| The current aeroplane design has called for the demand of new generation aluminum alloys with higher combined performance than the traditional ones,which means the new ones have higher strength and hardness,better plasticity,toughness,corrosion and tolerance resistance.The design of Al-Zn-Mg-Cu alloys aims to the direction of high main alloy contents,especially the zinc content has a content higher than 9.0wt.%.The high Zn-containing Al-Zn-Mg-Cu alloys have a relatively high alloying degree,which have a larger amount of precipitates during aging processes compared with the traditional alloys with a zinc content interval of 5.0?8.0wt.%.Besides,the precipitation dynamics and property variations of the high Zn-containing alloys have some differences with the traditional ones.In present,domestic studies on them lack sufficiency and seldom investigations focus on the influence of main alloy content on precipitation behavior and realated properties.Further more,a severe demand on combined performance has been inflicted on the aluminum alloys applied in the aeroplane field.However,the mechanism of alloy element content variations on fatigue performance is still ambiguous,which leads to an evidence-lacked alloy design corresponding to the request of high fatigue performance.In present work,one group of alloys with same Zn/Mg ratio(Al-8.02Zn-1.80Mg-2.02Cu and Al-9.00Zn-2.02Mg-2.02Cu)and another group of alloys with different Zn/Mg ratio(Al-9.32Zn-2.00Mg-1.76Cu and Al-9.78Zn-2.04Mg-1.76Cu)are designed.The two groups have different copper contents.Based on these alloys,it has been systematically investigated that the as-cast,as-homogenized and solution treated microstructure difference and the influence of element variation on microstructure and properties,so as to the fatigue performance with different aging tempers.The main results include the following aspects.The as-cast microstructure of the alloys is investigated.The common second phase of the alloys are Mg(Zn,Cu,Al)2 phase,Fe-rich phase and MgZn2 phase.The alloys with same Zn/Mg ratio include Al2Cu phase which contained a certain content of zinc and magnesium elements.Moreover,zinc and magnesium contents in Al2Cu phase of the alloy with higher zinc and magnesium contents are larger than those with lower ones.As for the alloys with different Zn/Mg ratio,Al2CuMg phase and Al2Cu phase are found in the 9.3Zn+2.0Mg alloy and 9.8Zn+2.0Mg alloy,respectively.Under same Zn/Mg ratio,the alloy with higher zinc and magnesium contents has a smaller distribution density difference of matrix precipitates inside grains and a larger amount of Mg(Zn,Cu,Al)2 phase.Under different Zn/Mg ratio,the alloy with a higher zinc content has the same characteristics of the alloy with higher zinc and magnesium content under same Zn/Mg ratio.The second phase dissolution of the alloys during homogenization/solution treatments is investigated.Homogenized with the two stage homogenization regime of 440?/12h+470?/24h,the main solidification phases have dissolved into the matrix except for the Fe-rich phase,leading to a sufficient homogenization effect.During the solution treatment,it is relatively hard for the Mg(Zn,Cu,Al)2 phase dissolution of the higher zinc and magnesium-containong alloy under same Zn/Mg ratio and the higher zinc-containing alloy under different Zn/Mg ratio.During the solution treatment processes,a transformation from Mg(Zn,Cu,Al)2 phase to A12CuMg phase is observed in the 8.0Zn+1.8Mg alloy while Mg(Zn,Cu,Al)2 phase directly dissolve into the matrix in the other alloys with zinc content no less than 9.0wt.%.When the solution temperature is fixed,a prolonging of solution time has a positive effect on Mg(Zn,Cu,Al)2 phase dissolution.The precipitation behavior and properties during one-stage aging treatment of the alloys are investigated.The change rules of hardening effect during single aging treatment are similar for the all alloys.The hardness needs longer time to reach a peak value with a lower aging temperature and the peak hardness planeu sustains a longer time interval while the hardness achieves a peak value rapidly with a higher aging temperature and then decreases continuously.The electrical conductivity for the all alloys is positively related to the prolonging of aging time.The main strengthening phases are GP zones and ?' phase for the alloys with peak aging temper.As agimg temperature is fixed,under same Zn/Mg ratio,the alloy with higher zinc and magnesim contents has a faster aging response,higher hardness and strength values,lower electrical conductivity values and a larger proportion of big size precipitates with the peak aging temper.Under different Zn/Mg ratio,the alloy with higher zinc content has a larger proportion of big size precipitates with the peak aging temper and a better resistance on over-aging treatment.The precipitation behavior and properties during two-stage aging treatment of the alloys are investigated.The alloys exhibit similar change rules of hardening effect during the second stage preservation stage of two-stage aging treatment.The alloys show a continuous decrement in hardness and ultimate tensile strength and a persistent increment in electrical conductivity with the prolonging of the second stage aging time.The yield strength increases at the initial stage of the second stage aging and then continuously decrease.The main precipitates of the alloys with different over-aging degrees are GPII zone,?' phase and ?phase.With the over-aging degree deepens,the amount of GPII zone and ? phase shows a decrement and an increment,respectively,and the precipitates have growed up and coarsened.As two-stage agimg regimes are fixed,under same Zn/Mg ratio,the alloy with higher zinc and magnesim contents has a faster aging response,higher hardness and strength values and lower electrical conductivity values.After the T76 aging treatments,the alloy with higher zinc and magnesim contents has a larger proportion of big size precipitates than the alloy with lower ones.Under different Zn/Mg ratio,the alloy with higher zinc content has higher strength value,a larger proportion of big size precipitates and a better resistance on over-aging treatment.The precipitation behavior and properties during three-stage aging treatment of the alloys with different Zn/Mg ratio are investigated.The change rules of hardening effect for the alloys are similar during the retrogression period.With the prolonging of the retrogression time,the alloys show a continuous decrease in hardness,ultimate tensile strength and yield strength and a persistent increase in electrical conductivity.After the three-stage aging treatment with the regime of 110?/16h+180?/45min+120?/24h,the main precipitates of both alloys are GPII zone and ?' phase.Besides,the alloy with higher zinc content has higher strength values and a larger proportion of big size precipitates compared with the alloy with lower one.The fatigue crack propagation behavior of the alloys with different two-stage and three-stage aging treatments is investigated.After T76 treatments,in terms of the alloys with same Zn/Mg ratio,the alloy with higher zinc and magnesium contents has a relatively lower fatigue crack propagation rate.As for the alloys with different Zn/Mg ratio,after T76/T77 treatments,the alloy with higher zinc content has a lower fatigue crack propagation rate.The enhancement of main alloying elements content results in the increase of precipitate amount and a skew of precipitate size distribution to the big size precipitates is observed,which raise difficulties for dislocations to cut precipitates,so as to improve the resistance to fatigue crack propagation.The fatigue crack propagation rates for the 9.3Zn+2.0Mg+1.8Cu alloy treated by T79,T74 and T73 tempers show a continuous decrease with the over-aging degree deepens.The ?' phase is still the main precipitate in the alloy treated by two-stage aging treatments and the cutting mechanisum still possesses an obvious role.As the aging degree deepened,the resistance to fatigue crack propagation is improved.The influence of Fe-rich phase on fatigue crack propagation in the Paris zone is investigated.Equiaxial dimples are formed around the continuously distributed Fe-rich phases with the fatigue crack propagated.Besides,the secondary cracks through each other have appeared and the path exhibit a certain defections.In the dimples,there can be multi paths of secondary cracks,some could propagate into other dimples and the other show a closure.As the fatigue crack propagated,some raise ridges appeared at the dispersedly distributed Fe-rich phases along thepropagation direction of main crack while no obvious influence on the fatigue crack propagation rate emerged. |
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