Study on prevention of welding defects for thick plates with high power CO2 laser

The authors have examined proper welding condition which enclosed good weld beads in terms of welding appearance for full penetration welding. For CO2 laser welding of large output, it is generally known that porosity may occur in the weld bead when a deep penetration welding in single pass is performed in the ambient atmosphere. Welding defects mainly including porosity also occur more frequently with increase of plate thickness even though in the case of using proper welding condition. In order to spread laser welding generally, porosity is one of the most important problems to be solved. It is thought that formation mechanism of porosity is expressed as follows. Bubbles are generated mainly at the bottom of keyhole, which are trapped in molten metal during floating up and remain as porosity in metal. In this context, to reduce welding defects in welded bead, the authors paid attention to the following two conceptions; the first is that long heat input time would enable to delay beginning of molten metal solidification; the second is that molten metal could be fluidified smoothly by changing direction of recoil pressure. The authors adopted use of twin spot condensing system and improvement for the radiation angle of laser to test piece to accomplish these conception. Twin spot welding was the method of welding with two spot lasers which were condensed two laser beams which were divided from an uncondensed laser beam so that heat input increased in spite of decrease of penetration depth. Forehand welding was performed using a method of inclined laser radiation so that molten metal could be fluidified smoothly by changing direction of recoil pressure. In this report, the authors discuss in detail the prevention of welding defects which are generated by mainly porosity for thick plates.