Modeling of microstructure formation of Al-Cu crystalline ribbons in planar flow casting

A numerical model has been proposed for the prediction of polycrystalline microstructure formation in planar flow casting. The present model is based on the coupling of the control volume(CV)method for macroscopic heat flow calculation and a two-dimensional cellular automaton(CA) model for treating microstructural evolution in planar flow casting. The CA model takes into account nucleation and growth kinetics. Heterogeneous nucleation can occur on nucleation sites both at the wheel surface and in the bulk liquid. Grains in the stage of nucleation are assumed to have random crystallographic orientations. The growth kinetics of a dendrite tip was evaluated using the Lipton-Kurz-Trivedi (LKT) model by which the relationship between the dendrite tip velocity and the local undercooling was calculated. The growth velocity dependent partition coefficient was integrated into the model to describe the growth velocity of a dendrite which grows from the undercooled melt. At each time interval, the latent heat released by the growing cells in the CA model was fed back into the control volume containing those cells in order to calculate the temperature distribution for the following step. The present model has been applied to predict the cooling curves and the resultant microstructures of AI-Cu polycrystalline ribbons spun by planar flow casting. The effects of wheel speed, alloy composition, and superheat of the melt on grain structures were investigated. The calculated grain structures were in good agreement with those obtained from experimentation.