Modelling of rheological behaviour of semisolid metal slurries Part 4 – Effects of particle morphology

Abstract

In this part of the series, the fractal dimension D is introduced to quantify the morphology of equiaxed dendrites and the concept of equivalent solid fraction Ψ to connect solid fraction to the effective solid fraction. Generally, dendrite formed during diffusion controlled solidification has a fractal dimension of 2.5, while for a perfectly spherical particle D=3. The rheological model developed in Part 1 is then used to deduce the D data from experimental results of continuous cooling in the literature. It is shown that, within the range of experimental conditions concerned, D is predominantly affected by solid fraction (or semisolid temperature), while shear rate and cooling rate only have a limited effect on D. The rheological model developed in Part 1 is applied to study the rheological behaviour of semisolid metal (SSM) slurries with non-spherical particle morphology. It is found that under constant morphology assumption the shear thinning effect is much more pronounced for SSM slurries with solid particles of lower fractal dimension (i.e. a more dendritic morphology), and that the degree of thixotropy increases with decreasing fractal dimension of solid particles. In addition, it has been demonstrated that as a diffusion controlled process, particle spheroidisation takes many thousands of seconds, and is much slower than the deagglomeration process (a few seconds) and the agglomeration process (a few thousand seconds).