Hot isostatic pressing is now an accepted material processing technique for the consolidation of metal powders to near-net-shape components. This thesis examines the use of coatings as in-situ envelopes to overcome the problems associated with traditional containerisation of powder. The application of metallic coatings by physical vapour deposition, involving resistive and electron beam evaporation and ion plating, onto green powder metal compacts has been studied as a potential method for encapsulating powder metal products prior to hot isostatic pressing. The coating structures are discussed in terms of processing conditions and surface roughness influence. The most promising approach is a combined sinter-hot isostatic, pressing cycle, which utilises the formation of a transient liquid phase to defect-heal the coating during the sinter cycle prior to the application of pressure. The influence of particle size distribution on densification has also been studied. This included both monosized and bimodal powders. The results of this study has been incorporated into a modified Ashby model computer program and it is shown that the model results in a shift of the dominance of the mechanism fields and gives good correlation between the predicted and measured values of density.

Studies have been made of both the solid state and supersolidus sintering characteristics of spherical cupronickel powders. Observations were made of the structural changes and shrinkage rates in specimens sintered in vacuum and in hydrogen. It was concluded that the early stage of solid state sintering (up to one hour at 1200°C) was dominated by Nabarro-Herring creep. Calculations of the stresses at necks during sintering were consistent with the proposed mechanism. No solute segregation to necks occurred during sintering, contrary to earlier observations by Kuczynski with other copper alloys. When pre-sintered cupronickel powder ( 68 μm) aggregates were heated to a temperature above the equilibrium solidus, melting was nucleated first at high angle grain boundaries (necks) and particle surfaces (voids). Most melting was intragranular, nucleated at interdendritic sites of above-average copper content. Solid-liquid equilibrium was established in less than one minute at the supersolidus temperature. The dihedral angle in the system was less than or equal to zero. Growth of solid grains during supersolidus sintering obeyed a parabolic rate law consistent with a model of growth due to phase boundary reaction-controlled solution and precipitation. Shrinkage during supersolidus sintering proceeded in several distinct stages. Prior to attainment of equilibrium; i.e. within the first minute above the solidus (Stage 1), contraction could be attributed to a melting and melt accommodation sequence, plus flattening by the the local operation of solution and precipitation. Beyond this (Stages 2 and 3) all densification was attributed to solution-precipitation, including grain growth. In the final stage of shrinkage (Stage 3) the rate of contraction was controlled by the rate of escape of gas from closed pores. Comparisons have been made between the supersolidus sintering of cupronickel and the liquid-phase sintering of iron-copper. The processes are seen to have little in common. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, July 2015 / ABSTRACT Solid titanium (Ti) and Ti-6Al-4V (wt.%) materials were fabricated from powders using spark plasma sintering (SPS), cold isostatic press (CIP) and sinter, layered (rapid) manufacturing, centrifugal and vacuum casing. ASTM Grade 4 Ti, Al and V, 60Al-40V (wt.%) and the pre-alloyed Ti-6Al-4V powders were used as starting materials. The solid Ti and Ti-6Al-4V materials produced by the SPS were compared to the CIP and sinter method on the basis of density, microstructure and chemistry. The materials produced by the CIP and sinter method were also compared to those produced by vacuum casting method on the basis of microstructure, oxygen pick-up, chemistry and room temperature tensile properties. Centrifugal casting was compared to the vacuum casting technique on the basis of microstructural homogeneity. Rapid manufacturing was compared to SPS and CIP and sinter on the basis of microstructural homogeneity, density and tensile properties. The tensile properties of all materials were also compared to their commercial counterparts to investigate the effect of interstitial oxygen. The technology resulting in materials with superior properties was finally identified as most promising for commercial production of Ti-based materials. On the basis of densification, the SPS method appears superior compared to the CIP and sinter and rapid manufacturing method due to the benefit of pressure aided sintering, while the rapid manufacturing method is superior to the CIP and sinter method due to the use of a high power laser resulting in high densification rates. In cases where microstructural homogeneity is the key requirement, the CIP and sinter and rapid manufacturing methods appear superior compared to the SPS method due to longer isothermal holding time and higher sintering temperature and the use of pre-alloyed Ti-6Al-4V powder, respectively. On the basis of oxygen pick-up and additional contamination, the vacuum casting route is inferior due to the tendency of melt-crucible interaction, resulting in the dissociation of ZrO2 and subsequent pick-up of O and Zr. Based on the homogeneity of the microstructure, centrifugal casting is better than vacuum casting. The ductility of vacuum cast Ti was better than that of CIP and sintered Ti, possibly due to limited diffusion of oxygen from the crucible compared to oxygen absorbed from the controlled atmosphere during CIP and sinter. The vacuum casting of the Ti-6Al-4V alloy resulted in dissolution of oxygen and Zr due to melt-crucible interaction. Hence the ductility was worse compared to the alloy produced by CIP and sinter. The rapidly manufactured Ti-6Al-4V specimens exhibited superior ductility and strength compared to all alloys produced by other methods due to the use of high purity starting powder. The tensile properties of these specimens were also comparable to standard requirements. The similarity of the tensile properties of wrought Ti-6Al-4V alloy reported in the literature was an indication of limited oxygen pick-up during rapid manufacturing. Therefore based on low oxygen pick-up, microstructural homogeneity, high density and superior tensile properties, the rapid manufacturing route appears to be the most promising approach for commercial processing of titanium based materials.

Increasing concern over the environmental impact of motor vehicles is driving the need for the development of lighter materials to reduce automobile weight and fuel consumption. Sintered aluminium alloys, with their high strength to weight ratios, have potential applications in the automotive industry, but conventional pressed-and-sintered materials have poor mechanical properties due to the presence of residual porosity in the sintered compact. Residual porosity can be eliminated by Hot Isostatic Pressing (HIPing) or combined sinter-HIPing, but these processes are expensive due to the high gas pressures involved, up to several hundred MPa, and also pose a significant safety hazard. There is a limited amount of evidence in the literature suggesting that applied gas pressures as low as a few MPa may be beneficial to the sintering of aluminium alloys, and it is this idea that the present work explores. Compacts of aluminium alloy 2712 (Al-3.8Cu-1Mg-0.7Si-0.1Sn) were prepared from elemental powders and sintered at 590ºC for up to 60 minutes in a horizontal tube furnace under constant flowing nitrogen or argon at pressures up to 600 kPa. Archimedes’ method was used to measure the density of sintered compacts, and the amount of open and closed porosity. Increasing the nitrogen pressure at the start of the isothermal holding stage to 160 kPa increased the sintering rate compared to standard atmospheric pressure sintering. Increasing the nitrogen pressure further, up to 600 kPa, had no additional benefit. The sintering rate was increased further by applying a 600 kPa nitrogen pressure during both heating and isothermal holding. The elevated nitrogen pressure had a negligible effect on the maximum sintered density achieved, and sintering in argon at elevated pressures had no measurable effect on the sintered density or sintering rate. It was shown that the elevated pressure aids in the closure of pores open to the specimen surface, contrary to HIPing and sinter-HIPing, where the pores must be isolated prior to the application of pressure. It was also shown that at 600 kPa nitrogen pressure, the sintered density was independent of the presence of tin in the alloy. The improvements to sintering seem to be related to the formation of aluminium nitride.

Thesis (Ph. D.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains xvi, 140 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 137-140).

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The current, commercially available, press and sinter Al-Mg-Si-Cu alloys are based on wrought or cast alloy compositions and have not been tailored for the press and sinter process. The limited development of the Al-Mg-Si-Cu alloys for the press and sinter process can be partly attributed to a poor understanding of the effects of processing conditions on the sintering behaviour. The primary objective of this work was to investigate and understand the effects of processing conditions on the sintering behaviour of Al-Mg-Si-Cu-(Sn) alloys. Dilatometry was used in conjunction with other experimental techniques to elucidate and understand the expansion and shrinkage events that occur during the liquid-phase sintering of Al-Mg-Si-Cu-(Sn) powder metallurgy alloys. Samples were uni-axially pressed from elemental metal powder blends, de-waxed, and then sintered in a horizontal push-rod dilatometer to record the dimensional changes in the pressing direction. The processing conditions examined included the alloy composition, temperature, green density and atmosphere. A liquid forms during heating due to reactions between the alloying elements and the aluminium. This liquid is initially non-wetting on the oxide layer of the aluminium particles, resulting in separation of the particles, which is manifested by expansion of the sample. The oxide is reduced as sintering progresses, alleviating the non-wetting conditions. When more liquid forms, further expansion occurs, despite the improved wetting conditions. It is proposed that atmospheric oxygen and/or nitrogen can react with the liquid, forming a solid phase (‘shell’) at the liquid-vapour interfaces. These shells prevent the liquid from wetting the particles, resulting in further expansion and preventing shrinkage. Unbalanced diffusivities (the Kirkendall effect) between the aluminium and silicon contribute to the expansion. A mechanism is proposed to account for the transition to shrinkage, whereby the shells at the liquid-vapour interface rupture when there is a rapid increase in the volume of contained liquid. The liquid then flows out and over the shells, onto the aluminium substrate, causing shrinkage. Magnesium and nitrogen delay the transition to shrinkage by facilitating nitride shell formation at the solid-liquid interface. Silicon and tin cause an earlier transition to shrinkage by increasing the liquid volume. In addition, tin promotes shrinkage by segregating to the liquid-vapour interfaces, limiting the thickness of the shells at the liquid-vapour interfaces. The two dominant liquid-phase shrinkage mechanisms during the sintering of Al-Mg-Si-Cu-(Sn) alloys are rearrangement and pore-filling. Contact-flattening is not a dominant shrinkage mechanism, but may occur concurrently with the other mechanisms. If contact flattening occurs, a decrease in the pressure of isolated pores increases the total shrinkage rate. Nitrogen increases the shrinkage rate during rearrangement by restricting grain-growth. Magnesium increases the shrinkage rate during rearrangement by reducing the solid-liquid interface energy. Magnesium and nitrogen are essential for the formation of nitride within isolated pores, which decreases the pore pressure and increases the contribution of contact-flattening on the total shrinkage rate. Silicon reduces the beneficial influence of magnesium during rearrangement by diluting the magnesium content in the liquid. Silicon increases the pore-filling rate due to an increase in the liquid volume. Magnesium increases the pore-filling rate by facilitating aluminium nitride formation within isolated pores and by increasing the pore-filling. Tin additions can decrease the pore-filling rate due to its segregation to the liquid-vapour interface, limiting the consumption of nitrogen within isolated pores.

Thesis (M.S.)--University of Nevada, Reno, 2008. / "August 2008." Includes bibliographical references (leaves 72-82). Online version available on the World Wide Web.