Environmental protection and resource conservation are the major challenges of this century. In a globalized world, the transportation of people and commodities plays a fundamental role in this topic. In order to make transport more efficient, lightweight construction is a valuable approach, since not only greenhouse gases such as CO2 can be saved during operation, but also material during production. Welding is a common joining method for lightweight constructions, since less mass can be used than in a comparable screwed or riveted joint. Among welded joints, modern solid-state joining processes are considered as particularly environmentally friendly because they consume less energy and produce less emission than conventional welding processes. In addition, solid-state joining processes can also be used to join materials that are typically considered non-weldable. But the welding process and its effects on the materials must be carefully understood to prevent structural failure.
One of these solid-state joining processes that is particularly suitable for joining large structures is refill friction stir spot welding (RFSSW). RFSSW is performed by a three-part tool consisting of: clamping ring, sleeve and pin. Under rotation, the sleeve penetrates into the material to be welded causing dynamic recrystallization of the microstructure due to temperature and deformation within the stir zone. In addition to the stir zone, other zones with different strengths are formed within the spot weld, i.e. thermo-mechanical affected zone and heat affected zone.
In order to investigate the influence of spot welds on the failure of the material, an experimental setup was designed and mechanical tests were carried out, including cyclic and quasi-static loading cases. During quasi-static loading, the local deformation was recorded using digital image correlation (DIC) and subsequently evaluated. In addition to DIC, residual stresses resulting from the welding process were measured in various ways. As a result of the tests, it was found that the angle at which the crack hits the spot weld has a large influence on the further crack propagation in the material.