Al-Li Alloy Complex Curvature Member Plastic-creep Composite Forming Process Research

With the creep aging forming technology from small curvature skin-type components to further expand the application of high tendons,large curvature complex structure of thin-walled components,the previous creep-based single deformation method to achieve small curvature components as a whole once formed is difficult to meet the requirements of such large curvature components large deformation;at the same time,the curvature of the component is large,in the early vacuum sealing loading paste mold is very difficult,very easy to occur vacuum bag rupture or air leakage phenomenon.Therefore,the introduction of pre-deformation such as roll bending can effectively solve the above problems faced by the precise forming of such components.At present,most of the research on preforming of complex curvature components with difficult deformation(e.g.tank melon petal)stays at the stage of single curvature roll bending preforming with the goal of reducing the difficulty of die attaching,and there is a lack of theoretical research on how to match the amount of roll bending preforming with creep deformation variables to improve the forming accuracy of components,and there is a lack of accurate preforming process determination methods.From the results of the preliminary research,it is found that the introduction of unreasonable preforming volume may lead to increased difficulty in subsequent accurate creep aging forming of the component.Therefore,there is an urgent need to carry out research on the matching law of pre-deformation introduction of plastic transformation and creep aging accurate composite forming.In this paper,we focus on the problem of "plastic transformation-creep composite forming process matching" in the process of forming and manufacturing of complex large curvature components of aluminum-lithium alloy,and take the melon flap components of aluminum-lithium alloy storage box bottom as the research object,and carry out its plastic transformation-creep aging composite forming process experiments,local mold construction,mold design,process optimization,finite element simulation and engineering applications,etc.The whole chain of research.To identify the influence of plastic transformation/pre-forming introduction on the creep aging behavior/forming accuracy of material/component;to realize the design and software development of plastic transformation-creep aging composite forming process for complex curvature components based on the material intrinsic model and mathematical model of process parameters;to complete the experiments of aerospace complex curvature typical components to verify the rationality of the process and the accuracy of the model;and to finally form a plastic transformation-creep composite forming process that meets the forming requirements.The process design method will provide the engineering basis for the complex curvature composite forming of aerospace components.The main research contents and conclusions of this paper are as follows.(1)The aging hardness,creep aging and mechanical properties testing experiments of 2195 aluminum-lithium alloy were carried out,and based on the manufacturing requirements of aluminum-lithium alloy melon flap formability,the basic aging regime of plastic-creep accurate composite forming was determined to be 180℃+9h,which reduces the creep amount by 14.83% shortening the forming time by 77.7%compared to the single creep aging forming 180℃+16h aging regime.(2)To identify the effect law of small plastic change introduction on the creep aging behavior of 2195 Al-Li alloy.The creep aging tests were carried out for different initial plastic variables and different stress levels,and it was found that the introduction of different initial plastic variables below 1.5% had a suppressive effect on the creep amount of the material.The creep amount decreases with increasing initial plasticity up to 1%,with a maximum decrease of 30.3%;above 1%,the inhibitory effect gradually decreases.Meanwhile,the inhibition effect was more obvious at higher stress level(280 MPa)than at lower stress level(160 MPa).The mechanical properties of the post-aging specimens were tested,and it was found that the introduction of plastic transformation did not have a significant effect on the yield strength and tensile strength of the material,but could improve the post-extension of the material.(3)The introduction of initial plastic transformation will generate dislocations inside the material,so a macroscopic intrinsic structure model for creep aging of 2195 aluminum-lithium alloy describing different initial plastic transformations and different creep stress levels was established by considering the initial dislocation density,and the model prediction curves were in good agreement with the experimental values with a maximum deviation of 4.26%.The established intrinsic structure model was compiled into CRPLAW subroutine and embedded into the finite element software MSC.MARC simulation calculation,and the prediction accuracy of the specimen scale constant stress creep aging simulation model was in good agreement with the test results.(4)In view of the slow convergence speed and error accumulation problems of the existing iterative compensation and point-to-point compensation weighting algorithms,a busbar compensation-rotation method is proposed for the design of the mold profile,based on the finite element method to identify the relationship between the curvature of the component,plastic variables and the amount of rebound,and the composite forming mold profile is constructed by the rotation method after compensating the busbar curvature through the above rebound law.(5)Establish the matching criterion of plastic volume,creep volume,total deformation volume and pre-forming plastic volume in the composite forming process of the component,and introduce the influence of stress and pre-forming plastic volume into it.Through finite element simulation and mathematical modeling to establish the connection between the roll bending process parameters and the target control quantity,the matching method of plastic and creep composite forming process for complex curvature members is proposed,and the visualization software interface window is developed using Matlab to form the composite forming matching system.(6)Based on the forming demand of 3.35 m aluminum-lithium alloy melon flap,the engineering application verification of the complex curvature member plastic transformation-creep aging composite forming process design method was carried out.The parameters of the roll bending process were determined according to the process matching system,and finite element simulation and engineering experiments of composite forming were carried out to compare the accuracy of the formed components.The deviation between the actual forming results and the finite element prediction results is 7.16%,which indicates that the established composite forming finite element simulation has good prediction effect.Compared with the single creep aging process,the total deviation is reduced from 15.21 mm to 5.59 mm,and the forming accuracy is improved by 63.25%,which illustrates the rationality of the proposed composite forming process matching method of plastic and creep forming for complex curvature components.