Microstructure, crack formation and improvement on Nickel-based superalloy fabricated by powder bed fusion

Nickel-based superalloy (Haynes 230) parts were fabricated by powder bed fusion using a laser - based system. Crack defects were unavoidable. In order to solve the cracking problem, it was necessary to understand the crack formation mechanism, and the hot isostatic pressing method was used to eliminate the crack defects. The results indicated that the microstructure displayed cellular crystal structure in XY direction and columnar crystal structure with {001} < 001 > cubic texture in XZ direction. Under the thermal stress and micro-segregation, M₂₃C₆ carbides and NiSi eutectic phases were precipitated at high angle grain boundaries, which promoted the formation of solidification and liquation cracks. Because the direction of the internal cracks in the horizontal tensile sample was perpendicular to the tensile direction and the microstructure was {001} < 001 > cubic texture, the tensile properties were anisotropic. After hot isostatic pressing treatment, the cracks were obviously eliminated, the ultimate tensile strength and elongation were significantly improved. The carbides transformed from Cr-rich M₂₃C₆ to W-rich M₆C.