Abstract: Due to its excellent mechanical and physical properties, the application proportion of CFRP/TC4 stacks in aerospace, automobile and other fields is increasing year by year. The application of longitudinal torsional ultrasonic vibration to the drilling of CFRP/TC4 stacks can obtain more ideal hole making effect than the traditional machining. The prediction of the thrust force in the longitudinal-torsional ultrasonic vibration assisted drilling process is very important for the low damage drilling of CFRP/TC4 alloy stacks. Therefore, the cutting force of the main cutting edge and the chisel edge of CFRP and TC4 alloy are modeled by considering the changes of cutting angle and thickness, as well as the effects of friction and ploughing characteristics on the cutting force during the drilling assisted by longitudinal and torsional ultrasonic vibration. Based on the relative position relationship between the longitudinal torsional vibration drill and the stack, the source of thrust force in different drilling stages is determined, and the full-stage theoretical prediction model of thrust force is established. The relationship between TC4 burr, hole wall damage and thrust force was further explored. The results show that the model can predict the thrust force of CFRP/TC4 alloy stacks in longitudinal torsional ultrasonic vibration assisted drilling, and the error between the theoretical value and the experimental value of the maximum thrust force of CFRP and TC4 alloy is within 18.5% and 14.86%. In addition, the thrust force is proportional to the burr height of TC4 and the damage degree of CFRP hole wall. When the thrust force is higher, the damage of TC4 burr, fiber pull-out and matrix loss is more serious.

Key words: longitudinal torsional ultrasonic vibration assisted drilling, CFRP/TC4 alloy stacks, thrust force, damage