Direct laser powder bed fusion of duplex-austenitic stainless steel bimetallic structures: Microstructural evolution and robust interfacial bonding

Bimetallic structures (bimetals) composed of duplex stainless steels (DSSs) and austenitic stainless steels (ASSs) are promising for various applications. The brittle phases and coarse grains tend to form at the interface of DSS/ASS bimetals fabricated by conventional techniques. In contrast, the laser powder bed fusion (LPBF) technique enables the direct fabrication of DSS/ASS bimetals with strong interfacial bonding and allows the production of complex components. However, owing to its high cooling rates, a ferrite-dominated microstructure is formed in the DSS part. Meanwhile, the high Ni and Mo contents in conventional DSSs significantly increase their fabrication costs. To address these limitations, a novel fabrication route was developed to directly fabricate DSS/ASS bimetals with desired interfacial bonding. Low-cost DSS part was fabricated by the in-situ alloying of Ni-free high-N DSS and 304L ASS powders. To explore the interfacial microstructure and properties of the bimetals, 316L ASS powder was used to fabricate the ASS part using different process parameters. After optimized LPBF process, the resulting DSS/ASS bimetals exhibited a dual-phase DSS part and a single-phase ASS part. Notably, the interface was dominated by fine austenite grains and exhibited a high fraction of Σ3 twin boundaries (∼52.3%), which induced grain boundary strengthening and effectively suppressed microcrack initiation, thereby promoting strong metallurgical bonding. The DSS/316L bimetals exhibited a nearly defect-free interface, and the failure of the bimetals occurred on the ASS side. Overall, these results demonstrate a cost-effective strategy for directly fabricating DSS/ASS bimetals with strong interfacial bonding via the LPBF.