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This paper concerns the dynamic variation in plume velocity and keyhole size during the laser welding process of 5083 Al-alloy. The keyhole size and plume velocity were measured from high-speed images through imaging process methods and the particle image velocimetry (PIV) method. The correlation between the oscillation in plume velocity and keyhole size was discussed based on the pressure balance inside the keyhole. Results show that the keyhole keeps opening and collapsing during the welding process. During the time the keyhole is open, the size of the keyhole continuously oscillates until it collapses. Plume velocity increases to a maximum at the initial stage of keyhole's opening, and then varies in synchronisation with the oscillation of the keyhole size. It is considered that local evaporation occurs on the humps of the keyhole front wall, which results in the oscillation in plume velocity. The oscillation in plume velocity then leads to the variation in the pressure balance in the keyhole and drives the keyhole to expand or shrink accordingly. [ABSTRACT FROM AUTHOR]

• 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]