Powder Metallurgy

A novel plasma spraying method, direct write thermal spray (DWTS), is demonstrated for strain gauge manufacturing. Sprayed and laser finished strain gauges were subjected to cyclic loading and performance was compared with that of commercial strain gauges. The main differences observed are in sensitivity and zero point drift and it is suggested that these differences can be significantly reduced. Thus, it is proposed that DWTS is a potential novel method for strain gauges integration within strained structures for monitoring purposes.

The increasing implementation of sintered steel parts for highly loaded engine and gear box applications is accompanied by the need for reliable fatigue design, especially in the pre-development phase. In most cases necessary characteristic values for the fatigue design of PM components, especially for sharply notched areas, are not available. In the present study, the locally endurable stress amplitude for different loading modes and mean stresses is derived for a diffusion alloyed 4%Ni sintered steel (Distaloy AE) via the highly stressed volume approach. The correlation of the locally endurable stress amplitude with the highly stressed volume, defined as V_x, is estimated for x = 80% and 90%, to evaluate both approaches with regard to PM steels. The applicability of assessing local cyclic behaviour by the highly stressed volume approach is demonstrated for a typical design detail of a highly loaded PM part. Characteristic values for the fatigue design of PM components are recommended.

It would be useful to be able to produce brass and bronze components made from prealloyed powders by supersolidus liquid phase sintering. The microstructures obtained in such alloys are sensitive to constituent alloying elements and small change in sintering temperature. Although the formation of liquid during sintering is potentially attractive for densification, the effects of gravity on the liquid phase can result in graded densification. Evaporation of alloying elements and their solubility in the base metal also affect the extent to which heterogeneous cross-sections are obtained. The aim of the present study was to examine the effect of alloying and sintering temperature on the mode of particle rearrangement, and consequently on graded densification, by microstructural and fractographic analysis. Comparing the fracture morphology from top to bottom of the fracture surface is also helpful in developing a model to describe the phenomena during sintering of similar alloys.

Previous studies have demonstrated that it is possible to compensate for the absence of Co in a W–Ni–Fe liquid sintered composite alloy under industrial conditions by appropriate control of the process parameters, to achieve mechanical properties close to those of Co containing alloys. The present investigation offers an optimised description of this Co-free system, using phase analysis following long term vacuum heat treatment in a laboratory furnace, with the aim of identifying potentially brittle phases. This optimised system has been evaluated in tests with a small industrial kinetic penetrator (APFSDST CT40) against a semi-infinite target and inclined armour plate. Results show no degradation of the ballistic performance relative to conventional Co containing alloys.

The low alloy steel powder Distaloy † , is today widely used in applications demanding high strength and wear resistance. Its basic properties and composition were designed half a century ago in the USA. The advantage lay in the fact that it was a partial prealloy, i.e. the alloying elements – copper, nickel and molybdenum – were bonded in particulate form to the basic iron particles, thus avoiding impairment of the compressibility. By balancing the contents of nickel and copper it was possible to minimize dimensional change on sintering. Bonding the alloy particles to the iron particles minimised segregation and also contributed to dimensional stability. Carbon was added conventionally as fine graphite. However, the new powder, marketed as Ancoloy, did not take off in North America, due to the lack of suitable applications, the cost of the alloying elements and – above all – the poor compressibility and high oxygen content of the iron powder then available. The high, variable oxygen content made it impossible to control the carbon content with the precision necessary to achieve the desired strength and hardness.

In the 1960s, demand for high strength precision parts emerged in the European car industry, initially at Citroen, which pioneered increased use of PM parts in European cars. The component was (and still is) the synchronising hub used in manual transmissions. Höganäs had in the mid-1960s developed a sponge iron powder with much higher compressibility, and this was taken as a raw material for an improved grade, later to be called Distaloy SA. This new powder had improved compressibility and very low oxygen and carbon contents, which made it possible to make the high strength precision parts that the car industry required. Distaloy was immediately accepted and used, first in the French car industry, then elsewhere in Europe and subsequently also in Japan. Some years later, when high compressibility atomised powder became available, the same basic technique was applied to these, to produce the Distaloy A grades, which now are most popular. Höganäs continues to improve and refine the production techniques and to come up with compositions for new applications.

The properties and the metallurgy of Distaloy-based materials have been thoroughly studied by metallurgists at Höganäs and at PM laboratories throughout the world, and new results are still being reported with respect to both applications and fundamental properties. A parameter of great relevance is of course the cost of raw materials and much effort is going into finding more cost effective ways of achieving the desired results.

The sintering behaviour of prealloyed powder compacts has been studied as a function of the sintering atmosphere in free sintering experiments. Atmospheres with different hydrogen/nitrogen ratios and even vacuum have been used in the sintering cycles. Powder compacts with and without diamond additions have been sintered. Three different grades of diamond were used in the experiments, all of them synthetic manmade diamond. Two had different levels of metallic inclusions and one was coated with Ti. The interaction between bond/atmosphere/diamond has been characterised analysing the density, microstructure, bend strength and degradation of the diamonds after dissolving the matrix. Diamonds from atmospheres with low hydrogen content show evidence of strong degradation. Moreover, any diamond additions strongly decrease the strength of the bonds, acting as defects. The strength is also affected by the sintering atmosphere and sintering temperature but not significantly by the type of diamond.

Rolls-Royce has successfully established a capability for the hot isostatic pressing (HIP) of austenitic stainless steels in nuclear plant. The focus has now been extended to HIP of nickel based alloys, whose good material properties make their use in plant increasingly attractive. A development programme has been undertaken, comparing the mechanical property behaviour of HIP Alloy 600, 690 and 625 with their wrought counterparts. The results are promising, showing acceptable tensile properties and Charpy impact results for the HIP test pieces that are comparable to, or better than, the wrought data. It is concluded that HIP offers an attractive alternative manufacturing route that is ideal for the production of a small number of high integrity components. Hot isostatic pressing microstructures have the advantage of being isotropic and equiaxed, with uniformly fine grain sizes; properties not normally found in thick-section forgings.

In the past few decades, stationary solid oxide fuel cell (SOFC) systems have been developed that can generate electricity and heat from the energy stored in hydrogen or hydrocarbons with total efficiencies up to 95%. While the mechanical cell support of stationary systems is commonly supplied by thick ceramic cell components (i.e. anode and electrolyte supported concepts), mobile systems demand a more robust design. This is ensured by a strong yet porous metallic substrate which serves as the mechanical backbone of thin film membrane electrode assemblies [metal supported cell (MSC) concept]. Porous PM Fe–Cr oxide dispersion strengthened alloys for use as MSC supports have recently been developed. These materials provide mechanical and chemical long term stability in typical SOFC atmospheres at operation temperatures up to 850°C. The substrates support a multilayer anode–electrolyte–cathode thin film assembly, constituting a high performance MSC repeat unit. These units are the building blocks for MSC stacks with superior properties for mobile applications.

The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets of high and low porosity specimens calibrated the model. Strain–life behaviour depicted that above the plastic strain limit of 0·002 mm mm⁻¹ in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain–life curves.

Silver based composites with varying concentration of graphite and/or MoS₂ were prepared by powder metallurgy method. Impacts of composition on the tribological performance of the composites in ambient air and vacuum were investigated. The lowest friction in air was achieved by Ag–20G (vol.-%) composite, while Ag–20MoS₂ exhibited the best lubricity in vacuum. XPS evaluation revealed the oxidation of MoS₂ in air and a decrease concentration of graphite on the surface of the wear tracks under vacuum. As the proportion of graphite to MoS₂ increased, the friction coefficient and the wear rates ascended gradually in air while decreased sharply under vacuum. As compared with other compositions, Ag–15MoS₂–5G exhibited a comparable stable and good tribological performance as the environmental condition changed for its friction coefficient and wear rate remained around 0·14 and 5×10⁻⁶ mm³ N⁻¹ m⁻¹.

Mg alloys are characterised by several promising properties, including a good biocompatibility. In this work, a commercial AZ91 powder was used to produce cylindrical specimens by spark plasma sintering (SPS), and the specimens were further consolidated by hot extrusion. The SPS materials were found to be quite brittle because of the low bonding between the original powders. After hot extrusion, however, they displayed a ductile behaviour as revealed by room temperature tensile tests and hot compression tests. The metallographic investigation showed that extrusion induced a dynamic recrystallisation with grain refinement, but also an increase in the β phase content, which reduced the corrosion resistance of the materials.

Tungsten nitrides were synthesised from NiO–WO₃ and NiWO₄ precursors at 973–1273 K in a flow of H₂–N₂ gas mixture. The reduction–nitridation reactions were carried out isothermally in fluidised bed reactor, and the off-gas from the reactions was continuously analysed by gas chromatography. The effect of reaction temperature and precursor composition on the rate of formation of Ni–W nitrides was studied. The different phases developed during the reduction–nitridation reactions were identified by X-ray diffraction analysis technique. The morphology and the grain structure of the precursors were examined by SEM, and the elemental composition in the structure was analysed by electron dispersive spectrometry. The results showed that the reduction of Ni–W–O precursors proceeded in a stepwise manner (NiWO₄→Ni–WO₃→Ni–WO₂→Ni–W). Tungsten nitrides (WN and WN₂) were formed from the reaction of the freshly reduced W metal with N₂ gas and WN was the predominant phase detected at higher temperatures. The reaction mechanisms were elucidated from the apparent activation energy values and the application of different formulations derived from the gas–solid reaction model at early and later stages of reactions. It was concluded that the interfacial chemical reaction is the rate determining step at initial stages, while a combined effect of gaseous diffusion and interfacial chemical reaction controlled the reaction at later stages. At final stages, the nitridation reactions contributed to the reaction mechanism leading to produce tungsten nitrides.

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m⁻²) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.