Microstructural influence on oxide-controlled wear behaviour of multi-pass friction stir processed AA6061-O aluminium alloy

An experimental investigation was performed to examine the microstructural evolution and tribological behaviour of friction stir processed AA6061-O alloy. The alloy was processed at different tool rotational speeds, traverse speeds, and numbers of passes. The specimen processed at 1000 rpm and 25 mm/min after the third pass exhibited the highest hardness of 63 HV0.3. Microstructural analysis showed that the average grain size was refined from 62 µm in the base material to 2.7 µm in the FSPed sample after the third pass. Dry sliding wear tests performed using a pin-on-disc tribometer under loads of 10–30 N and sliding velocities of 1–3 m/s exhibited a significant reduction in wear rate and a more stable coefficient of friction for the processed alloy. X-ray diffraction and microstructural analyses revealed lattice distortion accompanied by grain refinement and an increased proportion of high-angle grain boundaries, attributed to continuous dynamic recrystallisation. High-resolution scanning electron microscopy and energy-dispersive spectroscopy observations revealed a load- and velocity-dependent transition in wear mechanisms, establishing a direct correlation between friction stir processing, microstructural refinement, and wear behaviour of AA6061-O alloy.