Experimental study on high quality micro-holes with large aspect ratio in thick metal based on femtosecond laser focus shaping and plasma processing underwater

Aiming at the problems of insufficient aspect ratio and poor hole wall quality in femtosecond laser drilling of thick metal, this paper introduced a processing strategy involving femtosecond laser focus shaping and plasma micromachining. On one hand, through the multifocal diffractive optical element, the single-focus femtosecond laser was shaped into a dual-focus laser along the beam propagation direction, thus prolonging the length of the spatial filament and making it more conducive for deep hole drilling. On the other hand, by inducing plasma through femtosecond laser in a water environment, the wall of micro-hole drilled in air could be machined to improve its quality. And then the influence of dual-focus laser drilling parameters on micro-hole morphology and aspect ratio was studied and compared with the results of single-focus processing. At the same time, the effect of laser-induced plasma machining on hole wall morphology and quality was studied. After the experiments, the processed micro-holes were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), laser confocal microscopy, and other methods. The experimental results have shown that the maximum aspect ratio of micro-holes obtained by dual-focus laser was 22.8, which was 11.8% higher than that of traditional single-focus laser drilling. In addition, the maximum processing capacity increased by 5.76%. In terms of processing quality, the roughness of the hole wall after plasma modification reduced by 67.3% compared with the unmodified hole wall. And the EDS analysis results have shown that the oxygen content on the hole wall was greatly reduced after plasma machining, and the oxide particles deposited on the hole wall were basically eliminated.