Simulation of the columnar-to-equiaxed transition in directionally solidified Al-Cu alloys

A combined cellular automaton-finite difference model was applied to simulate the columnar-to-equiaxed transition (CET) during the directional solidification of Al-Cu alloys. This model provided a novel insight into the solutal interactions both within the advancing columnar dendritic network and within the equiaxed grains forming ahead of them. Simulations revealed that solute interaction among secondary and tertiary arms is strong, but the interaction at the columnar tips is weak. The region with the largest solute adjusted undercooling was found to be in the region between columnar dendrites, rather than ahead of their tips as assumed in prior CET models. In addition, it was found that prior simulations which neglect the solute built-up at the interface predict the CET at a significantly lower velocity for a given gradient. The effect of crystallographic orientation on CET was also simulated and was found not to be significant. The influences of thermal gradient and growth rate on CET were combined on a CET map, showing good agreement with prior theoretical models at low growth rates, while at high growth rates the current model predicts that CET will occur at lower gradients. Reasonable agreement with the limited number of experimental observations available was obtained.