Efficacy of Stationary Shoulder Friction Stir Welding to Minimize the Abnormal Behavior of Grain Growth in 7075 Aluminum Alloy

Friction stir welding (FSW) is an advanced solid-state joining technique with significant industrial potential. However, its application is limited due to the susceptibility of weld nuggets to abnormal grain growth (AGG) and degrading weld quality. This study investigates the efficacy of stationary shoulder friction stir welding (SSFSW) in mitigating AGG by reducing heat input and peak temperature significantly below the solutionizing temperature of base metal compared to conventional friction stir welding (CFSW). Weld joints of 7075-T651 aluminum alloy were produced with both welding processes and analyzed as-welded and post-weld T6 heat treatment for microstructural characteristics, dislocation density, and mechanical properties. The SSFSW process generates lower peak temperature, leading to finer grain structures, high dislocation density, and reduced propensity for AGG. In addition, grain sub-division and discontinuous grain growth were observed in SSFSW, while only the discontinuous grain growth was witnessed in CFSW. Consequently, AGG was mitigated in the SSFSW than in CFSW due to the domination of both mechanisms over discontinuous grain growth that occurred in CFSW. Electron backscattered diffraction analysis revealed higher dislocation density and energy stored in dislocations in SSFSW welds, contributing to high resistance against AGG. Mechanical testing indicated welds produced through SSFSW possess higher micro-hardness and ultimate tensile strength, demonstrating its potential to enhance weld quality by minimizing AGG.