Studies On Composition-constituent Design And Mechanical Properties Of Novel Heat-resistant Aluminum Alloys

Al-Cu-Mn system alloys have been widely used in structural parts for automobile and aerospace industries that work at higher temperature environment,because of its good heat-resistance and wielding and hot-working properties.But when their working temperature is over 250^oC,the main heat-resistant phase,Cu Al₂ phase in Al-Cu-Mn system alloy,will be coarsened quickly,and thus the high temperature strength is abruptly decreased.With the fast development of economy and science technology,the requirements for high temperature performances of heat-resistant aluminum alloys become increasingly rigorous,and hence there are huge challenges in development of new heat-resistant aluminum alloys.Alloying and micro-alloying combined with heat treatment can improve the high temperature strength of aluminum alloy.In this thesis,first,studies on the influence of Cu content in Al-x Cu-1Mn(x=2,5,7.5)alloy on microstructure and mechanical properties at room and high temperatures and their isothermal hot compression flow behaviors were carried out.Here,a shell-core structure of primary?-Al dendrites in solidified microstructure and uneven precipitation behavior during the solution treatment of Al-Cu-Mn alloys were described in details for the first time.It is found that there is a shell-core structure of primary?-Al dendrites in the solidified microstructure of the studied alloys,where the Cu content in shell is far more than that in core.The shell is Cu-rich and the core is Cu-poor.With the increase of Cu content in the alloy,the area of Cu-rich zone is increased.After two-stage solution treatment,large amounts of rod-like T_Mn(Al₂₀Cu₂Mn₃)and Al Cu₃Mn₂dispersoid particles are precipitated in or near the original shell(Cu-rich zone),and there are no precipitates formed in the central zone of the core.Moreover,at the same time,the combination of re-dissolving of most eutectic Cu Al₂ phase into Al-matrix and the fact that Mn is fairly poor in the eutectic boundaries results in the formation of precipitation-free bands.This precipitation behavior during the solution treatment is characterized as uneven precipitation.With the increase of Cu content in the alloy,the amount of precipitation particles is increased as well as the reduction in size,and the area of precipitation-free bands is increased.The precipitation of dispersed T_Mn and Al Cu₃Mn₂particles during the solution treatment make yield strength at 300^oC improve considerably.After ageing treatment,some metastable Cu Al₂ spherical particles are observed on the rods of T_Mn and Al Cu₃Mn₂ phases,however,they are re-dissolved or significantly coarsened during tensile test at 300^oC and thus has almost no contribution to high temperature strength.The simulation investigations of isothermal hot compression of the studied alloys indicate that the deformation flow stress of them at high temperature is small and they are suitable for hot deformation,and there is no obvious difference in deformation activation energy among the studied alloys.The characteristics of hot compression flow curves of the studied alloys during isothermal compression are classified into four types and the physic relationships of these flow characteristics with the dynamic recovery and dynamic recrystallization occurred during hot compression are established.Considering the alloy cost,flow stress of hot deformation and high temperature strength,Al-5Cu-1Mn alloy is selected as the base alloy for further study.Effect of Ni-alloying on the microstructure and mechanical properties of Al-5Cu-1Mn alloy was systemically investigated.Here,the phases that play a considerable role in high temperature strengthening of heat-resistant aluminum alloys,for the first time,are categorized into three types of heat-resistant phases formed during solidification,solution treatment and ageing treatment,respectively.And the corresponding contribution degrees of these heat-resistant phases in high temperature strengthening is clarified and discussed by comparing the change in yield strength at 300^oC of the studied alloys at different states of as-cast,T4(only after solution treatment)and T6(after solution and ageing treatments).For the Ni-alloyed Al-5Cu-1Mn alloy,it is found that the contribution to strength at high temperature from heat-resistant phases formed during the solution and ageing treatments is far more than that from solidification.Thus,a new point of view is pointed out that the role of heat-resistant phases formed during the solution and ageing treatments to high temperature strengthening should be paid much more attention in composition-constituent design of new heat-resistant aluminum alloys.TEM observation on samples after tensile test at 300^oC or exposed at 300^oC indicates that the fine???phase precipitated during the ageing treatment will transform into relatively coarse??phase.However,??phase is seen to strongly hinder the motion of dislocations during tension deformation at high temperature.This is the physic reason for that ageing treatment can significantly improve the high temperature strength.However,it is not consistent with people's previous realization on ageing precipitates.Effects of micro-alloying with Mg,Ni and Ni+Zr on the microstructure and high temperature strength were also studied deeply.Micro-alloying with 0.5%Mg in Al-5Cu-1Mn-0.5Ni alloy leads to the formation of large amounts of ridge-shaped Al Cu Mg phase slices(hexagonal structure with a=0.6,b=0.6,c=0.71nm)at the eutectic boundaries during solidification as well as the compacting of eutectic Cu Al₂ phase.This make the yield strength at 300^oC of the as-cast alloy improved by 18MPa.Micro-alloying with Mg has little influence on the dispersoid precipitation during the solution treatment,however,the re-dissolving of Mg-rich phase in this course results in 20MPa increment in yield strength at 300^oC of the alloy in T4 temper.During the ageing treatment of the alloy with Mg-micro-alloying,there are no definite Mg-rich precipitates observed,however a great amount of Cu-Mg clusters is observed that will be transformed into fine particulate Al₅Cu₂Mg phase(orthorhombic structure:a=0.61nm,b=0.79nm,c=1.84nm)during the subsequent tensile test at 300^oC.Just this microstructure evolution during the ageing treatment and subsequent tensile test at300^oC leads to a surprising rise in yield strength at 300^oC of this alloy in T6 temper.Micro-alloying with Mg makes the increments of the yield strength at 300^oC by the solution treatment and ageing treatment respectively raised significantly,exhibiting a very strong strengthening effect at high temperature,but it remarkably deteriorates the fracture elongation too.Compared with micro-alloying with 0.5%Ni,yield strength at300^oC of the alloy micro-alloyed with 0.3%Ni micro-alloying at the states of as-cast,T4 and T6,even the Ni addition is reduced by 40%,is almost little decreased,illustrating almost the same micro-alloying effect on high temperature strengthening.On the basis of Al-5.0Cu-1Mn-0.3Ni alloy,further micro-alloying with 0.15%Zr has no considerable influence on the aforementioned microstructure evolutions during solidification,solution treatment and ageing treatment,but,it is found that there are a large number of blocky LPSO(long-period stacking/order)phase particles formed during the solution treatment,which are embedded in the rods of T_Mn and Al Cu₃Mn₂phases.It is the first time to report the formation of LPSO phase in aluminum alloy.At the states of whether as-cast,T4 or T6,yield strength at both 300^oC and 350^oC of the alloy with Zr-micro-alloying is much higher than that without micro-alloying,exhibiting significant strengthening effect.Especially,at 350^oC,Zr-micro-alloying can considerably raise the increments in yield strength by the solution treatment and ageing treatment respectively,indicating its strong strengthening effect at higher temperature.This is thought to be due to the rise of thermal stability of??phase by Zr-micro-alloying.The above studies on micro-alloying effect further demonstrate the aforementioned important roles of the heat-resistant phases formed during solution and ageing treatment respectively in high temperature strengtheningOn the basis of above studies,the base alloy is adjusted into Al-6Cu-0.2Mn and micro-alloyed with 0.3%Ni+0.15%Zr.The microstructure of new alloy in T4 temper changes surprisingly and interestingly,where,the original rod-like T_Mn and Al Cu₃Mn₂phases are completely disappeared and a great number of new spherical Al₃Cu Zr particles(tetragonal structure with a=0.82nm,b=0.82 nm,c=0.26nm)are formed.Compared with Al-5Cu-1Mn-0.3Ni-0.15Zr alloy,yield strength at 350^oC of Al-6Cu-0.2Mn-0.3Ni-0.15Zr alloy at the states of as-cast,T4 and T6 are increased by 24MPa,51MPa and 45MPa,respectively.Ultimate tensile strength and yield strength at 350^oC of Al-6Cu-0.2Mn-0.3Ni-0.15Zr alloy in T6 temper reach up to 145MPa and 133MPa respectively as well as 6.5%of fracture elongation.This yield strength value is the highest one reported in the literatures up to now.