Understanding of asymmetric ablation in waterjet assisted ultrafast laser drilling

The heat accumulation and material damages in the process of high-power ultrafast laser ablation can be effectively restrained by waterjet. However, the moving water film formed by waterjet on the target surface interacts with ultrafast laser, resulting in irregular material removal behavior. A waterjet assisted ultrafast laser drilling system is established and the material removal evolution is summarized. Multiphase flow models of moving water film and multi pulse material plasma are built to obtain the size of cavitation channel formed in the water film. Combined with the simulation of cavitation channel contraction, the mechanism of asymmetric material removal caused by moving water film is proposed. Before the target is penetrated, the cavitation channel in the moving water film enables the laser to remove the material directly by ionization. After the target is penetrated, the cavitation channel shrinks unevenly inward under the pressure of the moving water film. The ionization of water in local position leads to the indirect removal of material, resulting in asymmetric morphology.