Influence of the temperature conditions on the joining quality during refill friction stir spot welding of aluminium

Refill friction stir spot welding (refill FSSW) is the subject of previous studies, which investigate the influence of welding parameters as well as tool wear on the welding results, and thereby prove its high potential in joining materials with limited joinability. For example, it is shown that although the parameter window for reproducible welds shifts depending on the wear condition of the tool, sound joint properties can still be achieved. However, in the studies regarding the tool wear, a change in temperature conditions always occurred in combination with the geometric and tribological changes on the tool. The change in tool properties was thus always superimposed on the temperature changes; a differentiated observation is still pending. Especially, the effect of varying temperatures of the welding tool as well as the aluminium sheets due to fluctuating ambient conditions on the process stability is unknown. Therefore, the aim of this study is to investigate the influence of the joining temperature on the weld quality and the process stability independently of the geometrical and tribological properties. For this purpose, both joining partners and the welding tool are specifically tempered and therefore, their start temperature is set at the beginning of the process. This specifically simulates the different conditions in a series production as they can be caused by different environmental influences. The welds are then characterized regarding their mechanical and metallographical properties and the process conditions are compared with the results. Finally, an evaluation regarding the influence of the tool and workpiece temperature on the welding result in refill FSSW is carried out. Based on the results generated in this study, a contribution to the understanding of the process robustness is given, which improves the applicability of refill FSSW for industrial manufacturing.