Research on Homogenization and Surface Integrity of Ti-6Al-4V Alloy by Longitudinal-Torsional Coupled Ultrasonic Vibration Ball-End Milling

This research studied the surface homogenization and morphology of a Ti-6Al-4V alloy using the longitudinal-torsional coupled ultrasonic vibration-assisted milling (UVAM) of a ball-end cutter, in which a method of continuous processing between the flat surface and the freeform surface connection was proposed. The cutting experiments compared the UVAM method with the conventional milling (CM) process by setting three parameters: the cutting speed, feeding rate, and depth of cut. The finished surface roughness, the in situ cutting force, and the topography of the surface are characterized in this paper. The surface roughness that resulted from UVAM ball-end milling was much better than the surface roughness of CM at high cutting speeds. In particular, there was a steep decrease of the average roughness rate from 0.668 μm at 4000 rpm to 0.161 μm at 5000 rpm. Moreover, the mean cutting force in ball-end milling using UVAM decreased by 20–40% compared to the CM process. The homogenization of the surface roughness also improved from 59.4 to 15.1% in the UVAM method compared to the CM method when the transverse-longitudinal ratio was taken into consideration. The cutting experiments have not only powerfully demonstrated the validity of the UVAM method but have also shown the beneficial effect that the UVAM process brings.