Understanding the bowl-bottom effect in ultra-short pulsed laser drilling of CFRP laminate

Ultra-short pulsed laser drilling endows CFRP laminate with superior processing surface quality, whereas the hole taper usually generates. Herein, a theoretical model is established to understand the mechanism of hole taper formation, thereby further developing the corresponding regulatory strategy. A bowl-bottom effect that is a sudden increase of hole taper close to the hole exit in pulsed laser drilling, is demonstrated by the comparative laser drilling experiments for the first time. The hole cross-section taper, heat-affected zone (HAZ) and drilling efficiency related to the bowl-bottom effect were investigated. It was found that the formation mechanism of the bowl-bottom effect includes initial formation stage of taper (Stage I), taper accumulation stage (Stage II) due to the expanding of the focused spot area and its projected area, and bowl-bottom formation stage (Stage III) due to the secondary processing. The formation of HAZ is concentrated at the last few plies close to the hole exit due to the effect of secondary processing. The drilling efficiency of Stage III could decreased by 83.1% compared with that of Stage I. Increasing the laser average fluence by 73.4% could reduce the maximum hole taper by 20.5%, and improve the drilling efficiency of Stage III by 303.1%.