Unravelling the influence of vibration on material removal and microstructure evolution in ultrasonic transversal vibration-assisted helical milling of Ti-6Al-4V holes

Helical milling (HM) is a developing hole-making technology in aircraft assembly. Nevertheless, fundamental problems, such as tool deflection and chip evacuation, when helical milling small-diameter or deep holes still exist. To solve these problems and improve the machining quality of Ti-6Al-4V holes, an ultrasonic transversal vibration-assisted helical milling (UTVHM) method is developed. The motion paths of the peripheral cutting edges are established, and the tool-workpiece contact behaviour is analysed to explore how transversal vibration affects the material removal of UTVHM. The hole surface strengthening mechanisms in the UTVHM were revealed and explored. Moreover, the verification experiments were conducted, and the results suggested that the transverse force and thrust force in the UTVHM reduced up to 38.1% and 13.9%, respectively, because of the intermittent cutting mode realised in the UTVHM compared to HM. Significantly improved surface integrity, excellent chip evacuation conditions, and narrow hole geometric tolerances were obtained using UTVHM. A maximum depth of 9.7 mu m for the plastic deformation layer was generated in the UTVHM, which was 6.5 times that of HM. The maximum increases in surface microhardness and residual compressive stresses in the UTVHM were 184.7% and 150.2%, respectively. Electron backscattered diffraction (EBSD) analysis showed that a higher plastic strain and smaller grain size occurred in the UTVHM. This study enhances the understanding of the surface and subsurface formation and material removal mechanisms of UTVHM, providing a theoretical basis and practical method for machining holes with high precision and excellent surface integrity in aircraft structures.