Strengthening of dissimilar spot welds of Al alloy and steel by friction stirring

In conventional friction stir spot welding (FSSW) applied to a dissimilar metal lap joint of an Al alloy and steel, it has been difficult to improve the weld strength because the FSSW tool has to be plunged only into the upper Al sheet and not into the lower steel sheet in order to prevent the tool from wearing or chipping. In the present study, therefore, we propose a novel spot joining process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics. When this process is applied to the lap joint of the Al alloy and steel, the tool can be plunged into the lower steel sheet, then a steel projection is formed and plays the role of an anchor against the upper Al sheet. As a result, this anchor effect of the steel projection can significantly increase the weld strength. This new spot joining was carried out for a dissimilar metal lap joint of A5052 and SPCC with the same thicknesses of 1 mm, accordingly, the tensile shear strength reached about 3.6 kN/point.