Experimental and numerical studies on the issues in laser welding of galvanized high-strength dual-phase steels in a zero-gap lap joint configuration

The automotive industry is increasing the usage of high strength steels in body structure manufacturing due to their high strength-to-weight ratio and good formability. In order to increase the resistance to corrosion, these steels are galvanized; however, this poses a series of problems for laser welding of galvanized high-strength steels in a zero-gap lap joint configuration. The boiling point of zinc is 907 ºC, which is significantly lower than the melting point of steel (around 1500 ºC). During the laser welding process, highly pressurized zinc vapor is easily generated at the faying interface and vents out through the molten weld pool resulting in spatter on the adjoining surfaces as well as blowholes and pores in the solidified weld bead. To date, there has not been published on a cost-effective, efficient, and easy-to-apply welding technique capable of laser welding galvanized high-strength dual-phase steels in a zero-gap lap joint configuration without pre- or post-weld procedure requirements.;In this study, various laser welding techniques combined with numerical modeling and monitoring methods were proposed to mitigate the effect of zinc vapor in welding of galvanized DP980 steels in a zero-gap lap joint configuration.;A feasibility study on the laser welding of galvanized high strength DP980 steels in a zero-gap lap joint configuration was carried out experimentally and numerically. An experimentally-based thermal finite element (FE) model was developed to predict the geometry of the keyhole and the weld pool. A feature parameter which represents the ratio of the faying surface area from which zinc was vaporized to the molten pool area at the faying surface of the two overlapped steel sheets was introduced in order to represent the quality of weld with respect to the laser welding parameters. The results showed that a lower welding speed or higher laser power can mitigate weld defects caused by the pressurized zinc vapor.;Two-pass laser welding of galvanized high-strength steels in zero-gap lap joint configuration was developed. During two-pass laser welding, the first pass consisted of applying a defocused laser beam to preheat the top sheet; the second pass applied a focused laser beam was applied to perform the welding. During the preheating process, the defocused laser beam burnt the zinc at the top surface, melted and partially vaporized the zinc coatings at the interface of the two overlapped steel sheets, and improved the absorption of the focused laser beam during second pass which created a stable keyhole through which any remaining zinc vapor formed at the faying interface of the two steel sheets will be vented out. The mechanical properties of the welded joints are also studied. An experimentally-verified 3-D FE thermal model was introduced to predict the temperature field during the preheating. The temperature distribution at the faying surface during the preheating process was analyzed to help understand the effect of the preheating parameters on the zinc coating vaporization and the final quality of the two pass laser-welded lap joints.;Laser welding with the application of a pressure wheel was introduced for galvanized high-strength steels in lap joint configuration. During the welding process, the highly-pressurized zinc vapor which was generated at the contact interface and its escape laterally through a gap at the faying interface of two overlapped sheets was controlled by the pressure wheel and an optimal range for defect free welding defined. Furthermore, the mechanical properties of the welded joints were studied in relation to the resulting weld quality. Experimental results demonstrated that defect-free lap joints of galvanized high-strength steels can be achieved by using the laser welding process assisted by the application of a pressure wheel.;As galvanized high strength steel sheets could be joined in a zero-gap lap joint configuration successfully, the influence of laser welding conditions / parameters on the mechanical strength of the DP980 steel lap joint was studied. Subsequently, an experimental-based FE model was developed to predict the equivalent strain distribution in the lap joints and the failure mode under a tensile shear condition.