Use of a mixed columnar-equiaxed solidification model to analyse the formation of as-cast structure and macrosegregation in a Sn-10 wt% Pb benchmark experiment

A solidification benchmark experiment based on a Sn-10 wt% Pb alloy was performed at the SIMAP Laboratory in Grenoble, France, to investigate the formation of as-cast structure and macro- and mesosegregation (Wang et al., 2009; Hachani et al., 2015). The current study simulated this benchmark using a mixed columnar-equiaxed solidification model. This model was developed by the current authors (Wu et al., 2006), and recently extended to consider the crystal fragmentation of columnar dendrites as the source of equiaxed grains. A satisfactory experiment-simulation agreement in the temperature evolution, the as-cast structure, and the distribution of macro- and mesosegregation, was obtained. By analysis of the modelling results, new knowledge regarding the formation of the as-cast structure (mixed columnar-equiaxed) was obtained: The fragmentation-induced equiaxed crystals are transported away from the front region of the columnar zone by the melt flow; those crystals continue to grow or re-melt depending on the condition of local constitutional undercooling or supersaturation; the density of the melt varies with the Pb enrichment in the melt, and the equiaxed crystals can settle down or float up through the balance of the flow-induced drag force and the gravity-driven sedimentation; it is the motion of equiaxed crystals that determines the final distribution of the equiaxed zone in this benchmark experiment; the interdendritic flow and its interaction with the solidification in the front region of the columnar zone lead to the formation of segregation channels.