Exploring the boundaries of refill friction stir spot welding: influence of short welding times on joint performance

Refill friction stir spot welding is a solid-state spot-welding technique suited to lap joining of thin aluminium sheets, including difficult-to-weld 2xxx series alloys that are prone to hot cracking during fusion welding processes. Long welding time is an ongoing challenge that hinders industrial adoption of the process. To address this, the present study explores much shorter welding times than those previously reported in the literature and assesses the impact on joint quality. Joints of 1.8 mm thick AA2024-T3 sheet were produced with welding times from 3 s, down to 0.75 s and rotational speeds of 1000 rpm to 2500 rpm. Defect formations within the welds were studied with the aid of optical microscopy. The mechanical properties were evaluated using tensile lap shear testing and microhardness mapping, and failure modes were characterised using scanning electron microscopy. Various weld defects were found at all welding times and rotational speeds, and the defects enlarged with decreasing welding time and increasing RS. The highest lap shear strength of 9.21 kN was achieved with a welding time of 3 s and rotational speed of 2000 rpm; lap shear strengths of 7.02 kN and 6.37 kN were achieved for 1.5 s and 0.75 s welds, respectively.