Modelling of rheological behaviour of semisolid metal slurries - Part 1 - Theory

In this paper, part of a series, a microstructural model is presented which describes the theological behaviour of liquid-like semisolid metal (SSM) slurries under a simple shearing flow. A liquid-like SSM slurry is considered as a suspension in which interacting spherical solid particles of low cohesion are dispersed in a liquid matrix. In a simple shear flow field, the dynamic interactions between solid particles result in the formation of agglomerates. Under viscous forces, collisions between agglomerates lead to the formation of new agglomerates of a larger size, and at the same time agglomerates also break up giving rise to agglomerates of a smaller size. At a particular time, the state of agglomeration is described by a structural parameter n, which is defined as the average number of particles in the agglomerates. It is believed that it is the state of agglomeration which determines the rheological properties of SSM slurries, whereas the external flow conditions, such as shear rate and shearing time, affect the rheological properties by changing the state of agglomeration. Based on such considerations, the time evolution of the structural parameter n has been derived analytically as a function of both microstructural parameters and the external flow conditions. Through effective solid fraction both viscosity and shear stress can be expressed as a function of n. As will be demonstrated in the following parts of this series, the present model can be used to predict the pseudoplastic, thixotropic, and continuous cooling behaviour exhibited by SSTM slurries. MST/4961.