A Study of Sliver in C-Shaped Grain Selectors during Investment Casting of Single-Crystal Superalloy

Abstract

In this study, the formation mechanism of sliver defects in C-shaped 2D grain selectors during investment casting of single-crystal superalloy was investigated by analyzing the effects of the C-shaped 2D grain selectors on the solidification behavior of superalloy. The physical field properties of the sliver formation and solidification characteristics of CM247LC nickel-based superalloy were determined. The temperature and stress fields were simulated using ProCAST. The results showed that the stress fields of solidification played an essential role in the formation of sliver, indicating that the solidification interval characteristics of the alloy and stress based on the geometry of the C-shaped 2D grain selector are crucial for the formation of sliver. In addition, the origin of sliver depended upon tensile stress during solidification, relying on the constraints of dendrite boundaries. The findings suggested that the joint sections of the starter block—i.e., selector and selector-casting joint of C-shaped selector sections—are stress-sensitive areas where sliver can form readily. Furthermore, sliver is formed in the final stages of solidification and especially originates in the grain selection part where the accumulated thermal stress is high, and there is only a small quantity of liquid phase with a low melting point between the dendrites. Therefore, the solidification and stress conditions generate thermal cracks, which can also cause sliver defects.