High-speed simultaneous observation of plasma and keyhole behavior during high power CO2 laser welding: Effect of shielding gas on porosity formation

Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily f ormed in the weld metal, but its formation mechanism has not been well unde rstood. Therefore, the authors have conducted systematic studies of the elu cidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size a nd shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the mol ten pool. However, there are few reports that deal with the simultaneous ob servation of keyhole and plasma dynamic behavior as well as the formation o f bubbles and porosity. In this study, therefore, the interrelationship bet ween keyhole and plasma behavior was examined by using two synchronized hig h-speed cameras and an x-ray transmission observation system. Especially, t he effect of shielding gas on porosity formation was investigated in terms of plasma and keyhole behavior. In the case of He shielding gas, metallic p lasma emanated from a keyhole, and the keyhole was open continuously. On th e other hand, in the case of N-2 shielding gas, a big gas plasma was formed above the weld bead periodically, and metallic plasma and keyhole disappea red just like in the pulsed laser welding. Such periodical interval and dut y were different depending on the materials used and exerted an effect on p orosity formation tendency. (C) 2000 Laser Institute of America. [S1042-346 X(00)00406-X].