Simultaneous top and root high speed imaging on droplet formation in laser welding

The regular formation of drops, called the humping effect in laser welding has been studied and analyzed by several research groups, however preferably for the top surface. Humping phenomena at the root have been much less investigated, even though they might have similar impact on the mechanical properties of the weld. Root humping is studied based on synchronized high speed imaging of the melt pool at the top and simultaneously at the root side of the weld. Important relations between the dynamics of the two melt pools can be identified, including characteristic dimensions and time scales. Besides explaining the mechanisms, these relations can be suitable for online process monitoring of root drop formation from the more accessible top of the weld. Opening and closure of the keyhole at the root and its accompanying dynamics from boiling is of importance. Finally a wider theoretical explanation for root humping is derived and measures for its suppression are proposed.