Interface-specific evolution in directed energy deposition-processed SS316L-CuSn10-NiTi joints

Abstract Directed Energy Deposition (DED) can join dissimilar alloys, but differences in thermophysical properties and interfacial chemistry make the process difficult. This work examines the interfacial microstructure and phase formation of DED-processed SS316L-CuSn10-NiTi joints, in which a CuSn10 bronze interlayer separates SS316L from NiTi. The two interfaces developed different structures. At the SS316L-CuSn10 interface, Fe and Cu redistributed and solidified in a fishbone pattern within an intermixed reaction zone of about 350 μm. The CuSn10-NiTi interface produced a narrower transition zone of 100 to 150 μm with overlapping Cu, Ni, Ti signals and a dendritic structure. Within the NiTi region, the microstructure evolved from fine dendrites through coarser dendrites to elongated feathery dendrites. XRD identified NiTi, Ti 2 Cu, Ni 3 Ti, CuTi, a Cu-Sn solid solution, Cu 3 Sn and γ-Fe reflections. No Fe-Ti intermetallic reflections were detected within the detection limit of XRD, which is consistent with the barrier role of the interlayer and with the local composition lying outside the Fe 2 Ti field of the Fe-Ni-Ti phase diagram. The CuSn10 interlayer therefore separates the two dissimilar reactions and prevents direct SS316L-NiTi contact, which makes it a practical choice for DED joining of these alloys.