Drilling strategies for metals with pulsed YAG lasers

Laser beam drilling with µs-pulses is widely used for production of turbine blades and sieves. However, there are gaps in understanding the dynamics of drilling, especially related to the quality features of the hole, like formation of recast, cracks and taper. This work describes the advances in fundamental physical modeling and mathematical analysis of the Free Boundary Problem as well as monitoring of time varying processes such as percussion drilling. The acceleration/deceleration of the melt film by evaporation/recondensation and the widening of the hole diameter compared with the beam diameter as well as the onset of recast are the physical phenomena relevant to the formation of quality features. The dynamic model predicts widening and recast sets in or is suspended depending on intensity distribution and pulse repetition frequency for the laser power. More detailed experiments with shaped and modulated pulses show improved drilling quality and efficiency compared to rectangular. These findings confirm the importance of a radiation energy input adopted to the dynamics of the drilling process.