Effects of bond strength on densification and flow of powder compacts

Abstract

Plastic flow surfaces for metal and metal/composite powder compacts with variable cohesive strength are derived using the Beltrami total strain energy criterion, modified to permit asymmetric yielding. The present theory thus includes the domains of both soil mechanics and powder metallurgy, covering both granular and porous microstructures of varying bond strengths over all possible densities. A quantitative theory of the expansion of non-bonded particle compacts under certain combinations of applied shear stress and pressure is given. Physical models of the failure mechanisms are provided, their applicability depending on the particle interface strength. If the particle interface lacks strength, the flow surface is identical to that of the ‘critical state’ criterion of soil mechanics. The flow ellipse lies entirely within the negative pressure domain and failure occurs at the particle interface by various mechanisms including frictional slip. At the other extreme, if the particle interfaces are as strong in shear as the particles, failure occurs by plastic shear of the particles and the flow ellipse is centred at the origin. Density–stress and density–aspect ratio maps are shown which define these domains. Theoretical predictions compare well with the results of data compiled from the literature as well as data from tests performed in this study on cold pressed Al and Al/SiC powder compacts.