Effect Of Extrusion And Heat Treatment On Microstructure And Properties Of Spray Deposited 2195 Al-Li Alloy

Al-Li alloy is a new structural material with low density,high elastic modulus and high specific stiffness and strength,which is widely used in the aerospace industry.The Al-Li alloy extrusion profiles are used as structural parts such as beams and stringers in aerospace equipment.By replacing the conventional aluminum alloy profiles,the equipment weight can be decreased,which can reduce fuel consumption.Although we have been able to independently prepare advanced Al-Li alloys,there is still a big gap with the high level in the production of profile components.We have been facing great challenges in the extrusion technology of high-performance Al-Li alloy profiles.The manufacturing processes of the Al-Li alloy profile are long and complex.There are many scientific problems in thermos-mechanical coupling deformation behavior,microstructure evolution,strengthening and toughening mechanisms,etc.At present,many researches have been carried out on the composition design and preparation of Al-Li alloys,hot plastic forming processes,and microstructural evolution during heat treatment.However,there are still some key issues to be studied and solved in the extrusion process of the spray deposited Al-Li alloy profile.The microstructure and property of the spray deposited Al-Li alloy and microstructural evolution in homogenization are still unclear.The precise constitutive model of the spray deposited alloy needs to be established.The evolutions of the microstructure and properties of the spray deposited alloy in the extrusion process are still unclear,and the effective control of profile forming quality lacks theoretical guidance.The strengthening and toughening mechanisms in the heat treatment processes of the spray deposited alloy after extrusion need to be studied and revealed.Focusing on the above issues,the spray deposited 2195 Al-Li alloy was taken as the research object in this paper.The microstructure evolution of the alloy during homogenization was studied.The evolutions of the microstructure and properties of the alloy during the extrusion process were revealed,and the prediction and control methods of the profile forming quality were proposed.The strengthening and corrosion mechanisms of the alloy after extrusion and heat treatment were clarified.The results provide theoretical guidance and data reference for the formulation of the extrusion and heat treatment process parameters of the spray deposited 2195 alloy.The main research contents are as follows:(1)To reveal the microstructure and performance characteristics of the spray deposited 2195 Al-Li alloy and the differences from the as-cast 2195 alloy.The microstructure and mechanical properties of the two alloys were characterized and tested.The differences in grain structure,precipitates,mechanical properties,and fracture mechanism between the two alloys were compared.It was found that the spray alloy has equiaxed grain structure,and the grains are more uniform and fine than that of the as-cast alloy.However,there are many micro-pores in the spray deposited alloy,which reduces the densification degree of the billet.Coarse plate-like T1 phase exists in both spray deposited and as-cast alloys,which is easy to cause brittle fracture and result in the low mechanical properties of the alloys.The mechanical property of the as-cast alloy is lower than that of the spray deposited alloy.(2)The microstructural evolution of the spray deposited 2195 alloy during homogenization was studied.The transient and long-term changes of the secondary particles in the homogenization process were revealed.The dissolution kinetics equations of secondary phases as well as the diffusion kinetics equations of the solute elements were established.The dissolution time of secondary particles and diffusion time of element were predicted.The influences of the homogenization on the precipitation of A13Zr dispersoid and recrystallization behavior of alloy were revealed and the influential mechanisms were also clarified.The homogenization with ramp heating can dissolve the particles with different melting points in the alloy uniformly and also increase the number of dispersoids,which more effectively inhibits the recrystallization and grain growth and thus improves the uniformity of the alloy.(3)The hot deformation behavior of the spray deposited 2195 alloy under different temperatures and strain rates were studied and the constitutive model was established based on the true stress-strain data obtained from the hot compression tests.The processing map was established based on the dynamic material model and the stable and unstable deformation areas were determined.The microstructure evolution mechanism under different deformation conditions was revealed.It was found that the softening mechanism of the alloy at the low deformation temperature is dy'namic precipitation and discontinuous dynamic recrystallization.At high deformation temperatures,the softening mechanism of the alloy changes into dynamic recovery and continuous dynamic recrystallization.According to the hot processing map and micro structure observation,the stable deformation range of the spray deposited 2195 alloy was determined to be 475?525? and 0.1?1 s-1.(4)The spray deposited 2195 Al-Li alloy was extruded,solution quenched,and aged with different parameters.The evolutions of microstructure and mechanical property during the hot extrusion and heat treatments of the alloy were studied systematically.The effects of grain and precipitation characteristics on the mechanical property and fracture behavior of the alloy were revealed.It was found that fiber grains and fiber textures of<111>and<100>along extrusion direction can be formed after the extrusion.A lot of T1 precipitates in the matrix after solution and aging treatment,which significantly improves the strength of the alloy.The precipitation density of the T1 phase can be effectively increased by pre-stretching and aging treatment,which brings a higher tensile strength than conventional aging treatment without pre-stretching.The pre-stretching before aging can effectively inhibit the width of PFZ and the size of GBPs,which improves the elongation of the alloy.(5)The extruded 2195 Al-Li alloy was treated with different pre-stretching and aging processes.The effects of pre-stretching degree and aging process on precipitates,boundary microstructure,mechanical property,and intergranular corrosion of the alloy were studied,and the strengthening and intergranular corrosion mechanisms were revealed.Increasing the amount of pre-stretching can increase the strain hardening effect,but reduce strengthening effect of the T1 phase,so the mechanical property of the alloy cannot be significantly improved by the increased pre-stretching degree.Pre-stretching can reduce the size of the PFZ and GBPs and thus effectively inhibit the intergranular corrosion behavior of the alloy.The mechanical properties of the alloy can be improved to a higher level by two-stage aging treatment after the pre-stretching,and the intergranular corrosion resistance can also be improved.(6)The extruded 2195 Al-Li alloy was preheated with different temperatures and holding times,and the preheated billets were subjected to secondary extrusion experiments under the same parameters.The influences of the preheating parameters on the microstructure of the billet,the secondary extrusion load,and the microstructure and properties of the secondary extrusion plates were studied.The results show that the preheating treatment at high temperature for a long time would increase the load during the secondary extrusion.Different preheating parameters have little effect on the microstructure and properties of the secondary extrusion plates.However,the mechanical properties of the extruded plates would decline in the subsequent solution and aging treatment when the billet is preheated with the high temperature and long time.According to the experimental results,the as-extruded billet is appropriate to be preheated under the temperature range of 430?500? for 4-8 h before secondary extrusion,which is not only beneficial to reduce the extrusion load but also can avoid the performance decline in the subsequent heat treatment.(7)The profile extrusion experiments were carried out on the as-extruded 2195 alloy billet under different process parameters.The effects of aspect ratio,extrusion temperature,and speed on the microstructure and mechanical properties were investigated.The evolutions of the grain morphology and orientation with the parameters are revealed,and the relationship between the texture evolution and mechanical anisotropy was revealed.The strong fiber textures of<111>and<100>along extrusion direction can be formed in the area with low aspect ratio,which makes the profile have higher strength along the extrusion direction and cause greater mechanical anisotropy.With the increase of aspect ratio,the fiber textures change to{011}<211>,which reduces the strength along extrusion direction but decreases the anisotropy degree.The increase of extrusion speed can promote the formation of the texture of {011}<311>,which decreases the anisotropy.As the extrusion temperature increases,more recrystallized textures can be formed,which reduces the mechanical anisotropy.However,the excessive temperature can lead to serious recrystallization and grain coarsening,which greatly decreases the plasticity of the profile.(8)The effect of extrusion temperature and speed on the cracking behavior of the 2195 Al-Li alloy profile was studied,and the cracking mechanism under different extrusion parameters was revealed.It is found that the profile temperature at the die bearing and the accumulation of tensile plastic work when the material flowing through the bearing are the main reasons for ductile cracking.When the surface temperature of the profile is too high and the accumulation of plastic work exceeds the critical value,it will cause the profile to crack.A prediction criterion for the extrusion cracking was established by taking into account profile temperature and accumulation of plastic work.The accurate prediction for the cracking behavior of the profile was realized by the criterion combined with numerical simulation.The boundary conditions for safe extrusion of the profile were investigated,and the extrusion limit diagram of the as-extruded 2195 Al-Li alloy billet was constructed.