A comprehensive model of fatigue life prediction of zerogap lap-jointed galvanized DP980 steel with considering laser welding induced residual stress.

摘要

The application of advanced lightweight materials in automotive industry has been driven by the increasing need for lightweight cars to reduce energy consumption and emissions. Hence, the weight reduction of cars as well as compliance with the car body safety standard has become imperative. In light of this, galvanized high-strength dual phase (DP) 980 steel is increasingly being used as a lightweight material in automotive industry due to its high strength-to-weight ratio and good surface corrosion resistance.;A large number of the steel structures in engineering applications are fabricated by different laser welding techniques. Recently, laser as a heat source has been finding a broader application in joining different types of materials in diverse industries such as aerospace, shipyard, aviation and automotive. The ultimate driving force for the selection of laser for joining steel structures in automotive industry, compared to the other joining methods like riveting, adhesive bonding, resistance spot welding, is that laser welding has the capability of low heat input, high welding speed, high penetration, easy to automate, and high accuracy. However, the laser welding is characterized with high cooling rate that generates high level of residual stress in the welded structures.;Previous studies note that welding induced residual stress has a direct influence on the fatigue strength and the number of cycles to failure of welded structure. However, there is little evidence on investigation of the fatigue life prediction of the laser lap welded steel sheets with considering the residual stress. Hence, it becomes inevitable in this research to establish a comprehensive method of predicting the fatigue life of DP980 steel in a lap joint configuration under constant load spectrum based on induced residual stresses by laser welding.;It is time-consuming and costly to completely depend on experimental methods to investigate the evolution of residual stress in laser welded structure. Numerical analysis affords the most cost-effective way to achieve this investigation. Therefore, the finite element method is introduced along with the experimental verification to determine the fatigue number of cycles to failure of a laser welded DP980 steel sheets in lap joint configuration as a function of residual stress. This comprehensive method contains three fundamental multiphysics procedures -- the thermal (as discussed in chapter 2), the mechanical (as discussed in chapter 3), and the fatigue life prediction (as discussed in chapters 4 and 5).;A 3-D FE thermal analysis was performed to study the temperature field in the laser welding of DP980 steels in a lap joint configuration. The numerical simulation was validated by thermocouples and macrographs of the weld cross-sections. The corresponding micro-hardness test is performed to study the micro-hardness distribution across the base metal and weld zone in the laser welded DP980 joint.;The experimental and numerical simulation of the residual stress distribution field of the joint was carried out. The thermal data obtained from the thermal model are input into the mechanical model to calculate the residual stresses by the application of von Mises method. The tensile shear test, the hardness test, and the X-ray diffraction test were carried out to verify the numerically simulated results. (Abstract shortened by UMI.).