Study On Microstructure And Mechanical Behavior Of Cast Al-Li-Cu-Mn Alloys

Al-Li alloys are regarded as ideal structural materials in aerospace field since they offer an impressive combination of specific mechanical properties,such as low density,high strength/stiffness-to-weight ratio,good anti-fatigue performance and corrosion resistance.By replacing conventional Al alloys with Al-Li alloys,the components weight could be decreased by 10%²0%,and the stiffness improved by 15%²0%.However,at present,most researches on Al-Li alloys have been confined to wrought alloys.Casting is not only a technology to manufacture products with complex shapes,but also an effective way to mitigate the anisotropy of mechanical property because of their isotropic texture.Moreover,the weight reduction could be much more effective in cast Al-Li alloys,since the limitation of Li addition could be much higher than that in wrought alloys.Combining the unique advantages of casting technology with the good performance of Al-Li alloys,the cast Al-Li alloys will exhibit a broad development prospects.Therefore,study on cast Al-Li alloys is of great importance,which might hopefully promote the development and applications of cast Al-Li alloys in the aerospace field.Recently,the Al-3Li-0.2Zr base alloy has received much attention due to its extremely low ensity?about 2.26g/cm³?and high stiffness?about 85GPa?.But the strength and toughness of Al-3Li-0.2Zr alloy is poor since it was almost strengthened by Al₃Li particles,which could not satisify the requirements in the aerospace field.Alloying is an effective way to improve the mechanical properties of cast Al-Li alloy,especially,Cu addition could introduce the??-Al₂Cu and T₁-Al₂CuLi precipitation,resulting in improved strength and toughness.However,at present,the mechanical properties of the studied cast Al-Li-Cu alloy were still low.The advantages of cast Al-Li alloy over other conventional cast Al alloys were not obvious,leading to some difficult in large-scal applications.Therefore,a kind of cast Al-Li alloy with good mechanical properties is waiting to be developed to satisify the increasing requirements for the usage of low density,high strength and high stiffness cast components in the aerospace field.The present work systematically investigated the effect of Cu,Mn and Sc addition on the microstructure and mechanical properties of cast Al-3Li-0.2Zr alloys.A multi-stage solution treatment was put forward to increase the alloy supersaturation degree.The precipitation behavior and microstructures evolution of Al-3Li-xCu-yMn-0.2Zr-?0.2Sc?alloy was revealed.The mechanism of Cu and Mn element on the alloy strength and toughness was verified.The addition values of Cu and Mn element was optimized,and a cast Al-3Li-2Cu-0.3Mn-0.2Zr-0.2Sc alloy with excellent mechanical properties was developed.Moreover,the effect of hot-expossure on cast Al-3Li-2Cu-0.2Zr alloy was clarified.In the case of Sc-containing alloy,a“pre-ageing”treatment was put forward to enhance the number density of Al₃Sc particles.The strengthening and toughening mechanisms of cast Al-3Li-xCu-0.2Zr alloys was clarified via studying their as-cast microstructure,microstructural evolution and mechanical properties.The Cu element is significantly enriched in grain boundaries to form Al₂Cu and Al₂CuLi,etc.non-equilibrium phase.The as-cast grain size of Al-3Li-xCu-0.2Zr alloy was reduced by Cu addition due to the constitutional undercooling contribution caused by Cu segregation during solidification.When the Cu content beyond2wt%,a certain amount of residual phases was remained after solution and decreased the alloy mechanical property.Cu addition could promote the precipitation of??-Al₂Cu and T₁-Al₂CuLi phase,resulting in a significant improvement of strength.Meanwhile,the precipitation of??and T₁ phase is also beneficial to ductility because they act as unshearable obstacles which could disperse the planar slip.The Al-3Li-2Cu-0.2Zr alloy possesses the best combination between strength and ductility.The microstructural evolution and mechanical properties of Al-3Li-2Cu-0.2Zr alloy subjected to various ageing treatments was studied comparatively.With the increase of single ageing temperature,the number density and size of??,??and T₁ precipitate was increased due to the improved diffusion rate of Cu and Li solutes,which increase the strength but decrease the ductility.Comparing to the single ageing treatment,low-to-high duplex ageing treatment could improve the alloy ductility without sacrificing strength.This is because that the first low temperature ageing stage increase the driving force for precipitation and hence,promote the uniform precipitation of??and T₁ phase,giving rise to an increased number density of??and T₁ precipitate within the alloy matrix after duplex ageing treatment.In addition,the mechanism in microstructure and mechanical properties evolution during low temperature exposure was revealed.TEM results shows that the improvement in strength and decrease in ductility of Al-3Li-2Cu-0.2Zr alloy during exposure at 70^oC was the result of continued precipitation of fine??particles;while the change in property of Al-3Li-2Cu-0.2Zr alloy during exposure at 100^oC was owing to the precipitation of much fine??phase.The 0.3wt%Mn content was optimized through studying the microstructure and mechanical properties evolution of Al-3Li-2Cu-0.2Zr alloy with different Mn content.The role of Mn in Al-Li-Cu alloy was clarified.The addition of Mn could react with Al to form primary?-Al particles in the high temperature,resulting in grain refinement.Moreover,Mn addition could also neutralize the detrimental effect of Fe element.A large amount of Al₂₀Cu₂Mn₃ dispersiods were formed in alloys containing more than 0.5wt%Mn content.The number density of Al₂₀Cu₂Mn₃ dispersiods increased with further increasing the Mn contents,and primary Al₂₀Cu₂Mn₃ phase might form at grain boundaries when the Mn content beyond a certain level.The formation of primary Al₂₀Cu₂Mn₃ phase and Al₂₀Cu₂Mn₃ dispersiods will inevitably consume the Cu solute in the?-Al supersaturated solid solution,decreasing the number density of??and T₁ phase that precipitated from the matrix.Therefore,the strength of Al-3Li-2Cu-0.2Zr alloy decreased with the increase of Mn addition.The Al₂₀Cu₂Mn₃ dispersiods formed during solution could disperse planar slip caused by??phase,which is beneficial to alloy ductility.But on the other hand,the Al₂₀Cu₂Mn₃ dispersiods might cause stress concentration itself,which has harmful effect on alloy ductility.The influence of Mn content on the ductility of Al-3Li-2Cu-0.2Zr alloy was determined by the above two factors.Therefore,the ductility evolution of Al-3Li-2Cu-yMn-0.2Zr alloy varies under different ageing condition.By adding trace amount of Sc into Al-3Li-2Cu-0.3Mn-0.2Zr alloy and applying the“pre-ageing”treatment,the alloy ductility was significantly improved.The results showed that the as-cast grain size was reduced from 130?m in the Sc-free alloy to 50?m after Sc addition.Sc addition decreased the slip-length of dislocation,resulting in improved ductility.Moreover,Sc addition will not only form coherent Al₃Sc particles during solution,but also promote the precipitation of Zr solute,therefore enhancing the number density of Al₃Sc?Al₃Zr and Al₃?Zr,Sc?particles.During ageing,the??phase will nucleate and grow on these Al₃M?M=Zr,Sc,Zr/Sc?particles to form Al₃?M,Li?composite particles.The Al₃?M,Li?composite particles are non-shearable,and hence could disperse the planar slip,resulting in increased ductility.The“pre-ageing”treatment before solution could promote the precipitation of Al₃Sc particles,which will further improve the alloy ductility.A kind of enviormentally friendly cast Al-Li alloy with low density and good mechanical property was developed.By optimizing the Cu and Mn content in combine with trace addition of Sc element in the Al-3Li-0.2Zr base alloy,a cast Al-Li alloy with composition of Al-3Li-2Cu-0.3Mn-0.2Zr-0.2Sc was successfully developed.After solution treating at 500^oC-32h+560^oC-24h and ageing at 175^oC for 32h,the yield strength?YS?,ultimate tensile strength?UTS?and elongation?EL?of Al-3Li-2Cu-0.3Mn-0.2Zr-0.2Sc alloy was 326MPa,438MPa and 5.5%,respectively.As a result,the developed cast Al-Li alloy will show a broad application prospect in the aerospace field.