Research On Mechanical Properties And Corrosion Resistance Of 7A04 Aluminum Alloy With Two-stage Stress-aging

7xxx series high-strength aluminum alloy are typical heat-treatable aluminum alloys,they are widely used in the automotive and aerospace industries due to their low density,high specific strength,good fracture toughness and excellent stress corrosion cracking resistance(SCC).In recent years,with the rapid development of China's aerospace and transportation industry,the demands for the 7xxx series high-strength aluminum alloy with excellent performance have been continuously increasing.Simultaneously,higher requirements have been put forward for the mechanical properties and corrosion resistance of the material itself.The traditional heat treatment process can not make the alloy obtain high strength and excellent corrosion resistance at the same time,hence the development of new heat treatment process is of utmost importance.The stress-aging process combines the effects of temperature and stress fields,to change the alloy microstructure and improve overall performance.The research on the mechanical properties and corrosion resistance of7A04 aluminum alloy with two-stage stress-aging can provide new ideas and theoretical guide for the preparation of high performance aluminum alloys,which is of great significance.In this thesis,the effect of stress-aging process on the mechanical properties and corrosion resistance of 7A04 aluminum alloy with two-stage aging was systematically studied by optical microscopy(OM),scanning electron microscopy(SEM),and X-ray diffraction(XRD)methods,combined with mechanical test,exfoliation corrosion and electrochemical corrosion experiments.The optimum single-stage solid solution process is 480?/2h,and the best two-stage stress-aging process is 120?/2h/200MPa+160?/3h/200 MPa.The effect of stress-aging process on the mechanical properties of 7A04 aluminum alloy with two-stage aging was studied.Compared with the condition of no-stress aging,the yield strength,tensile strength and hardness are obviously improved with the aging stress of 100~300MPa.When the first-stage agingtemperature is 120?,the tensile strength,yield strength,and hardness of the alloy are higher than 100? and 140?.The stress applied on the aging of first and second stage can improve the strength and hardness of the alloy,and the effect of the first-stage stress-aging is more significant.The effect of stress-aging process on the fracture behavior of 7A04 aluminum alloy with two-stage aging was studied.With the increase of aging stress,the fracture mode of the alloy changes from the ductile shear failure without aging stress to the transgranular dimple fracture with low aging stress,and then to the intergranular fracture with high aging stress.The fracture of stress-free aging specimens are mainly ductile shear fractures under different first-stage stress-aging temperature,and the stress aging of 200 MPa are mainly the fracture of the transgranular dimples.The large size dimples are mainly formed by the aggregation of the secondary particles and the aging precipitates in the process of alloy stretching,while the small dimples are formed by the separation of the precipitates and the matrix.The effect of stress-aging process on the exfoliation corrosion resistance of 7A04 aluminum alloy with two-stage aging was studied.The exfoliation grade of 200 MPa aging stress has improved to EA,compared with no stress-aging condition,which is EB.The stress-aging specimen has better exfoliation corrosion when the first stage aging temperature is 120?,while it is 100? for the no-stress aging specimen.The aging stress applied in the first and second stage have increased the exfoliation corrosion resistance of the studied alloy.The exfoliation corrosion of aluminum alloys formed pitting corrosion at first,and then developed to intergranular corrosion,turned into exfoliation corrosion at last.The effect of stress-aging process on the electrochemical corrosion resistance of7A04 aluminum alloy with two-stage aging was studied.The electrochemical corrosion resistance of stress aging alloy is higher than that of no-stress aging alloy.The aging stress applied in first and second stage all increased the electrochemical corrosion resistance of the studied alloy.The electrochemical corrosion process of7A04 alloy is formed by pitting corrosion of the secondary particles and precipitates.With the expansion of pitting corrosion,intergranular corrosion is gradually formed.The stress aging treatment improves the discontinuity of the precipitates at the grain boundaries of the alloy,thereby improving the electrochemical corrosion resistance of the alloy.