Modelling of rheological behaviour of semisolid metal slurries - Part 2 - Steady state behaviour

In this part of the current series, the rheological model presented in Part 1 is applied to study the steady state behaviour of semisolid metal (SSM) slurries. Experimental results on the steady state viscosity of Sn - 15 wt-%Pb slurries in the literature are used to determine the model parameters. Application of the developed model to Sn - 15 wt-%Pb slurries has revealed a number of microstructural and rheological characteristics of SSM slurries. Under shear flow, the steady state represents the dynamic equilibrium between two opposite processes, agglomeration and deagglomeration. Under steady state, the microstructure of a SSM slurry is characterised by spherical particle morphology, uniform particle size, constant average agglomerate size and constant effective solid fraction. The theoretical analysis shows that there is an strong coupling between the microstructure and the steady state viscosity. Shear rate affects viscosity of SSM slurries by changing the amount of entrapped liquid between solid particles in the agglomerates and altering therefore the effective solid fraction. It is shown that steady state viscosity of a SSM slurry decreases with increasing shear rate and approaches an asymptotic viscosity. The asymptotic viscosity is only a function of solid fraction and corresponds to a microstructure in which there is no particle agglomeration. Finally, the current model predicts that particle size has no significant effect on apparent viscosity when particle size is either less than a few micrometres or greater than a few hundred micrometres. However, in the intermediate particle size range, apparent viscosity decreases with increasing particle size. This theoretical prediction is in good agreement with experimental results in the literature. MST/4962.