Hook formation and mechanical properties in AZ31 friction stir spot welds

Hook formation and the failure load properties of AZ31 friction stir spot welds are investigated in friction stir spot welds made between wrought and heat-treated AZ31 sheets having widely different average grain sizes. Tool shoulder penetration into the surface of the upper sheet and pin penetration into the lower sheet during the dwell period provides the driving force for the displacement of lower sheet material upwards, the formation of hook regions and increasing stir zone and bonded widths. Once formed, hook regions are displaced outwards from the axis of the rotating tool in AZ31 spot welds made using dwell times from 1 s to 4 s, and are located at the boundary between the thermal-mechanical affected zone (TMAZ) and the heat affected zone (HAZ). Particularly poor failure load properties are found in AZ31 spot welds made without the application of a dwell period, since the bonded width is extremely small (about 280 μm). The failure load properties are highest in AZ31 spot welds made using a dwell time of 1 s and are decreased when the dwell period is further extended, since the beneficial influence of increasing bonded width is outweighed by the detrimental effects resulting from the increased distance from the sheet intersection to the tip of the hook region, angle of curvature of the hook region and shoulder penetration depth values.