Zinc transport phenomena in laser welding of coated sheet steel in overlap configuration

Consumer demand for longer car lifetimes has led to an increased usage of zinc coated sheet steels in the automotive industry; furthermore, the usage of lasers is still increasing in this industrial sector. These factors together lead to a strong interest in the weldability of zinc coated steels in an overlap configuration, without the need for additional processing steps or tooling to vent the zinc vapours during welding. In general overlap welding of these steels is considered to be extremely difficult, and has only been achievable when a gap was introduced between the sheets prior to welding, or when welding in a pulsed mode with well defined pulse shapes, with tilted or split laser beams. Such techniques have the disadvantage that an extra processing step is introduced in the case of a preset gap or a decrease of the welding speed must be accepted in the case of pulsed laser welding. Tilting of the beam increases the interaction area and splitting the beam involves a division of the available power over the beams, both resulting in a decreased energy density and thus a decrease in beam speed. The results in this paper show that overlap welds are feasible without deliberately introducing a gap between the sheets before welding. Continuous wave laser welds are described, which were performed on zinc coated steels in an overlap configuration without an intentionally introduced gap to vent the zinc vapour formed during welding. It is shown in this paper that during the welding process liquid zinc flow occurs between the sheets driven by pressure differences and capillary forces.