Effect of ultrasonic rolling coverage rate on surface integrity and strengthening mechanisms of Ti–6Al–4V alloy by the trajectory interference experiments

The Ti-6Al-4V alloy faces challenges in wear and fatigue resistance despite its high specific strength and specific stiffness. The ultrasonic surface rolling process (USRP) has been shown to effectively improve performance by creating gradient nanostructures, residual compressive stress layers, hardened subsurface layers and reduced surface roughness. However, the influence of trajectory interference behavior during USRP on material flow and strengthening mechanisms remains unexplored. This study established a theoretical relationship between the feed rate and coverage rate during USRP and investigated the material flow behavior through the dual-track interference scratch experiment and the multi-track interference USRP experiment. The effects of different coverage rates on the friction force, surface morphology, surface roughness, microstructure, hardness field and residual compressive stress field of Ti-6Al-4V workpieces after experiments were investigated. The findings showed that increasing the coverage rate significantly enhanced material flow uniformity, decreased surface roughness by filling micro-valleys and suppressing the adhesive wear. The surface roughness was reduced to Ra 0.022 μm, the maximum subsurface hardness value could reached to 364.17 HV, and the maximum residual compressive stress in the subsurface reached to 610.1 MPa with the effective layer depth of 700 μm under optimal USRP parameters. Furthermore, the amorphous layer and the highly oriented deformed grain layer were formed after USRP. While the high-density dislocation was observed within the highly oriented deformed grain layer, which contributed to the maximum residual compressive stress which appeared at the topmost surface of Ti-6Al-4V.