Effect of Alloy Elements on Solidification Behavior and Castability in a Directionally Solidified Ni-Based Superalloy

IN792, a Ni-based superalloy, exhibits excellent mechanical properties at medium and high temperatures (600~900oC). However, it is known to be very susceptible to casting defects such as hot cracking and hot tearing during casting. This study investigated the effect of alloy elements such as Hf, Zr, and B on the solidification behavior and castability of IN792. Four types of ingots were manufactured by adjusting the contents of Hf (0~1.0wt%), Zr and B, and directional solidification and castability tests were performed. As a result of directional solidification, as the Hf content increased, there was little change in dendritic spacing, which is a measure of segregation behavior, but the mushy zone increased. The volume fraction of residual liquid in the mushy zone was also measured to determine the change rate during solidification. The measurement results showed the 0.8wt%Hf alloy had the largest change rate. It is predicted that rapid changes in the residual liquid during solidification will lead to a higher strain rate. The castability test was quantified using the length and frequency of cracks after casting, to determine the effects and causes of grain boundary strengthening elements and process variables on casting cracks. The results indicated the specimen containing 0.8wt% Hf had the worst castability. Mapping results showed that Zr content around cracks was highest in the 0.8wt% Hf sample. It was concluded that castability is determined by the complex effects of not only Hf but also Zr and B.