The Formation of Downward Freckles in Nb-Containing Superalloy Remelt Ingots

A revised mechanism for the formation of downward frecklesFreckle is proposed. The mechanism accounts for the observations of freckleFreckle formation in industrial ingot production. In particular, the association of freckling with process instability is emphasized and is part of the proposed mechanism. Additionally, we suggest that the shrinkage which takes place as the final eutectic solidifies can explain the formation of freckling when the basic gravitational force produced by negative buoyancy in the solidifying liquid appears to be insufficient for the initiation of interdendritic fluid flow. We conclude that the key factor in preventing freckleFreckle formation in industrial ingot remeltingRemelting is the maintenance of process stability.