Three-dimensional Numerical Simulation of Channel Segregation in Directionally Solidified Sn-20 mass% Bi Ingot

Abstract

Channel segregation behavior of a lateral directional Sn-20mass%Bi ingots was examined by X-ray computing tomography (X-ray CT) and three-dimensional numerical analysis. The channel segregation with string shape from near the cold wall side was observed by the X-ray CT and those segregation patterns could be reproduced by the numerical analysis. Low concentration regions also appeared above the channel segregation in both X-ray CT images and numerical analysis results. These results suggest that the proposed numerical method is a reasonable model to simulate the channel segregation. Detailed investigation of the effects of convection on the generation of channel segregation showed that a solidification-accelerated region (SA-region) and solidification-retarded region (SR-region) were formed by the fluid flow. At the SA-region, solidification was accelerated by the inflow of lower Bi concentrated liquid. On the other hand, in the SR-region, solidification was retarded by the inflow of the liquid with high Bi concentration from the SA-region. It was found that the SA-region (called donor part) and SR-region (called acceptor part) group into pairs, and generate the channel segregation.