Laser absorptivity on wavy molten metal surfaces: Categorization of different metals and wavelengths

The absorptivity of a laser beam interacting at grazing incidence with the processing front during laser welding, cutting, or drilling depends on the metal to be processed and on the laser wavelength. The absorptivity on a smooth processing front is homogeneously governed by the angle-dependent operating range of the corresponding Fresnel-curve. In contrast, a wavy processing front strongly modulates the absorptivity across the surface. The calculated absorptivity modulation is presented for five different metals and four laser wavelengths. A discussion based on the respective optical constants and Fresnel-curves leads to a categorization of the absorption behavior, mainly dividing into low and high electrical conductivity metals as well as into short and 10.6 mu m long laser wavelength. Six categories are distinguished. In particular, highly conductive metals remain highly reflective for short laser wavelength, even for a wavy processing front. Exceptions of higher overall absorptance are Cu for 532 nm and Al for 808 or 532 nm. (C) 2014 Laser Institute of America.