Evaluation of Surface Crack in Resistance Spot Welds of Zn-Coated Steel

摘要

The development of the automotive industry is now focused not only on improving basic vehicle performance but also on reducing weight and enhancing safety and durability. Various automotive high-strength steels are being developed, and Zn-coated steels are being manufactured to prevent corrosion of the external white vehicle body. The most commonly used welding method in the car body assembly process is resistance spot welding (RSW), which has been extensively studied worldwide. In this process, the work piece is basically heated according to the contact resistivity of the interfacial between the electrode and the material as well as the bulk resistivity of the material itself. At this point, if the metal is Zn, which has a lower melting point than the Fe base metal on the surface, it is mainly melted in the temperature range of 400-900°C. It becomes easy to penetrate the grain boundary of the HAZ during welding. Also, the tensile stress in such a state decreases the ductility of the grain boundary and causes liquid metal embrittlement (LME). Cu_5Zn_8, an intermetallic compound, can be formed from the reaction of the alloy with the Cu material electrode in the expulsion current range at a high temperature. Its formation is likely to be facilitated by LME or a surface crack.In this study, the fatigue characteristics of a tensile shear specimen during spot welding was investigated with the welding parameters that occur in the surface crack of welds on Zn-coated steel. Finally, a controlled spot welding condition was suggested to prevent surface cracks,