Microstructural characteristics and crack formation in additively manufactured bimetal material of 316L stainless steel and Inconel 625

This research illustrates the rationale of adopting a preferred printing sequence by examining crack generation predominated by resultant interfaces and microstructural inhomogeneity, through underlying governing mechanisms in directed energy deposition of 316L stainless steel/Inconel 625 (SS316L/IN625) bimetals. For this purpose, microstructural and crystallographic characterizations augmented by numerical simulations were employed on additively manufactured two distinct interfaces, i.e. Type-I (IN625 deposition on SS316L) and Type-II (SS316L deposition on IN625). Changing the printing sequence generated these two types of interfaces with unique morphologies, which was found attributable to the compositional variations and mismatch in thermal properties. Type-I interface was typified by gradual-change composition in the transition zone, causing the IN625 grains to grow epitaxially on the grains of SS316L. Type-II interface was characterized as a compositional sudden-change zone (CSCZ) adjacent to SS316L, leading to merging bidirectional nucleation and grain growth from the bottom IN625 and upper CSCZ, and lack of epitaxial growth. Additionally, high cracking susceptibility occurred near the Type-II interface rather than the Type-I interface, which was related to solidification and liquidation cracking, and further promoted ductility dip cracking. This research will provide a guideline for the additive manufacturing of bimetals with the consideration of printing sequence to control interface formation for a crack-free structure.