Fatigue crack growth and residual stress in simultaneous double-sided friction stir welded aluminum alloy AA6061-T6

Friction stir welding has demonstrated significant efficacy as a solid-state welding methodology for aluminum alloys, including AA6061-T6, and is extensively utilized within automotive and aerospace engineering domains. Nonetheless, conventional FSW methods often lead to uneven residual stress distributions, compromising the material's resistance to fatigue cracking. Simultaneous Double-sided Friction Stir Welding (SDFSW) was introduced to overcome this limitation, offering enhanced welding quality by welding from both sides. This study examines the influence of tool rotational velocity on the fatigue crack growth and the distribution of residual stresses in the SDFSW process applied to AA6061-T6 aluminum. Several rotational velocity combinations were employed to assess their effect on joint quality, encompassing residual stress distribution and cyclic load performance. Based on previous experiments, the SDFSW process uses upper and lower tool speeds. These are 965/965 rpm, 967/1251 rpm and 965/1555 rpm. Fatigue crack growth testing complied with ASTM E647 standards, and the residual stress distribution was assessed through the X-ray diffraction cos α method. Additional mechanical property assessments were performed, including radiographic analysis, examination of the macrostructure and microstructure, microhardness testing, evaluation of tensile strength, and fracture characterization. The findings reveal that the rotational velocity of the tool significantly impacts the weld zone's microstructure, influencing mechanical properties, residual stress distribution, and crack growth behaviors. Among the tested conditions, the tool's rotational speed of 965/1555 rpm yielded the highest tensile strength of approximately 179.82 MPa, representing about 53 % of the strength of the base material and the greatest microhardness of 85 HV. This velocity combination also demonstrated a low fatigue crack growth rate, with Paris law coefficients C and n measured at 2E-08 and 3.6931, respectively, along with a more favorable residual stress distribution.