Powder Metallurgy

Process modelling, based on finite difference methods, is used to show that the thermal conductivity increases, which typically attend the martensite transformation in steel, affect the cooling rate in the Jominy end quench test. A one-dimensional model, which includes the effects of material property variations, is presented that predicts slightly increased cooling rates with increases in the M _s temperature for fully dense steels and significantly increased rates for powder metallurgy (PM) steels. The model is based on earlier studies of the end quench test that initially showed increased cooling rates in PM steels versus fully dense ones and then went on to show water penetration of the pores as a causative mechanism. In the present study, it is shown that by combining a simple theory of this mechanism with the aforementioned M _s effects, it is possible to obtain cooling curves that display a marked resemblance to the experimentally observed ones of these earlier studies.

The grey iron microstructure Fe–2C–2Si powder based compact is tailored by different kinds of in situ and post sintering processing. This has been achieved by combining thermodynamic and kinetics modelling of microstructure development with sintering and controlled heat treatment experiments of tensile test specimens die compacted at 600 MPa. Applying optimised sintering conditions led to a grey iron like microstructure with 95% relative sintered density. Sinter hardening the compacts led to 500 MPa in yield strength and 600 MPa in ultimate tensile strength in combination with ductile fracture. Quenched and tempered condition showed the same strength values, but combined with brittle fracture due to martensitic structure. Pore rounding and partial pore filling by graphite were obtained by austenising isothermal hold during the cooling of the sintering cycle.

In the present work the tensile and axial fatigue behaviour of sintered hardened Fe–1·5Mo–2Cu–0·5C at three density levels (6·8, 7·0 and 7·2 g cm^–3) have been studied. The materials were tested under the as sintered condition, and after tempering at 180 and at 240°C. The results show that steels under the as sintered condition posses a high hardness but a brittle tensile deformation and fracture behaviour. Tempering at 180 and 250°C induces the disappearance of brittleness and tensile fracture is thus ductile although very localised at the necks. Fatigue strength is determined by the resistance of the materials to the internal damage evolution due to the nucleation of small cracks at the pores edges, and their coalescence into a long crack. Tempering induces an increase in the fatigue resistance. The greatest fatigue strength at 2 × 10⁶ cycles is displayed by the steel with a density of 7·2 g cm^–3 and tempered at 180°C.

The objective of this work was to investigate the effects of hot and cold swaging on the density and mechanical properties of a commercial Al–Zn–Mg–Cu powder metallurgy alloy known as Alumix 431D. To do so, as sintered samples of the PM alloy were swaged under a variety of conditions and characterised. For comparison purposes, equivalent characterisation tests were completed on the chemically similar wrought alloy 7075-T6. Cold swaging was moderately successful provided the as sintered billets were annealed or solutionised before densification. Here, modest improvements in density and tensile properties were noted. Hot swaging proved to be a more effective approach. Optimal properties were achieved when samples were preheated to 470 ± 10°C. When processed in this manner, a density of 99·6% of theoretical was realised while the tensile and fatigue properties exceeded those of the wrought 7075-T6 alloy tested for comparison purposes.

Few Ti alloys have been designed for ease of sintering. This paper considers the design of alloys for processing using the mixed elemental technique, in which powders are mixed, cold pressed in a die to near net shape and sintered under vacuum at high temperature. The authors describe steps in the process of developing a Ti–Ni–Sn alloy, able to be sintered to near full density at a sintering temperature as low as 1100°C without requiring unusually fine powder or high compaction pressure. Higher sintering temperature allows the Ni content of the alloy to be reduced, but swelling of the alloy probably imposes an upper limit on practical sintering temperature. The increase in green density, and hence sintered density, conferred by Sn in Ti–Sn alloys, and the increase in sintered density due to the high diffusivity of Ni in Ti–Ni alloys, are combined in Ti–Ni–Sn alloys.

In earlier work the authors examined the sintering of Ti–Ni alloys by means of dilatometry of mixed elemental powders. Some notable differences were observed when heat treatments were carried out using a vacuum tube furnace rather than the dilatometer: higher sintered density was achieved due to a combination of lower heating rate and lower residual pressure, and swelling during liquid phase sintering was greatly reduced. This observation is consistent with the idea that gas pressure within closed pores causes swelling during liquid phase sintering and retardation of shrinkage in solid state sintering. In addition to the results of measurements of density and open and closed porosity as a function of Ni content and sintering temperature, macrographs and optical micrographs of the sintered compacts are presented, and the effects of heating rate and compaction pressure are described.

The attractive physical and mechanical properties that can be obtained with Ti metal matrix composites (Ti-MMCs), such as high specific modulus, strength or wear resistance have been documented extensively. Interest in MMCs for use in the automotive industries, aerospace, and other structural applications has increased over the past 25 years as a result of several processing routes being developed and availability of reinforcements. Additionally, Ti powder metallurgy offers the possibility of creating net shape or near net shape parts without the material loss and cost associated with machining intricate components from wrought stock material, Ti being notoriously difficult to machine. This work focuses on research of a suitable Ti based metal matrix which will be reinforced by particles. First an adequate sintering assembly was designed and studied. Next, in order to achieve desired levels of mechanical properties, different Ti powders (either pure Ti or Ti hydride) were tested as metal matrix. These compacts were handled in different atmospheres (Ar and air) and compacted at different pressures followed by sintering at different conditions (temperature, time). Characterisation of the samples included hardness, density, oxygen content and microstructural analysis. This showed the influence of impurities in the raw powder as well as of the powder size and the processing parameters. Finally after the characterisation results, the two most suitable starting powders were selected for the fabrication of the Ti-MMCs. Such Ti based matrices have been reinforced with particles (TiB₂) at varying contents. Conclusively, the characterisation results of the compacted Ti based composites showed the importance of the adequate selection of the starting powder and the processing parameters simultaneously to achieve the desirable levels of mechanical properties for the Ti based composites.

One of the most economic ways in production of diamond composites is cold compaction of diamonds and metallic powder, and sintering of the compacted mixture. Complex loading situation of individual diamonds, which has physical contact with the die wall during the compaction, results in damaging of the press die. Replacing of press die is costly and time consuming. In order to increase the life time of the press tools, different hard PVD coatings were deposited on the inner surface of the die. Sintered bodies were produced using coated and uncoated moulds to analyse the effect of the hardness and friction coefficient of the die wall on the characteristics of the sintered body. The result evidenced that the lower friction coefficient and higher hardness of the die wall increase the hardness and the porosity of the sintered diamond composites.

A (Ti,V)C/Fe composite was produced by self-propagating high temperature synthesis combined with powder metallurgy using Ti, Fe, FeV and carbon powder. The microstructure of the Fe–(Ti,V)C composite was studied by scanning electron microscopy and X-ray diffraction; with the help of differential thermal analysis, the reaction mechanism of the Fe–Ti–V–C system was discussed. The results show that the production of an iron matrix composite reinforced by (Ti,V)C particulates using the process is feasible. (Ti,V)C particles exhibit homogeneous distribution in the α-Fe matrix. The reaction mechanism is as follows: first, allotropic change Fe _α →Fe _γ and the reaction FeV+C→Fe+VC at 765·7°C; second, formation of the compound Fe₂Ti at 1058·5°C because of the eutectic reaction between Ti and Fe; third, reaction between carbon and melted Fe₂Ti causing formation of TiC at 1140·4°C; finally, formation of (Ti,V)C due to the solubility between TiC and VC.

Powder injection moulding (PIM) carried out with the use of low viscosity feedstocks offers numerous benefits for manufacturing small complex shape parts. Unlike typical high pressure metal injection moulding (HPIM) viscous feedstocks, soft tooling can be employed for prototyping and small volume manufacturing. Compared to HPIM, there are very few studies on the rheology of low viscosity feedstocks. The objective of this paper is to clearly determine, using a statistical method, optimal models which define viscosity as a function of three parameters: shear rate, temperature and solid loading for low viscosity feedstocks. With the statistical method employed, it was found that the models of Herschel–Bulkley, Arrhenius, and Maron and Pierce can be used respectively to effectively model each of the three parameters stated previously. Moreover, the combination of these three models in one global model is proposed to predict the combined effect of the three parameters on low viscosity PIM feedstocks.