Investigations On Integrated Reversion Ageing And Forming(IRAF) Process And Its Mechanism In T4-tempered High-strength 7xxx Series Aluminium Alloy

Vehicle lightweighting is one of the effective means to achieve "carbon peak" and "carbon neutrality" strategies.Material lightweighting is the most direct and effective way to achieve automotive lightweighting.High-strength Al-Zn-Mg-Cu(7xxx)alloys have become one of the promising lightweight materials in the automotive industry due to their low density,high strength and excellent age-hardening potential.However,poor formability and springback of high-strength Al-Zn-Mg-Cu alloys at room temperature severely limit their applications in the automotive structural parts.The warm/hot forming process of aluminum alloy is a lightweight manufacturing process to improve the formability and forming accuracy of high-strength aluminum alloy,and its final mechanical properties can be further improved by subsequent bake hardening treatment.In order to solve the problems of high cost and low efficiency of current warm/hot forming process,this paper proposes a new warm forming process that combines the reversion aging treatment of naturally aged aluminum alloy with warm stamping forming: Integrated reversion ageing and forming(IRAF),which can still ensure the high formability and high final mechanical properties in the Al-Zn-Mg-Cu alloy component.In this paper,the microscopic mechanism of reversion aging process parameters on the natural aging clusters and shape/property synergistic manufacturing of high-strength AA7075 aluminum alloy is researched.A large number of Zn-rich natural aging clusters forms with the assistance of supersaturated vacancies of as-quenched AA7075 aluminum alloy,which begin to dissolve significantly at an aging temperature higher than 150 °C,also called as reversion aging.It’s found that the hardness of AA7075-T4 aluminum alloy can be reduced by 40 HV and the yield strength value can be reduced by about 100 MPa when reversion aged at 180℃ and 210℃ for 10-45 s.The higher reversion aging temperature corresponds to the lower reversion hardness value,and the time to reach the lowest hardness value is earlier,but the natural aging clusters cannot be completely dissolved to the quenched state during the reversion aging process.The reverted samples exhibited obvious secondary natural aging,but its effect on the bake hardening response of the high-strength Al-Zn-Mg-Cu alloy was negligible.The yield strength of samples after bake hardening increase significantly to 520 MPa.The GPII zone and η’/η2 precipitates of 4~8nm were dispersed in the grains and the grain boundary precipitates were discontinuous,and the grain boundary precipitates free zone was narrow(<50nm),which indicates that the reverted sample after artificial aging can obtain higher strength and excellent stress corrosion resistance.The effects of reversion aging process parameters on the formability and post-forming mechanical properties of naturally aged high strength 7xxx aluminum alloy sheets were systematically studied through warm tensile experiments.At a forming temperature of 180 °C,the higher heating rate(>5°C/s)reduced the yield strength of the material by suppressing reprecipitation during reversion ageing.When the AA7075-T4 material was heated from room temperature to 150-300°C at a heating rate of 50°C/s,its yield strength decreased from 400 MPa to 322MPa-135 MPa.However,a large number of coarse η phases completely incoherent with the matrix are formed when the forming temperature is higher than 240 °C,which consumes too much solute elements and leads to low bake hardening strength.When rapidly heated(50°C/s)to a forming temperature lower than 240°C,short-term holding can further dissolve natural aging clusters to improve sheet formability.Holding at 180°C for 10-30 seconds or at 210°C for 5-15 seconds results in a good combination of high formability and bake hardening strength by reducing the yield strength.The yield strength evolution modeling during reversion process shows that the dissolution of the clusters and the formation of precipitates occur simultaneously during the rapid heating process.The formation of precipitates dominates when the temperature is higher than 240 °C while the dissolution of NA clusters dominates when the temperature is lower than 240 °C.The fitting results of the unified visco-plastic constitutive equation show that when AA7075-T4 aluminum alloy is plastically deformed at 150-300 °C(>50℃/s)at a strain rate of 0.1s-1,the strain hardening exponent n1 of the material decreased from 6.07 to 2.57,and the main plastic deformation mechanism was elasto-viscoplastic deformation dominated by dislocation motion.The three-point bending test shows that the springback height of the aluminum alloy sheet can be reduced from 12 mm at room temperature to 6mm under the warm forming process,the springback angle can be reduced from 37° to 10°,and the springback angle ratio can be reduced from 47.4% to 12.8%.The experimental results and simulation results are in good agreement.The finite element simulation modeling and the sample trial production test verify that the constitutive model can accurately predict the warm deformation behavior and property evolution of the alloy.The results of this paper have important guiding significance for the development of high-efficiency,high-performance and low-cost warm forming technology,which may expand the practical application of high-strength 7xxx aluminum alloys in the field of automotive industry.