Development of high power CO₂ laser welding process

A deep penetration CO₂ laser welding process has been developed as a low heat input, high efficiency fabrication route for advanced structural steels. To improve fundamental understanding of the process and facilitate the elimination of welding defects, the laser–plasma interaction has been characterised using a power probe, infrared spectroscopy, and high speed photography of monochlomatic images. The extent of the laser induced plasma was found to increase on raising the laser beam focal point from below the surface to the surface position. Plasma absorptivity was strongly affected by plasma composition, and hence by vapour generated by inhomogeneous melting of the keyhole wall; this effect exerts an important influence on keyhole instability. Variation in the keyhole wall profile as a result of inhomogeneous melting has been found to lead to porosity through the generation of bubbles that fail to escape from the weld pool. It is shown that appropriate pulse modulation of the laser beam can improve keyhole stability, thereby greatly reducing porosity in the welded joint.