Surface integrity and strengthening mechanism of Ti-6Al-4V subjected to high-speed ultrasonic vibration-strengthening milling

Titanium alloys have become indispensable in various fields, including aerospace systems, naval architecture, automotive engineering, offshore technologies, and biomedical implant production, owing to their superior mechanical characteristics. However, machining often results in poor surface integrity, compromising the safety and reliability of components. Despite the progress in machinability, studies on surface integrity improvement, particularly for Ti-6Al-4V under high-speed conditions, are scarce. Ultrasonic vibration-strengthening milling (UVSM) has emerged as a promising solution. This study evaluated the effectiveness of longitudinal UVSM for Ti-6Al-4V at 40–160 m/min and compared the surface morphology, roughness, microhardness, residual stress, and subsurface microstructure with those of conventional milling (CM). A dynamic response model was established based on the subsurface microstructure to elucidate the strengthening effects. The experimental results showed that UVSM significantly enhanced surface integrity compared with CM. The work-hardening layer was less affected by the cutting speed under UVSM (27.2 % reduction across the tested speed range), and the surface compressive residual stress at 160 m/min increased by 36.2 % compared with CM. In addition, the UVSM samples exhibited better surface quality and a thicker plastic deformation zone (>31.56 μm) than the CM samples. These findings demonstrate that integrating high-frequency ultrasonic vibration with high-speed milling significantly improves surface integrity compared to conventional methods.