Modeling of microstructure formation in alloy solidification with convection

The effects of melt convection and the transport of solid, in the form of free equiaxed crystals, on the formation of grain structure and macrosegregation in dendritic alloy solidification are studied. A recently developed multiphase model is presented that accounts for nucleation and grain growth mechanisms taking place at various microscopic length scales, as well as for the flow, heat transfer and solute redistribution at the system scale. Representative results of several numerical simulations of equiaxed solidification of an AI-Cu alloy are presented. In addition, comparisons with experiments are carried out for the case of solidification of a transparent ammonium chloride-water alloy analog in a square test cell. Several important features of the solidification process are successfully predicted, including the dendrite growth in the presence of liquid flow, the sedimentation of equiaxed crystals, the formation of a crystal sediment bed, and a bottom zone of negative segregation resulting from the counter-current solid-liquid multiphase flow. Qualitatively good agreement between the measured and predicted cooling curves and evolutions of the crystal sediment bed is achieved. The results indicate that considerable additional research is required to quantify the generation of grains in the presence of convection, due to fragmentation and nucleation mechanisms, before accurate predictions of the grain structure are possible.