Effect Of Pre-aging In RRA On Corrosion Resistance Of7050Aluminum Alloy Hot-rolled Plates

In the nineteen seventies,Alcoa developed the third-generation aluminum alloys with high strength,high fracture toughness and good stress corrosion resistance including7050aluminum alloy,which were widely used in aerospace industry.But the localized corrosions in the alloy have often taken place,such as exfoliation corrosion and stress corrosion,which limit its application.In this thesis,the effect of pre-aging temperature in retrogression and re-aging treatment(RRA)on stress corrosion resistance of7050aluminum alloy was investigated by means of hardness test,conductivity test,exfoliation corrosion test,slow strain rate tensile test(SSRT),electrochemical test, transmission electron microscopy(TEM) and scanning electron microscopy(SEM) on the hot-rolled plates from a company in china.The main conclusions are as follows:(1) Effect of the single-stage aging time on exfoliation corrosion of7050aluminum alloy plates has been investigated.It is shown that the resistance to exfoliation corrosion is improved with increase of the single-stage aging time,especially under over-aged condition.After the single-stage aging at140℃for24h,the samples immersed in the EXCO solution for48h,reach the exfoliation corrosion degree of EA+(2) The corrosion resistance of7050aluminum alloy plates is characterized by electrochemical measurement. It is shown that the open circuit potential(corrosion potential) rises,the corrosion current calculated by polarization curves reduces,the radius of capacitive arc in the impedance spectroscopy increases,the value of the impedance gets higher with the increase of the single-stage aging time at140℃.The above results show that the resistance to corrosion of7050aluminum alloy plates is improved with the increase of the single-stage aging time.(3) The pre-aging temperature of7050aluminum alloy plates has been optimized and its effect on stress corrosion resistance has been studied.It is shown that the stress corrosion resistance first increases and then decreases with the increase of pre-aging temperature,when the pre-aging temperature is80℃,as in under-aged condition at the pre-aging stage of RRA,the aluminum alloy plates show better stress corrosion resistance.Specifically,the tensile strength,elongation and stress corrosion index (ISSRT) are473.5MPa,10.67%and0.05824in the stress corrosion test with slow strain rate (10-6S-1) and corrosion medium of3.5%NaCl solution.(4) The mechanism of stress corrosion resistance changing with the pre-aging temperature is explained based on the microstructures. When the pre-aging temperature increases,the main intragranular precipitates transform to η’phases from GP zones after RRA,which is difficult to cause stress concentration,and beneficial to the stress corrosion resistance.Meanwhile the grain boundary precipitates become coarser and discontinuous,which is difficult to lead to the continuous anode dissolution,and reduces the sensitivity of alloy to stress corrosion.But when the pre-aging temperature increases further,the precipitate free zone (PFZ) appears and becomes wider,whose width goes up to140nm when the pre-aging temperature rises to120℃.The wide PFZ is harmful to the stress corrosion resistance of alloy,because it can cause serious stress concentration and anode dissolution.(5) The pre-aging time of7050aluminum alloy plates has been optimized further based on the optimized pre-aging temperature. It is shown that the stress corrosion index (ISSRT) of the7050aluminum alloy plates aged by RRA first decreases and then increases with increase of the pre-aging time at80℃,and the stress corrosion index (ISSRT) reaches the minimum (3.127×10-2)when the pre-aging time is24h.The change regulation of stress corrosion index (ISSRT)sHows that the stress corrosion resistance of7050aluminum alloy plates first increases and then decreases with increase of the pre-aging time.When the pre-aging is80℃/24h,as in under-aged condition at the pre-aging stage of RRA,the alloy plates show better stress corrosion resistance.Specifically,the tensile strength, elongation and stress corrosion index (ISSRT) of the plates are494.1.5MPa,9.749%and0.03127in the stress corrosion test with slow strain rate (10-6s-1) and corrosion medium of3.5%NaCl solution.