Mechanical properties and microstructure of aluminum alloy 7075-T6 spot-welding joint

In this study, an orthogonal experiment was conducted to investigate the effects of various process parameters on the mechanical properties of resistance spot welding joints in AA7075-T6 aluminum alloy. A numerical simulation was also performed to determine the optimal parameter combination. The mechanical properties, macro-morphology, and microstructure of the joints were characterized and analyzed through a series of tests including tensile testing, super depth-of-field testing, metallographic observation, and microhardness testing. The results show that the welding parameters have a significant effect on the mechanical properties of the joints with the following order of influence: welding time > welding current > electrode pressure. The combination of 60 ms welding time, 17 kA welding current, and 0.22 MPa electrode pressure yielded the best comprehensive mechanical properties with an average tensile load of 2364N and an energy absorption value of 6.8 J. The numerical simulation results are consistent with the nugget size of this parameter combination. During spot welding, spatter defect occurs, shrinkage cavity exists at nugget interface, strengthened T-phase and finer equiaxed grain structure precipitate in nugget center. After welding process and microstructure change, the hardness of the nugget zone is lower than that of the base material while the hardness of the nugget center is higher than that of the edge. The failure mode of the joint is interface fracture.