Experimental Analysis and Simulation of Freckle Formation at Different Edge Angles During Directional Solidification of Ni-Base Superalloys

Freckles are casting failures occurring at the surface of superalloys as an elongated trace of equiaxed grains. Preliminary work shows that freckles also form at corners of rectangular cross-section samples during directional solidification of CMSX-4. Criteria formulating the requirements of freckle formation do not provide a clear explanation for this edge phenomenon, since corners are expected to cool faster than the remaining volume and therefore freckles should not form according to the requirement of a concave shaped liquidus line. Therefore, in this study the edge effect is focused and experiments are carried out with specimens of different edge angles of 30, 45, 60 and 90 deg. In addition, the solidification process was described numerically to visualize the gradients during solidification for the different edge angles and in order to derive a general trend. Overall, the experimental and simulation results show that the tendency to form freckle decreases with increasing edge angle.