Analysis of laser surface absorptivity modification for selective laser hardening

Laser hardening is a thermal process that improves the surface mechanical properties of metallic materials. Efficiency enhancement of the thermal process of laser hardening is a long-term challenge. The article develops an approach called selective laser hardening that utilizes an absorptivity modification of the surface areas of the part to be processed by laser hardening. Laser surface treatment is used to modify the surface absorptivity. Subsequently, standard laser hardening can be performed on the entire surface. The change of material microstructure occurs only in the areas with increased absorptivity while the areas without surface modification remain structurally unchanged. The methods of laser absorptivity modification including subsequent thermal treatment and analysis of optical properties are described. The results of spectral absorptivity and absorptivity for wavelengths of commonly used hardening lasers (808, 1030 and 1064 nm) are presented for carbon steel EN 10083-2: C45. An important increase in the absorptivity of the laser-modified surface has been found up to 136% of the value of the basic material. This absorptivity difference decreases with increasing temperature of heat treatment, for examples to 7% at 800 °C for a laser wavelength of 808 nm. Further, the influence of laser surface modification parameters on the resulting absorptivity and the ability of increased absorptivity to withstand the elevated temperature during laser hardening are also discussed.