Thermo-mechanically affected zone in AA6111 resistance spot welds

A hardened thermo-mechanically affected zone (TMAZ), measuring a few millimeters in width, was identified between the weld nugget and base metal in AA6111 resistance spot welds (RSW). The spatial distribution, microstructure, hardness and evolution of the TMAZ, and the impact on fracture morphology of spot welds were investigated through micro-hardness mapping, optical metallography, transmission electron microscopy, welding process simulation and quasi-static tensile tests. Strain hardening induced by the compressive force applied by the welding electrodes, as opposed to precipitation hardening induced by heat, was determined to be the dominant hardening mechanism in the development of the TMAZ in AA6111 RSW.