Research On The Mismatched Formability And Mechanical Properties Of Double-pulse MIG Welded Joints In Aluminum Alloy Sheets For Automotive Applications

Under the development of new energy vehicles integrating new materials,intelligent connection and mobile internet,lightweight materials such as aluminum alloys are favored in the manufacture of new energy vehicles.However,the application of aluminum alloy in auto body structure depends not only on advanced forming process,but also on reliable jointing technology.The Double-pulse Metal Inert Gas Welding(DP-MIG)has become one of the main jointing technologies for aluminum alloy auto body structures,due to its automation,high welding efficiency,cost-effectiveness and etc.Nevertheless,the welding process leads to the multisource heterogeneity in the geometry characteristics,chemical composition,microstructure and mechanical properties of the welded joints,which leads to the formation of mismatched aluminum alloy welded joints.In addition,the mismatched welded joint affects the load-bearing capacity of the welded structure,which brings difficulties to the development of the aluminum alloy welded structure in auto body structure.In this paper,a series of research on the mismatched formability and mechanical properties of aluminum alloy welded joint was conducted,on the basis of experimental data and by means of DP-MIG technique.The performance of mismatched aluminum alloy welded joint was controlled by designing the welding process parameters.Besides,combined with the experimental and simulation results,the mechanical properties of the mismatched welded joints and the influence of the mismatched joints on the bearing capacity of the welded sheets were studied,which provides a theoretical basis for the development of aluminum alloy welded structures in the auto body.The details are as follows:Firstly,the welding thermal cycle of DP-MIG welding of 6061-T6 aluminum alloy sheets and the mismatched property of each welding zone,including geometric characteristic,microstructure and mechanical properties,were analyzed.The welding thermal cycle,geometric characteristic,microstructure and mechanical properties of each welded zone were studied by experiments.In addition,the evolution law of the welding thermal cycle on the microstructure of different welded zones was explored.Also,the influence mechanism of the microstructure characteristics on mechanical properties of materials was investigated.Therefore,the relationship among the welding thermal cycle,microstructure and mechanical properties of aluminum alloy welded joints was established.Secondly,the reliability optimization design of the welding process parameters for mismatched aluminum alloy welded joints was carried out using a hybrid optimization strategy.According to experimental results,the influence of welding process parameters on the performance of the aluminum alloy welded joint was analyzed systematically,and the reliability of its performance was evaluated.The reliability-based optimization design of the welding process parameters was conductive to produce high-performance aluminum alloy welded joints with high probability.Moreover,the optimal welding process parameters were determined considering the load-bearing characteristics of the auto body structure under typical service conditions,which enable the deterministic and robust control over the performance of the aluminum alloy welded joint and the auto body structure.Thirdly,the mechanical properties of the mismatched aluminum alloy welded joints was controlled by adjusting heat treatment parameters.The effects of preheating temperature,post-weld aging temperature and post-weld aging time on the microstructure and mechanical properties of aluminum alloy welded joints were studied by controlled variable experiments.Furthermore,the response surface model between heat treatment parameters and the mechanical properties of the joint was fitted by mathematical statistic method.The optimization design based on the approximate model can directly improve the mechanical properties of the welded joint,and the optimal heat treatment parameter was determined according to the load-carrying characteristics of the auto body structure under typical working conditions,which realized the integrated optimization of design and application.Then,the static mechanical properties and failure mechanism of the mismatched aluminum alloy welded joint were investigated by the experimental and simulated results.Based on the mismatched formability of aluminum alloy welded joints,a sequentially combined thermo-mechanical and mechanical simulation modeling method was developed.Firstly,the thermo-mechanical simulation was carried out to analyze the welding thermal cycling and residual stress distribution,which are conductive to build mechanical simulation model with detailed geometric features,the corresponding local material properties as well as attached residual stress.And then,the mechanical properties of the mismatched welded joints were analyzed by numerial simulation.Combined with the experimental and numerial simulation results,the effect of welding process on the mechanical properties of the aluminum alloy welded joint was explored,and the reasons for the fracture of the joints was revealed.Therefore,the prediction of the mechanical properties and failure fracture characteristics of mismatched aluminum alloy welded joints was achieved.Finally,the influence mechanism of the mismatched geometric characteristics and mechanical properties on the bearing capacity of the aluminum alloy welded sheet was revealed emphatically.According to the hardness distribution of the welded structure,the tetra-material equivalent model was established,and the effects of mismatched geometric characteristics and mechanical properties of different welded zones on the performance of the welded sheets were analyzed by numerical simulation.Through the statistical simulation results,the main effect,interactive effect and contribution of each parameter to the bearing capacity of the welded structure were investigated,and the empirical model for evaluating the relative strength of the mismatched welded sheets was developed,which can predict the ultimate load-bearing capacity of the welded structure.It provides theoretical basis for the application of mismatched aluminum alloy welded sheets in the auto body structure.