Mechanism governing nitrogen absorption by steel weld metal during laser welding
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10/3/04
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Mechanism Governing Nitrogen Absorption by Steel Weld Metal during Laser Welding WEI DONG, HIROYUKI KOKAWA, SUSUMU TSUKAMOTO, YUTAKA S. SATO, and MAKOTO OGAWA The existence of monatomic nitrogen in the plasma just over the keyhole during CO2 laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO2 laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO2 laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO2 laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO2 laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.
I. INTRODUCTION
GENERALLY, the equilibrium solubility of nitrogen by the reaction 1/2N2 N in a liquid metal is given by Sieverts’ Law, which states that the concentration of this species is proportional to the square root of the molecular nitrogen partial pressure at any temperature.[1] However, the equilibrium between the monatomic nitrogen gas and the weld metal should be considered during fusion-welding processes of steels, such as arc and CO2 laser welding, because a plasma phase, which contains a significant quantity of electrons, ions, and excited atoms, exists just above the weld pool. Several researchers have reported that the nitrogen absorption by a weld metal is enhanced above the equilibrium solubility of nitrogen predicted by Sieverts’ Law and that the existence of monatomic nitrogen dissociated from molecular nitrogen in the plasma is responsible for the enhancement of nitrogen absorption in a low-nitrogen steel weld pool during arc welding.[2–14] In welding applications, the two most common types of lasers are the CO2 gas laser, with a wave length of 10.6 m, and the neodymium:yttrium aluminum garnet (Nd:YAG) solid-state laser, with a wave length of 1.06 m. Previous studies have shown that the nitrogen content of weld metal during CO2 laser welding is higher than that during YAG laser welding, but lower than that during gas tungsten-arc (GTA) welding.[15,16] During laser welding, a plume occurs over the molten pool. This plume is the result of the ejection of materials from the area of the weld which is impinged on by the laser beam.[17] The absorption coefficient of the WEI DONG, formerly Ph.D. student, is Research Fellow, Department of Materials Processing, Graduate School of Engineering, Tohoku University. HIROYUKI KOKAWA, Professor, and YUTAKA S. SATO, Research Associate, are with the Department of Materials Processing, Graduate School of
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