Bake-Strengthening of Resistance Spot Welded Aluminum Alloy 6061

Three different heat treatment sequences, which alter the solute redistribution state in as-welded (AW) Aluminum Alloy 6061 resistance spot welds, were designed to investigate the effect of welding-induced microsegregation on the bake-strengthening ability of the joints. These sequences were postweld baking (AW-B), preweld solution treatment and postweld baking (SW-B), and postweld solution treatment and baking (WS-B). Vickers hardness and lap-shear test results showed that, compared with the AW samples, both the AW-B and SW-B samples experienced limited weld strengthening, while the WS-B sample welds experienced significant strengthening after baking (the strength was found to be comparable to that of the base metal). Calculation of the solidification path in the fusion zone using the Scheil-Gulliver model showed the effective solutes (Mg and Si) are segregated at the interdendritic regions in the AW samples (to form the beta-Mg2Si phase and eutectic), as confirmed by transmission electron microscopy. This reduced the degree of solute supersaturation, thereby significantly weakening the bake-strengthening ability of this site. Only the postweld solution treatment, which caused the effective solutes to diffuse into the Al-rich dendrites, could make the bake-hardening ability of the weld comparable to that of the base metal (confirmed by scanning transmission electron microscopy). Finally, digital image correlation analysis was conducted on a specially designed lap-shear test sample to reveal the local strain in the joints. It was found that, for the same weld size, the strain concentration occurred in the fusion zone of the AW and AW-B samples, and at the load-bearing site in the base metal of the SW-B and WS-B samples. Thus, the application of postweld solution treatment could improve the bake-strengthening ability of Aluminum Alloy 6061 resistance spot welded joints and increase the tendency of the joints to experience pullout failure.