Microstructural features and mechanical properties of AM60 and AZ31 friction stir spot welds

The microstructural features and mechanical properties of AM60 and AZ31 friction stir spot welds are investigated in joints made using different tool designs (threaded and three-flat/threaded tools) and dwell time settings. Since the hook regions are curved inwards towards the keyhole periphery in AM60 friction stir spot welds made using threaded and three-flat/threaded tools and different dwell time settings, the distance from the tip of the hook region to the keyhole periphery mainly determines their failure load properties. In contrast, the hook regions are curved outwards from the axis of the rotating tool in AZ31 friction stir spot welds and their failure strength properties are determined by the bonded width, the distance from the tip of the hook region to the sheet intersection, the depth of tool shoulder penetration into the surface of the upper sheet and the distance from the tip of the hook region to the top of the welded joint.The highest failure load properties are found in AM60 friction stir spot welds made using a three-flat/threaded tool and dwell times of 3 and 4 s, since the distance from the tip of the hook region to the keyhole periphery is highest in these joints. The highest failure load properties are found in AZ31 friction stir spot welds made using the three-flat/threaded tool and a dwell time of 1 s, since these joints have the largest bonded widths, distances from the tips of the hook regions to the sheet intersection, and limited penetration of the tool shoulder into the surface of the upper sheet (about 0.5 mm).