Numerical study of the dynamics of the hole formation during drilling with combined ms and ns laser pulses

•The impact of shock waves in drilling with ms and ns laser pulses was studied.•A numerical model was set up and compared to experimental findings.•A hole forms in drilling with combined laser pulses while not with a single ms pulse.•Melt ejections are induced by the shock waves generated by the ns pulses.•Drilling stops due to the defocusing of the ns laser beam with increasing hole depth. Improved drilling efficiency has been experimentally observed during laser drilling with combined ms and ns laser pulses, which is believed to be the contribution of the enhanced melt ejection induced by the ns pulse generated shock waves. A numerical model to generate a fundamental understanding of the involved physical phenomena is needed in order to be able to optimize hole quality or drilling efficiency in the practical applications. Based on the Navier-Stokes equations such a numerical model was established with the goal to investigate the impact of shock waves on the melt flow and the dynamics of the formation of the hole itself. It was validated by comparing experimentally obtained hole depths, the extension of molten pools and hole geometries under different processing parameters. The results show that melt is expelled out of the outlet of the holes within a few microseconds after the generation of the shock wave. The melt then gradually flows back to the bottom of the hole before the next ns pulse is applied. Although deeper holes can be achieved by increasing the average power and the duration of the ms pulse, the decrease of pressure due to the defocusing caused by the surface of the interaction zone that is moving away from the focal plane of the beam providing the ns pulses leads to an accumulation of melt at the bottom of the hole, and thus a saturation of the hole depth.