Effect of SiC/TiB₂ Nanoparticles and MPFSP on the Microstructure and Mechanical Properties of the Aluminum Alloy AA7050

This study investigates the influence of multi-pass friction stir processing (MPFSP) on the microstructure and mechanical properties of AA7050 aluminum surface composites reinforced with a hybrid mixture of 50% SiC and 50% TiB 2 (weight %) nanoparticles. The processing was conducted at a tool rotational speed of 1000 rpm and a welding speed of 70 mm/min. The effect of up to six FSP passes was analyzed to optimize nanoparticle dispersion and mechanical performance. The ultimate tensile strength (UTS) improved significantly from 429.56 MPa in the single-pass condition to 436.64 MPa in the second pass, reaching 458.09 MPa after four passes and peaking at 495.64 MPa after six passes due to refined grain structures and homogeneous reinforcement distribution. Similarly, the strain increased from 10.15% (1 pass) to 12.88% (2 passes), peaked at 15.83% (4 passes), but decreased to 11.59% (6 passes), indicating potential embrittlement with excessive processing. Microhardness in the stir zone also followed this trend, rising from 146.00 HV in the single-pass sample to 155.33 HV (2 passes) and reaching a maximum of 164.33 HV after four passes, before slightly reducing to 149.67 HV in the six-pass sample. X-ray diffraction (XRD) confirmed the presence and refinement of the phases such as TiB 2 , SiC, and Mg 2 Si with increased FSP passes, contributing to grain refinement and improved mechanical properties. Fractographic analysis revealed ductile failure with decreasing dimple size and depth at higher passes. The results underscore that four FSP passes provide the best combination of strength, ductility, and hardness, making MPFSP a promising approach for fabricating high-performance aluminum nanocomposites.