Fresnel absorption of 1 μm- and 10 μm-laser beams at the keyhole wall during laser beam welding: Comparison between smooth and wavy surfaces

► Laser welding: direct absorption at the keyhole surface is modelled. ► Surface waves heavily modulate direct absorption compared to a smooth capillary. ► Waves rapidly cause local shadowing regions, redistributing the energy. ► 10 μm-lasers loose much of their stable high direct absorption at wavy surfaces. ► 1 μm-lasers experience an absorption increase at wavy surfaces.