Powder Metallurgy

The Particulate Engineering Committee (PEC) of IOM3 held its long anticipated workshop on hot isostatic pressing (HIP) of materials for offshore (energy) applications on 16 April in Aberdeen, UK. Hosted by TWI and supported by EPMA, the one day meeting attracted about 30 delegates that filled comfortably the premises to its capacity. It was an opportunity for end users from the oil and gas sector and those involved in powders, consumables, engineering services and other end user communities to come together to capture the latest developments in the field of PM-HIP for offshore and energy applications.

The fibre space holder (FSH) method combines powder metallurgy and lost-foam casting with the aim of producing novel ultra-thin materials with micro-porous structures. A fabric, used as a spaceholder, is coated with metal powder (stainless steel, copper, nickel or titanium) in a water-soluble polymer binder. During sintering, the spaceholder is removed by a high temperature treatment to obtain the final porous metal sheet. This new material combines the advantages of fabric and porous metal. Its large specific surface area, minimum thickness of <30 μm, maximum porosity of >95% and strong liquid absorbency make it suitable for various application, in particular electrodes in hydrogen fuel cells and medical applications. The metal sheet can be further functionalised by coating with nanopowders.

As part of a project to develop a small batch metal atomisation system, based on free fall atomisation and close-coupled atomisation, the influence of the process parameters on various powder quality features has been investigated, using copper–tin alloys as feedstock material. Particle size distribution, appearance of satellite particles, particle circularity and the flowability were recorded as criteria of particle quality. From the data obtained in these experiments, the effects of the main process parameters (atomisation pressure, mass melt flow, and height of the spray chamber as well as the atomiser system) have been evaluated with respect to powder quality features. The atomisation system was optimised to produce high quality powder with narrow particle distributions (d_84·3/d_50·3 = 1·6 for free fall) and high circularity with mass melt flows in the range of 100 kg h⁻¹ using melt volume between 100 and 1000 mL.

The most important use of fine spherical iron powders is for metal injection moulding (MIM). For many applications, the high costs of powder based on the carbonyl or atomising production route are a limiting factors. An alternative two-step hydrogen reduction process using a granulated hematite powder, which is a recycling product from steelmaking, has been developed to produce <25 µm spherical powder. The morphology and properties of the powder have been found to depend strongly on the second temperature step of the reduction process. A further important step is enclosed powder processing by milling and sieving to remove agglomerates. The powder properties and sintering behaviour as a function of heat treatment and processing parameters are reported and discussed.

Ti–6Al–4V and stainless steel 316L have been processed by selective laser melting under similar conditions, and their microstructures and mechanical behaviours have been compared in details. Under the investigated conditions, Ti–6Al–4V exhibits a more complex behaviour than stainless steel 316L with respect to the occurrence of microstructural and mechanical anisotropy. Moreover, Ti–6Al–4V appears more sensitive to the build-up of internal stresses when compared with stainless steel 316L, whereas stainless steel 316L appears more prone to the formation of ‘lack of melting’ defects. This correlates nicely with the difference in thermal conductivity between the two materials. Thermal conductivity was shown to increase strongly with increasing temperature and the thermophysical properties appeared to be influenced by variations in the initial metallurgical state.

Additive manufacturing is a novel way of processing metallic cellular structures from a powder bed. However, differences in geometry have been observed between the CAD and the produced structures. Struts geometry has been analysed using X-ray microtomography. From the 3D images, a criterion of ‘mechanically efficient volume’ is defined for stiffness prediction. The variation of this criterion with process parameters, strut size and orientation has been studied. The effective stiffness of struts is computed by finite element analysis on the images obtained by X-ray tomography. Comparison between the predicted stiffness and the effective one tends to show that the efficient volume ratio leads to a slight underestimation of the stiffness. Finally, the effective stiffness is used at the scale of a unit cell. This can help define the build orientation and loading direction that lead to the highest stiffness.

In recent years, many efforts have been made to obtain more environmentally acceptable powder injection moulding processes. In this sense, the purpose of this study is to optimise an eco-binder based on polyethylene glycol (PEG) as a water soluble component and cellulose acetate butyrate (CAB) as a natural backbone polymer derived from cellulose for powder injection moulding of zirconium silicate powders until a solvent debinding stage. Four different feedstocks have been investigated. As well as, a volume fraction of PEG and CAB 70/30 (vol.-%) and a solid loading of 57·5 (vol.-%) were maintained, molecular weights of polymers were combined in order to minimize distortion during binder solvent extraction. Water solvent debinding was carried out at three temperatures stepwise during 5 h. As a result, efficient removal of the PEG as well as free defects samples were obtained after solvent debinding for binder systems based on low molecular weight of PEG.

Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.

Ordered B2-NiAl intermetallic compound powder was successfully synthesised by mechanical alloying after 20 h in an attritor mill, starting from elemental Ni and Al powders and without subsequent heat treatment. NiAl powder obtained was homogenous and had a nanocrystalline microstructure. It was consolidated by field assisted hot pressing (FAHP), in a novel configuration with a Gleeble 3800 thermomechanical simulator. The powder was also processed by hot isostatic pressing (HIP) in order to compare both methods. The consolidation was successful by both methods obtaining above 98% of NiAl theoretical density (5·86 g cm⁻³). The results showed that the consolidation process by FAHP technique is effective and uniform throughout the sample as indicated by homogenous hardness values, obtaining microstructure and properties similar to those obtained with HIP technique, with certain advantages over it. The achieved room temperature yield strength of 850 MPa and fracture strain 26–28% corresponds to the bulk values of NiAl intermetallic.

Intermetallic as well as (carbides and nitrides) interstitial compounds present functional and structural properties which make these materials necessary for advanced technologies. Meanwhile fabrication routes based on melting and casting or plastic deformation, are usually far to compete with powder metallurgy processes likely to provide near net shaped parts and components. The present study is devoted to a model system exemplified by nitrided iron and steel powders for which the thermal stability of nitrides is severely decreasing when temperature exceeds critical values during densification treatments. Thanks to the analysis of the thermal treatment induced transformations mainly characterised by Mössbauer spectroscopy, the consequences and impact on driving forces for densification under critical thermal conditions are discussed in order to achieve an optimised sintering process and an actual development.