By: Hongping Gu and Boris Shulkin

Stronach Centre for Innovation (SCFI)

With the advancement of high power fiber delivery lasers, remote laser welding becomes a reality and furthermore cost reduction on a new laser system in recent years drives adoption of laser welding process into production lines. Remote laser welding takes the advantages of less mechanical movement and better accessibility of the beam to the workpiece, thus much faster processing speed can be achieved. In most cases, remote laser welding involves lap welding. While the sheet metal components in automotive industry are mostly zinc coated for better corrosion resistance. One typical concern involved in laser lap welding of zinc coated materials is the potential porous weld caused by the zinc vapour. Since zinc has a vaporizing temperature that is lower than the melting point of steel, zinc vapour generated during laser welding will expel the molten metal out of the weld pool, resulting in rough weld. Therefore, laser beam lap welding of zinc coated steel components is not a straightforward process and it requires a special procedure to provide proper venting of the zinc vapour. Although there are several approaches to address this issue, many of the approaches are either impractical or too costly to apply to remote laser beam welding. Currently, the only practical solution for remote laser welding is to maintain a small gap between the joining materials. It has been proved through trials that a gap around 0.15 mm in size is optimum for such welding. The open question remains, however, how to generate such a small gap consistently and effectively at the welding location?

Laser dimpling has been proved to be a viable technique for producing a protuberance or a hump on the surface of one component. This protuberance serves as a spacer to make a small gap at the welding location. Laser dimpling requires a moving laser pulse to generate the dimples at a typical welding speed. At SCFI, Magna International, we developed a new technique to make laser dimples using the high speed “humping effect”. As is well known, “humping effect” in laser welding is a phenomenon of weld pool instability that occurs when a certain welding speed is exceeded. It characterizes by the formation of periodic droplets on weld bead. We found that the height of the humps at extreme high speed, for example 60 m/min, is in the optimal range for gapping and the height is adjustable by varying the speed. Therefore, a welding technique utilizing the humping effect has been developed to achieve remote laser beam lap welding. This welding technology only introduces a minimal additional cost for processing time and no additional cost for tooling or equipment. The same laser source and processing device can be used for both dimpling and welding. No precision requirement for coincidence of laser dimples and welds. The process is highly flexible, consistent and efficient, and can be introduced on any welding lines suitable for remote laser weld process.