Influence of oxygen on weld geometry in fibre laser and fibre laser–GMA hybrid welding

The effect of oxygen on weld geometry during keyhole mode welding has been investigated by adding a small amount of oxygen to the shielding gas during fibre laser and fibre laser–gas metal arc hybrid welding. The results indicate that the penetration depth increases and the weld width decreases with increasing oxygen concentration. This effect is attributed to the formation of a deeper keyhole when oxygen is present. The addition of sulphur up to 1500 ppm in the molten pool has no significant effect on the penetration depth. This behaviour indicates that the Marangoni convection and surface tension are not the main reasons for the deeper weld penetration when oxygen is added to the shielding gas. The increase in the penetration depth owing to oxygen addition is consistent with the formation of CO by reaction between dissolved carbon and oxygen. Rapid generation of CO in the keyhole expands the keyhole and results in deeper weld penetration.