Effect of Inert Gas Cover on the Static and Fatigue Behavior of AA6061-T6 Aluminum Alloy Friction Stir Spot Lap-Shear Welds.

Friction stir spot welding (FSSW) technology relies on the generation of frictional heat during the rotation of the welding tool in contact with the workpiece as well as the stirring effect of the tool pin to produce solid-state spot joints, especially for lightweight materials. Although FSSW offers significant advantages over traditional fusion welding, the oxidation of the interfacial bond line remains one of the most challenging issues, affecting the quality and strength of the joint under both static and cyclic loading conditions. In this experimental study, inert argon gas was employed to surround the joint, aiming to prevent or minimize the formation of the interfacial oxides. Two welding processes were conducted with identical welding process parameters and welding tool geometry: the conventional process and another that employs an inert gas cover. Micrographs of as-welded specimens were analyzed using a computerized optical microscope to characterize the interfacial bond lines and an energy-dispersive spectroscope (EDS) was used to quantify the interfacial oxides. Specimens from both welding conditions were tested under static and cyclic loads to investigate the static and fatigue behaviors, respectively. The fatigue tested specimens were examined under different load levels to investigate the fatigue crack behavior and the modes of failure at low-cycle and high-cycle fatigue conditions. The optical micrographs showed significant improvement in bond line morphologies (33% enlarged fully bonded area) and both static and fatigue strengths (35% reduced partially bonded area) when the inert gas cover was used. The EDS analysis revealed a maximum reduction of the interfacial oxide of 41% in the bond line achieved in the argon-surrounded joints compared to specimens of the conventional welding process. Accordingly, an improvement of 14% in the static strength was reached, along with 60% and 26% in the fatigue strengths at low- and high-cycle fatigue conditions, respectively.