In Situ Observations of the Strain Competition Phenomenon in Aluminum Alloy Resistance Spot Welding Joints during Lap Shear Testing

The real-time evolution of the deformation and strain field of non-heat-treatable aluminum alloy 5754 and heat-treatable aluminum alloy 6061 resistance spot welding joints during the lap shear test was extracted using the digital image correlation (DIC) technique. The strain competition phenomenon between the nugget and its peripheral metal was quantitatively analyzed by applying 2D and 3D DIC analyses. The quantitative data show the tensile strain concentrated in the peripheral metal of the AA5754-O joint, which fractured in the pull-out mode. In comparison, a significant shear strain appears in the nugget of the AA6061-T6 joint, leading to its fracture in the interfacial failure mode during the lap shear test. The phase evolution of the nugget was analyzed using the thermodynamics database JMatPro, which was further used to calculate the local strength of the joints. The results indicate that the nugget strength of AA5754 is 223 MPa, the nugget strength of AA6061 is 178 MPa, and the heat-affected zone (HAZ) strength of AA6061 is 263 MPa. By inputting the local strength data, the calculated result of the analytical load-bearing competition model is in accordance with the experimental data of the lap shear test.