The Study Of Isothermal Quenching Properties Of Al-Zn-Mg-Cu-Zr Alloys Containing Sc

Al-Zn-Mg-Cu-Zr alloys are widely used in the aerospace industry because of its high strength,good corrosion resistance and combined with sufficiently high toughness.With the development of the aviation industry,the maximum thickness of conventional aviation slab and forgings are increasing.Due to increasing cross-section of the aircraft component,the cooling rate is inconsistent in quenching process along the direction of the cross-section,resulting in large difference of mechanical properties after aging.The aluminum alloy behaves quench sensitivity.How to get high mechanical properties and low quench sensitivity of the Al-Zn-Mg-Cu-Zr alloys is a hot topic in fields of materials science and engineering.Therefore,the study of Al-Zn-Mg-Cu-Zr alloys has theoretical and practical significance.The sheet of Al-Mg-Cu-Zr alloys containing 0.2%Sc were produced through homogenization,extrusion,hot rolling,intermediate annealing,cold rolling and other treatments.After heat treatments of solution,step quenching and aging,quenching conductivity and aging hardness would be measured for drawing TTT(Time–Temperature–Transformation)curve and TTP(Time-Temperature–Properties)curve.Microstructure and composition has been investigated by using differential scanning calorimetry(DSC),X-Ray diffraction(XRD),transmission electron microscopy(TEM).Moreover,this paper studied on the precipitation kinetics of Al-Zn-Mg-Cu-Zr-0.2Sc alloy during the isothermal heat process by analyzing the Johnson-Mehl-Avrami equation.Main conclusions are listed in the following:Firstly,the conductivity and hardness of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy was determined by an interrupted quench method.The results show that the conductivity increased and hardenability decreased on the whole while prolonging isothermal holding time.Secondly,it can be seen that the TTT curve and TTP curve of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy are “C” shape,and the nose temperature are both 330 ?.The quench sensitivity of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy is high because the incubation period is very short at the tip of the nose temperature less than 4s.The Avrami parameters k fitted by Origin 8.5 reaches the maximum at 330?.Therefore the rate of phasetransformation is the fastest at 330?.Thirdly,the results show that the quench sensitive temperature range of TTT and TTP curves of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy are 270?~390?.The aluminum alloy has high quench sensitivity,and high precipitation rate.After heat treatments of solution,slower cooling rate could be adopted during quenching in the temperature range of >390?,which can lead to high mechanical properties and low residual stress can be obtained.Fourthly,the second phase particles during isothermal holding were decomposed in supersaturated solid solution of Al-9.0Zn-2.5 Mg-1.5Cu-0.15Zr-0.2Sc alloy.With extending the isothermal holding time,the quality of large equilibrium particles could be increased during quenching.Consequently,less volume of hardening precipitates during subsequent aging was obtained due to less amounts of solutes,and it would result in the loss of hardenability.The feature of the precipitate free zone(PFZ)is that its size increases combined with continuous distribution when the holding time increasing.Thus,it seems that the PFZ is mostly related to the loss of solute caused by the quench-induced precipitation.Finally,the equilibrium ?(MgZn2)phase nucleates on the Al3(Sc,Zr)dispersoids heterogeneously and precipitates from the supersaturated solid solution of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy within the critical temperature range.The heterogeneous precipitation leads to a low concentration of solutes,and consequently reduces the amount of solutes,and consequently reduces the amount of the strengthening phase ?' during aging,resulting in a lower hardness.