Study on the mechanism and performance of longitudinal-torsional ultrasonic vibration assisted drilling CFRP/Ti stack

The CFRP/Ti stack structure composed of Carbon fiber-reinforced polymer (CFRP) and Titanium alloy (Ti) is intensively used in the assembly manufacturing of aircraft key structures. The difference in physical properties between CFRP and Ti leads to the poor cutting matching performance of the two materials during the one-time drilling of CFRP/Ti stack. Longitudinal-torsional ultrasonic vibration assisted drilling (LTUVD) technology is considered to have excellent application prospects due to its excellent processing characteristics when machining difficult-to-machine materials. In this research, longitudinal-torsional ultrasonic vibration was applied to the drilling of CFRP/Ti stack structure. This research contributes to understand the machining mode mechanism of CFRP/Ti stack processed by LTUVD. Based on the thrust force, entrance morphology, exit morphology, interface morphology and hole wall morphology obtained by LTUVD and CD experiments, the action mechanism and drilling performance of longitudinal-torsional composite ultrasonic vibration in stack drilling were further expounded. The existence of longitudinal-torsional ultrasonic vibration makes the maximum thrust force of CFRP and Ti lower than that of CD by 39.6 % and 24.1 % respectively. The damage degree of CFRP entrance, Ti exit and interface obtained by CD machining was more serious than that of LTUVD. Among them, the entrance damage factor and interface damage factor of CFRP processed by CD were 10.21 % and 15.36 % higher than those of LTUVD, respectively. The morphology of CFRP hole wall formed by CD was slightly worse than that of LTUVD, and has higher surface roughness.