Effect of Tool Shoulder Diameter on Mechanical Properties of Friction Stir Spot Welded Joints

The effect of tool geometry on microstructure and tensile shear strength in friction stir spot welds of 6061 aluminium alloy sheets was studied. Tools with three different shoulder diameters were evaluated. The shape parameters dn and tu were defined from the weld structure observed on the cross section of the weld centre, which represent the actual nugget size and the thickness of the upper sheet under tool shoulder circumference, respectively. These parameters and the shape of the boundary between the upper and lower sheets varied depending on shoulder diameter, tool rotational speed and tool holding time. The tensile shear strengths at the fixed tool rotational speed were dependent on tool holding time and shoulder diameter. Two fracture modes were recognized under tensile shear loading at the shoulder diameters of 10 and 15 mm. The dependence of static strength on shoulder diameter was discussed based on the shape parameters and fracture modes.