Abstract
Water-assisted ultrafast laser processing was proposed to solve the problem of surface quality in metal during laser micro-hole drilling. The percussion drilling process with different single pulse energies was tested under 10 mm water layer and in air. The morphologies of the laser induced plasma (LIP) were observed and recorded. The drilling depth and micro-hole radius were characterized by microscope and 3D surface profilometer. Experimental results show that the debris, spatter recast or others caused by the laser energy deposition are quickly transferred away, but the material removal performance caused by LIP under water is different from that in air. It is noteworthy that the drilling depth has been significantly reduced at all pulse energies owing to the shielding effect of water and LIP. But the micro-hole radius has increased several times after a sufficient pulse number, which is attributed to the defocusing effect of LIP. In order to explain the mechanism of underwater ablation characteristics, a model combining the LIP formation and evolution was developed to analyze the energy attenuation effect and the beam refraction effect by LIP underwater. Moreover, the annular morphology at the hole bottom under high pulse energy can also be explained by these two effects of LIP.
