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JWJ > Volume 24(2); 2006 > Article
Journal of KWJS 2006 April;24(2) :71-78.
Published online July 18, 2006.
The Weldability of 6㎜<SUP>t</SUP> Primer-coated Steel for Shipbuilding Using CO₂Laser (Ⅱ) - Dynamic Behavior of Laser Welding Phenomenon and Composition of Porosity and Vaporized-particle -
Jong-Do Kim, Hyun-Joon Park

Correspondence:   Published online July 18, 2006
ABSTRACT
  It has been reported that good quality weld beads are not easily obtained during the CO2 CW laser welding of primer coated plate. However, by introducing a small gap clearance in the lap position, the zinc vapor can escape through it and sound weld beads can be acquired. Therefore, this study examines for keyhole behavior by observing the laser-induced plasma and investigates the relation between keyhole behavior and formation of weld defect.   Laser-induced plasma has accompanied with the vaporizing pressure of zinc ejecting from keyhole to surface of primer coated plate. This dynamic behavior of plasma was very unstable and this instability was closely related to the unstable motion of keyhole during laser welding.   As a result of observing the composition of porosity, much of Zn element was found from inner surface of porosity. But Zn was not found from the dimple structure fractured at the weld metal. By analyzing of vaporizing element in laser welding, a component ratio of Zn was decreased by introducing a small gap clearance. Therefore we can prove that the major cause of porosity is the vaporization of primer in lap position.   Mechanism of porosity-formation is that the primer vaporized from the lap position accelerates dynamic behavior of the key hole and the bubble separated from the key hole is trapped in the solidification boundary and remaines as porosity.
Keywords: CO₂ Laser welding;Primer-coated steel;Laser induced plasma;Porosity | Keyhole;Gap clearance
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