Study On Quenching Sensitivity Of Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-(Cu)alloy is a kind of age-hardenable aluminum alloy,which is widely used in aerospace,transportation and other fields as important lightweight structural materials because of its excellent mechanical properties,especially high specific strength and specific rigidity.With the development of lightweight in the aerospace industry,the size demanding of the astronautic aluminum alloy structure have become larger and larger,which results in the microstuctures and properties inhomogeneous between the surface and the center layer,which is known as the quenching sensitivity.In this work,quenching sensitivity of Al-Zn-Mg-Cu Alloy is investigated by means of hardness test,end quenching test,transmission electron microscopy(TEM)and differential scanning calorimetry analysis(DSC).(1)the effects of multi-stage aging treatment on the microstructure and properties of 7A09 aluminum alloy at different quenching rates were investigated;(2)the effect of pre-deformation on the microstructure and properties of 7050 aluminum alloy at different quenching rates were studied.(3)the influence of quenching rate and Mg content on the microstructure and properties of Al-Zn-Mg-Cu alloy were systematically studied.The main conclusions are as follows:(1)This study proposes an aging heat treatment process for reducing the hardness inhomogeneity caused by quenching of large-size 7A09 aluminum alloys.This method is a natural ageing combined with a two-stage artificial aging treatment process after solution quenching.The hardness of the core is greatly increased,without significantly reducing the hardness of the surface of the alloy,thereby achieving the purpose of reducing the unevenness of hardness of the large-size aluminum alloy caused by quenching.Compared to the T6 peak treatment,after natural aging combined with two-stage artificial aging,the hardness retention values of the air-cooled alloy relative to the water-cooled alloy was increased from 68.6%to 81.2%,with an increament of 18%.TEM results show that the natural aging combined with two-stage artificial aging post treatment can provide more nucleation particles in the slow quenching state,as a result,the size of the precipitates in the slow quenching state was refined,which improves the precipitation strengthening effect of the alloy under slow quenching conditions.(2)Combining deformation and aging,this work proposes a process for reducing the hardness inhomogeneity of large size 7050 aluminum alloys.By introducing 20%pre-deformation after solution quenching of large-size aluminum alloys,the precipitation strengthening effect of the alloy under slow quenching conditions is greatly enhanced without significantly reducing the hardness surface of the alloy,the precipitation strengthening effect of the alloy under the condition of slow quenching is greatly increased,which improves the mechanical strength of the alloy in the slow quenching state.The depth of the hardened layer is increased by 30%compared with the conventional method.(3)In the traditional T6 treatment,with the increase of the distance from the quenching end,not only coarse quenching-induced precipitates and obvious PFZs are generated in the grain boundaries and grains,the η’ phase inside grains also coarsened,which greatly weaken the precipitation strengthening effect of the alloy.With the introducing of pre-deformation,the size of the intragranular η’ phase is coarser than that of T6 treatment.With the decrease of the quenching cooling rate,the size of η’phase remains basically unchanged,and the introduction of dislocations promotes the precipitation strengthening phase during slow quenching,which provides a better precipitation strengthening effect for the alloy,due to the combined effect of better precipitation strengthening of the η’ phase and dislocation strengthening,the hardness non-uniformity of the alloy due to the quenching rate is greatly reduced.This process provides a new idea for reducing the quench sensitivity of the alloy.(4)With the decrease of the quenching cooling rate,the aging process of the alloy is slowed down and the peak hardness of the alloy is reduced.Compared to the water-cooled state,the peak hardness of 1-1 and 1-2 alloys under air cooling conditions was reduced by about 10%and 17%,respectively.As the Mg content increases,the alloy’s quench sensitivity decreases.