Prevention of porosity by oxygen in partial penetration laser and laser-GMA hybrid welding

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Prevention of the porosity has been attempted in partial penetration laser and laser/arc hybrid welding of heavy section steel plates. Bead on plate welding was carried out on 0.10C-0.30Si-1.33Mn steel (JIS SM490A) using 7 kW fibre laser and pulsed GMA. Laser power modulation with triangular waveform has been attempted for various frequencies to prevent the porosity. Small amount of oxygen addition in the shielding gas has also been attempted to prevent the porosity. Stability of the keyhole and porosity formation process was observed during welding using an in-situ X-ray transmission imaging system. Laser power modulation is effective to prevent the porosity in fibre laser welding. However, a lot of porosities are formed in hybrid welding with power modulation, since the welding phenomenon is mainly controlled by the frequency of pulsed GMA in hybrid welding. On the other hand, the porosity is effectively prevented by small amount of oxygen addition to the molten pool both in single laser and hybrid welding. It is attributed to stabilization of the keyhole by oxygen.Prevention of the porosity has been attempted in partial penetration laser and laser/arc hybrid welding of heavy section steel plates. Bead on plate welding was carried out on 0.10C-0.30Si-1.33Mn steel (JIS SM490A) using 7 kW fibre laser and pulsed GMA. Laser power modulation with triangular waveform has been attempted for various frequencies to prevent the porosity. Small amount of oxygen addition in the shielding gas has also been attempted to prevent the porosity. Stability of the keyhole and porosity formation process was observed during welding using an in-situ X-ray transmission imaging system. Laser power modulation is effective to prevent the porosity in fibre laser welding. However, a lot of porosities are formed in hybrid welding with power modulation, since the welding phenomenon is mainly controlled by the frequency of pulsed GMA in hybrid welding. On the other hand, the porosity is effectively prevented by small amount of oxygen addition to the molten pool both in single laser and hybrid weld...