Hot cracking behavior and mechanism of a third-generation Ni-based single-crystal superalloy during directed energy deposition

Abstract

Hot cracking is a frequent and severe defect that occurs during the directed energy deposition (DED) of single-crystal superalloys. Understanding the cracking behavior and mechanism is key to avoiding these defects. Thus, in this study, a third-generation single-crystal superalloy, CMSX-10, was investigated. Hot cracking occurred at high-angle grain boundaries and especially at low-angle grain boundaries. Cracks were formed beyond a critical misorientation angle of 6.9°. Hot cracking was determined to be caused by a stable liquid film, stress concentration, and Re-rich precipitates. The stability of the liquid film depended on dendrite coalescence undercooling which was related to the misorientation angle. The dendrite coalescence undercooling at low-angle grain boundary (misorientation angle 6.9°) was 178 K, which was far higher than the vulnerable temperature interval 38 K for hot cracking within a single dendrite. Stress concentration provided the driving force for crack initiation and propagation. Re-rich precipitates promoted crack initiation by a pinning effect on the liquid feed. These findings provide technical support for achieving high-quality additive manufacturing and repair of non-weldable Ni-based single-crystal superalloys.