Mechanisms of laser drilling of metal plates underwater

Several metal plates with different thickness including copper, iron, aluminum, and stainless steel have been drilled in the surroundings of air and water, respectively, by a Q-switched pulsed Nd:yttrium-aluminum-garnet laser. It is observed that for the same metal plate less energy is needed to drill a hole in water than that in air, and the surface morphology of hole drilled in water is improved greatly than that in air by comparison of the scanning electron micrographs. The underlying mechanisms behind the efficiency and quality enhancement in water are further investigated by means of optical beam deflection technique. The experimental results show that due to the water confinement the peak amplitude and duration of the laser-ablation-generated impact underwater is much larger than that in air. During the underwater laser drilling, besides laser ablation effect, both the first and second liquid-jet-induced impulses by cavitation bubble collapse in the vicinity of a solid boundary are also observed and their amplitudes are, respectively, about 12.4 and 5.2 times that of the laser ablation impact in air. Cavitation bubbles are the special dynamic phenomenon occurring in liquids. Therefore, it is concluded that in-air-drilling laser ablation-produced impact is a dominant mechanism; while during laser underwater drilling, it is the result of a combination of ablation-produced impact effect and liquid-jet-induced impact, especially the latter. Thus, the efficiency and quality of laser processing in the surrounding water can be greatly increased and improved compared with that in air. (C) 2004 American Institute of Physics.