High-Speed Simultaneous Observation of Plasma and Keyhole Behavior during High Power CO_2 Laser Welding -Effect of Shielding Gas on Porosity Formation-

摘要

Laser welding can produce a deeply penetrated head at high speed. However, in high power CW CO_2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of porosity formation. They have revealed that many bubbles are formed mainly from the bottom tip of a keyhole by intense evaporation of metal. It has been also revealed that the keyhole fluctuates frequently and changes its size and shape corresponding to the intermittent bubble formation. And the majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined by using two synchronized high-speed cameras and X-ray transmission observation system. Especially, the effect of shielding gas on porosity formation was investigated in terms of plasma and keyhole behavior. In the case of He shielding gas, metallic plasma emanated from a keyhole, and the keyhole was always open continuously. On the 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 disappeared just like in the pulsed laser welding. Such periodical interval and duly were different depending on the materials used and exerted on effect on porosity formation tendency.