Laser Drilling of Tool Steel under Plasma Atmosphere

Laser drilling of microholes into tool steel (1.2379) with a diameter of about 100 microm and 3 mm depth still constitutes a challenge. Expulsion of the molten metal and its accumulation around the drill hole necessitates a post processing of the surface. Melting processes within the drill hole causes a hole geometry diverging cylindrical shape and the energy input results in microstructure defects and fissures. Concerning this matter, the influence of gas pressure within the scale of vacuum pressure and the use of various process gases (active gas: O2, air; passive gas: Ar, He, N2) to the drilling process was analyzed within this thesis. Furthermore, the impact of an additional cold low-pressure plasma, which was generated above the opening of the drill hole, on the drilling process was examined. The aim was to achieve a reduced accumulation of molten metal on the surface, an improved cylindricity of the drill hole as well as a lowered thermal stress on the component through these procedures. For this purpose, microholes were drilled in a vacuum chamber into tool steel by percussion drilling method at different pressure levels and process gases. The expulsion of molten metal as well as the annealing colors on the sample surface was measured with an optical microscope and laser scanning microscope. By cross-section preparation the drill hole was visualized and measured.