D. Lin, M. S. Yang, M. W. Huang, M. F. Kong, P. D. R. Kovacevic, Southern Methodist University, Dallas, TX; R. Ruokolainen, D. X. (. Gayden, General Motors Corporation, Warren, MI
Zinc, particularly at the interface of two pieces of galvanized steels in the overlap-joint welding, creates a problem since its boiling point of 907 ¢ªC is much lower than the melting point of the steel (around 1500 ¢ªC). High power laser welding has advantages in welding steels over traditional arc welding since it can work in a high speed. However, since laser produces extremely high heat input over a short period of time, metallic zinc vapor trapped between two overlapped sheets can quickly increase its pressure. The zinc vapor will be evacuated through the molten pool generated by the laser beam producing blowholes, spatters, and porosities in the weld. In addition, the strength of an as-welded joint is relatively low due to the reduced cross-sectional area. To solve the problems mentioned above, the Center for Laser-aided Manufacturing at Southern Methodist University has developed a low cost technology based on a commercially available high power fiber laser combined with a GTAW system. In this hybrid process, the electric arc generates a molten pool at the upper piece while the laser beam further enlarges and deepens the molten pool and penetrates through the interface opening the vent for the zinc vapor and providing sound joints. The strong forces in the molten pool caused by the electric arc further enhance the evacuation of the zinc vapor. Monitoring of the welding processes via a machine vision system and the final inspection of the welded specimens has demonstrated the elimination of blowholes and high pressure spatters during the welding process. A metallographic analysis has not shown the presence of the porosity across the weld bead. In addition, mechanical testing has shown that the as-welded joint in the gap-free overlap configuration is strong and the fractures occur at the heat-affected zone.
Summary: Currently, there is no low-cost effective solution for the problems of blowholes, spatters, and porosities caused by the evaporation of pressurized zinc vapor in the laser welding of galvanized steels at gap-free overlap-joint configuration. The technology developed at Southern Methodist University under the sponsor by General Motors Corporation solves these problems by integrating two heat sources, one is a commercially available fiber laser and the other is a GTAW process. This technology can be extended to apply on welding any galvanized steel in order to achieve a high quality weld in overlap- and fillet-joint configuration.
