Powder Metallurgy

The precipitation mechanism of small hafnium carbides in the sintered and thermo-mechanically processed molybdenum based alloy MHC (Mo–0·65Hf– 0·65C (at.-%)) is reported. Light and scanning electron microscopy revealed hafnium oxides, large hafnium carbides and molybdenum carbide layers at the grain boundaries in the as-sintered material. Additionally, atom probe tomography showed a residual dissolved content of 0·12 at.-%Hf, but no carbon in solid solution. After thermo-mechanical processing of the as-sintered material in a deformation dilatometer, transmission electron microscopy revealed small hafnium carbides with diameters of 10–100 nm. These carbides were preferentially located at dislocations and dislocation networks. Without deformation prior to aging, no formation of small hafnium carbides was observed. X-ray diffraction confirmed that decomposition of molybdenum carbides had occurred, which delivered the carbon for the formation of strain induced precipitates.

Current production and consumption of copper and copper alloy powders in China is reviewed. China is now the largest producer of copper powder globally and many of the technological gaps with other producers have been closed or eliminated. Priorities for further development are proposed.

β alloys based on the Ti–Nb alloy system are of growing interest to the biomaterial community. The addition of small amounts of Fe and Cr further increases β-phase stability, improving the properties of Ti–Nb alloy. However, PM materials sintered from elemental powders are inhomogeneous due to restricted solid state diffusion and mechanical alloying provides a route to enhance mixing and elemental diffusion. The microstructural characteristics and bend strength of Ti–Nb–(Fe–Cr) alloys obtained from elemental powder mixture and mechanical alloyed powders are compared. Mechanical alloying gives more homogeneous compositions and particle morphology, characterised by rounded, significantly enlarged particles. In the sintered samples α and β phase are observed. The α phase appears at the grain boundaries and in lamellae growing inward from the edge, and is depleted in Nb. The β phase is enriched with Nb, Fe and Cr. The addition of Fe and Cr significantly increases the mechanical properties of Ti–Nb alloys, providing increased ductility.

Ti–Nb alloys are attractive as biomaterials because of their excellent combination of low elastic modulus, high strength, corrosion resistance and enhanced biocompatibility. The effect of a post-sintering quenching treatment on the microstructure and mechanical properties of powder injection moulded Ti–17Nb has been investigated. Tensile test specimens were produced using a feedstock based on blended elemental powders, some of which were solution-treated in the β phase field and water quenched. Both as-sintered and quenched alloys had densities 95·5% of theoretical. The as-sintered material showed an α−β structure, whereas after quenching a fully martensitic α″ structure was obtained. The modulus of elasticity of the water-quenched alloy (∼45 GPa) was about 40% lower than that of the as-sintered alloy (∼76 GPa).

The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterisation. Experimental results point out a strong strength decrease associated with localised corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.

Currently, commercial biodegradable implants are mainly made from degradable polymers, such as polyglycolic acid or polylactide acid (PLA). These polymer implants, produced by injection moulding technique, suffer from long degradation times between 18 and 36 months, poor mechanical properties and acidic degradation behaviour. On the other hand, magnesium alloys are drawing increasing interest as biodegradable medical implant material for orthopaedic applications in bone tissue; thus, a replacement of polymers by Mg would be attractive. The production of biomedical and biodegradable Mg alloy parts and implants by powder metallurgy and metal injection moulding (MIM) respectively offers the opportunity for economic manufacturing of parts with mechanical properties matching those of cortical bone tissue, as well as the provision of porous surface structures beneficial for cell ingrowth and vascularisation. Furthermore, the technique guarantees a homogenous microstructure being crucial for a predictable degradation process. This study shows how magnesium powder can be processed successfully by MIM. Recent magnesium alloy implant prototypes and tensile test specimen, produced by MIM technique, provide strength and stiffness twice as high compared to modern polymer based implants. Ultimate tensile strength (UTS) of 131 MPa, yield strength of 64 MPa, residual porosity of 2–6% and elastic modulus of 46 GPa, measured by dynamic method, were achieved under application of special sintering technique and sintering atmosphere control. The paper is focussing on sintering methods and porosity control and measurement.

There are many applications where compound materials can be of interest, for example when different properties are needed in different parts of a component. Compound materials can be produced by hot isostatic pressing (HIP) of powder metallurgical materials. One aspect that should be considered in the design is the quality of the interface between the two different material compositions. Diffusion during HIP can cause formation of brittle phases in the interface or deteriorate properties by diffusion of alloying elements. The present work shows results from a study where different steel types were joined (quench and temper steel/air hardening steel/bearing steel with a tool steel/corrosion resistant martensitic steel). The evaluation was performed by computational predictions and by small scale HIP experiments that were evaluated by microstructure analysis and chemical analysis.

Materials that combine metallic and ceramic properties are of interest for various applications, such as surgical instruments or grinding tools. Powder technology offers the possibility to mix metals and ceramics in form of their powders and process them to complex shapes by means of powder injection moulding. Thus, different material combinations were chosen in order to demonstrate a broad applicability of this approach. The produced composite materials were characterised regarding their densities, mechanical properties and microstructures. It could be observed that the powder injection moulded samples showed rather typical densities for specimens produced by this process, up to 98·3% relative density. The mechanical properties varied strongly mainly dependent upon the materials used in the composites. Thus, the range of mechanical properties is wide and leads to various possibilities to adjust certain properties to a desired level. In combination with the possibilities of the powder injection moulding process to produce near net shape parts cost-efficiently in large quantities, these versatile composite materials can be made accessible for various applications.

Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure–property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels, all obtained from commercially available powders but presenting different microstructures, are examined: a ferritic–pearlitic Fe–C steel, a bainitic prealloyed Fe–Mo–C steel and a diffusion alloyed Fe–Ni–Cu–Mo–C steel, with more heterogeneous microstructure (ferrite, pearlite, upper and lower bainite, martensite and Ni rich austenite).

Radio frequency identification (RFID) applications are becoming more and more important in logistics, especially for the purpose of tracking and managing items. Real time tracking of medical instruments during a surgical operation belongs to the state-of-the-art applications of this technology. The metallic environment in the operating room represents a big challenge, hampering the readability of the RFID tag. Currently, transponders are joined to metal instruments by bonding or clamping them inside a polymer shell, containing as little metal as possible. Assembling the tags is a time consuming process, which changes the usual shape of the instrument. It also represents a point of failure because of the possibility of losing the tag during the instrument's life cycle. Selective laser melting (SLM) offers a way to integrate the entire RFID tag in a metallic instrument while keeping the original shape. Instrument manufacturing by SLM allows small wall thicknesses and a reduced temperature impact on the RFID tag during the building process. The production of trial components from IN718 nickel alloy is reported.

The effective use of alloying elements in powder metallurgical steels requires a deep understanding of their redistribution kinetics during sintering. In this work, interrupted sintering trials of Fe–2Cu and Fe–2Cu–0·5C compacts were performed. Moreover, diffusion simulations of Cu in γ-Fe using Dictra were performed. It is found that transient liquid phase penetrates the Fe interparticle and grain boundaries in less than 3 min of holding time. However, C addition limits the penetration of liquid Cu, particularly into grain boundaries of large Fe particles. The results also show that the mean diffusion distance of Cu in γ-Fe in the C added system is slightly lower than that in the C-free system at 3 min of holding time; however, after 33 min, the mean diffusion distance is similar in both systems. The diffusion distances of Cu in γ-Fe, predicted by Dictra, are in good agreement with the measured values.

In this study, nickel alloy powders were consolidated by spark plasma sintering. Experiments were performed between 700 and 750°C temperature range under 50 MPa pressure with holding times from 5 to 10 min. In addition to these main spark plasma sintering parameters three different heating rates ranging from 100 to 235°C min⁻¹ and two different particle size ranges (75–106 μm narrow size distribution and −45 μm wide size distribution) were used for the experiments. After sintering, the sliding wear behaviour of the samples was investigated. The results revealed that the density of the material increased with raising the sintering temperature and holding time. However, heating rate and particle size also played an important role in the densification and these parameters were investigated in detail.

Porous Ni₃Al–Mo electrodes were obtained by reactive synthesis methods with Mo contents of 2, 5, 10, 15 and 20 wt-% respectively. The microstructure, surface morphology and composition of the prepared porous Ni₃Al–Mo electrodes were analysed by X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). Hydrogen evolution reaction (HER) on the porous Ni₃Al–Mo electrodes in 6 mol L⁻¹ KOH solution was investigated by polarisation measurements and electrochemical impedance spectroscopy (EIS) technique. It was shown that the hydrogen evolution activity of the porous Ni₃Al–Mo electrodes was related with the Mo content. The porous Ni₃Al–Mo electrode with optimum catalytic activity for the HER was found to contain the maximum Mo content of 10 wt-%.

Sizing is studied in this work as a post-sintering operation aimed at improving the dimensional and geometrical precision of sintered parts. The required dimensional and geometrical characteristics are obtained by the plastic deformation due to sizing, which is related to the applied stress. In this work, the relationships between applied force, resulting deformation, attainable geometrical characteristics have been investigated. By means of the data recorded by a hydraulic press, force–displacement curves have been derived. The analysis of these curves allowed identifying the elastic deformation of part and tool, as well as the plastic deformation of the part. The plastic deformation has been correlated to the actual dimensional changes measured on the part, as well as to the change in the required geometrical characteristic (conicity). On the basis of these relationships, a design procedure to optimise the sizing strategy has been proposed.