| Al-Zn-Mg-X alloys are widely used in structural components in aerospace and other fields due to their excellent comprehensive properties.With the continuous expansion of its application in marine,industrial and atmospheric fields,the corrosion resistance of traditional Al-Zn-Mg alloy has gradually been unable to meet the requirements of component design.While maintaining high strength and high toughness,good corrosion resistance has become a key issue for the development of Al-Zn-Mg alloys.Therefore,this paper studies the new Al-Zn-Mg-X aluminum alloy to meet the comprehensive performance requirements of strength and corrosion resistance for aerospace applications.The effects of Ti and Cu on the microstructure and properties of the alloy were studied.The relationship between microstructure,mechanical properties,and corrosion resistance was revealed,and the theoretical basis was provided.The Al-Zn-Mg-X alloy after solution treatment was characterized by EBSD,and the aging precipitation behavior was analyzed by TEM and DSC.The results show that there is a large deformation energy storage in the hot rolling process of the alloy.After adding the Ti element,the resistance of the L12 phase containing Ti decreases due to dislocation and subgrain boundary migration.The microstructure of the alloy further evolves in the solution process,the degree of recrystallization increases,and the grain size is refined.Promoting the transformation of theη′phase during aging has little effect on the size and distribution of intragranular precipitates.The size of the width direction of the grain boundary precipitates increases,thereby increasing the width of the grain boundary non-precipitation zone and reducing the axial ratio of the precipitates,which has little effect on the grain boundary composition.After adding the Cu element,the degree of recrystallization is reduced,and the grain morphology is still refined,and the full recovery occurs.It promotes the aging process and the growth of theη′phase.The width of grain boundary precipitates is reduced and the axial ratio of the phase is increased.In addition,the addition of Cu affects the composition distribution of grain boundary precipitates and increases the content of Mg.Through Ti and Cu microalloying,the strength of the alloy is greatly improved compared with that of the 7050 alloys.The enhancement of strength by adding Ti and Cu mainly comes from fine grain strengthening,the second phase strengthening,and precipitation strengthening.The contribution value of the Ti element to the precipitation strengthening at the early aging stage is the largest.With the aging process,the contribution value decreases gradually,because the Ti element can promote the transformation of theη′phase at the early aging stage.Cu element has a certain contribution value to the precipitation strengthening of the alloy at various stages because Cu element has a promoting effect on the transformation of the precipitated phase at various aging stages.The contribution of the Cu element to the mechanical properties is mostly due to the increase of the volume fraction of the precipitated phase,rather than the change of the size.The addition of the Ti element increases the intergranular corrosion sensitivity of the alloy,but its exfoliation corrosion resistance is better.The addition of the Cu element decreased the intergranular corrosion resistance and exfoliation corrosion resistance of the alloy,mainly because after the addition of Ti,the recrystallization degree of the alloy was promoted,and the width-thickness ratio of the grain was reduced.Even if the corrosion occurred in the early stage,it was only the drop of a certain grain,rather than the development of exfoliation corrosion.After the addition of the Cu element,the recrystallization degree of the alloy was reduced,and the fine and long grain were obtained.The grain boundary was pinned and cannot release the strain energy After the corrosion occurred,the product accumulation caused exfoliation corrosion.In addition,after adding the Ti element,the axial ratio of the precipitated phase is about 1.8,and the spacing between the precipitated phases is greater than the other two.After adding the Cu element,the axial ratio of the precipitated phase of ASTC alloy is further increased.The axial ratio of the precipitated phase is particularly critical for the intergranular corrosion resistance and exfoliation corrosion resistance of the alloy.When the axial ratio is about 1.8,or the smaller the axial ratio is,the better the corrosion resistance of the alloy is.With the increase of the axial ratio,the corrosion resistance gradually deteriorates.In this paper,the AST alloy treated by Ti microalloying at 140°C T6 can have excellent exfoliation corrosion resistance and high yield strength（564 MPa）,which is a high-strength and corrosion-resistant aluminum alloy with great potential. |
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