Study On Microstructure And Mechanical Properties Of Softening Zone Of Aluminum Alloy Welded Joint For High Speed Train

With the rapid development of the high-speed rail industry,the lightweight of the car body has become a development trend,and aluminum alloys have shown certain advantages in all aspects and are widely used in vehicle manufacturing.Welding is one of the most important processing methods for aluminum alloy structures,and fusion welding is the most widely used and most important in aluminum alloy welding processing technology.In heat-treatable-strengthened aluminum alloys,due to the heat of welding,the properties of each area of the joint change with the change of its microstructure,which is one of the main reasons for joint softening.Therefore,the analysis and accurate determination of the softening behavior of welded joints and their mechanical constitutive relationship during the welding of aluminum alloys is of great significance for the control of welding deformation and the optimal design of welded structures in the future.With the continuous development of machine vision measurement technology,the testing applications of material properties have gradually increased,and various methods in the field of machine vision have achieved good test results.The three-dimensional digital image correlation method（DIC）is widely used in various mechanical property testing experiments due to its high degree of automation,non-contact,and full-field surface deformation measurement.In this paper,the tensile specimen of A6N01 aluminum alloy is analyzed based on the full-field strain measurement system,and the overall stress-strain field distribution is obtained,combined with the material parameter inversion technique of the system,the constitutive model parameters of the material are obtained.The softening behavior of the welded joint is systematically analyzed,and the overall and thermal parameters of the specimen are obtained.The true stress-strain curve of the affected zone portion during stretching.After that,the in-situ tensile test was carried out on the part of the heat affected zone,and the resulting pictures were processed by DIC to analyze the strain field distribution,and combined with the test results,the softening behavior of the joint is analyzed.The plasma-MIG composite welded joints of A6N01 aluminum alloy were subjected to microstructure analysis,energy spectrum measurement,hardness measurement of welded joints,and grayscale analysis of the micrographs of the heat-affected zone under different heat input conditions.The results show that the softening behavior of the heat affected zone is due to the larger size of the precipitates and the complete incoherence relationship with the aluminum matrix,which reduces the solid solution strengthening effect,and the softening behavior becomes more obvious with the increase of heat input.Due to the phenomenon of joint softening in A6N01 aluminum alloy,the fracture mode is ductile fracture.During the stretching process,the heat-affected zone part will have mesoscopic damage behavior,and the mesoscopic damage will be accompanied by the nucleation,growth and aggregation of microscopic pores.In this paper,the GTN damage model is used,and the parameters of the GTN model are obtained by grayscale analysis according to the microscopic images obtained from the experiment,and the microscopic damage simulation of the tensile process is carried out by the ABAQUS finite element simulation software,and the holes of the tensile parts under different stretching stages are obtained.The variation law of volume percentage?,pore nucleation volume percentage?_nucleation and pore growth volume percentage?_growth.The results show that the change of the volume fraction of pores is mainly concentrated in the heat-affected zone,the nucleation of pores is mainly concentrated in the early stage of stretching,and the nucleation of pores is slowed down or even stagnant in the later stage,while the growth of pores is mainly concentrated in the later stage of stretching,and the pores grow and occur together.Polymerization until necking to fracture occurs.At the same time,the strain field distribution during the stretching process is obtained,and the strain field results are compared with the test results to verify the accuracy of the results.