Research on the machining quality and damage suppression of Ti/CFRP stacks by longitudinal torsional ultrasonic vibration assisted drilling

The stack structure of titanium alloy and carbon fiber reinforced polymer (Ti/CFRP) is prone to multiple defects during conventional drilling (CD), which severely restricts its practical application. Considering the favorable machining characteristics of longitudinal torsional ultrasonic vibration assisted drilling (LTUVD), the feasibility and damage suppression mechanism of this method for Ti/CFRP stacks were investigated through combined theoretical analysis and experimental validation. Crack propagation in CFRP and deformation behavior of Ti under longitudinal torsional vibration were analyzed theoretically. Drilling experiments were conducted to systematically evaluate machining performance using thrust force, temperature, damage morphology and hole wall integrity as key indicators. The results indicate that ultrasonic vibration effectively suppresses crack propagation in CFRP and deformation in Ti. Compared with CD, thrust force in Ti and CFRP decreased by 19.3% and 10.5% respectively. Increasing spindle speed leads to a pronounced rise in interface temperature during LTUVD, whereas feed rate shows no consistent influence on temperature. Higher feed rates exacerbate mechanical and thermal damage at the CFRP interface, while the interface damage factor increases with spindle speed. Relative to CD, interface damage in CFRP was reduced by 15.5%, and exit damage factor decreased by 15.6% under ultrasonic vibration assistance. Under the combined influence of mechanical loading and thermal effects, defects such as fiber bundle deformation and pits were observed on the CFRP hole wall.