Surface and sub-surface analysis of rotary ultrasonic elliptical end milling of Ti-6Al-4V

Ultrasonic vibration has received widespread attention for its dramatic effect on grain refinement and microstructure modification during casting, additive manufacturing, cold rolling, and cutting, changes which can significantly improve the mechanical and physical properties of components. As a novel ultrasonic vibration cutting method, rotary ultrasonic elliptical milling (RUEM) has been introduced to mill the alloy Ti-6Al-4V. However, the effects of ultrasonic elliptical vibration on the microstructures of machined surfaces in end milling of Ti-6Al-4V are still unclear. A comprehensive study on the surface characteristics and sub-surface microstructure in RUEM of Ti-6Al-4V was conducted. The results show that the uniform textures, in the form of ridges mapped on the machined surface in RUEM, varied with the cutting speed. Compared with conventional milling, microchip debris adhesion on the machined surface was significantly reduced by using RUEM. Moreover, intense plastic deformation in the sub-surface was obtained, and nanocrystalline layers, with grain dimensions of 10 nm to 100 nm, were fabricated on the processed surfaces of RUEM. Additionally, the improvement in sub-surface microstructure increased the surface micro-hardness from 21.22% to 33.84%. This study allows an in-depth understanding of sub-surface deformation and surface nanocrystallization in RUEM of Ti–6Al–4V.