Mechanisms of vapour plume formation in laser deep penetration welding

• Vapour plume formation during laser welding is studied by high speed imaging.
• Inclination and elongation of the vapour plume depend on the keyhole geometry.
• A model relating the shape of the vapour plume to keyhole geometry is proposed.

We analysed the dynamic shape of the metal vapour plume during deep penetration laser welding by means of high speed imaging. Our studies show that besides the inclination also the elongation of the vapour plume is determined by the evaporation processes inside the keyhole. When welding stainless steel sheets, the shape of the vapour plume as well as the shape of its origin, the keyhole, significantly vary with laser power and feed rate settings in a consistent way. This rather deterministic relationship allows for estimating how evaporation is distributed along the keyhole surface in different welding regimes. Based on the evaluation of the high speed sequences and corresponding microsections we propose a model of the vapour plume formation. The model indicates that, depending on the keyhole shape, the generation of the vapour plume is either governed by the vapour formed at the bottom of the keyhole or by the vapour formed at the inclined front keyhole surface, resulting in a very different shape of the plume in each case.