Cold compaction

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Cold compaction is a process that powder materials is compressed in a temperature region where high temperature deformation mechanics like dislocation or diffusional creep can be neglected. Cold compressing is the most important compaction method in powder metallurgy. It starts from bulk powders containing very small amounts and sometimes even no lubricant or binder additions.

One usually distinguishes between axial (die) and isostatic pressing. In axial pressing, the powder is compacted in rigid dies by axially loaded punches. The axial compaction pressure P_ax, which is defined as the punch load divided by the punch face area is the main process variable. In isostatic pressing, the powder is sealed in an elastic mould and exerted to the hydrostatic pressure P of a liquid pressure medium.

The compaction behavior of powders, expressed by their overall pressure density relations is shown in Fig 3. The controlling parameters are mainly particles size and the ability for plastic deformation. Densification starts form the apparent density, which is similar for the coarse iron and alumina powder, and which is not too far away from random dense packing for both of them. The fine powders exhibit a significantly lower starting density, due to hindered packing. With increasing pressure, the average density of the compact increases. The slope of the curves differs significantly for the ductile metal and non ductile alumina. This is due to the filling of inter-particle voids by large amount of plastic deformation. The inter-particle friction and bridging effects increases with decreasing particle size.