Research On Cryogenic Deformation Behaviors And Deep Drawing Technique Of Al-Li Alloy Sheet

Thin-walled components account for more than 50%of the weight of aircraft structure,which is the main goal to achieve aircraft weight reduction.Al-Li alloys with high specific strength/specific stiffness are the preferred material for thin-walled components to achieve weight reduction.However,the plasticity of Al-Li alloys is poor,and cracks usually occurred during the forming process when using traditional forming techniques to fabricate complex-shaped components.Meanwhile,there are various types and multi-scale second phases in Al-Li alloys.The microstructure evolution mechanism is complex during the deformation process,and the material properties significantly decreased after forming.To solve the above problems,cryogenic forming technique was studied in this work.The plasticity of Al-Li alloys was significantly improved at cryogenic temperatures because of the excellent work hardening ability and deformation uniformity,which led to the improvement of component accuracy.Meanwhile,the multi-scale microstructures of Al-Li alloys were regulated by controlling the cryogenic deformation parameters and heat treatment process,which improved the fatigue performance and strength-plasticity of components.Finally,a complex-shaped component with high precision and high performance was successfully manufactured.The main work is as follows:（1）The low-temperature flow characteristics and the plastic enhancement conditions of Al-Li alloys were obtained.A physical constitutive model based on dislocation density and dynamic strain aging（DSA）was established.It is found that the ductility of annealed 2060 Al-Li alloy gradually increased with the decrease of temperature from 25℃to-196℃.As for the water-quenched alloy,DSA occurred in the range of-60-25℃,which led to the occurrence of serrated flow and a decrease of plasticity.When the temperature was below-120℃,the DSA was inhibited and the plasticity was improved.The uniform elongation of annealed and water-quenched 2060 Al-Li alloy reached 24.1%and 32.6%at-196℃,respectively.The plasticity of T4 and T8 aged alloys was slightly improved at cryogenic temperature.The strain rate has little effect on the flow curves and plasticity of Al-Li alloys at cryogenic temperatures.The traditional K-M dislocation constitutive model was modified,and a dislocation constitutive model considering DSA was established,which accurately described the negative strain rate sensitivity and increased flow stress caused by DSA.（2）The microstructure evolution of Al-Li alloys during the cryogenic deformation process was studied,and the mechanism of plasticity enhancement was revealed.The annealed and water-quenched alloys were dominated by intragranular deformation.With the decrease of the deformation temperature,the stacking fault energy of the Al-Li alloy decreased,which inhibited the cross-slip of dislocations.The dislocation density quickly increased at cryogenic temperatures,leading to the formation of refined substructures in the interior grains and an increase in work hardening ability.Meanwhile,since dislocations were hard to slide through cross-slip after encountering resistance,the formation of coarse slip bands was suppressed,thereby reducing the formation and propagation of micro-cracks in the slip bands.On the other hand,the strain was more uniformly distributed in the grains with different orientations at low temperatures,which was attributed to the increased resistance of dislocation movement at low temperatures.The uniform deformation in different grains suppressed the formation of high lattice distortion regions inside the grains.The above factors led to a substantial improvement in the macro/micro deformation uniformity at cryogenic temperatures,resulting in excellent plasticity.Many precipitation phases existed in the aged Al-Li alloy crystal,which improved the strength of interior grains.Consequently,dislocations were easy to accumulate around the grain boundaries and the strengthening phases,resulting in stress concentration and an insignificant increase in plasticity.（3）The forming limit curves and microstructure evolution of Al-Li alloys under complex thermal-force loading conditions were obtained.Based on the Nakajima experiments,it is found that as the deformation temperature decreased from 25°C to-160°C,the formability of 2060 alloy was significantly improved under different stress loading conditions,and the FLC₀ point of the forming limit curve increased by 40%.The microstructure analysis revealed that strain concentration and dynamic recovery occurred in the local area of the sample deformed at room temperature,which was reflected by the existence of many small-angle grain boundaries and Cube recovery texture.When deformed at cryogenic temperature,more deformation textures such as Goss and Brass were retained in the alloy,leading to the increase of work hardening ability and deformation uniformity.The uniform deformation during cryogenic deformation can inhibit local thinning and increase formability.Compared to the annealed alloy,the second-phase particles were dissolved in the aluminum matrix for the water-quenched alloy,which inhibited dislocation dispersion slip and substructure rotation.The water-quenched alloy has a higher content of deformation texture in the alloy and better resistance to local thinning.（4）The effect of cryogenics deformation and heat treatment on damage tolerance and strength-plasticity of Al-Li alloys was studied.The heat treatment method suitable for cryogenic forming was obtained.The influence of different precipitation phase distributions on the damage tolerance performance of the 2060 alloys was studied.The formation of various types of precipitation phases interior grains by under-aging T6treatment and coarse T₁ phases by T8 over-aging treatment both decreased the crack growth rate.The existence of fine and dense T₁ phases in the T8aged alloy improved the strengths as compared to that of T6 aged alloy.The effects of different forming routes on the microstructure and properties of the 2060 alloy sheet were compared.For the sample deformed at cryogenic temperature,it is found that recrystallized grains with large-angle grain boundaries and refined second phase particles were formed during the subsequent solid solution treatment,which decreased the crack growth rate of 2060 alloys.When the alloy was directly artificially aged after solution treatment and cryogenic forming,dense dislocations and T₁phases formed in the alloys,which improved the strength but increased the crack growth rate.It is concluded that the optimized forming route for thin-walled Al-Li alloys was"cryogenic forming+solution and quenching+T6 under-aging",and excellent comprehensive properties（crack growth rate,strengths,and plasticity）were obtained.（5）Based on the cryogenic deep drawing technique,the aircraft cabin component of 2060 Al-Li alloy was successfully fabricated.The modulus and tooling structure with local liquid nitrogen cooling has been established.First,the second phases in the alloys were dissolved into the aluminum matrix by solution and quenching pretreatment.Then the region of components that easily cracked during the forming process was cooled to cryogenic temperatures.The above two measures significantly improved the deep drawing formability of 2060 alloy sheet.The aircraft cabin component was fabricated without intermediate annealing treatment.The maximum forming depth reached 97.5 mm,and the average thinning rate was 11.7%.The after-formed component was subjected to solution treatment and under-aging T6 treatment,which induced the formation of multiple strengthening phases（T₁,θ’,δ’phases）interior of grains.Meanwhile,the formation of PFZ and coarse phases at grain boundaries was inhibited,and the performance of fatigue propagation was improved,with a value of 1.65×10^-3mm/cycle(ΔK=30 MPa·m^1/2).Meanwhile,the strength value of the components was larger than 85%of the original sheet.