Coordinating Mechanism of Deformation and Properties Tailoring of 7050 Aluminum Alloy in the Whole Process of Creep Aging

Author:Lei Chao

Supervisor:yang he li heng


Degree Year:2018





High strength aluminum alloy integral panel components are in urgent needed in the high-end equipment manufacturing of aerospace field.Creep age forming(CAF)technology using the characteristics of creep deformation and aging strengthening in aluminum alloy can synchronize material forming process and heat treatment process,and then the goal of integrate on shape forming and properties tailoring should be realized naturally.Thus,as an advanced sheet metal forming technology,CAF is urgently needed to be studied and developed.However,CAF is carried out in the complex thermal-mechanical coupling field.There is a strong interaction between creep and aging,which is manifested as the interactive influence among material deformation,microstructure evolution and properties change.As a result,it is difficult to obtain accurate dimensional shape and excellent material performance at the same time.To this end,the creep aging tests,microstructure characterizations,mechanical property and corrosion resistance tests are conducted.Based on the experimental results,combined with theoretical analysis and numerical modeling,the deformation and propertirs evolution laws of 7050 aluminum alloy in the whole process of creep aging under various influencing factors are analyzed,and the corresponding coordinating mechanisms of deformation and properties tailoring are revealed.The main research contents and results are as follows:Using five different heat treatment states as the material initial tempers,viz.,solution,peak-aging,over-aging,retrogression and re-solution,the creep aging behavirs of 7050 aluminum alloy with different initial tempers are compared.It is found that the fine and uniformly distributed intragranular precipitates form in the alloy with initial temper of re-solution,leading to the difficult dislocation movement,increasing mechanical strength,and steady creep deformation.The grain boundary precipitates in the alloy with initial temper of re-solution are coarsened and discontinuously distributed after creep aging,resulting in the satisfactory corrosion resistance.Therefore,re-solution is the optimal material initial temper to obtain the coordination of deformation and properties tailoring in creep aging under the given thermal-mechnical condition.The evolution of deformation and properties of 7050 aluminum alloy in the whole process of creep aging under different thermal-mechanical loading sequences are comparatively studied.The heating stage under the sequence of loading prior to heating(LH)corresponds to a non-isothermal creep aging process,in which creep deformation occurs and even accounts for 44.4% of the final deformation.While the heating stage under the sequence of heating prior to loading(HL)is merely a non-isothermal stress-free aging process with no creep deformation.As a result,the final deformation of LH specimen is 1.55 times of that of HL specimen.Besides,the applied stress in the heating stage under LH could promote diffusion of solute atoms and movement of dislocations,and accelerate the nucleation and growth of the intragranular and grain boundary precipitates,leading to more evenly distributed intragranular precipitates and more discontinuous grain boundary precipitates after creep aging.The LH specimen shows higher mechanical strength and better corrosion resistance than the HL specimen.Therefore,the LH sequence is more favorable for coordinating of deformation and properties tailoring in creep aging.A non-isothermal high-efficiency creep aging(NICA)method of 7050 aluminum alloy is proposed.Unlike the isothermal creep aging(ICA)which contains short heating and cooling stages and long holding stage,the NICA only has slow heating and cooling stages.In the NICA process,the precipitates nucleate and grow up in the first half of heating stage,resulting in the reduction of creep rate and enhancement of mechanical strength.The coarsening of precipitates occurs in the second half of heating stage,elevating the creep rate and significantly improving the corrosion resistance.Nearby the peak temperature,the primary precipitates partially dissolve,and the creep rate,mechanical strength and corrosion resistance decrease.Distinctive from the ICA process,a secondary precipitation phenomenon takes place in the cooling stage of NICA,leading to the recovery of high mechanical strength.Apparently,the higher heating-cooling rate is not conducive to the accumulation of creep deformation,while lower one results in excessive dissolution of the precipitates.Under the moderate heating-cooling rate,larger creep deformation,higher mechanical strength and better corrosion resistance can be achieved in the alloy,and the time consumption is only 52.9% of the ICA process.The damage generation mechanism and evolution law of 7050 aluminum alloy during creep aging under high stress level are studied.The larger the applied stress,the more likely the creep damage occurs;the larger the growth rate and size of the microvoid,the faster the mechanical strength declines,and the main creep rupture mechanism will change from shear to microvoids coalescence.Based on the continuous damage mechanics constitutive model,combined with finite element software,the CAF processes of 7050 aluminum alloy integral panel are simulated.The simulated and experimental results show that the creep damage mainly occurs on the curved ribs with severe stress concentration.The higher the ribs,the greater the creep strain and damage degree,and the smaller the springback after unloading.The tension-compression asymmetry in the whole creep aging process of 7050 aluminum alloy is revealed.The existence of misfit between precipitate and matrix is proved by stress-free aging test,which produces local strain around the precipitate,leading to a reduction of macroscopic size of the alloy.The deformation in creep aging process under compressive stress is always greater than that under tensile stress.By removing the misfit induced strain,it is found that the tensile and compressive creep strains are almost same.Therefore,the root cause of tension-compression asymmetric deformation is the misfit between precipitates and matrix.The strain under compressive stress is the superposition of compressive creep strain and misfit strain,while the strain under tensile stress is the offset of the two.Since more deformation occurs under compressive stress state,more dislocations will be introduced in the alloy,thus the compression creep aging specimen shows higher strength and faster growth rate of the intragranular precipitates.Based on the internal variable method and unified modeling theory,a unified constitutive model considering tension-compression asymmetry is established for the whole process of creep aging of 7050 aluminum alloy.By introducing three internal state variables of microstructure,viz.,precipitate radius,precipitate volume fraction and dislocation density,the relationship among deformation,microstructure and mechanical properties can be described.The material constants of constitutive model are determined by genetic algorithm.The tensile and compressive creep aging tests of 7050 aluminum alloy are simulated using ABAQUS with user subroutine CREEP.The maximum errors between the calculated and experimental values of strain and yield strength are 4.3% and 4.8%,respectively,indicating that the established constitutive model is suitable for simulating the evolution of deformation and properties of 7050 aluminum alloy in the whole process of creep aging.