Investigation of heat accumulation effects during deep hole percussion drilling by high power ultrashort pulsed laser radiation

In the last few years, commercially available ultrashort pulsed (usp) laser systems have reached average powers of several 100 W, which makes them also interesting for enhancing traditional applications. For example, laser drilling, a conventionally melt-dominated process, would benefit from the advantages of an usp ablation process. Due to the small processing area in laser drilling, substantial heat accumulates already at a few Watts of average power. This heat accumulation creates melt but also increases the mean ablation rate at least 1 order of magnitude and could lead to a productive drilling process. In this study, the increase of the mean ablation rate in percussion drilling due to heat accumulation is examined for various metals and sample thicknesses for high average powers of up to 300 W and a pulse energy in the milliJoule range. Those investigations have not yet been performed in such detail. It is shown that by doubling the pulse energy the drilling time can be decreased by 2 orders of magnitude due to heat accumulation. This behavior is valid for various metals like steel or aluminum, despite their varying material parameters. By analyzing the influence of different repetition rates and focal diameters, it is shown that the predominant parameter to characterize the heat accumulation is the average power. No significant difference is observed if the average power starting from 20 W is set up by a high pulse energy or repetition rate. Stainless steel has a different behavior compared to the other investigated metals when the pulse duration is changed from 2 to 20 ps, which is caused by a modified plasma generation. For stainless steel, the drilling time is found to be describable with one empirical formula for the entire range of examined average power and sample thickness.