Diffusion welding of CoCrNi Medium-entropy alloy (MEA) and SUS 304 stainless steel using different interlayers

With the emergence of entropy alloys, the scientific community has been persuaded to explore its joining issues for some stimulating and un-explored engineering applications. Currently, CoCrNi Medium-entropy alloy (MEA) is considered to be an excellent cryogenic material which can retain highest strength and ductility even at cryogenic temperature (i.e. 77K). With such extravagant properties, authors compelled to explore the joining issues of this alloy. Therefore, the motivation of this research was to examine the weldability of CoCrNi MEA with the commercially available SUS 304 stainless steel using different interlayers. This research work was mainly concerned to investigate the effect of Ni, Cu, and Nb interlayers on bond formation and interface reaction during vacuum diffusion welding process. Results clinched that Ni-interlayered joints were free from the formation of Intermetallic Compounds (IMCs) and offered maximum shear strength (425.5MPa). Cu-interlayered joints displayed the formation of Cr-C IMCs at Cu-SUS interface while Nb-Co, Nb-Ni and Nb-Cr-Ni phases were formed at Nb-MEA side. Formation of microvoids, cracks and presence of IMCs was observed in Nb-interlayered welded samples which caused lowest shear strength (238.12MPa). Energy dispersive X-ray (EDX) and electron probe micro analysis (EPMA) were used to examine the diffusion thickness, diffusivities of constituent elements and other microstructural features across the welded joints. Scanning electron microscopy (SEM) scans and X-ray diffraction (XRD) was also executed on fractured surfaces to comprehend the joint formation mechanism.