Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure
evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ’s width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.