Manufacturing of Dissimilar 6061–7075 Aluminum Alloy Joints Via Friction Stir Welding

The aim of this study was to investigate the macrostructure, microhardness, and basic mechanical properties of AA6061–AA7075 FSW butt joints produced at different tool rotational speeds (600, 1000, and 1400 rpm) and with offsets toward the AA6061 alloy (0, 1, and 2 mm). The results of the study made it possible to demonstrate that sound, defect-free dissimilar AA6061-T6/AA7075-T6 FSW joints can be produced within the investigated parameter set. The HAZ of the lower-strength alloy was identified as the most critical region of the joint, as confirmed by both microhardness measurements and the fact that all tensile-tested specimens fractured in the HAZ of the 6061-T6 alloy. At the same time, the minimum hardness in this region remained essentially unchanged (approximately 60 HV0.1) across the investigated welding parameters. For joints produced at a constant tool rotational speed of 1000 rpm, increasing the tool offset toward AA6061-T6 reduced weld nugget microhardness due to the increased fraction of the softer alloy in the stirred region, while simultaneously improving elongation at fracture without a noticeable change in tensile strength. Overall, the produced joints exhibited highly repeatable mechanical properties, with joint efficiency in the range of 65–67%.