Absorption peaks depending on topology of the keyhole front and wavelength

By high speed imaging, wavy patterns were observed at the keyhole front in Yb:fibre laser welding. Despite a regular appearance of downstreaming wave flow in the movies, deeper image analysis shows that the pattern structure is complex. The observed grayscale levels of the flow pattern are likely to correspond to a combination of temperature, topology, and emissivity of the keyhole front, in turn originating from variations of the beam absorption, temperature, boiling, ablation pressure, and melt acceleration across the keyhole surface. Bright domains can fluctuate on a time scale of 10–100 μs. For interpretation of the fundamental mechanisms, the evidence from high speed imaging analysis is accompanied by mathematical modeling of the absorption modulation, combining local absorptivity, and beam projection. The calculated results show that for a wide range of keyhole topology parameters, lasers with a wavelength of about 1 μm induce high absorption peaks at the wave shoulders while CO2-lasers have a more homogenizing behavior. Different regimes can be distinguished.