The present study used underwater friction stir welding (FSW) to join WE43 alloy and AA-7075 alloy, offering better thermal control than conventional FSW. This technique resulted in grain refinement and the formation of dual phases with a more uniform distribution of beta (ß) and gamma (γ) phases. These microstructural improvements enhanced the tensile strength and ductility in the stir zone (SZ). Specifically, in Air-FSW, the strength was 51 MPa with 35 % joint efficiency, whereas in Water-FSW, it increased to 103 MPa with 64.3 % joint efficiency. The transverse Water-FSW joint exhibited a 7.36 % increase in elongation. The reduction in residual stress in Water-FSW enhanced ductility compared to traditional joints. The evenly distributed hard intermetallic compounds also contributed to the improvements in strength and elongation.