Proposed Diffusion Mechanism and Microstructural Evolution in MIG Braze-Welded Cu–Austenitic Steel Joints

Dissimilar joints between copper and austenitic stainless steel are relevant to marine and shipbuilding systems in which corrosion resistance, thermal conductivity, and structural integrity are required. This study investigated the formation, microstructure, and mechanical performance of Cu–austenitic steel joints produced by MIG braze welding, with particular emphasis on wetting, interfacial diffusion, and the asymmetric response of the two base materials. The joints were produced within a process window of 150–170 A, 19–21 V, and 220–320 mm/min. Their quality and structure were evaluated using visual testing (VT), penetrant testing (PT), tensile testing, light microscopy (LM), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), and electron backscatter diffraction (EBSD). The applied process parameters affected filler-metal spreading, joint continuity, interfacial morphology, and the occurrence of welding imperfections. Among the defect-free joints selected for tensile testing, the ultimate tensile strength ranged from 123 to 131 MPa. The highest UTS of 131 MPa was obtained for the joint produced at 170 A, 19 V, and 320 mm/min. Microscopic and compositional analyses showed that joint formation proceeded differently at the Cu-side and steel-side interfaces, reflecting differences in the thermal and metallurgical behavior of the base materials and resulting in an asymmetric interfacial structure. Based on the LM, SEM/EDS, and EBSD observations, an integrated joint-formation mechanism combining wetting, diffusion, solidification, and side-dependent microstructural evolution was proposed. The results demonstrate that MIG braze welding can produce continuous Cu–austenitic steel joints within a defined process window and that joint quality and mechanical performance depend on the specific combination of current, voltage, and travel speed.