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 formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation 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 and shape, corresponding to the intermittent bubble formation. 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 with 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 an 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 open continuously. On the other hand, in the case of N2 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 duty were different depending on the materials used and exerted an effect on porosity formation tendency.