Optimization On Structure Strength Of Resistance Spot Welding For Ultra High Strength Steel

Lightweight is the trend in automotive industry. With the advantages of high strength and hardness, the ultra-high strength hot-stamped steel plays a more and more important role in automotive industry. Since the production of ultra-high strength steel is still in the development stage, it is only used in the key position of the vehicle, which makes it connect to other material by the resistance spot weld (RSW) method. However, due to the invisibility of RSW process the finite element method is usually used to grasp the changes in the process of resistance spot welding and obtain the strength information of spot welding structure. In this study, the thermal physical properties of two different steel materials, low carbon steel and ultra-high strength steel, were obtained by JMatPro software. A finite element model was built to simulate the RSW process of ultra-high strength steel and the low carbon steel. This model can achieve thermal-electricity-structural three direct coupling calculation for the process of RSW. The influences of welding parameters on the change of the temperature field were studied through simulation. Researches on the welding nugget size, the metallographic structure, the micro-hardness and the mechanical properties of spot welded joint were performed. The experimental outcomes showed the size of nugget agreed well with the finite element simulation results. Under the optimized welding parameters, the mechanical properties of spot welding joint was greatly improved.Finally we studied the influence of nugget size on the static strength and the influence of three kinds of spot welding arrangement types on static strength and fatigue life. The simulation results showed the increase of nugget size is helpful to improve the strength of the welded joint. The arrangement of one-row four joints perpendicular to the loading axis has been introduced as a best recommended welding arrangement in view point of fatigue life. In addition, the maximum equivalent stress and the smallest fatigue life are located close to the low carbon steel side, and fracture position is consistent with the shear experiment results.