Drilling of Cooling Holes by Using High Power Ultrashort Pulsed Laser Radiation

One of the main applications for laser drilling is the manufacturing of cooling holes with diameters of e.g. 500 µm in turbine components such as turbine blades and vanes or combustion chambers. Nowadays, these cooling holes are drilled by using flash lamp pumped Nd:YAG laser radiation and partially by using QCW fiber laser radiation with pulse durations in the range of 200 µs up to a few ms. The main deficit of these conventional technologies is the appearance of recast layers with thicknesses up to a few 100 µm at the hole wall. These recast layers arise due to the melt-dominated drilling process. During operation of the drilled components, the recast layers can chip off or can be the initial point for cracks. Both effects can lead to a shorter life-time.Within this work, the drilling process has been investigated by using high power ultrashort pulsed laser radiation. The aim is to decrease or avoid the recast layer by using a vaporization dominated drilling process. The pulse duration is smaller than the electron-phonon interaction time, so that a “cold ablation” is possible. An Amphos ultrashort pulsed laser system with a maximal average power of 450 W and pulse durations between 700 fs and 7.5 ps is used for this purpose. The challenge is the implementation of a suitable beam deflection so that the pulse overlap is small. The recast layer could be decreased to a few µm at the wall of holes with a diameter of 500 µm in 3 mm thick stainless steel.