zenner and Liu is recommended. Furthermore, mean relative damage sums are derived, which can be used to correct the fatigue life calculation. For PM steel, these values are very similar to compact steels, whereas they are higher for PM aluminium.

Fe₈₂Si₂B₁₄C₂ amorphous powder cores with low core loss and relatively high magnetic flux density were fabricated by cold pressing, and their superior magnetic properties were discussed by comparing with traditional Fe₇₈Si₉B₁₃ amorphous powder cores. At f = 100 kHz and B_m = 0·1 T, the Fe₈₂Si₂B₁₄C₂ cores exhibited core loss ∼449 kW m⁻³, a value comparable to that of traditional Fe₇₈Si₉B₁₃ ones. The permeability roughly kept constant, giving a value of ∼70, at frequency up to 1 MHz. Moreover, superior characteristics of permeability under dc bias field were also attained. The values of Fe₈₂Si₂B₁₄C₂ cores were ∼10% higher than those of Fe₇₈Si₉B₁₃ at H_dc = 100 Oe because of the higher magnetic flux density of ∼1·64 T at 800 A m⁻¹ (B₈₀₀) for the Fe₈₂Si₂B₁₄C₂ alloy. Consequently, the above superior magnetic properties of Fe₈₂Si₂B₁₄C₂ amorphous powder cores indicate they could be potentially used to deal with the high output current in electrical power supplies.

In this study, the modified preparation method of combining planetary and vibratory ball milling was proposed to prepare Mg based hydrogen storage alloy powders. The comparison of micromorphology and hydrogen storage behaviour between Mg₂Ni prepared using the modified and conventional preparation methods were investigated experimentally. The comparison results showed that the combination of first planetary and then vibratory ball milling has more favourable effect on improving both the kinetics and the thermodynamics of ball milled Mg₂Ni alloys. The sample synthesised by first planetary milling for 40 h and then vibratory milling for 30 h has faster hydrogen absorption kinetics and lower dehydriding onset temperature than those prepared by the single method of planetary or vibratory milling and hydriding combustion synthesis owing to its popcorn-like microstructure. Moreover, this kind of modified method reduces the reaction enthalpy and activation energy by up to ∼18 and 22% respectively.

Induction heating is an attractive technique to sinter metal powders in a short time and with limited energy. A series of direct induction sintering experiments has been performed with a micron size nickel powder in a dedicated set-up with 50 or 150 kHz current frequency and several heating rates, up to 900°C min⁻¹. With a view to better catching the specific outcome of induction sintering, conventional sintering tests have also been achieved and their results in terms of densification have been depicted by adjusting a Master Sintering Curve model. The main conclusion of this study is that nickel specimens with high density, reasonably low grain size and homogeneous microstructure can be obtained by direct induction sintering with processing times much smaller than typical conventional sintering times. The obtained data also show that powder densification is accelerated during induction sintering, which is supposedly due to the enhancement of diffusion under electric current.

Dilatometry experiments have been carried out to investigate the shrinkage kinetics on cold isostatic pressed iron specimens in the 550–730°C temperature range, showing that dimensional contraction is much higher than that predictable on the basis of the shrinkage kinetics models, which neglect the effect of the prior cold compaction. The greater shrinkage is due to an enhanced diffusivity which may be attributed to the large density of structural defects accumulated in the powder particles during compaction (structural activity). A time depending effective lattice diffusion coefficient was determined, with an Arrhenius type dependence on temperature.

Impulse atomisation in helium and nitrogen and water atomisation have been utilised to produce powders of D2 tool steel. It was determined that higher cooling rates result in a lower percentage of eutectic. Scanning electron microscopy image analysis, along with coarsening model, was used to predict eutectic and primary phase undercooling of particles. Small particles exhibited a higher amount of undercooling. The particles exposed to a He atmosphere during atomisation had a larger amount of eutectic undercooling. The fraction of primary phase that solidified during the recalescence was then calculated based on the amount of primary phase undercooling under adiabatic conditions. In smaller particles, there was a larger amount of primary phase solidified during recalescence due to a higher amount of primary undercooling. Based on primary phase undercooling values, critical nuclei radius of austenite and assuming homogenous nucleation, the number of austenite unit cells in the stable nucleus was calculated.