| In this research,A7N01S-T5 aluminum alloy,which is used in high speed trains,was taken as the investigated subject.After solution heat treatment,different heat treatments were carried out on the alloy,including peak aging?T6?,over aging?t74?,regression re-aging?RRA?.The mechanical and corrosion properties of the alloy,which in different aging conditions,were studied.At the same time,the influence of temperature and time during the regression process of A7N01 aluminum alloy on the mechanical properties and corrosion properties were studied.The influence mechanism of aging treatment on the mechanical properties and corrosion properties of A7N01S-T5 aluminum alloy were analyzed by the observation of microstructure,especially grain and precipitate phase.The test results showed that after T6 treatment,the tensile strength of A7N01 aluminum alloy reached a peak value of 440MPa and the yield strength was 356 MPa;After RRA treatment,the tensile strength of the alloy was 470.47MPa,which is improved by 6.85%compared with T6 state,and the tensile strength is improved by 18.69%;and the tensile strength of the alloy is 409 MPa in T74 state,which is reduced by 7.03%compared with T6state,and the tensile strength is reduced by 4.83%;After T74 treatment,the tensile strength of the alloy is reduced by 12.99%compared with that of RRA,and the yield strength is reduced by 19.81%.The grains of the three aging alloys are banded along the rolling direction,the grain in T74 state is the finest and the T6 state is the largest.The grain size of RRA state is slightly smaller than the T6 state,and large particle black impurity phase is distributed along the grain boundary.The percentage of small angle grain boundaries of T74and RRA samples is less than that of T6 samples,and the ratio of large angle grain boundary is similar.The size of the precipitated phase in the T74 state alloy is the largest,and the distribution density of the precipitates is the smallest,which makes the strength decrease significantly.A series of electrochemical tests show that the order of corrosion resistance is T74>RRA>T6,the Icorr value of T74 state alloy is the smallest,which is 4.8749×10-6A/cm2,and the Icorrorr value of T6 state alloy is the largest,which is 4.1138×10-5 A/cm2,and the order of corrosion current density of the alloys in different states is T6>RRA>T74.In the electrochemical impedance spectrum,it can be seen that the impedance spectrum of T74alloy has no inductance arc,which indicates that the corrosion time of T74 state alloy is later than that of T6 and RRA alloy,and its Rt value is the largest,and the Rt value of T6 alloy is the smallest,so the corrosion resistance of T74 alloy is the best and the corrosion resistance of T6 state is the worst.The time domain spectrum of electrochemical noise shows that the rupture and repair of the aluminum alloy oxide film happened alternately in the intergranular corrosion solution.Pitting occurred firstly,and then pitting changed into intergranular corrosion.During the period of corrosion,the accumulation of corrosion products in the corrosion hole of the sample reduced the ion exchange rate between the metal and the solution,which made the reaction speed decrease.The corrosion products fell off in the later stage,and the corrosion reaction rate increased.The corrosion state of T6 alloy is the most serious,cracking is occurring in this stage.The grain boundary precipitation phase of T6state alloy is continuous chain distribution,and the grain boundary precipitate free zone?PFZ?on both sides of grain boundary is very narrow.Due to the electrochemical difference between the grain boundary and the matrix,corrosion first occurs at grain boundaries.A corrosion channel formed along the precipitate phase in the grain boundary to accelerate the occurrence of grain boundary corrosion.The grain boundary precipitation of T74 and RRA alloys is larger and intermittently distributed,and there is a clear PFZ on both sides of the grain boundary,which makes the corrosion channel on the grain boundary difficult to form,and the corrosion current density decreases as well.For the RRA state of A7N01 aluminum alloy with different regression temperature and time,when the regression temperature is the same,the hardness and strength of A7N01aluminum alloy decreases gradually with the extension of the regression time.And the higher the regression temperature is,the faster the decrease of strength and hardness.At the same time,the larger recrystallized grains become larger,and the number of small angle grain boundaries becomes more and more.The average size of precipitated phases inside the grains increases,too.When the regression time is the same,with the increase of the regression temperature,the average size of the precipitated phase in the grain increases slightly,and the distribution density decreases,which significantly reduces the strength of the alloy.After soaking 96h,the noise resistance Rn of 200°C-1h sample is less than200°C-2h sample,indicating that the longer the regression time is,the lower the corrosion activity of the specimen in the later stage of corrosion.The Rn value of 230°C-40min sample is less than 250°C-40min sample,which shows that the higher the regression temperature is,the better the corrosion resistance of the specimen after the re-aging.The higher the regression temperature and the longer the regression time,the larger the precipitation phase,the gap and the PFZ width on the grain boundary of the alloy,the corrosion channel for intergranular corrosion can't form on the grain boundary.The corrosion current density is reduced because of the existence of PFZ,which makes the corrosion resistance of the alloy better. |
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