2017 Volume 103 Issue 12 Pages 730-737
A numerical model was developed to predict solidification grain structures and macrosegregation based on a three- dimensional cellular automaton finite difference method coupling with flow calculation of natural convection. For validation of the proposed model, simulations of unidirectional solidification cooled on the bottom surface of mold were performed at Al -10wt.%Mg alloy. Columnar grain structures have formed from the bottom to the top in alloy melt. During solidification, Mg-rich plumes rising in the melt were seen due to a subsequent upward flow, which caused by the thermosolutal buoyant force. Once the plume occurred in the melt above the mushy zone, the morphology of columnar grains varied and the grains became coarse. Mg-rich channels forming in the mushy zone were observed. Such region could delay solidification inside liquid-rich channels and results in a freckle defect. Examined the average Mg concentration profiles of the cross section from the bottom to the top, the negative segregation occurred in the middle of solidification and the positive segregation occurred at the end of solidification. From these results, it was confirmed that the proposed model was effective to predict the macrosegregation coupling with the grain structure formation. Moreover, the influence of the anisotropic permeability on macrosegragation formation was investigated in the present simulations. As the results, it was confirmed that the anisotropic permeability should be considered to predict quantitatively macrosegregation.
