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• Keyhole and plume features were automatically extracted from the image processing method; • Effects of Mg content was helpful to keep the opening of keyhole; • Effects of Mg content leaded to the frequent swelling generation at the rear weldpool. This paper concerns the effects of Mg content on keyhole behaviours during the laser welding process of 5754, 5083 and 5A06 Al-Mg alloys. The features of the vapour plume and keyhole area are extracted from high-speed images with image processing methods. The increase of Mg content leads to the increase of hydrodynamic pressure caused by metal evaporation and decrease of surface tension acting on the keyhole wall. The strong metal evaporation and decrease of surface tension help the formation of keyhole, which both result in a large size of keyhole with deep penetration and low frequency of keyhole collapse. However, the impact of metal vapour could be enhanced by the increase of evaporation and the decrease of both surface tension and viscosity, which leads to the frequent generation of swelling at the rear weldpool and poor formation of the weld bead. [ABSTRACT FROM AUTHOR]