This study focuses on optimizing the underwater friction stir welding (UFSW) process parameters for AA8011 reinforced with 5 wt% silicon carbide (SiC) particles. The primary goal is to enhance the mechanical properties of the weld joints by systematically investigating the influence of rotating speed (A), traverse speed (B), and tool pin profile (C). The experimental setup utilized a Taguchi L9 orthogonal array, and optimization was performed using multi-criteria decision-making (MCDM) techniques, specifically MEthod based on the Removal Effects of Criteria (MEREC) and Measurement of Alternatives and Ranking according to COmpromise Solution (MARCOS). Mechanical properties such as tensile strength (TS), hardness (HV), and impact strength (IS) were evaluated alongside microstructural analysis. The ideal welding parameters—1000 rpm rotational speed, 20 mm/s travel speed, and hexagonal tool pin—were determined using MCDM approaches (MEREC and MARCOS), resulting in a maximum tensile strength of 108.88 MPa, hardness value of 98, and impact strength of 10 J. ANOVA indicated that the tool profile was the most significant factor, accounting for 81.44% of the joint quality, followed by rotational speed at 13.19% and traverse speed at 4.69%. Microstructural analysis revealed enhanced grain refinement and uniform material flow under optimal conditions, contributing to the improved mechanical performance. The findings underscore the critical role of parameter optimization in achieving high- performance weld joints and demonstrate the effectiveness of UFSW for producing durable and reliable composite materials. These insights provide a valuable foundation for advancing welding practices in aerospace, marine, and automotive industries.

Keywords: Underwater friction stir welding, Tool profiles, MEREC, Tensile strength, Hardness.