Underwater Friction Stir Fabrication and Performance Analysis of <scp>AA2024</scp> Composites Reinforced With In Situ <scp> AlB ₂ </scp>

ABSTRACT This study investigates the microstructural evolution and mechanical performance of AA2024/AlB 2 composites fabricated via underwater friction stir processing with rotational speeds of 600–1200 rpm. Boron powder (15 ± 2 μm) was in situ transformed into AlB 2 during processing. Increasing rotational speed elevated the strain rate from 21.15 s −1 (600 rpm) to 31.05 s −1 (1200 rpm) and the peak temperature from 416°C to 487°C. Optimal reinforcement dispersion and microstructural refinement were achieved at 800 rpm, producing an average grain size of 2.3 ± 0.1 μm, a hardness of 139.5 ± 2.9 HV 0.1 , an ultimate tensile strength of 596.4 ± 10.1 MPa, and a toughness of 85.8 ± 1.4 MJ/m 3 . At lower speed (600 rpm), fine grains (1.9 ± 0.2 μm) were offset by less uniform particle distribution, limiting mechanical gains. Higher speeds (1000–1200 rpm) caused grain coarsening up to 5.2 ± 0.2 μm, hardness reduction to 101.1 ± 4.9 HV 0.1 , and UTS decline, attributed to particle clustering and thermal softening. Fractography revealed ductile fracture with microvoid coalescence, though high‐speed conditions promoted localized brittle zones from clustered reinforcements. The results identify ˜800 rpm as the optimum condition, balancing thermal and mechanical inputs to maximize grain refinement, reinforcement dispersion, and mechanical performance.