Study On The Effect Of Precipitated Phase On The Fracture And Corrosion Behavior Of 7075 Aluminum Alloy Friction Stir Welding Joint

With the lightweight and high-speed of the transportation tools,the demand for the safety and reliability on the welded structure of train bodies has become more urgent.7075 aluminum alloy has been widely used in the field of transportation tool manufacturing due to the advantages of high specific strength,good toughness and corrosion resistance.Friction stir welding（FSW）is one of the most promising welding technologies for the aluminum alloy.The flat butt welding was carried out on the thickness of 6 mm 7075 aluminum alloy using friction stir welding under different parameters.The precipitated phase among the WNZ,TMAZ,HAZ and BM of the joints under different parameters were characterized by the microscopic characterization methods.The phase analysis of the joint was measured by XRD,the high resolution image of the precipitated phase of the joint was observed by TEM,and the lattice fringe spacing was calculated to characterize accurately the species of precipitated phase.On this basis,the influence of the parameter on the precipitation behavior of precipitated phases in different micro-zones was explored.The flat tensile test of the joints under different speeds were carried out by the MTS,the microcrack ncleation and extended fracture process during the tensile process were studied to explore the weak link and fracture mechanism of the joint.The finite element model of the mechanical properties mismatch joints was established by finite element software ABAQUS,and the stress state of the joints during the tensile process was calculated.Finally,the electrochemical corrosion on the joints under different parameters in the acidic environment was tested by the electrochemical workstation,and the corrosion behaviors of the joints were studied.The results show as follows:（1）The microstructure of the 7075 aluminum alloy FSW joint distributes extremely uneven,and the micro-hardness presents"W"shape.The precipitated phases in the joint mainly contain AlCuMg with elliptical shape,Al₂CuMg with capsule shape and MgZn₂ with rod shape.The strengthen effect of the AlCuMg and Al₂CuMg are better than that of the MgZn₂.The precipitated phase in WNZ mainly includes AlCuMg and Al2CuMg.The number of the AlCuMg and Al2CuMg in TMAZ is lesser and the number of the MgZn₂ is more compared with the WNZ.The number of the MgZn₂ in HAZ is relatively more.（2）When the welding speed is constant,with the increasing of the rotation speed,the size of the precipitated phase in WNZ increases and the number increases.The number of the AlCuMg and Al₂CuMg increases obviously,the microhardness increases first and then decreases.The size of the precipitated phase in TMAZ increases and the number increases.The number of the AlCuMg and Al₂CuMg increases,the number of the MgZn₂ increases slightly,and the microhardness increases first and then decreases.The size of the precipitated phase in HAZ increases slightly and the number increases.The number of the AlCuMg and Al₂CuMg increases slightly.The number of the MgZn₂ increases obvious relatively,and the microhardness changes little.When the rotation speed is constant,with the increasing of the welding speed,the number of the precipitated phase in WNZ,TMAZ and HAZ increases and the size reduces.The number of the AlCuMg and Al₂CuMg in WNZ and TMAZ increases relatively more,the number of the MgZn₂ in HAZ increases relatively more,and the microhardness increases.（3）With the increasing of strain rate,the yield strength of the 7075 aluminum alloy FSW joint increases slightly,the tensile strength changes little,the yield strength ratio increases,and the fracture strain of the joint decreases.Compared with the AlCuMg and Al₂CuMg,the MgZn₂ causes the uncoordinated deformation of the joint to form microvoid,the microvoid coalescence and grows to the microcrack.When the increasing of the strain rate,the microcracks nucleate at the junction of the BM and HAZ,the crack propagation path develops from meandering to straightness,the angle between the fracture path and the loading direction decreases gradually,and the fracture mode is mainly changed from dimple polymerization fracture to low plastic shear fracture.（4）With the increasing of the heat input,the yield strength and tensile strength of the 7075 aluminum alloy FSW joint increase first and then decrease.The width of equivalent model under different heat inputs is uniformed to 46 mm.The equivalent model of the 7075 aluminum alloy FSW joint can replace the fine model of the joint for subsequent vehicle finite element simulation.（5）When the welding speed is constant,the corrosion resistance of the 7075aluminum alloy FSW joint in the sulfuric acid solution at pH 6.5 increases first and then decreases with the increasing of the rotation speed.When the rotation speed is constant,the corrosion resistance of the joint increases first and then decreases with the increasing of the welding speed.The most corrosion resistance parameter:the rotation speed 800 r/min and the welding speed 300 mm/min.The micro-zone corrosion resistance of the 7075 aluminum alloy FSW joint is:WNZ>TMAZ>HAZ.The corrosion originated from the precipitated phase.Compared with the AlCuMg and the Al2CuMg,the MgZn2 has a deeper influence on the corrosion degree of surrounding Al matrix.