| DP steel has been widely applied as the body components of cars due to the need of light weight and safty of the car in the field of automobile manufacturing. The matching accuracy of the car body size and the joint strength can be improved by laser welding technology, and meanwhile, heterogeneous welding and welding of steel sheet with different thickness can also be realized by laser welding. Hence, laser welding technology presents great advantages in improving structural stiffness of car and the flexibility of structure design. Nowdays, laser welding technology is one of the important candidates for replacing traditional welding technology. As an important component of the car, the welding joints are always subjected to dynamic loadings with high strain rate (approximately 500?1000 s-1) during manufacturing and car crash. Therefore, it is full of practical significance to investigate the dynamic mechanical properties of the welded joint. The service reliability of the welded components will be decided by the strain rate effect on the deformation and fracture of DP steel and its welding joint.In this thesis, DP780 steel for automotive application was butt welded by pulsed Nd:YAG laser. The influences of laser parameters on the quality of welding joint were investigated. Furthermore, the strain rate effects on the tensile deformation behavior and fracture mechanism were also investigated to reveal the characteristics of the mechanical property change under the dynamic loadings.The results of laser welding process showed that the surface morphology, the shape and dimension, microstructure, hardness distribution and the mechanical properties of the welding joints were significantly influenced by welding speed, pulse duration and welding current. The optimized laser parameters can be concluded as following:the welding speed of 300?500 mm/min, the pulse duration of 8.6?9.4 ms, and the welding current of 86?94 A. DP780 welding joint samples were prepared which presented excellent formability and no weld defects, such as cracking or pores.The results from tensile tests at various strain rates revealed that the tensile deformation behavior and tensile properties for both of DP780 base metal (BM) and the laser welded joint (WJ) were sensitive to the strain rate. DP780 BM and WJ presented the same changing trend of tensile property via strain rate. Compared with that of the DP780 BM, the tensile properties and deformation behavior of WJ presented more sensitive to the strain rate. The yield strength and ultimtated tensile strength increased with increasing of strain rate. However, the ductility presented a decreasing first and then increasing trend with increasing of strain rate. The ductility reached the maximum value when the strain rate increased up to 500s-1, and then decreased again with increasing of strain rate. The overall trend in the ductility of DP780 steel was decreasing after laser welding process. The fracture location of DP780 WJ became closer to the weld centerline with increasing of strain rate, indicating that the fracture location transferred from the BM to the softened heat affected zone (HAZ).The mechanism of strain rate effect on the deformation and fracture behavior of DP780 BM and WJ were analyzed. The results showed that the significant strain rate effect on the fracture location of DP780 WJ can be attributed to the different sensitivity of various microstructures in the different zones acorss the welded joint under various strain rates. During tensile deformation, the sensitivity of macroscopic mechanical behavior of DP780 WJ to the strain rate depends mainly on the change of deformation behavior and fracture mechanism of DP780 BM under various strain rates. |
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