Microstructure and mechanical properties of resistance spot welded AZ31/AA5754 using a nickel interlayer

Dissimilar joining of aluminum (AA5754) and magnesium (AZ31) alloys was investigated in the current study via resistance spot welding. A commercial pure nickel interlayer was inserted between the two base metals to prevent the formation of Al–Mg intermetallic compounds. Microstructural investigations, via SEM/EDS and XRD methods, were carried out to characterize the Mg/Ni and Al/Ni interfaces. It was found that the Al–Mg reaction was successfully prevented and continuous submicron intermetallic layers (through Al–Ni and Mg–Ni reactions) were formed at the separate interfaces. The mechanical test results indicated substantial improvement in the joint strength compared to a direct joint without using an interlayer. The application of excessively high welding current (42kA) caused microstructure deterioration at the Mg/Ni interfacial region. Investigation of the fracture surface revealed that microstructure defects in the latter region should take responsibility for the reduction in the joint strength at high welding current (42kA).