Modelled dependency of the direct absorption on the keyhole wall waviness during 1 micron-and 10 micron-laser welding

During keyhole mode laser welding the absorption of the laser beam at the vapour capillary, the keyhole, takes directly place by Fresnel absorption. From ultra high speed imaging it was observed that waves stream down the keyhole front. By introducing a wavy surface to the modelled keyhole front, the dependency of the absorption on the slope of the waves was studied. Even waves of low roughness rapidly cause shadowing, particularly at low welding speed. For 1 µm-lasers the direct absorptance increases for higher waviness while CO₂-lasers follow the opposite trend. The CO₂-laser operates close to the Brewster-maximum for smooth keyholes, while waves introduce reflective domains. The sensitivity to waviness decreases with increasing speed for the 1 µm-wavelength lasers.