Optimizing the mechanical properties of synergistic double-sided friction stir welding joints of aluminum alloy 2024 by utilizing asymmetric tool geometries

Our previous study identified a reduction in the ultimate tensile strength (UTS) of the synergistic double-sided friction stir-welded joints of aluminum alloy 2024, which was on the average half of the UTS of the base material (BM) due to defects in the joint's microstructure. This study addresses it by improving the microstructure and mechanical properties of the joints by modifying the tool with two distinct designs. As a result, there are no typical defects throughout the weld cross-section, and the grain size is reduced to ultra-fine grains in the nugget zone (NZ). The UTS measured exceeded 400 MPa, at a slightly longer elongation than ever, with a mixed-mode fracture where fine-equiaxed dimples with shallow depth and quasi-cleavage facets were present due to the brittle-to-ductile transition of the fractured surface. The softened zone is distributed in the heat-affected zone (HAZ), where averagely 25% decreased from its BM. The NZ shows distinct alignment of the grains producing a reduction of high-angle grain boundaries. The peak engineering strain calculated is between 14% and 17% at the maximum, which was measured around the thermomechanical affected zone and HAZ, and not in NZ. This outcome was achieved using two welding speeds of 100 and 150 mm/min at a rotation speed of 800 rpm.