Melt ejection during single pulse drilling and percussion drilling of micro holes in stainless steel and nickel-based superalloy by pulsed ND:YAG laser radiation

Single pulse drilling and percussion drilling of micro holes (diameter 100 µm, depth 2 mm) with Nd:YAG laser radiation (pulse duration 100 - 500 µs) in stainless steel (X5CrNi18-10) and nickel-based superalloy (CMSX-4) are reported. The development of the hole geometry, especially the hole diameter depending on the focal diameter including caustics and the thickness of the melt at the hole wall, is investigated. The experimental results are compared to the calculations of a physical model describing the motion of the melt front, the phase boundary between liquid and solid. Closures resulting from melt in the hole are detected by recording the expansion of plasma using high speed photography (50000 fps). The quality of drilled holes, e.g. conicity, thickness of recast and closures, is investigated by metallography and optical microscopy. Timescales of physical processes as the onset of melting and vapourisation, the time to stationary melt flow and geometrical scales like the hole depth, the recast depth, the closure depth and the hole diameter are determined experimentally and numerically.