Carbon fiber reinforced plastics (CFRP)/Titanium alloy (Ti) stacks are being extensively applied in the aerospace industry for excellent mechanical properties. However, their poor machinability poses great challenges to the aircraft manufacturing industry. In this study, longitudinal-torsional ultrasonic vibration drilling (LT-UVD) is innovatively introduced to improve the quality of CFRP/Ti drilling. First, the separation mode of LT-UVD was analyzed by kinematic equation. Then, an experimental platform was built based on LT-UVD vibration actuator to carry out CFRP/Ti drilling experiments. The thrust force, interface temperature, hole wall quality, hole defects, Ti chip morphologies and tool wear in conventional drilling (CD), Longitudinal ultrasonic vibration drilling (L-UVD), and LT-UVD were compared in the experiment. The experimental results show that compared with CD and L-UVD, the thrust force of CFRP in LT-UVD decreases by 20.36%-40.55% and 2.04%-14.61%, and the thrust force of Ti decreases by 19.08%-24.83% and 1.95%-9.34%. At the same time, a relatively low maximum interface temperature is achieved in LT-UVD. In addition, the hole size accuracy, surface roughness for hole inner surface, and delamination factor are improved in LT-UVD. Due to the existence of torsional vibration in LT-UVD, the cavity and fiber pull-out defects, chip breaking performance, and tool wear of CFRP are improved. Finally, it is observed by high-speed camera that the damage forms of the interface area are different when drilling CFRP/Ti stacks with different drilling sequence.

Carbon fiber reinforced plastics (CFRP)/Titanium alloy (Ti) stacks are being extensively applied in the aerospace industry for excellent mechanical properties. However, their poor machinability poses great challenges to the aircraft manufacturing industry. In this study, longitudinal-torsional ultrasonic vibration drilling (LT-UVD) is innovatively introduced to improve the quality of CFRP/Ti drilling. First, the separation mode of LT-UVD was analyzed by kinematic equation. Then, an experimental platform was built based on LT-UVD vibration actuator to carry out CFRP/Ti drilling experiments. The thrust force, interface temperature, hole wall quality, hole defects, Ti chip morphologies and tool wear in conventional drilling (CD), Longitudinal ultrasonic vibration drilling (L-UVD), and LT-UVD were compared in the experiment. The experimental results show that compared with CD and L-UVD, the thrust force of CFRP in LT-UVD decreases by 20.36 %-40.55 % and 2.04 %-14.61 %, and the thrust force of Ti decreases by 19.08 %-24.83 % and 1.95 %-9.34 %. At the same time, a relatively low maximum interface temperature is achieved in LT-UVD. In addition, the hole size accuracy, surface roughness for hole inner surface, and delamination factor are improved in LT-UVD. Due to the existence of torsional vibration in LT-UVD, the cavity and fiber pull-out defects, chip breaking performance, and tool wear of CFRP are improved. Finally, it is observed by high-speed camera that the damage forms of the interface area are different when drilling CFRP/Ti stacks with different drilling sequence.

With the gradual promotion and the application of difficult-to-machine materials such as titanium matrix composites in the aerospace field, high-quality hole-making technology has become a major demand in aviation manufacturing. In order to improve the hole-making quality of TiBw/TC4 composites, asynchronous mixed frequency vibration assisted hole-making (AMFVAHM) method is proposed. The process consists of two steps which are base hole drilling assisted with ultrasonic vibration (UVAD) and hole expansion by helical milling assisted with low-frequency torsional vibration (LFTVAHM). Based on this process, the cutting trajectory modeling is established, and the hole-making experiment on TiBw/TC4 composites is conducted. The experimental data shows that the maximum XY-plane average milling force decreases by 30.96 % and the maximum axial average milling force decreases by 24.49 % compared with conventional helical milling (HM) when the torsional vibration frequency and the milling frequency are the same in LFTVAHM. The hole-making experiment shows that AMFVAHM can reduce the chip size, tool wear and some other defects such as entrance/exit burrs, scratches and fractures of the hole wall. Comparing with HM and UVAD, the verticality of hole wall increases by 71.43 % and 86.21 %, the inlet damage decreases by 27.98 % and 31.60 %, the outlet damage decreases by 2.80 % and 14.47 %, the hole wall roughness (Ra) decreases by 36.29 % and 63.43 %, and the maximum white layer thickness decreases by 19.99 % and 67.66 %. Meanwhile, AMFVAHM process not only reduces the cutting force and cutting temperature, but also improves the hole-making quality due to the fretting friction effect of LFTVAHM in secondary hole expansion, which meets the need of high-quality hole-making of difficult-to-machine materials in practical engineering applications.

With the gradual promotion and the application of difficult-to-machine materials such as titanium matrix composites in the aerospace field, high-quality hole-making technology has become a major demand in aviation manufacturing. In order to improve the hole-making quality of TiBw/TC4 composites, asynchronous mixed frequency vibration assisted hole-making (AMFVAHM) method is proposed. The process consists of two steps which are base hole drilling assisted with ultrasonic vibration (UVAD) and hole expansion by helical milling assisted with low-frequency torsional vibration (LFTVAHM). Based on this process, the cutting trajectory modeling is established, and the hole-making experiment on TiBw/TC4 composites is conducted. The experimental data shows that the maximum XY-plane average milling force decreases by 30.96 % and the maximum axial average milling force decreases by 24.49 % compared with conventional helical milling (HM) when the torsional vibration frequency and the milling frequency are the same in LFTVAHM. The hole-making experiment shows that AMFVAHM can reduce the chip size, tool wear and some other defects such as entrance/exit burrs, scratches and fractures of the hole wall. Comparing with HM and UVAD, the verticality of hole wall increases by 71.43 % and 86.21 %, the inlet damage decreases by 27.98 % and 31.60 %, the outlet damage decreases by 2.80 % and 14.47 %, the hole wall roughness (Ra) decreases by 36.29 % and 63.43 %, and the maximum white layer thickness decreases by 19.99 % and 67.66 %. Meanwhile, AMFVAHM process not only reduces the cutting force and cutting temperature, but also improves the hole-making quality due to the fretting friction effect of LFTVAHM in secondary hole expansion, which meets the need of high-quality hole-making of difficult-to-machine materials in practical engineering applications.