Effect Of Post-weld Heat Treatment On Microstructure And Properties Of 7N01-T4 Aluminum Alloy Friction Stir Welded Joint

The 7N01 aluminum alloy serves as the structural material to fulfill the lightweight requirements owing to its high strength and excellent weldability.Friction stir welding is a high-efficiency green welding technique commonly used in 7N01 aluminum alloys,However,7N01 aluminum alloy has certain stress corrosion sensitivity,and the mechanical properties of the welded joint have a large decline.Approprnate post-welded heat treatment process can improve the mechanical properties and corrosion resistance of 7N01 aluminum alloy friction stir welded joint,which has important engineering practical value and theoretical significance.In this paper,the thick-plate 7N01-T4 aluminum alloy was welded by FSW technique.Firstly,the optimized solid solution process and aging process were obtained by investigating the metallographic structure,microhardness and electrochemical performance of the joint using different post--weld heat treatment process parameters.Secondly,the effects of post-weld heat treatment on the microstructure and mechanical properties of joint were compared.The results of the electron backscatter diffraction analysis(EBSD)test show that the proper post-weld heat treatment process can change the grain orientation of the welded joint,so that the internal texture orientation and the texture volume ratio of the joint change.For the weld nugget zone,the grain size of the joint becomes larger after heat treatment.Due to the release of the heat dislocation energy of the joint during the heat treatment,the dislocation density decreases,and the equiaxed grains grow further.The grain size order in the RRA process is that welding nuclear zone>two-stage aging>single-stage aging,GNDs of RRA follow the order that welding nuclear zone<two-stage aging<single-stage aging.The reason for this phenomenon may be related to the heat input during the heat treatment process.As the heat input becomes larger,the dislocation density gets lower and the grain size is larger.By analyzing the microhardness of different micro-areas of the different post-weld heat treatment joint,it is found that the welded joint with different post-weld heat treatment processes have lower hardness along the middle of the plate thickness directionThe tensile strength of single-stage aging joint reachs 312 MPa,and the elongation decreased from 15%to 10.6%.The two-stage aging joint resulted in an increase in tensile strength due to precipitation of the n,phase,reaching 332 MPa,which was 4.7%higher than that of the as-welded sample.The RRA sample also resulted in an increase in tensile strength due to a large amount of precipitation of theη’phase and the η phase,reaching 341 MPa,which was 7.6%higher than that of the as-welded sample.The tensile strength of natural aging joint is 363 MPa,which is 14.5%higher than that of the as-welded joint.The increase of the tensile strength during the natural process is mainly caused by the large amount of precipitation in the GP zone.Finally,the differences in corrosion resistance of joint after different post-weld heat treatment processes were explored.The results of chemical immersion experiments show that the corrosion resistance properties are summarized that RRA>corrosion resistance of two-stage aging>corrosion resistance of single-stage aging>corrosion resistance of as-welded samples.The results of electrochemical experiments show that the corrosion resistance develops an order of RRA>corrosion resistance of two-stage aging>corrosion resistance of single-stage aging>corrosion resistance of as-welded samples.The difference in corrosion resistance of various processes is mainly due to the size,distribution and morphology of the precipitated phase.At the same time,it can be seen that the corrosion resistance of the joint heat affected zone is worse than the weld nugget zone for all processes.Intergranular corrosion experiments were carried out on as-welded sample.two-stage aging sample and RRA sample.The experimental results show that the joint do not present significant grain boundary corrosion.The order of the stress corrosion resistance is that RRA>stress corrosion resistance of two-stage aging>stress corrosion resistance of single-stage aging>stress corrosion resistance of as-welded joint.For two-stage aging.single-stage aging,and as-welded joint,there are obvious intergranular fractures in the slow-stretching fractures in 3.5%NaCl solution,indicating that the stress corrosion cracking mechanism conforms to the anodic dissolution theory.However,for the RRA joint,there is no edge fracture phenomenon in the fracture,showing the characteristics of the quasi-cleavage fracture.