The Influence of Particle Size on Viscosity in Thixo Material

Abstract

In thixo casting and forming processes, the forming alloy is in a semi solid state - a suspension of solid particles in a liquid matrix. As known from suspension rheology, the solid fraction, particle size distribution, particle shape, surface properties, state of agglomeration etc. determine the complex flow behaviour of the material. In thixo forming, the flow behaviour determines the quality of the produced parts, and it is essential to quantify the flow behaviour to optimize the process route. The goal of the present work was to evaluate the effect of particle size and state of agglomeration on the flow properties in a thixo material-analogue suspension, and to establish an easily applicable model to describe the non-Newtonian behaviour of a thixo material. For this purpose, experiments were conducted applying synthetic suspensions (built up of mono-disperse glass spheres in silicone oil) with behaviour analogue to thixo material. The advantage of synthetic suspensions is that parameters such as particle size and shape can be controlled in a simple manner and hence the number of unknown parameters is decreased. The experimental results show influence of shear rate, solid fraction, particle size and liquid viscosity. A model to determine the shear dependent viscosity was set up. This model was based on the Krieger-Dougherty equation for the relation between viscosity and solid fraction, and on the generally accepted theory that the state of agglomerations is influenced by shear. The model could successfully display the degree of shear thinning and takes into account the influence of particle size. However, it was found that the relation of the viscosity of the suspension and the liquid viscosity was non-linear. This does not correspond to the Krieger-Dougherty equation.