Tailoring the Surface Integrity of Ti-6Al-4V Alloy by Ultrasonic Surface Rolling Process: A Review.

Ti-6Al-4V alloy is widely used in aerospace and other high-performance engineering components, but its service reliability is often constrained by surface-initiated fatigue, fretting damage, wear, and corrosion. Ultrasonic surface rolling process (USRP) couples a static rolling force with high-frequency mechanical impacts to introduce severe plastic deformation while retaining relatively low surface roughness, thereby producing a gradient-strengthened surface layer. This review systematically summarizes advances in USRP strengthening of Ti-6Al-4V alloy within a "process-microstructure-surface integrity-service performance" framework. The effects of static load, ultrasonic amplitude and frequency, feed rate, spindle speed, processing passes, treatment temperature, and lubrication conditions are first compared. Particular attention is then paid to the mechanisms governing dislocation multiplication and rearrangement, grain subdivision, gradient nanostructure formation, the responses of the α and β phases, deformation-induced phase transformation, and the evolution of depth-dependent residual compressive stress. The intrinsic relationships between these mechanisms and surface roughness, hardness, strengthened layer depth, wear and corrosion resistance, fatigue performance, and fretting fatigue performance are subsequently clarified. Control strategies involving electropulsing, laser/temperature assistance, deep cryogenic treatment, and coating combinations are further reviewed, together with methods for contact dynamics analysis, residual stress prediction, and data-driven optimization. The combined evidence indicates that the performance gains from USRP are jointly controlled by surface defects, gradient microstructure, and residual compressive stress; excessive load, processing passes, or heat input may weaken or even reverse the fatigue benefit because of defect accumulation, gradient mismatch, and residual stress relaxation. Current limitations include inconsistent reporting of process parameters, difficulty in quantitatively separating the contributions of different strengthening mechanisms, insufficient investigation of residual stress stability, and limited validation on complex components.