Effect Of Heat Treatment On Microstructure And Properties Of Al-4.5Zn-1.2Mg-0.3Mn Welded Joint

Al-Zn-Mg aluminum alloys are often used in transportation,aerospace and other fields due to their high strength,excellent plasticity and good welding performance.However,after the alloy is welded,due to heat input and welding stress,its mechanical properties are lowered and its corrosion resistance is affected,so the application of the alloy is affected.It is well known that heat treatment changes the microstructure of the material and improves the mechanical properties and corrosion resistance of the material to meet the application under different conditions.In this paper,Al-4.5Zn-1.2Mg-0.3Mn alloy was used as the research object.The alloy was tungsten-argon-welded by ER5356 and ER4043 welding wire,and then heat treatment was carried out in different processes.The welded joints were studied by metallographic microscope and scanning electron microscopy.Microstructure;its mechanical properties were tested by hardness and tensile properties experiments;the macroscopic corrosion morphology,polarization curves and intergranular corrosion depth of the materials were tested by exfoliation corrosion experiments,electrochemical experiments and intergranular corrosion experiments.Corrosion resistance of welded joints under different heat treatment systems.Finally,through the comparative analysis of the mechanical properties and corrosion properties of the materials,the best heat treatment process and the matching best welding wire were determined.The experimental results show that the best heat treatment process for improving the comprehensive performance of Al-4.5Zn-1.2Mg-0.3Mn alloy welded joints is 450?/1.5h+480?/0.5h+120?/24h,that is two-stage solid solution+single stage aging process;post-weld heat treatment can improve the microstructure of the welded joints,refine the grains,and precipitate the strengthening phase MgZn₂,Al₃Mg₂,Mg₂Si and Mg₃₂Al₄₉;compared with the ER4043 welding wire,Al-4.5Zn-1.2Mg-0.3Mn alloy welded joint with ER5356 welding wire has good comprehensive performance;after the 450?/1.5h+480?/0.5h+120?/24h heat treatment process,the hardness of the weld joint of ER5356 welded joint reaches 100HV,the hardness of the base metal area is 143HV,and the tensile strength,yield strength and elongation after fracture are respectively 367MPa,274MPa and 17%,the fracture mode is ductile fracture;the maximum depth of intergranular corrosion is 64?m;the self-corrosion potential is-1033.772mV,the self-corrosion current density is 6.143?A·cm^-2;The peeling corrosion of the welded joint is finally rated as N⁺,and the exfoliation corrosion grade of the base metal is PC.The welded joint is in the corrosive solution,from pitting corrosion to pitting corrosion steady state and then transitioning to intergranular corrosion,and continuing to prolong the corrosion time will enter the exfoliation corrosion phase.