Observation of Keyhole and Molten Pool Behaviour in High Power Laser Welding : Mechanism of Porosity Formation and Its Suppression Method

概要

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In high power pulsed and CW laser welding, characteristic porosity is frequently formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of porosity formation in relation to keyhole dyanamics. 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 frequently fluctuates 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 the simultaneous observation of keyhole and plasma dynamic behaviour as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behaviour was examined by the optical and X-ray transmission observation methods. The systematic observations of keyhole, molten pool and laser induced plasma have revealed that the keyhole instability is enhanced by the localized intense evaporation of metal on the front keyhole wall and this unstable behaviour leads to the formation of characteristic porosity. It has been also revealed that the porosity can be suppressed successfully by the use of pulse modulation laser and pure nitrogen shield.