Study on process and mechanism of laser drilling in water and air

An Inconel 718 sample was drilled in air and water using a diode-pumped nanosecond pulse laser with a pulse width of 8 ns and a wavelength of 532 nm. The surface, cross-section, and in-wall morphology were analyzed with a confocal laser scanning microscope and a scanning electron microscope. The pinnacle at the bottom of holes was more prone to exposure during water-assisted drilling. However, the in-wall of those holes was more rough. To better understand the mechanism of underwater drilling, the effects of pulse number, laser energy density, and water layer thickness were evaluated. An increase in the diameter of holes and a decrease in the debris and spatters on the surface were observed compared with the holes drilled in air. Also, an increase in the etching rate was observed under the high-energy condition. The physical effects of high pressure, fabricated by the bubbles and plasma in a water layer, were evaluated.