Research On Structures And Properties Control Of Creep-aged 2050 Al-Li Alloy For Aircraft

Creep age forming can realize the manufacturing of large and complex aircraft components achieving both shape and property.However,the overall deformation is small,which greatly limits its further application.In addition,the aircraft components are faced with complex conditions of high and alternate load during service,so it is necessary to solve the collaborative problems of strength,toughness,formability and fatigue resistance of 2050 Al-Li alloy.In this work,the effect of creep aging parameters on the collaborative of shape and property and microstructure of 2050 Al-Li alloy was investigated,and the corresponding prediction constitutive model was established.The effects of grain size and cold-rolling reduction on creep strain were studied by adjusting the initial grain structure.Finally,the 2050 Al-Li alloy components with different grain structures were prepared by creep aging forming.By comparing their formability,mechanical anisotropy,fatigue crack propagation resistance and fracture toughness,the creep age forming technology of the 2050 Al-Li alloy components with high comprehensive properties was explored.The main conclusions are as follows:（1）The law and mechanism of temperature control on the creep behavior and property of creep-aged 2050 Al-Li alloy were investigated.The creep strain of the alloy increases with increasing creep temperature,the peak aging time decreases with increasing creep temperature,and the peak aging strength firstly increases and then decreases with increasing creep temperature,reaching the maximum value at 160℃.The tensile strength and yield strength of the alloy are 523.9 MPa and 497.4 MPa,respectively.The creep temperature will significantly affect the evolution with time of dislocation,precipitates and concentration of solute atoms in the matrix of 2050 Al-Li alloy,resulting in four-stage"double primary feature"at 160℃,three-stage"slow deceleration"at 185℃and classical two-stage characteristics at other creep temperatures.（2）A variable temperature creep aging constitutive model for 2050 Al-Li alloy was established.Based on the mechanism of collaborative of shape and property of 2050 Al-Li alloy,the evolution models of dislocation density,solute atoms and precipitates with time were established.At different temperatures and applied loads for 24 h creep aging,the error between the yield strength calculated by the constitutive model and the actual yield strength is within 5 MPa,and the error between the yield strength calculated by the constitutive model and the actual yield strength is within 0.01%.The predicted values are in good agreement with the experimental results.（3）The law and mechanism of grain size control on the creep behavior and property of creep-aged 2050 Al-Li alloy were investigated.The creep strain of the sample with band-shaped grains of 53.4μm average grain size increases from 0.073%to 0.14%during 24 h creep aging at 160℃under175 MPa compared with the sample with equiaxed grains of 12.2μm average grain size,while their strength and elongation are basically the same.Dislocation motion contributes the largest proportion to creep strain,and the increase of grain size promotes dislocation motion.Although refining grain is beneficial to Harper-Dorn creep and diffusion creep,the contribution of these two mechanisms to creep strain is too small.Therefore,increasing grain size makes the alloy obtain greater strain during 24 h creep aging.（4）A method to simultaneously increase the creep strain and strength of 2050 Al-Li alloy by increasing the cold-rolling reduction without changing the creep aging process parameters was proposed,and its mechanism was expounded.With the cold-rolling reduction increasing from 5%to 75%,the creep strain of the sample creep aged at 160℃/175MPa for 24 h increases from 0.136%to 0.243%,and the yield strength and tensile strength increase to 500.8 MPa and 548.1 MPa respectively,but the elongation decreases from 11.1%to 6.6%.After the large strain cold-rolling reduction,the cell structure composed of dislocation in the matrix evolved into lamellar structure.The lamellar cell structure promotes dislocation motion during creep aging.Moreover,the work hardening degree of the alloy is improved by the large strain cold-rolling reduction,so the creep strain and strength of 2050 Al-Li alloy increases simultaneously.（5）A method for controlling microstructure and properties of creep-aged 2050 Al-Li alloy with high comprehensive properties was proposed,and the correlation between the initial microstructure and the mechanical properties,fatigue crack propagation properties and fracture toughness of the creep aging formed component was established.The results show that their properties are greatly affected by the grain structures.The large-strain microstructure has the smallest spring-back of 43.4%,and the highest yield strength and tensile strength of 548.5 MPa and 586.5 MPa,respectively,but its anisotropy is much greater than that of the other two kinds of grain structures.The fine-grained component has the highest elongation of13.8%.The coarse-grained component has the slowest fatigue crack growth rate,and the maximumΔK of fracture is 33.3 MPa·m^1/2,It also has the highest Kahn tear fracture toughness,with the tear strength and UIE values of 657.4 MPa and 182.1 N/mm,respectively.Figures:92;Tables:17;References:214...