HIP of Bi-Modal Powder Mixtures: Modeling and Experiment

Abstract

Models of the HIPing process have often assumed a uniform, spherical powder. However it is known that ″inhomogeneities″, in the form of particles of differing composition and/or size, will alter both initial powder packing and subsequent HIPing behavior. We have investigated these effects using the simplest form of mixture; a bi-modal powder with various size ratios and particle fractions. Modeling is based on the Arzt/Ashby et al. HIP map concept, with appropriate modification to produce a determinate and selfconsistent model for this more complex case. Experiments have been performed with a 316L stainless steel powder, with particle size ratios varying from 1:1 to 6:1 and fractions from 0 to 100% small particles. Packing experiments confirm the existence of ″optimum″ packing fractions for which the density is maximized over both monosized and the as-received (continuously distributed) powders. During HIPing these powder mixes remain relatively more dense, but densify more slowly than monosize packings. The implication of these results with respect to problems such as differential densification/distortion and the modeling of composite powder mixes is discussed.