Double Aging Peaks Phenomenon Of Al-Si-Mg Alloys And Electrochemical Characterization On Age Of Al-Cu Alloys

Al-Si-Mg cast alloys and Al-Cu cast alloys can be strengthened by heat treatment,there are obvious changes of mechanical properties during the aging process,the effects of Aluminum's age-hardening is dependent on the hardening precipitation of aging process.System of electrochemical measurement,optical microscope,scanning electronic microscope(SEM)and energy disperse spectroscopy(EDS)and many kinds of other testing technology were used to research the age-hardening effect and electrochemical capability of cast Al alloys series.This paper consists of two parts:the first one is,age-hardening of cast Al-Si-Mg alloys and the phenomenon of bimodal aging-curve.The second one is,the correlation between the electrochemical corrosion and the aging progress of ZL201 alloy.(1)The phenomenon of two peaks in age-hardening curve is found in Al-Si-Mg cast alloy after adding Cu.There are two main precipitation sequences in aging process:(â…°)α_sss→GP zones→β′→β,(â…±)α_sss→GP zones→θ″→θ′→θ. The two precipitation sequences are independent and take place one after another.The first peak of the curve is result from strengthen of metastable phase β′,and the second peak is result from strengthen of metastable phaseθ′.The first peak is slightly lower than the second peak.(2)The phenomenon of two peaks in age-hardening curve is also found in Al-Si-Mg cast alloy after adding Zr.Zr plays a great role in grain refinement and the metastable phase Al₃Zr plays a role in dispersion hardening,Zr can also change the age hardening behavior of the alloy,making it ahead of schedule. The first peak of the curve is result from strengthen of metastable phaseβ′,and the second peak is result from grain refinement strengthening and dispersion hardening of Zr.The first peak is slightly higher than the second peak.(3)During the early aging progress of Al-Si-Mg cast alloy,age-hardening curve goes down at first and then goes up rapidly.After quenching,theα_sss gathers a large residual stresses.In the begining of age treatment,the residual stresses become to release,and the solute ofα_sssstarts to be precipitated.During the forepart of Aging,the attenuation of solution strengthening and stresses relaxation is more than hardness enhancement of GP zones,so alloy's hardness goes down.The decrease amplitude is due to quenching operation,solution strengthening,stress of quenching,DAS and so on.Because the strenthen effects of GP zones andβ′phase become advantaged,alloy's hardness goes up soon. The same phenomenon also exists in the Al-Cu alloys,and it is a common phenomenon in aluminum alloy which can be strengthened by aging treatment.(4)The correlations among the mechanical properties,ageing precipitation behavior and electrochemical corrosion parameters in ZL201 have been established.The ageing precipitation behavior is reflected by measuring electrochemical corrosion parameters such as E_k and I_corr.When the maximum E_k and the minimun I_corroccur,the alloys stay at solution treatment,the hardness is the least,and the microstructure of alloy is composed of supersaturatedαsolid. When the minimun E_k and the maximum I_corroccurs,the alloys stay at peak aged, the hardness is the least,and the microstructure of alloy is composed of metastable phaseθ′.