Formation mechanism of the weld defects has been investigated by observing dynamic keyhole behavior and analysis of gas compositions in the weld metal. Full penetration laser welding was carried out on 11 and 15mm thick steel plates in various back surface atmospheres. A shielding box attached on the back surface was used to control the back surface atmosphere. Supersaturation of nitrogen, which is supplied from back surface plasma, forms bubbles in the molten pool and then causes the porosity. The critical nitrogen concentration to form the porosity shows constant value for various back surface atmospheres. Oxygen enhances porosity formation due to enhancing nitrogen dissolution in the molten pool. In the inert gas back shielding, the indissoluble inert gas enters the keyhole from the back surface and perturbs it significantly, although the porosity can be suppressed. It causes a hot cracking susceptible weld section. Coating of aluminum on the back surface prior to the welding is effective in preventing the porosity and hot cracking due to denitrification of the molten pool.

Topics
Chemical elements, Supersaturation, Fracture mechanics, Welding
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