Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Fundamental aspects of hot isostatic pressing: An overview

Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2

Materials with fine microstructures by advanced powder metallurgy

Evolution of grain boundary network topology in 316L austenitic stainless steel during powder hot isostatic pressing

Rapidly solidified γ-titanium aluminide powders were characterized and subjected to consolidation by hot isostatic pressing (HIP). Primary solidification of h.c.p.-α phase followed by formation of interdendritic γ-segregate prevails in both REP (Ti-50% Al-1.8% Nb) and RSR (Ti-48% Al-2.4% Nb-0.3% Ta) powders, except for a minor fraction of fine powders that form b.c.c.-β phase from the liquid. The evolution of these microstructures and their subsequent solid state transformations are in line with previous observations on binary titanium aluminide droplets solidified with and without supercooling. In the consolidation experiments, the influences of time, temperature and pressure on final densification were experimentally determined and compared with predicted HIP maps using best estimates of input data on material properties. Experimentally it was found that the densification rate is independent of average mean particle diameter. Best agreement between predictions and experiments were obtained for power-law creep as the dominant consolidation mechanism at high pressure, i.e. 182 MPa. Modifications of the model are suggested for consolidation during the pressurization.

Densification of rapidly solidified titanium aluminide powders. I, Comparison of experiments to hiping models

Prediction of deformation behaviour during hipping is often difficult because data on complex material properties are required but are not usually available from the literature. From the present work, measurement of the deformation of a single sphere is proposed as a direct and simple method of predicting hipping behaviour. For C1018 steel, copper, and API nickel superalloy, the predictions made from deformation experiments with single spheres were in good agreement with measured hipping behaviour at hipping densities <93% theoretical. In addition, the creep parameters, such as the exponent n for the power law creep relation, of the powder material itself were measured during single sphere deformation. During hipping of C1018 in the density range above 90%, the use of lower pressures, which is favoured for economic reasons, showed that microstructural development and densification during the final stage of hipping are determined by relatively large pores. These large pores may result from irregul...

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Mirjana Mitkov

Papers

Microstructural development and densification during hipping of ceramics and metals