Fundamental aspects of hot isostatic pressing: An overview
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Fundamental Aspects of Hot Isostatic Pressing: An Overview H.V. ATKINSON and S. DAVIES 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.
I. INTRODUCTION
THE basics of hot isostatic pressing (hipping) have been summarized previously.[1,2,3] Hipping is largely concerned with the removal of pores. Pores may originate from, for example, the packing of powder particles, from gas evolution or shrinkage during the solidification of castings, from the agglomeration of vacancies generated by creep, and by interdiffusion during the bonding of dissimilar materials. The driving force to achieve densification is associated with the reduction in surface area and, hence, surface energy of the pores. The isostatic pressure in hipping arises from molecules or atoms of gas colliding with the surface of the object. Each gas atom is acting as an individual “hot forge.” Under particular processing conditions, the gas atoms may be moving at a velocity of around 900 ms21, and approximately 1030 collision events are occurring per square meter per second. These tiny atomic forges reach all surfaces of the component, including re-entrant angles, and act reliably and consistently, independent of shape. On average, the number of gas atoms moving through a unit area, and their velocities, are the same in all directions. Thus, for every surface of a component that is being processed, the pressure is the same and acts in a direction normal to the surface. Isostatic pressing must be distinguished from the more conventional unidirectional pressing. Pressure is applied along a single axis by a ram in unidirectional pressing, and the component is contained in a die. No intervening fluid is used to transmit the pressure; rather, it is transmitted by contact between the solid surfaces of the ram and
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