Experimental investigation of longitudinal-torsional ultrasonic vibration-assisted micro-milling of TC4 titanium alloy

TC4 titanium alloy is widely used in aviation, electronics, and medical fields owing to its exceptional properties but remains difficult to machine due to high cutting forces, severe tool wear, and poor surface integrity. This study presents an experimental investigation of longitudinal-torsional ultrasonic vibration-assisted micro-milling (LTUVAMM) for improving the machining performance of TC4. Comparative experiments between conventional micro-milling (CMM) and LTUVAMM were conducted by systematically varying spindle speed, feed per tooth, depth of cut, and ultrasonic amplitude. Compared with CMM, LTUVAMM significantly reduced the cutting force in all directions, with feed (F_x), radial (F_y), and axial (F_z) forces decreasing by 21.8–33.3%, 8.6–16%, and 18.5–27.2%, respectively. In addition, surface quality was improved with the formation of a uniform fish-scale microstructure and a 3.8–17.7% reduction in surface roughness. Both cutting force and surface roughness exhibited nonlinear responses to amplitude, with minimum values et al./A_T = 3.3/1.65 μm. Tool wear analysis confirmed that LTUVAMM substantially reduced the cutting-edge wear-band width, flank wear, and adhesive material buildup compared with CMM. These results demonstrate that LTUVAMM is an effective process strategy for improving cutting performance in the micro-milling of TC4 titanium alloy.