In situ optical observations of keyhole dynamics during laser drilling

To better understand the laser drilling process and especially to clarify keyhole dynamics in metal drilling, a quasi-two-dimensional drilling assembly was set up with a thin sandwich structure. Keyhole dynamics coupling multiple physical processes were recorded using high-speed photography, and clear images were obtained. The formation of keyholes was found not to be a single unified process, and the whole drilling process could be divided into five stages: an initial melt ejection, mild melting, rapid drilling, hole expansion, and backflow and recasting. As the keyhole evolved, the removal of material changed.