Particle packing, compaction and sintering in powder metallurgy

This thesis focuses on the improvement of density based on the controlling of particle packing and the subsequent densification processes. First, the two-sphere model for monosized powder to determine the lattice diffusion contribution in sintering is considered. Based on Eadie's model, two new solutions are developed and presented, an upper limit solution with surface curvature at a maximum all along the neck and a more accurate solution in which the form of the surface curvature at the neck can be varied as appropriate. Producing bimodal powder distribution powder metallurgy (P/M) parts is next considered. It is shown that this technique is capable of producing parts with excellent final densities and mechanical properties. In order to explain the above experimental results, theoretical research about bimodal powder is focused on particle packing. The first theoretical tool is the coordination number of the particles and the second theoretical tool is the concept of percolation. A bridge is established between packing density, microstructure and leading eventually to macroscopic properties by using coordination number and percolation. A new model (modified saturated model) for coordination number in bimodal packing is developed and presented. It is concluded that the structures that arise in bimodal powder distributions he somewhere between the previously proposed random structure and the saturated structure. The important role of the coarse particle network which can shield fine particles from the compaction forces is emphasised. Some comments about parameters influencing sintering and suggestions for future work are included.