Research On Mechanical Properties And Dynamic Deformation Behavior For Laser Welding Joints Of Dual-phase Steels

Laser tailor welded blanks(TWBs) for dual phase steel can effectively reduce the number and weight of car body parts, which being an important concern in the automotive lightweight process. Due to the effect of welding thermal cycle, the microstructure and properties of laser welded joint is not uniform and different from the parent metal. In addition, TWBs will be affected by dynamic loads during the car collision and the manufacturing process. Meanwhile, the dynamic deformation behavior of steel sheet has great differences with the quasi static, so the research on the local mechanical properties and dynamic deformation behavior of laser tailor welded joint has important guiding significance in engineering applications. In this thesis, the main conclusions are as follows.By analyzing the microstructure and properties of DP600 laser welding joints, the weighted average method is presented to describe the relationship among the heat affected zone(HAZ) of laser tailor welded blanks. Based on the weighted average method, the model to solve homogeneous HAZ performance parameters is established. By comparing the finite element analysis(FEA) simulation results and the test, a reasonable weighting coefficient is determined, and the model for solving homogeneous HAZ reliability is also proved.For DP980 laser welding joints, the presence of softening zone will lead to the laser welding joints performance degradation. In order to study the performance of the softening zone, the mixing rule is adopted. According to the improved mixing rule, the theoretical model for solving the softening zone is established. Based on the uniaxial tensile test, the mechanical properties of the base material and the weld are obtained. Then combined with the FEA, the reasonable of the softening zone performance is verified.By comparing the tensile deformation behavior of DP980 laser welding joints, the strain rate range from 10 3 to 103 s 1, and the main effects are as follows. First, all the tensile specimens are broken at the softening zone, and with increasing strain rate, the yield and tensile strength of tensile samples are increased. In addition, the elongation of the specimen is shown as remain unchanged beginning, then increases, and finally decline. Second, the stretched process of tensile specimens is accompanied by the emergence of micro holes, which show different forms of fracture morphology of dimple. When the low speed stretch, tensile specimen fracture showed equiaxed dimples, and high speed tensile fracture is the existence of a certain percentage of the shear dimples. Third, compared with the low speed stretch, the fracture cross section of high speed tensile specimen is change. Low speed tensile specimen fracture is “cup cone” shape, but high speed tensile specimen fracture is “inclination angle” shape.