High power laser is a promising tool for the application of high quality one path welding of thick plates. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. Especially, if the backside is not shielded in the full penetration laser welding, large porosities are likely to occur near the root due to supersaturation of nitrogen absorbed from the backside atmosphere. In the present study, the effect of the bottom surface atmosphere on the porosity formation has been investigated. Suppression of the porosity has also been attempted.
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