| With the rapid development of light weight of automobile,higher requirements are put forward for the comprehensive performance requirements of 6xxx series aluminum alloy,especially including the formability,paint bake hardening increment and bending performance.In this paper,in order to solve the problems of insufficient paint hardening increment of traditional 6xxx alloys,the novel Al-Mg-Si-Cu-Zn-Fe-Mn alloys were designed and developed based on the calculation of phase diagram.Combined with the control of pre-aging or pre-aging+pre-deformation inducing the homogenous nucleation or homogenous/heterogeneous synergistic nucleation,the age hardening response rate and ability of the alloy and the anti-natural aging stability have been significantly improved through the study of the aging precipitation behavior of the alloys at different temperatures,simple pre-aging process,solid solution-quenching/pre-aging and pre-aging/pre-deformation.The related mechanisms of rapid age hardening response,anti-natural aging stability,and homogeneous/heterogeneous nucleation were also further systematically studied.The following conclusions were drawn:The introduction of multi-scale iron-rich phases and Zn can improve the age hardening response rate and capacity during non-isothermal or isothermal aging process in Al-Mg-Si-Cu-(Zn)alloys.With raising isothermal aging temperature,the peak aging time is decreased significantly,but the yield strength can be increased to 370MPa.Zn can participate in the nucleation and growth of the precipitation phase,which does not change the precipitation sequence,maintaining the phase structure appeared in traditional 6xxx series aluminum alloys,but the lattice parameters of the precipitates have been changed to a certain extent.In addition,multi-scale iron-rich phases can also promote intragranular precipitation and refine the grain to inhibit the segregation of solute atoms on grain boundaries,thus significantly improving the strength and ductility of the series alloys in the peak aging state.The anti-natural aging stability and age hardening response rate of new AlMg-Si-Cu-(Zn)alloys with 0.2wt.%Fe were significantly changed by nonisothermal pre-aging process with different cooling rates.Reducing the cooling rate of non-isothermal pre-aging to 2.2℃/h can effectively inhibit the formation of unstable solute clusters and improve the age hardening response rate after long-time natural storage.With the addition of Zn,the complex interaction among Mg,Si and Zn can further inhibit the formation of fine solute clusters during long-term natural storage,and only the enrichment of Si atoms to the initial fine solute clusters can occur,reducing Mg/Si ratio of the clusters.Finally,the Zn-containing alloy exhibits excellent anti-natural aging stability,and the paint bake hardening increment of the Zn-containing alloy can reach 94MPa after natural storage of 2.5 years,which is similar with the value of pre-aged traditional 6xxx series aluminum alloys with natural storage of 14 days.The paint bake hardening increment of Al-Mg-Si-Cu-(Zn)alloys with 0.5wt.%Fe can be significantly improved via coupling solution-quenching and isothermal pre-aging.If the quenching rate was reduced appropriately,the aggregation of solute atoms during the quenching process can be slowly formed,and the number density of vacancies can be decreased.The stable solute clusters with high number density can be formed by the following pre-aging control.Based on this,treated by 555℃/2min solution+20℃ oil quenching+80℃/2h pre-aging,the bake hardening increment of Zn-containing alloy with the natural storage of 14 days can reach 145 MPa.Compared with the traditional AI-Mg-Si-Cu alloys without additions of Fe and Zn,with the positive influence of multi-scale iron-rich phases,the precipitate precursors with the higher number density and stability can be formed in the novel Al-Mg-Si-Cu-3.0Zn-0.5Fe(wt.%)alloy with the coordinated control of solutionquenching-pre-aging treatment,which is beneficial to improve the age hardening response rate.Meanwhile,the produced dislocations can be uniformly distributed in the alloy during pre-deformation with the effect of multi-scale iron-rich phases.Furthermore,based on the accelerated homogeneous nucleation rate of Mg-Si precipitate phases by Zn addition,the heterogeneous nucleation rate of the corresponding precipitate phase can be further improved.Finally,the yield strength increment of the alloy can reach 159MPa after paint baking treatment by the synergistic control of homogeneous/heterogeneous nucleation,while the ductility can still maintain a high level,and the comprehensive properties are much higher than those of the traditional 6xxx series aluminum alloy. |
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