A new double spiral scanning method for CFRP step blind hole drilling by nanosecond UV laser

Carbon fiber reinforced polymer (CFRP) composites used in aerospace under extreme conditions will inevitably leads to the occurrence of delamination, pits, and other defects. High-precision step blind hole drilling is the key process for eliminating damaged defects and preparing for subsequent repairs. A new double spiral scanning mode (DSSM) method was proposed to achieve five-layer step blind hole drilling with low damage and high efficiency. By analyzing the morphologies at the bottom of the blind hole under different laser frequencies, scanning intervals and scanning angles, it was found that the material between the scanning tracks was mainly removed by thermal ablation and expansion impact of the plasma plume, and the stable removal scanning intervals were related to the fiber arrangement angles. When the scanning interval was too large, the sector residue (SR) appeared on the ablative surface. When the traditional scanning method was used, there was no SR when the spacing was less than 0.04 mm. When the spacing was greater than 0.04 mm, the SR increased with the spacing. By changing the scanning angle, the curve mapping relationship between the scanning angle and the maximum scanning interval without SR was obtained. Compared with SSM, the variable curvature characteristic of the DSSM reduced the influence of the beam occlusion effect on processing. The maximum scanning spacing corresponding to no SR was increased to 0.12 mm, and the removal rate of single-layer blind hole material was increased by 34.77 % (2.045 mm3/s to 2.756 mm3/s). The processing efficiency was increased by 26.91 % (7.985 s to 5.836 s). Finally, DSSM was used to achieve high-precision drilling of five-layer equal-depth step blind hole structure with single layer depth of 125 +/- 15 mu m.