| Heat-treatable 6xxx series(Al-Mg-Si(-Cu))alloys are widely applied in many fields,such as aerospace,automobile,high-speed rail and subway,due to their low density,good comprehensive mechanical properties,excellent formability,weldability and so on.Generally,the corrosion resistance of Cu-free 6xxx series alloys is good,but that of Cu-containing 6xxx series alloys is poor.In the process of actual service,Al-Mg-Si-Cu alloys would be imposed by all kinds of stress,and would be easily corroded by corrosive media when they work in the coastal areas or in foggy weather.As a result,Al-Mg-Si-Cu alloys are prone to intergranular corrosion and stress corrosion cracking,which can seriously affect the service life of the equipment and endanger the safety of construction personnel.With the rapid development of transportation field,people require better corrosion resistance of the Al alloys.Therefore,it is very important to explore some methods to improve the corrosion resistance of Al-Mg-Si-Cu alloys.In the present study,the influence of traditional T6 process,deformation and aging process,retrogression and re-aging(RRA)process,two-stage aging(T78)process on the stress corrosion cracking resistance,intergranular corrosion resistance and microstructure in Al-Mg-Si-Cu alloys was systematically investigated by hardness measurement,the slow strain rate technique,intergranular corrosion test,and transmission electron microscopy(TEM).The results are as follows:(1)The alloy treated with traditional T6 process has severe stress corrosion cracking and intergranular corrosion susceptibility.Compared with T6 process,the deformation and aging process not only can greatly improve the strength of the alloy but also can effectively improve its stress corrosion cracking and intergranular corrosion resistance.The RRA process and T78 process can effectively improve the stress corrosion cracking and intergranular corrosion resistance of the alloy while ensuring its strength.(2)For the traditional T6 treated alloy,needle-like ?? phases are mainly formed in the matrix.Due to the fact that ?? phases almost do not contain Cu,the Cu atoms uniformly distribute in the matrix or in the grain boundary precipitates,which leads to a high potential of the matrix and grain boundary precipitates and increases the electrochemical potential difference between the matrix and the PFZ.Therefore,the alloy holds severe corrosion susceptibility.(3)In the deformation and aging process,pre-aging can tune the hardness and corrosion resistance of the alloy.The artificial peak-aging treatment results in the optimal combination of hardness and corrosion resistance.The high density of dislocations efficiently enhance the hardness of the alloy.Numerous nano-sized Cu-rich lath-like Q" precipitatesor curved continuous precipitates distribute in the matrix,which eliminates the formation of grain boundary precipitates and greatly reduces the electrochemical potential difference between the matrix and the PFZ,consequently significantly improving the corrosion resistance of the alloy.In the alloys pre-aged by natural aging,artificial under-aging and artificial peak-aging,the degree of precipitation in the matrix increases in sequence,and the corrosion resistance of the alloy is improved in sequence.(4)In the deformation and aging process,the effect of re-aging temperature on the corrosion resistance of the alloy is not obvious.The main reason is that all the aging times in the different re-aging tempers were selected to be peak-aging times,so that the degree of precipitate on in the matrix is similar,thus the corrosion resistance of the alloy is similar.(5)After RRA and T78 treatments,the precipitates of the alloy are mainly lath-like precipitates and a small amount of needle-like precipitates.The lath-like precipitates consumea large number of Cu atoms from the matrix,obviously reducing the electrochemical potential difference between the matrix and the PFZ,consequently improving the corrosion resistance of the alloy. |
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