Weld zone and residual stress development in AA7050 stationary shoulder friction stir T-joint weld

This paper shows the potential of stationary shoulder friction stir welding (SSFSW) for producing higher quality T-section joints relative to a conventional friction stir welding (FSW) approach. The residual stress distributions and their relationship to the weld zone microstructure and hardness distributions in SSFSW T-joints were investigated, as a function of welding travel speed. The final longitudinal residual stress fields were asymmetric, although individual weld zones showed similarities to those for conventional butt SSFSWs. The thermal excursion and plastic strain arising from the second pass lowered the residual stresses seen from the first pass, so that the largest tensile stresses (similar to 160 MPa) were found close to the nugget from the second weld pass. The asymmetry in hardness distribution was caused by the thermal field of the second pass which thermally treated material in the first pass and resulted in areas of age hardening and increased over ageing, depending on the position of overlap of the thermal fields. The effects of the second weld pass on the first pass were more apparent when a lower travel speed was used owing to the increase in heat input and duration of the thermal cycle.