Microstructure and mechanical properties of aluminum surface composites fabricated via multi‑pass FSP with B ₄ C and Al ₂ O ₃ reinforcements

This study focuses on the fabrication of AA7475/B4C/Al2O3 hybrid surface composites (HSCs) using multi-pass friction stir processing (FSP) to enhance their microstructure and mechanical properties. The study utilized four numbers of FSP passes with constant tool rotational speed (1300 rev/min), feed rate (45 mm/min), and reinforcement ratio (50% B4C and 50% Al2O3) and investigated the ultimate tensile stress (UTS), tensile elongation (El), and hardness distribution of HSCs. The study identified that multi-pass FSP resulted in significant grain refinement, fragmentation of reinforced particles, and uniform distribution throughout the stir region of the HSCs. For a single-pass FSP, the average grain size at the stir region was 6.5 ± 0.20 µm, whereas for the four-pass process, it decreased to 2.2 ± 0.18 µm. The XRD results revealed that as the number of passes increased, the peak intensity of B4C and Al2O3 phases was consistently reduced, which suggests fragmentation of nanoparticles. The UTS recorded for 1 pass, 2 passes, 3 passes, and 4 passes were 447 MPa, 464 MPa, 481 MPa, and 501 MPa, respectively. The hardness measurements at the SZ were recorded as 162, 174, 183, and 192 HV for 1, 2, 3, and 4 FSP passes, respectively. After reinforcing B4C and Al2O3, AA7475 showed percentage improvements in UTS, El, and hardness of 22.79%, 69.01%, and 38.12%, respectively. Field emission electron microscopy (FESEM) image of tensile fractured surfaces exhibit honeycomb-like coarser dimples, resulting in brittle-ductile mode of fracture for one-pass FSP, while HSCs produced through four-pass FSP display finer dimples, demonstrating ductile fracture mode.