Modelling bulk viscosity of powder aggregate during sintering

A model of sintering by coupled surface diffusion and grain-boundary diffus ion is presented, which is based on the numerical simulation of interpartic le neck changes. This model is used to investigate the bulk viscosity of pa rticle aggregates through densification. The results obtained suggest that a sintering powder has a Newtonian behaviour. The neck size is found to be the parameter that better controls the viscosity. However, as density is th e most convenient parameter to be introduced in constitutive equations, two expressions of viscosity against density are proposed for the cases when s urface diffusion is faster or slower than grain-boundary diffusion. It is s hown that the models proposed in the literature for the initial sintering s tage predict the same evolution as numerical results in the case of slow su rface diffusion, even at high density. On the other hand the models propose d for the intermediate stage predict the same evolution as numerical result s in the case of fast surface diffusion. even at low density.