Power attenuation and focus shit of solid state laser beam caused by laser induced plume

Abstract

Currently, remote laser welding using a solid-state laser is widespread in the industry. Meanwhile, it is well-known that laser-induced plume blown up from the processing point affects the penetration in laser welding, through the attenuation and the refraction of the laser beam. These phenomena in carbon dioxide laser welding have been investigated well and it is widely recognized that using the shielding gas flow to blow away the laser-induced plume is very important. However, at remote laser welding, it is not easy to maintain the shielding gas flow to processing point. By the way these phenomena depend on the wavelength of the laser. So, the quantitative knowledge of the attenuation and the refraction of the solid state laser beam are necessary in achieving a stable penetration in remote laser welding with this laser. This study was made to determine the attenuation coefficient and the amount of the effective focus shift caused by refraction of the laser beam in the plume, through the melt run experiments with YAG laser. The attenuation coefficient of the laser beam was estimated to be 0.00090 mm-1, from the dependence of the cross-sectional area of weld metal on the laser power and the plume length. This value is about one twentieth of the attenuation coefficient of the carbon dioxide laser beam at welding, found in the literature. The amount of focus shift was estimated to be 0.67 mm per 100 mm length plume, from the dependency of penetration depth on the defocusing distance and the plume length. Comparing the 3 mm plume of length, this value is centesimal of the estimated value by Beck et al in CO2 laser welding. Therefore, the solid-state laser like YAG laser is considered to be suitable laser source for remote laser welding.