Study on machining of high-quality micro-holes by laser trepan drilling

The influences of two key factors, geometrical morphology and recast layer, on hole quality in micro-hole laser drilling are investigated by using high power Nd:YAG millisecond pulsed laser on two different materials - 304 stainless steel and directionally crystalized nickel-based alloy DZ445. Trepan drilling technique is specially studied, and three related key factors are analyzed for their influences on hole quality, including trepanning path, trepanning velocity and trepanning times. The results show that the edge-started laser path will lead to a notch on the hole edge, but this can be avoided when the center-started laser path is employed; trepanning velocity and orbiting times both have important influences on the thickness of recast layer, and lower velocity and more trepanning times will produce holes with thinner recast layer. The mechanism lies in that the recast layer on the hole wall may remelt under subsequent radiation of laser in the trepanning process, resulting in melt migration by overcoming the viscous force under combined force of the vaporization pressure and the assistant gas pressure, and finally the melt is ejected from the hole exit end. The thickness of recast layer is inversely proportional to the time duration of melt migration when other experimental conditions remain unchanged.