Cracking mechanism of Hastelloy X superalloy during directed energy deposition additive manufacturing

Metal additive manufacturing (AM) offers promising potential in the production of components with geometric complex or customized structure and outstanding properties, but severe defect like cracking remains a significant concern. The debate has long prevailed as to the mechanisms that lead to cracking in AM parts of some nickel-based superalloys like Hastelloy X. In this study, we investigated the underlying cracking mechanism of Hastelloy X fabricated via directed energy deposition (DED). The cracks in DED Hastelloy X were confirmed to be solidification cracking based on extensive observations of the inner crack surface and fracture surface. The origins of the solidification cracking were mainly attributed to thermal stress/strain level, grain boundary (GB) characteristics (GB misorientation and GB density), and micro-alloy elements. Our results showed that the plastic strain rate during the terminal stage of solidification is one of the most critical factors affecting cracking susceptibility when AM processing conditions change. We found that the fraction of S-HAGB (crack susceptible - high angle grain boundaries) is another critical factor in affecting cracking susceptibility. More than 75% of cracks occurred preferentially in the range of GB angles of 25°–45° (defined as S-HAGB) owing to its high GB energy rather than high GB misorientation angle. Besides, GB density could affect the cracking susceptibility by adjusting the thermal stress/strain level and S-HAGB fraction. Micro-segregation of C and Mo is a necessary condition for solidification cracking in DED Hastelloy X, which promotes the formation of low-melting liquid films. These new insights gained from this study could be used to instruct the preparation of crack-free metals and alloys during AM.