Numerical Simulation of Macrosegregation Formed Due to Solidification Shrinkage and Bridging of Solidification Structures

Direct simulations of macrosegregation were performed using a numerical model to predict solidification grain structures and macrosegregation based on a three-dimensional cellular automaton finite difference method coupling with flow calculation of shrinkage flow and natural convection. In order to investigate the relationship between solidification structures and macrosegregation, the simulations for a special mold used in model experiments of Sato el al. [Tetsu-to-Hagane, 99(2013), 101], which can form macrosegregation in the central region of the small ingot, were carried out. In the simulations, the bridging of columnar grains formed in the center of ingot during solidification, and then the positive segregation was generated in the region below the bridging. On the other hand, the negative segregation was generated in the region above the bridging. The primary factor of this macrosegregation was the shrinkage flow with the formation of bridging, and the degree of positive and negative segregation was enlarged by the presence of natural convection. As the results, it was confirmed that the shrinkage and the bridging of solidification structures played an important role for the formation of macrosegregation.