Local absorptivity modulation of a 1 μm-laser beam through surface waviness

In laser materials processing operations such as deep penetration laser welding, the laser beam often interacts with a wavy, molten surface. This wavy topology strongly modulates the local angle of incidence of the laser rays across the surface and in turn the local absorptivity, which is described by the Fresnel-equations. In this paper this modulation of the absorptivity is modelled over a two-dimensional steel surface for prescribed wavy topology for high power lasers with a wavelength of about 1 μm, like disc, fibre, Nd:YAG or diode lasers. It has become apparent that even a topology with regular peaks and valleys causes a complex absorptivity distribution at the surface, including significant shadowing domains due to the grazing angle of incidence during processes like laser welding, drilling or cutting. In contrast to a smooth melt, the waviness tends towards an angle-averaged absorptivity, of 33% for 1 μm-lasers and steel. The high sensitivity of the absorptivity and of shadowing on the surface topology significantly modulates the local absorptivity of the beam power density and in turn the local process mechanisms, such as boiling accompanied by recoil pressure.