Freckles, a significant issue encountered during the directional solidification of superalloys, are recognized by a trail of equiaxed grains parallel to the direction of gravity accompanied by the local eutectic enrichment. In the current study, a mixed-columnar-equiaxed multiphase volume-average solidification model is employed to study the formation of freckles based on a superalloy casting. Fragments produced via flow-driven and capillary-driven fragmentation mechanisms are considered as the source of spurious grains. The transport of as-formed fragments, and the simultaneous growth and remelting of the fragments are solved in the model. According to the simulation results, some segregation channels develop at the corner of the casting. The flow-driven fragments are produced in/around the segregation channels, whereas the capillary-driven fragments are produced at a certain depth of the mushy zone across the whole section of the casting. In this experiment, the flow-driven fragmentation mechanism is evidently dominant. After the formation of the fragments, some of them remelted in the channel and some can be further transported into the bulk liquid and remelted there, while most of the fragments continue to solidify in the channel and grow into spurious grains. Formation of freckles are explained, and a satisfactory simulation-experiment agreement is achieved.

Moderated by: Andre Phillion / Matt Krane