Modeling of combined melt convection and solid transport during equiaxed dendritic solidification

The effects of thermosolutal 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. Numerical simulations are performed using a recently developed multiphase model that accounts for the nucleation and crystal growth mechanisms taking place at various microscopic length scales, as well as for flow, heat transfer and solute redistribution at the system scale. The predictions are compared to experiments carried out in a square test cell using the transparent NH₄Cl-H₂O alloy analog. Several important features of equiaxed dendritic solidification are identified, including the grain generation and growth behaviors 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. Finally, several areas for future research are indicated.