PREDICTION OF COLUMNAR TO EQUIAXED TRANSITION IN ALLOY CASTINGS WITH CONVECTIVE HEAT TRANSFER AND EQUIAXED GRAIN TRANSPORTATION

摘要

A macroscopic, non-equilibrium model of the Columnar to Equiaxed Transition (CET) in alloy shape casting is presented. Convective heat transfer in the liquid metal and equiaxed grain transportation by fluid flow is included in the model. Nucleation from mould walls is used as the mechanism for columnar grain initiation. Nucleation from inoculants in under-cooled liquid-ahead of the columnar front is considered for equiaxed grain formation. The front tracking model computes the advancement of the columnar front while the average growth of the equiaxed grain envelopes is simultaneously simulated. Latent heat release is incorporated in the model. The columnar mush and the coherent equiaxed dendrites are treated as porous media for convective flow. When equiaxed fraction is sufficient, no further advancement of the columnar front is permitted and the CET position is determined. CET is simulated for solidification of an aluminum-silicon alloy along with predictions of average equiaxed grain sizes.