Failure modes of friction stir spot welds in cross-tension specimens of dissimilar aluminium sheets

Failure modes of friction stir spot welds in cross-tension specimens of dissimilar aluminium 5754-O and 7075-T6 sheets are investigated. Experimental results show that the failure loads of friction stir spot welds in cross-tension specimens are much lower than those in lap shear specimens. Optical micrographs of the 5754/7075 and 7075/5754 welds before and after failure under quasi-static loading conditions are examined to understand the failure modes of the welds in cross-tension specimens. The micrographs show that the 5754/7075 and 7075/5754 welds in cross-tension specimens mainly fail from the fracture along the interfacial surface and then from the fracture surface through the upper sheet material. The failure modes of the welds in cross-tension specimens are quite different from those in lap shear specimens. Finally, the paper presents discussion of the load carrying capacities and fatigue data of the welds in lap shear and cross-tension specimens based on the linear elastic stress intensity factor solutions.