On the role of rotational speed in P-FSSW dissimilar aluminum alloys lap weld

One of the most promising solid-state methods for joining high-strength aluminum alloys, friction stir spot welding (FSSW), remains hindering due to formation of the keyhole defect. This paper presents its promising variation as a probeless-friction stir spot welding (P-FSSW) assisted by flat featureless shoulder. Based on the prior arguments on the feasibility of this method in joining aluminum alloys, present work provides in-depth study on the effect of rotational speed on microstructure, phase transformations, and final properties of dissimilar aluminum plates, studies using scanning electron microscopy (SEM), occupied by electron backscattered diffraction (EBSD) method and energy-dispersive X-ray spectroscopy (EDS) analysis. The resulting properties was examined through microhardness measurements and tensile-shear testing. Results showed that rotational speed plays a crucial role in determining the intensity of material softening, its flow, and mechanical properties. Uniform heating and circumferential material flow helped eliminate edge hook defects using a featureless tool during P-FSSW. The microstructure development followed progressive plastic deformation was accompanied by a gradual transition between continuous dynamic recrystallization (CDRX) and dynamic recovery (DRV) process. The re-precipitation of the Al2CuMg phase restricted grain growth by pinning recrystallized grains within the stir zone (SZ). Mechanical tests indicated improved joint strength, suggesting that P-FSSW could produce high-quality, lightweight joints suitable for automotive and aerospace applications.