Defect suppression mechanism and experimental study on longitudinal torsional coupled rotary ultrasonic assisted drilling of CFRPs

Longitudinal torsional coupled rotary ultrasonic assisted drilling (LTC-RUAD) technology is introduced to improve the surface roughness of the hole wall and solve the tear, burr and delamination of carbon fiber reinforced polymers (CFRPs) induced by large thrust force and torque during conventional drilling (CD) in this study. First, the defect suppression mechanism of CFRPs hole was proposed via the analysis of the types and forming mechanism of CFRPs hole defects using a tapered drill-reamer (TDR) in CD and the characteristics of LTC-RUAD. Then, the corresponding drilling experimental platform was built based on the novel designed LTC-RUAD vibration actuator, while drilling experiments involving T700S-12K/YP-H26 CFRPs specimens with different process parameters were performed by adopting the different ultrasonic vibration amplitude (UVA) in the longitudinal and torsional direction. The thrust force and torque, quality of the hole wall and holed defects obtained in both CD and LTC-RUAD were compared at the same process parameter. The experimental results show that compared with CD, the maximum average thrust force and torque can be reduced by approximately 30% and 40%, respectively. The surface roughness, defects at the exit of the drilled hole show a "concave" trend with the increase of UVA, and the burr factor, tear factor and delamination factor are reduced by 62.56%, 76.6%, 69.67% when the longitudinal UVA is approximately 7-9 mu m, respectively. The forming mechanism of hole defects in CD and the defect suppression mechanism of CFRPs hole in LTC-RUAD are verified to be correct.