Study On Ageing Hardening And Corrosion Resistance Mechanism Of Al-Mg-xSi-Cu-Zn Alloys With High Formability For Automotive

Al-Mg-Si alloys have been used as excellent materials for automobile body panels due to their high strength-to-weight ratio,good surface quality and recycling potential.However,the limited improvement of deep drawability,bake-hardening response(BHR)and insufficient natural ageing resistance of Al-Mg-Si alloys still cannot meet the demanding requirements.With the development of automobile lightweight,it is urgent to develop Al-Mg-Si alloys with excellent comprehensive properties.Based on the previous work of our group,the following researches were carried out for the designed Al-Mg-xSi-Cu-Zn alloys.Firstly,the alloy sheets with high formability were prepared by thermal-mechanical processing.Subsequently,the influence of cooling pre-ageing(PAC),per-deformation(PD)and natural ageing(NA)(single factor or multiple factors)on the precipitation behavior and its related mechanisms were systematically studied,and the alloy sheet with the ultra-fast agehardening response and excellent natural ageing resistance was developed.Meanwhile,the relevant corrosion resistance behavior and its related mechanism were studied.The main research results are as follows:(1)Based on the influence of the secondary particles on the recrystallization microstructure and texture of Al-Mg-Si-Cu-Zn alloys,the Al-Mg-xSi-Cu-Zn alloys with high formability were prepared by the designed thermal-mechanical processing.Subsequently,the effects of PAc and Si content on the precipitation behavior were systematically studied.The results show that PAc and Si content can induce the formation of multi-scale solute clusters,which are easy to nucleate and grow in the subsequent isothermal ageing process,and the distribution characteristics of the precipitates inherit the distribution characteristics of the initial solute clusters(genetic characteristics).Accordingly,the Al-0.70Mg-0.55Si0.20Cu-0.52Zn alloy with multi-scale precipitates and its control process were developed,and the elongation of the peak-aged alloy is improved while the strength remains unchanged.In addition,the yield strength of peak-aged alloys increases first and then remains unchanged with increasing Si content,but it is independent of the ageing routes.(2)The effect of PAc+PD on the precipitation behavior of Al-Mg-xSi-Cu-Zn alloys was systematically studied.The results show that PD can significantly affect the precipitation behavior of Mg,Si,Cu,Zn solute atoms.Among them,PD enables the segregation of Zn and Si atoms along dislocation to promote the heterogeneous nucleation of precipitates along dislocation,and induces the formation of Mg-Zn clusters in the dislocation-free region to promote the homogenous nucleation of precipitates.Accordingly,the synergistic regulation of homogenous/heterogeneous nucleation can be achieved by introducing dislocations with appropriate density,so as to the alloy has a rapid age-hardening response.Based on this,the Al-0.70Mg0.76Si-0.20Cu-0.54Zn alloy with the ultra-fast age-hardening response and its control process were developed.The BHR of the pre-aged alloy can reach 167 MPa and maintain excellent ductility(EI=14.3%),which much higher than those of traditional Al-Mg-Si alloys.Simultaneously,the coupling control based on PD and Si content can reduce the continuity of grain boundary precipitates and inhibit the formation of the precipitation-free zone in the peak-aged alloy,and finally maintain the strength and ductility while improving the intergranular corrosion resistance.(3)Based on the influence of NA and PD on the precipitation behavior of solute clusters,it is shown that high-density Si-rich clusters can be formed by the coupling effect of long-term NA and Si content,and PD can significantly affect the nucleation and growth of solute clusters.Accordingly,based on the coupling regulation of PAc+long-term NA+PD,the nucleation and growth of Si-rich clusters can be promoted while the synergistically regulation of homogenous and heterogeneous nucleation can be realized by the introduction of dislocations with appropriate density,so that the alloy has excellent natural ageing stability.Based on this,the Al-0.67Mg-1.08Si-0.19Cu-0.53Zn alloy with excellent natural ageing stability and its control process were developed,and the BHR of the natural aged alloy can reach 151 MPa,resulting in excellent strength and ductility in the bakehardening state(YS=319 MPa,EI=18.9%).(4)Based on the influence of the secondary particles on the formability of AlMg-Si-Cu-Zn alloy and the synergistic mechanism of PAc+PD on the precipitation behavior,the formability,BHR and corrosion resistance of the industrial alloy sheet were integrated regulated.The alloy sheet with excellent comprehensive properties was obtained.The properties of the pre-aged alloy sheet are as follows:r=0.673,Δr=0.062,YS=127 MPa,EI=33.5%.The properties of the alloy in the bakehardening state are as follows:YS=287 MPa,EI=15.7%,the BHR is 161 MPa,and the maximum depth of intergranular corrosion is 90 μm.