Rheological model for short duration response of semi-solid metals

A thorough understanding of the rheological behavior of the semi-solid metal (SSM) alloys is of utmost importance for modeling and simulation of rheocasting and thixoforming processes. Since the duration of these processes is very short-fraction of a second-the primary focus should be on fast transient flows. We present here a relatively simple engineering model for unsteady state shear stress of semi-solid metal suspensions. The time dependent character of thixotropic semi-solids is introduced through scaling by a structural parameter, which represents the degree of connectivity or aggregation of particles in the fluid. The kinetic equation for change in structural parameter as a function of time incorporates both breakage and agglomeration of suspended entities. The proposed model simulates the fast transient, tracks the evolution or healing of structure with holding time in absence of applied shear, and computes the steady state structure for a given shearing rate after long shearing time. Moreover, it explicitly incorporates the shear yield stress as well as is in agreement with important rheological characteristics of semi-solid materials, namely, shear thickening behavior under isostructural conditions and power law relationship between steady state viscosity and shear rate. The model is validated with published data for short time measurements of Sn-15%Pb thixotropic systems with near step change in shear rate and steady state shear viscosity. It could have practical utility in simulation of flow of semi-solid materials in different die cavities. (C) 2004 Elsevier B.V. All rights reserved.R