The microstructures and mechanical properties of underwater bobbin tool friction stir-welded 6082-T6 aluminum alloy

The aim of this work is to investigate the evolution of the peak temperature of thermal cycle, microstructure, and mechanical properties of the joint during underwater bobbin tool friction stir welding (underwater BT-FSW) of 6082-T6 aluminum alloy with the butt joint by comparison with atmospheric bobbin tool friction stir-welded (atmospheric BT-FSW) joints. The results show that the peak temperature of advancing side (AS) and retreating side (RS) at 3 mm from the center of the weld reduces to 440 degrees C and 461 degrees C respectively because of the cooling effect of the aqueous medium. On the cross-section of the joint, an "S" line and a distinct gray-white texture can be seen. The stir zone (SZ) has much smaller equiaxed recrystallized grains. The ranges of the heat-affected zone (HAZ) and the thermal-mechanically affected zone (TMAZ) of underwater BT-FSW joint are smaller than those of atmospheric BT-FSW joint. The microstructures of the SZ, TMAZ, and HAZ of underwater BT-FSW joint are alpha-Al + GP zones, alpha-Al + GP zones, and alpha-Al + beta' + Q' phases respectively. The Vickers hardness of underwater BT-FSW joint distribution profiles exhibit a W-shape. The SZ of underwater BT-FSW joint has the highest Vickers hardness at 94-103 HV and the minimum hardness of underwater BT-FSW joint appears at the transition zone of HAZ and TMAZ and is 79 HV. The tensile strength of underwater BT-FSW is 308 MPa, which is 95% of the strength of the base metal (BM).