Effect Of Sm Alloying On Microstructure And Properties Of Al-Zn-Mg-Cu-Zr Alloy

With the rapid development of aerospace industry,the performance requirements of Al-Zn-Mg-Cu-Zr series ultra-high strength aluminum alloy is increasing,and the desire to develop new alloys is becoming stronger and stronger.Although Al-Zn-Mg-Cu-Zr series alloy has high comprehensive mechanical properties,its exfoliation corrosion and stress corrosion cracking are more serious when serving in corrosive medium and stress environment.In order to solve this problem,a new type of Al-Zn-Mg-Cu-Zr-Sm alloy is designed and studied in this paper,which aims to improve the strength as well as corrosion resistance of the alloy.In this paper,the effect of Sm content on the microstructure of as-cast Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr alloy was studied systematically,the homogenization process and deformation process of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy were optimized,and the effects of Sm content on recrystallization behavior,precipitation behavior,room temperature and high temperature mechanical properties and corrosion properties of extruded alloy were studied.The strengthening mechanism and corrosion resistance mechanism of the alloy with Sm addition were revealed.Finally,the solid solution process of the alloy with optimal composition was optimized,and the effect of aging process on the microstructure,mechanical properties and corrosion resistance of the alloy with optimal composition was studied.The results are as follows:The addition of Sm can significantly refine the microstructure of as-cast Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr alloy,and the minimum grain size was 108μm when the content of Sm was 0.3%.Furthermore,the addition of Sm could decrease the content of second phase at the grain boundary and form the Al₁₀Cu₇Sm₂ phase at the same time.The grain refinement is attributed to the composition undercooling caused by Sm element and the inhibition of grain growth by the formation of Al₁₀Cu₇Sm₂ at the grain boundary.After pretreatment,a large amount of Al₃Zr phase was precipitated in the alloy.After homogenization,the low melting pointηphase in the alloy dissolved into the matrix to form a small amount of S（Al₂Cu Mg）phase,while the high melting point Al₂₃Cu Fe₄ and Al₁₀Cu₇Sm₂ phase can not be eliminated in the homogenization process.The optimum homogenization process of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy is 400°C×10 h+470°C×24 h.By using the linear regression method,the deformation activation energy of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy was calculated to be 182.762 k J/mol,and the constitutive equation of hot deformation was determined.The hot working diagram of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy was constructed,and the optimum hot working parameters were determined as follows:deformation temperature 360°C～440°C,strain rate 0.001s^-1～0.01s^-1and deformation temperature 430°C～450°C,strain rate 0.13s^-1～1s^-1.The microstructure analysis of hot compression shows that only dynamic recovery occurs when the deformation temperature is lower than 420°C When the deformation temperature is 450°C,dynamic recrystallization has occurred.Sm alloying can improve the as-extruded microstructure of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr alloy,and the sub-micron Al₁₀Cu₇Sm₂ phase formed during extrusion can inhibit recrystallization.Sm can promote the nucleation ofη’phase in the alloy,increase the dispersion ofη’phase,and restrain the transition fromη’phase toηphase.At the same time,Sm can coarsen grain boundary precipitates and reduce the width of grain boundary non-precipitation zone.Sm alloying can improve the mechanical properties and corrosion properties of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr alloy.The optimal addition of Sm is 0.3%.The addition of Sm can improve the strength of the alloy mainly by refining grains,strengthening substructure and increasing the dispersion ofη’phase.After Sm alloying,the microstructure characteristics such as fining of fiber structure,coarsening of precipitates at grain boundaries and narrowing of non-precipitation bands at grain boundaries would reduce the corrosion sensitivity of Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr alloy.Solution treatment can eliminate the residualηand S phases after Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm extrusion,but Al₂₃Cu Fe₄ and Al₁₀Cu₇Sm₂ phases are not soluble.The T6 state Al-6.7Zn-2.6Mg-2.0Cu-0.1Zr-0.3Sm alloy has higher strength,but the corrosion resistance is relatively poor,while the strength of the alloy treated with T74 process is sacrificed greatly,but it has strong intergranular corrosion,exfoliation corrosion and stress corrosion resistance.In addition,RRA aging process can obtain better resistance under the condition of small strength loss.Among them,the comprehensive properties of RRA190°C×30 min aging process is the best,the tensile strength is 723 MPa,the yield strength is 674 MPa,the elongation is 9.8%,the intergranular corrosion depth is 68μm,the exfoliation corrosion grade PB,and the maximum fracture strength and fracture time of slow strain rate tension are 658 MPa and 24700 s,respectively.