Dynamic behavior of plume and keyhole in CO2 laser welding

Dynamic behavior of plume and keyhole in CO2 laser welding mild steel sheets was analyzed experimentally by using two high speed video cameras and multi-photosensor system consisting seven photo-diodes. At lower flow rates of Ar assist gas, the heating of the keyhole wall by the transfer from the plasma lump produced near the keyhole opening cannot be neglected. As the flow rate increases, the plasma lump becomes almost transparent and the keyhole is predominantly heated by the Fresnel absorption. Fluctuation of light intensity from the keyhole becomes more significant in full penetration welding than partial penetration welding, since the plasma produced in the keyhole can escape from the rear side of the keyhole. In process monitoring technique has been developed to recognize the degree of full penetration welding from the mean square value of the AC component of the light emission.