Impact of Backing Plate and Thermal Boundary Conditions for High-Speed Friction Stir Welding of 25-mm Thick Aluminum Alloy 7175-T79

This study focuses on the challenges with butt joining of 25 mm thick aluminum alloy 7175-T79. High-speed (150 mm/min) single-pass friction stir welding was employed as an effective technique for this purpose. The influence of quenching and cooling rate on critical performance level indicators such as joint strength and microstructure was investigated. A series of friction stir welding (FSW) trials, conducted both in air and with a trailing water spray using steel and composite backing plates (BP) revealed distinct hardness distribution in the nugget, heat-affected zone (HAZ), and HAZ minimum hardness. The influence of trailing water spray (TWS) on joint efficiency proves more significant than the impact of BP combinations, owing to their markedly different contributions to the quenching process. The ultimate tensile strength of TWS welds exhibited a notable 14% increase compared to the air welds. TWS also introduces multifaceted effects on FSW, including a reduction in processing temperature, an increase in X and Z forces, and decrease in Y force, and a narrowing of the HAZ. Lastly, the study employs digital image correlation (DIC)-based fracture mode analysis and grain size measurements, establishing correlations with the observed micro-hardness distribution.