Prevention of porosity by oxygen addition in fibre laser and fibre laser–GMA hybrid welding

Prevention of porosity in partial penetration fibre laser and fibre laser–gas metal arc hybrid welding is investigated. It is found that modulation of laser power prevents porosity formation in fibre laser welding, but that this method is not effective in hybrid welding. However, the addition of a small amount of oxygen to the molten pool can prevent porosity formation in both fibre laser and hybrid welding. This is attributed to stabilisation of the keyhole. During welding, oxygen reacts with dissolved carbon to form carbon monoxide (CO) in the keyhole. The CO partial pressure in the keyhole prevents the intense interaction between laser beam and molten metal, thus stabilising the keyhole. The role of CO formation is confirmed by the enhancement of porosity suppression with increased carbon content of the base metal.