In order to understand the mechanism of different geometries for various oxygen contents, the effect of oxygen on the keyhole and fluid flow behaviour has been investigated in both fibre laser and fibre laser- GMA hybrid welding. The results indicate that the penetration depth increases with increasing the oxygen content of the shielding gas in laser and hybrid welding. This is not caused by change in the Marangoni convection direction by oxygen. Deeper keyhole formed in high oxygen shielding results in deeper penetration depth. One of the possible mechanisms is formation of CO by decomposition of oxide in the keyhole. It can enlarge the keyhole by increase in the keyhole static pressure, resulting in increasing the keyhole depth.
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