Mechanism of Zn vapour suppressed by Al in laser lap welding of high strength zinc coated sheet steel

The basic feasibility of laser fusion welding for prefabrication and assembly of zinc-coated sheet steel auto body components has been demonstrated in a significant number of prior studies. For trouble-free high volume manufacturing, however, one leading key technical concern remains incompletely resolved. In lap or similar weld joint geometries, venting of trapped Zn vapour can seriously disturb the molten weld pool and give rise to gross fusion defects. In this paper, the technical status of the issue has been reviewed, and various experiments were planned to explore Al foil as an interlayer and dual beam laser for suppressing Zn vapour. The results have shown that the welding processing is very sTable when the lap 1+1 mm sheet steel is welded by using Nd-Yag laser at 3KW power and 4m/min travel speed (defined as single beam) with 75 mu m Al foil as interlayer. However, if first using 2KW power and 4m/min travel speed, secondly using 3KW power and4m/min travel speed (defined as dual beam), only 25 mu m Al foil can stabilize welding fusion pool. It is found that Al foil just store the Zn vapour in the single beam method and cannot mix with Zn to form Al-Zn binary alloy, because the laser heating is too fast that Zn has been vaporized before Al was melted. But Al can mix with Zn to form Al-Zn binary alloy in the dual beam method, therefore, the boiling temperature is raised to about 1600 degrees C. So that, in laser lap welding of Zn coated (galvanized) sheet steel, the Al-Zn alloy will be more effective than Al metal for stabilizing welding process.