Study On Interface Structure And Fatigue Damage Of Dissimilar Welding Joints Of 6061 Aluminum/DP980 Galvanized Steel

Based on the current rapid development of the automotive industry,energy andenvironmental issues have become more prominent.Gasoline consumption and vehicle exhaust emissions have brought more challenges to energy and environmental issues.Therefore,it is imperative to incorporate the theme of energy conservation and environmental protection into its development.In order to achieve this policy,it is extremely advantageous to implement a lightweight body structure.The idea of light weight is to use lighter materials to make the body on the premise of ensuring the safety of the automobile structure.Therefore,high-strength aluminum alloys have become the materials of choice for lightweight vehicles due to their advantages of low density and high strength.Considering that the body structure must ensure safety,the use of steel is also essential.Then the other direction of weight reduction is to use higher strength steel instead of ordinary structural steel to achieve the purpose of weight reduction.Based on this realistic background and significance,this paper conducts weldingtests on 6061 aluminum alloy and DP980 dual-phase steel,and studies the performance of heterogeneous joints,in order to provide some theoretical guidance for the lightweight implementation of aluminum steel.The specific research contents of this article are as follows:In the first chapter of this article,the background and practical significance of thealuminum-steel connection are explained by combining the existing literature.Of course,it has also been found that studies on joint fatigue performance in aluminum-steel connections are rare.Therefore,in this paper,while studying the microstructure and shear properties of 6061 aluminum alloy and DP980 galvanized steel heterogeneous joints,the focus is on the fatigue performance of joints.The second chapter of this paper details the material composition,equipment model and test method of this test.The third chapter of this paper mainly analyzes the macro morphology andmicrostructure of 6061 aluminum alloy and DP980 galvanized steel heterogeneous joints.The study found that with the increase of welding heat input,the joint macroscopically showed that the weld width increased and the weld residual height decreased.The energy spectrum analysis and phase analysis（XRD）of the interface show that the interface layer is composed of intermetallic compounds Fe₂Al₅ and Fe₄Al₁₃.Observing the microstructure of the cross section of the weld,it was found that the microstructure of the joint can be roughly divided into the melting zone（FZ）,zinc-rich zone,interface layer（IMC）,and fusion zone（FL）.The melting zone is mainly composed of dendriticα-Al and reticular Al-Si eutectics.With the increase of welding heat input,the size of dendritic crystals will increase slightly.Al-Fe intermetallic compounds were formed in the interface layer,and their thickness increased with the increase of welding heat input.The zinc-rich region is composed of Al-Zn eutectic andα-Al-Zn solid solution.When the heat is high,the zinc-rich area will burn,and even the interface was missing.The fourth chapter of this paper studies the hardness and shear properties of 6061aluminum alloy and DP980 galvanized steel.Studies have shown that the hardness of the intermetallic compound in the interface area of the joint was extremely high,reaching375HV,and the softening phenomenon appeared in the fusion zone.During shear tests on joints at different welding speeds,the author found two different modes of fracture.Among them,in the interface fracture mode,due to the defect between the interface and the grains on the aluminum side,the stress is concentrated there,and it becomes the origin of microcracks when subjected to external force.In the fusion zone fracture mode,stress concentration caused by defects near the fusion zone makes the vicinity of the fusion zone a weak link of the joint.The fifth chapter of this paper studies the fatigue damage behavior of 6061、aluminum alloy and DP980 galvanized steel dissimilar joints.The study found that with the increase of welding heat input,the average fatigue life of the sample was significantly improved.By comparing the hysteresis loops of the first cycle under different heat inputs,it is found that with the increase of welding heat input,the amount of strain in a single stress loading cycle is reduced,and the fatigue life is improved.Analysis of the hysteresis loop during fatigue cycling shows that the specimen undergoes cyclic hardening and cyclic softening during fatigue.Sometimes the specimens also undergo cyclic hardening at high stress levels,resulting in increased joint life.The analysis of the fatigue fracture found that the segregation of impurities at the grain boundary position of the zinc-rich region makes it easy to initiate fatigue cracks in the region,and that the coarse columnar crystals in the zinc-rich region are subject to the same force,which is also an important reason for the weakness of the zinc-rich region.The subsequent crack propagation process proceeds along the interface layer with severe lattice mismatch.Distortion can make dislocations easier to move,so it presents an interface fracture mode.The fatigue fracture is characterized by greater brittleness.