Melt ejection during laser drilling

For many years, solid state lasers have been used successfully in machining processes such as drilling cooling holes in jet engine components. Nevertheless, industry demands improvements in the quality of machined features. More specifically, the drilled holes have to be more consistent in geometry, and should exhibit minimal recast layers and heat-affected zones. The application of hard-to-machine materials makes laser drilling even more challenging. Nanosecond laser pulses with a short wavelength are highly suitable for processing of various types of material [1, 2], mainly as a result of a higher absorption by the material and a lower absorption by the plume [3, 4]. To achieve the highest processing speed and to control the quality of the drilled holes, it is essential to achieve a fundamental understanding of the underlying drilling process. In the laser drilling process, material is removed by vaporisation and by melt ejection [5, 6]. The latter is the expulsion of molten material, driven by the recoil pressure generated by vaporisation of the material. Experimental studies about material removal during drilling and about the transparency of the plume can be found in literature for two applications. One application is evaporation of material for vapour deposition of thin metal and dielectric films [7]. In this case melt droplets have to be avoided. The other application is drilling, in which melt ejection is helpful because the energy required for melting is significantly less than for vaporisation [8]. However, melt ejection during drilling of metals with an excimer laser is not well understood for lack of detailed experimental investigations. Excimer lasers with a pulse length of 20-30 ns have been used only occasionally for the applications as studied in this paper.