Numerical study on the formation of spurious grains and freckles during the directional solidification of superalloys

Abstract

Freckles, a significant issue encountered during the directional solidification of superalloys, are recognised by a trail of equiaxed grains parallel to the direction of gravity accompanied by local eutectic enrichment. In the current study, a mixed-columnar-equiaxed multiphase volume-average solidification model was employed to study the formation of freckles in superalloy casting. Fragments produced via flow-driven and capillary-driven fragmentation mechanisms are considered as the source of spurious grains. The transport and the growth/remelting of the fragments are considered. According to the simulation results, some segregation channels develop at the corners of the casting. Flow-driven fragments are produced in/around the segregation channels, whereas capillary-driven fragments are produced at a certain depth of the mushy zone across the entire section of the casting. The fragmentation rate caused by the flow-driven mechanism is several orders of magnitude larger than that caused by the capillary-driven mechanism, i.e. the flow-driven fragmentation mechanism is dominant for the currently investigated sample. After the solidification process, four freckles formed at the casting corners on the shadowed side, whereas it was freckle-free on the bright side.