Study on laser drilling induced defects of CFRP plates with different scanning modes based on multi-pass strategy

There are two kinds of scanning modes based on multi-pass strategy, including concentric scanning mode (CSM) and spiral scanning mode (SSM), which can realize 3D hole drilling of CFRP plates by a nanosecond UV laser. The mechanism of the CFRP laser drilling and defects formation under these two different scanning modes has been described. Several experiments are conducted to explore and compare the drilling defects including heat-affected zone (HAZ), dimension error, and machined surface micro-defects under these two different scanning modes. The results show that SSM can significantly reduce the width of HAZ compared with CSM. Under the most obvious reduction of parameters, the use of SSM (84.76 mu m) is 33.42% less than the HAZ of CSM (127.3 mu m). For MR, it is reduced by 24.83% (45.75-34.39 mu m). Due to the special spiral trajectory of SSM, the dimension errors are increased when using SSM, including the taper error (maximum 26.11%) and slight circularity error. The micro-defects of laser processed surfaces with CSM are more serious than those with SSM, especially the obvious delamination, cracks, matrix loss, and fibre tip expansion formed fibre fusion caused by the significant thermal effect at a high laser repetition rate. This is consistent with the influence on HAZ at the hole edge.