Theories of hot-pressing : plastic flow contribution

The contribution of plastic flow to overall densification of a powder compact during hot-pressing has been analysed. The basis of this analysis is the incorporation of hot-working characteristics of materials at elevated temperatures into an equation applicable to hot-pressing conditions. The empirical equation relating steady state strain rate to stress is ἐ = Aσn and for the densification of a powder compact, the strain rate [formula omitted] The particles are assumed to be spheres and four different packing geometric configurations: cubic, orthorhombic, rhombic dodecahedron and b.c.c. are considered. Taking into consideration the effective stress acting at the points of contact, the equations for the strain rate can be combined and arranged into another equation which is shown below:[formula omitted] where α₁ and ϐ are geometric constants and can be calculated from the packing geometry. 'A' and 'n' are material constants. D is the relative density of the compact, and ‘R’ is the radius of sphere at any stage of deformation in arbitrary units. Computerized plots of D vs t were obtained for lead-2% antimony, nickel and alumina. Experimental verification of these plots was carried out using hot-pressing data for lead-2% antimony, nickel and alumina spheres. The hot-compaction experiments were carried out over a range of temperatures for each material and under different pressures. The experimental data fitted well with the theoretical prediction for the orthorhombic model. However, a deviation at the initial stage of compaction was encountered in most cases. This deviation was explained on the basis of the contribution to densification by particle movement or rearrangement at the initial stage, which could not be taken into account in the theoretical derivation. The stress concentration factor i.e., the effective stress acting at necks between particles has been calculated. This was found to be very much higher than that previously used by other workers. The theoretical equation for the effective stress is [formula omitted]. This equation predicts an effective stress, which is more than an order of magnitude higher than that predicted by several empirical equations used previously.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat