Product Code: PIC2008_607

Effect of Zinc Coating Thickness in Gap-Free Laser Welding of Galvanized Sheet Steels
Authors:
Ian M Richardson, Delft University of Technology; Delft Netherlands
Yu Pan, Netherlands Institute for Metals Research; Delft Netherlands
Presented at PICALO 2008

Laser beam welding has been used for joining zinc coated steel in the automotive industry. However, overlap laser welding of these steels is considered to be difficult because of the low boiling temperature of the zinc compared with the melting temperature of the steel. To date welding is normally undertaken when a gap is present between the plates, introduced prior to welding, or when welding with a modified laser beam configuration, such as a twin spots arrangement. These techniques have the disadvantage that an extra processing step is introduced in the case of the preset gap or decreases the welding speed in the case of modified beam configuration.

The research presented in this paper reveals the feasibility to produce good quality welds without the use of predefined gap or any other special processing conditions. The results are potentially very useful for the application of laser welding in the automotive industry.

This paper focuses on the understanding of process mechanisms through the study of zinc behaviour at the interface of the two sheets, which accounts for the defects produced during laser welding of zinc coated steel in overlap configuration. It is shown that during the welding process, there is a liquid zinc transport at the interface driven by a pressure difference and capillary force. A transient gap between two sheets caused by thermal distortion provides the space for such a liquid zinc transport. It is also shown that the clamping configuration and zinc coating thickness play important roles in the stability of the welding process.