Analytical modeling of the rheological behavior of semisolid metals and composites

The thixotropic nature of semisolid alloys and composites has been shown to be advantageous in semisolid metal processing. This effect can be highlighted by hysteresis experiments in the semisolid state where the shear rate is increased from zero to a maximum value and then decreased back to zero in a specific time period. The results presented in this article show that the rheological behavior of a semisolid material subjected to such a cycle depends strongly on the initial state of its microstructure. It is shown that the rheological behavior of the dissociated structure is reversible, while that of the agglomerated state is strongly thixotropic. The agglomeration of the structure is favored by a rest time and a low initial shear rate prior to the shear cycle. A model based on the assumptions of Moore was developed to describe our experimental results. It takes into account all the variables that influence the form of the hysteresis loops, i.e., the time to reach the maximum shear rate, the maximum shear rate attained during a cycle of shearing, the initial state of the microstructure, and the number of shearing cycles imposed. The results of our simulations show that we are able to reproduce the experimental results under various conditions in an analytical model, which take into account a set of experimentally determined constants and by varying the value of a structural parameter describing the initial state of agglomeration of the semisolid metal slug.