Numerical Simulation On Marcosegregation Formation During Binary Alloy Solidification Processes

Macrosegregation is an inevitable defect in solidification process, which has great influence on ingot qualities. Its formation is mainly due to the solute redistribution in front of the solid/liquid interface and the solutal flow in mushy zone. The natural convection, which is caused by thermo-solutal buoyancy, is one of the most important factors that cause solutal flow during solidification.In this paper, in order to investigate the influence of natural convection which is induced by thermo-solutal buoyancy on macrosegregation especially on channel segregation during the solidification, a model coupling macroscopic fluid flow, heat transfer and solute transport was developed for binary alloy’s solidification process. A FORTRAN program based on finite volume method was developed for simulating the solidification of binary alloys such as Sn-5wt%Pb and Pb-48wt%Sn alloy. The results show that:(1) Without considering solidification shrinkage, the macrosegregation in ingot is mainly caused by natural convection, which is induced by thermo-solutal buoyancy. The direction of buoyancy depends on the sign of thermal and solutal expansion coefficients, but is dominated by the magnitudes of them. In the present case, the cavity is cooled down from the left lateral side When the thermal and solutal buoyancies are in the same direction, the convection in liquid zone is counterclockwise; otherwise, the thermal/solutal buoyancy in large magnitude determines the convection direction of the melt.(2) The channel segregation is mainly caused by solutal buoyancy. When the magnitude of solutal expansion coefficient is one order larger than that of thermal expansion coefficient, the solutal buoyancy is dominant in the mushy zone. As a result, the channel segregation is obvious.(3) The larger the secondary dendrite arm spacing, and the smaller the dynamic viscosity, the less the flow resistance and consequently the more serious the macrosegregation.(4) Macrosegregation is more serious in the cases that the partition coefficient of solute is far away from1, and that the liquidus slope is steep and the solidification interval is larger. In addition, it is more prone to form macrosegregation when there is large difference between solute and solvent density.(5) Thermal diffusivity of alloys affects the temperature gradient internally. The smaller the thermal diffusivity, the larger the temperature gradient of ingot and the narrower the mushy zone, the less the segregation. However, the change of ingot size and cooling conditions influence the temperature gradient externally. The decrease of ingot size and the increase of heat transfer coefficient lead to the increase of the temperature gradient in ingot, which results in weak macrosegregation.