Microstructural simulations of the influence of solidification velocity on freckle initiation during directional solidification

The initiation of freckles by thermosolutal convection during the directional solidification of Pb-Sn alloys was studied numerically at the microstructural level. The model predicts the detailed dendritic structures, microsegregation and interdendritic convection in three dimensions. The onset and sustained growth of solutal channel (freckles) was simulated as a function of casting speed. The predictions were compared with experimental measurements via the introduction of a Rayleigh number. Good agreement was achieved, suggesting that such models might be used to predict the critical Rayleigh number for more complex alloy systems with further development. It was found that the large density variation of Pb-Sn alloys induces strong upward convection of segregated liquid and promotes the formation of solutal channels. However, the competition between upward solute transport and dendritic growth determines both the initiation and sustained growth of these channels. These open solute channels become the defect structure known as a freckle upon final solidification. © 2010 ISIJ.