Optimization of hardness and wear behavior in AA6061/(Al2O3-TiC-MoS2) hybrid composites fabricated by friction stir processing

This study investigated the microstructure, hardness, and tribological properties of friction stir processed (FSPed) AA6061 aluminum alloy-based hybrid surface composites reinforced with various combinations of Al₂O₃, TiC, and MoS₂ particles. Three FSP passes were performed at a rotational speed of 950 rpm and a traverse speed of 30 mm/min. Microstructural characterization revealed pronounced grain refinement, improved particle distribution, and extensive fragmentation of the reinforcing particles. In the C/(33Al₂O₃-33TiC-33MoS₂) composite, the average particle sizes of Al₂O₃, TiC, and MoS₂ decreased from 4.7, 16.9, and 11.9 μm (as-received powders) to 0.25, 0.41, and 0.48 μm, respectively. The maximum microhardness of 119 HV was obtained for C/(50Al₂O₃-50TiC), representing an approximately 27% increase over the base material (BM). The highest wear resistance was achieved in sample C/(50Al₂O₃-50MoS₂), with a volume loss of approximately 0.59 mm^3, corresponding to a ~62% reduction compared with the BM. This improvement was attributed to the load-bearing and strengthening effects of Al₂O₃ and the solid-lubricating effect of MoS₂; as its average friction coefficient (AFC) was lower than that of the FSPed alloy by about 30%. The C/(33Al₂O₃-33TiC-33MoS₂) and C/(50Al₂O₃-25TiC-25MoS₂) samples exhibited the most balanced improvements, with hardness increases of at least 20% and volume-loss reductions of at least 45% compared with the BM, demonstrating the synergistic effects of hybrid reinforcement. Examination of worn surfaces indicated a transition from severe plastic deformation (SPD), extensive delamination, and abrasive wear in the BM to mild delamination and abrasive wear in the C/(50Al₂O₃-25TiC-25MoS₂) composite.