| Lightweight structure design is an urgent demand to improve the flight and transportation capacity of aircraft and reduce the energy consumption,and it is also an important measure for social sustainable development.Al-Li alloys have been widely used in aerospace and other fields due to their excellent properties such as low density,high specific strength and stiffness,high elastic modulus and high damage tolerance,etc.,and have gradually become one of the main alternative materials for 2xxx and 7xxx series aluminum alloys.However,the strict service conditions in the aerospace field and the rapid development of composite materials have put forward higher requirements and challenges for the accuracy and performance of Al-Li alloy components.Although we have the ability to manufacture advanced Al-Li alloy materials independently,there is still a big gap between us and developed countries in the manufacturing technology and application of high-performance Al-Li alloy profiles.At present,many researches have been carried out on composition regulation,thermal simulation test,constitutive model construction and precipitation behavior during heat treatment of Al-Li alloys.However,there are still some key issues to be solved in the extrusion manufacturing and heat treatment strengthening of the new third-generation Al-Li alloys.For example,the effect of homogenization process parameters on elemental segregation and dendrite structure in the as-cast Al-Li alloy has not been investigated.The macroscopic deformation behavior and microstructure evolution of the new Al-Li alloy during hot compression and hot extrusion have not been revealed,which leads to the lack of theoretical guidance for the determination and optimization of the process parameters.The precipitation behavior and strengthening mechanism of Al-Li alloy profiles during pre-deformation heat treatment lack of theoretical explanation.The formation mechanism and dominant factors of mechanical properties anisotropy of profiles are still unclear.How to effectively control the anisotropy and corrosion resistance of Al-Li alloy profiles through heat treatment process is lacking of theoretical guidance.In view of the key issues mentioned above,the third-generation 2196 as-cast Al-Li alloy was taken as the research object in this paper.The deformation behavior and microstructure evolution of the alloy in the thermal compression and extrusion process were systematically studied.The effects of heat treatment processes on the microstructure,mechanical properties,anisotropy and corrosion behavior of the alloy were revealed,which provided data reference and theoretical guidance for the formulation of extrusion and heat treatment process parameters,as well as the coordinated control of profile shape and properties of 2196 Al-Li alloy.The main research contents are shown as follows:(1)In view of the dendritic segregation phenomenon in the as-cast 2196 Al-Li alloy,the single-stage and double-stage homogenization treatment with different process parameters were carried out.The effects of homogenization temperature and time on the dendrite structure and secondary phases were revealed,and the homogenization process system suitable for the alloy was formulated.It is found that low temperature and short time lead to insufficient diffusion of elements.Increasing the homogenization temperature could accelerate the diffusion of segregated elements and save the homogenization time,but too high temperature might lead to over-heating or over-burning phenomenon.Combined with homogenization kinetics analysis,the optimal single-stage homogenization treatment parameter of 2196 Al-Li alloy was determined to be 500?/24 h,which effectively eliminated the dendritic structure formed during the casting process and provided billets with uniform element distribution for the subsequent extrusion tests.(2)The Arrhenius constitutive model for predicting the heat flow behavior of 2196 Al-Li alloy was established based on the true stress-strain data obtained from the hot compression tests.The constitutive model was modified by comprehensive considering the compensation of deformation temperature,strain rate and strain,which further improved the prediction accuracy of the model.The accuracy of constitutive model in predicting metal flow behavior was verified by compared the shape and sectional deformation of the profiles obtained by numerical simulation and extrusion experiment.The evolution of microstructure during hot compression were systematically studied.It is found that the softening mechanism of 2196 Al-Li alloy during crompression was mainly DRV accompanied by a small amount of continuous DRX.The processing map of 2196 Al-Li alloy was established and the optimal deformation parameters were determined as 460?520 ? and 0.01?1 s-1.(3)The effects of extrusion temperature,extrusion speed and extrusion ratio on the deformation behavior of alloy and the microstructure and properties of extruded profiles were investigated.The evolution of microtexture,grain orientation and DRX of the alloy under different extrusion parameters were clarified,and the relationship between microstructure and mechanical properties of the profiles was revealed.The results indicated that with the increase of extrusion temperature and extrusion speed,the average misorientation as well as the degree of DRV and DRX were increased,and the average grain size was decreased,and the hardness and strength of the profiles were first increased and then decreased.Appropriate increase of extrusion ratio was beneficial to grain refinement and texture strengthening,which improved the mechanical properties of the alloy.The main textures in the extruded profiles were Y,Cube and some other {111} fibers.The tensile fracture mode of the profiles is mainly ductile fracture,and the fracture morphology exhibited a large number of dimples with uneven size and different depth.(4)The evolution of precipitate characteristics and the strengthening mechanism of mechanical properties of 2196 Al-Li alloy profiles during heat treatment were systematically studied by means of solution quenching,pre-stretching deformation and artificial aging with different process parameters.The mechanism of dislocation propagation induced by pre-stretching and its promoting effect on aging strengthening of the alloy were revealed.The results showed that the evolution of precipitates in 2196 Al-Li alloy during heat treatment was as follows:supersaturated solid solution?GP zone+?'/?'(Al3(Li,Zr))?+?' +?'(Al2Cu)+T1(Al2CuLi)??'+T1.The pre-stretching introduced a large number of dislocations,which provided conditions for the precipitation of phases in the aging process,and further promoted the uniform precipitation of T1 strengthening phases.It significantly improved the effect of aging strengthening,and effectively shortened the aging time.The optimal heat treatment process parameters of 2196 Al-Li alloy were determined as:515 ?/90 min solution treatment+3%pre-stretching+170?/16 h artificial aging.(5)In view of the mechanical property anisotropy of 2196 Al-Li alloy profiles,the microstructure of extruded profiles in different directions were observed and analyzed,and the causes of mechanical property anisotropy was revealed.The effects of solution treatment,artificial aging and pre-stretching on the mechanical property anisotropy of the alloy were clarified.It is found that the anisotropy was mainly caused by the differences of grain morphology,microtexture,recrystallization degree and secondary particles characteristics in different directions.The strengthes of the specimens in 0° and 90° directions were high,but the elongations were poor,while the specimen in 45° direction had the best ductility.The anisotropy of elongation was greater than that of the strength.The number density and distribution uniformity of the main strengthening phase T1 increased after aging treatment,and the anisotropy of the mechanical properties of the profiles decreased.Besides,the elimination of precipitate free zone(PFZ),the increase of grain boundary strength and DRX as well as the decrease of grain size were conducive to the decrease of anisotropy.The dislocations introduced by pre-stretching was beneficial to the uniform precipitation of fine phases,resulting in the optimal mechanical property homogeneity of the alloy.(6)Aiming at the difficulty in simultaneously considering the strength and corrosion resistance of Al-Li alloy profiles,a re-solution and re-aging treatment which can effectively improve the corrosion resistance of single-stage peak-aged alloy was proposed.The re-dissolution and re-precipitation behaviors of precipitates during the re-solution and re-aging processes were studied,and the corrosion mechanism of 2196 Al-Li alloy was revealed.The results demonstrated that the corrosion resistance of the initial T6 alloy was poor,and the corrosion mode changed from pitting to intergranular corrosion with the increase of corrosion degree.The coarsening and discontinuous distribution of grain boundary precipitates as well as the increase of PFZ width could inhibit the generation and propagation of stress corrosion cracking and thereby improved the corrosion resistance of alloy.After 515?/90 min re-solution and 170?/24 h re-aging treatment,the conductivity and the resistance to intergranular corrosion and electrochemical corrosion of the alloy were improved without sacrificing the strength of the alloy. |
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