The progress of microhole formation during laser percussion drilling of steel observed by highspeed synchrotron x-ray imaging

The formation of microholes during the percussion drilling of metals using ultrashort laser pulses remains poorly understood. The quality of a microhole can be affected by heat accumulation, the development of side channels, and microhole bulging due to plasma formation. Highspeed synchrotron X-ray imaging was applied to capture the space- and time-resolved evolution of the microhole shape during laser percussion drilling of stainless steel. The recorded images show that heat accumulation leads to the formation of a melt film on microhole walls, which dynamically fluctuates during drilling. Additionally, the formation of side channels was observed in the region of the maximum drilling depth where the overall fluence on the microhole walls drops below the threshold fluence. The experimentally captured quantities agree well with existing models and will significantly improve their prediction accuracy.