The effect of particle morphology on metal powder compaction

Abstract

A study has been made of the effect of particle morphology (shape) on the elastic parameters and failure criteria for unsintered, ductile metal powders during compaction. A comprehensive series of quasi-static uniaxial and conventional triaxial compression tests were conducted on loose powder samples of contrasting morphology. The study showed that bulk density alone is an inadequate descriptor of the state of the granular body when determining its elastic and deformation properties. The results of the tests indicate that the effect of particle shape is to alter the contributions of particle rearrangement and deformation during compact densification. It appears that the densification mechanisms, which are dependent on both material density and particle properties, such as morphology, dictate the compact response. It was found that, for compacts with densities in the range where particle plastic deformation is prevalent, Poisson's ratio increases with increasing density. For compacts with lower densities, corresponding to the compaction phase where densification is through localised particle deformation and/ or relative particle movement, Poisson's ratio decreases or remains unchanged with increasing density. The modulus-density relationship also reflects a dependence on the densification mechanism.The general form of the failure criteria for these materials, regardless of particle morphology, is shown to be similar to that of porous, ductile metals. It is distinct, however, in that the consolidation surfaces are bounded by points which designate the shear failure of the material. Accordingly, the failure criteria is not symmetrical about the deviatoric stress axis. The functional dependence of the consolidation surfaces on density are, like the elastic parameters, associated with the densification mechanisms promoted by the initial particle morphology. It was found that, consolidation through particle deformation requires surfaces of progressively increasing aspect ratio. Densification, on the other hand, through particle rearrangement may be described by elliptical surfaces of approximately constant aspect ratio. Powders which possess particle characteristics that retard densification, such as internal porosity, require consolidation surfaces at higher stress levels for a given relative density. The results of this study imply that, for accurate modelling of the compaction of loose, ductile, powder systems, the functional relationships used to describe the elastic parameters and failure criteria should consider both particle morphology and densification mechanisms as well as material density.