This study aims to examine how friction stir welding parameters, such as welding speed and rotational rate, aect the microstructure, defects, and mechanical properties of AA7075-T651 aluminum alloy joints. In addition, it assesses the relationship between the defects and microstructures and the mechanical properties. Microstructural investigations using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) indicated remarkable variations in grain structure among dierent welding zones. It was found that the interface between Welding Nugget Zone (WNZ) and Thermo-Mechanically Aected Zone (TMAZ) was a dominant determinant of the mechanical properties of joints. The signicance of the interface arises from the fact that it is the most prone region to cracks, micro-cavities, and tunneling defects. The WNZ and TMAZ interfaces, as well as their grain structures, can be in uenced by the heat generated from the friction between the rotating tool and work-piece material. Therefore, coarser grain structures observed at the WNZ-TMAZ interfaces of the samples welded at higher rotational rates or lower welding speeds result from the greater heat generated in such cases. Besides, microstructural variations in the weld zone aect the hardness and mechanical properties of weld joints. Thus, samples with coarse-grained structures display lower values of yield stress and microhardness.

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