Investigation on local cooling in reducing freckles for directionally solidified superalloy specimens with abruptly varying cross-sections

Abstract

In order to suppress the freckle formation in specimens with abruptly varying cross-sections, superalloy CMSX-4 was directionally solidified, and the local cooling method was investigated, in which a graphite block was inserted into the bottom of platform of specimen. The experimental and simulation results were coupled to evaluate this method in terms of freckling number, primary dendrite arm spacing (PDAS), dendrite morphology, and solidification interface shape. The results show that, with the application of local cooling method, freckles appearing in the specimens were absolutely removed. Dendrite microstructure was refined with reduced PDAS, and a dendritic to cellular transition was promoted. In addition, the concave solidification interface was flattened. Therefore, the local cooling method with the application of graphite block can be employed to reduce the freckling tendency in directionally solidified CMSX-4 superalloy specimens with abruptly varying cross-sections.