Wrinkling Behavior And Deformation Uniformity During Hydroforming Of 2219 Aluminum Alloy Curved Shell

Sheet metal shells with curved surface and very thin thickness have been widely applied in many areas such as aerospace and automobile industries.With the increasing requirements of lightweight,large size and integral forming,plastic instability has become a critical problem during the forming of these shells.In order to avoid the plastic instability and uneven deformation during sheet hydroforming process(SHP)of thin walled aluminum alloy shells,focusing on the integral dome of propellant tank of carrier rockets,the stress variation and wrinkling behavior during SHP of AA2219 curved surface shell were studied,the influence of loading methods of liquid pressure on uniform deformation were revealed.The current results can be theoretical guides for integral SHP of tank domes.In order to build a theoretical model for wrinkling prediction considering reverse bulging effect,the mechanical analysis during SHP of a hemisphere shell was carried out.Combining the geometrical model and stress model during forming process,the loading path of liquid pressure containing reverse bulging effect was obtained.The influence of experimental,geometrical and material parameters on the reverse bulging pressure were analyzed.Results show that the depth-height-ratio and thickness-diameter-ratio have great influence on reverse bulging liquid pressure.The critical wrinkling stress in reverse bulging area was calculated,with the increasing of liquid pressure and thickenss-diameter ratio,the critical wrinkling stress was improved.Using the obtained critical wrinkling stress,the forming window of liquid pressure for wrinkling-free shell was designed,in which the magnitude of circumferential stress is lower than critical wrinkling stress.In order to obtain the variation of reverse bulging shape and stress distribution during SHP,a simulation model basing on Hill 1990 yield criterion was built.The mechanism of wrinkling suppression by increasing liquid pressure was revealed.The improvement of liquid pressure leads to the decreasing of reverse bulging area and the increasing of principle strains.The peak value of strains locates at the boundary region of reverse bulging area.During the SHP,the circumferential stress is compressive when applied with lower liquid pressure at the initial period.Then,the compressive stress is largely increased in the later period,which will lead to wrinkling defect.Applying enough liquid pressure is beneficial for the decreasing of circumferential stress.The compressive stress even can be changed into tensile stress using the reverse bulging liquid pressure.The material is in a biaxial tensile stress state.Furthermore,there are peak values of radial and circumferential stress in bulging area,which can be improved with increased liquid pressure.Experiments of AA2219 hemisphere shells using SHP were conducted to illustrate the wrinkling behavior of curved surface shell under different loading paths of liquid pressure.Increasing liquid pressure and thickness-diameter ratio are positive for the suppression of wrinkling defect.Forming window of AA2219 curved surface shell with ratio of thickness to diameter 2.3‰ was built basing on theoretical analysis and numerical simulation results.In the forming window,the liquid pressure is improved linearly during the former forming period,then the pressure keeps unchanged during the later period to suppress wrinkling and avoid ruptures.Based on this theoretical method,a forming window was built foucusing on a dome of propellant tank of carrier rockets with diameter of 3m,the forming window can be regarded as a theorectical guide for the forming process.By pre-bulging hydroforming and double sided hydroforming,the influence of loading mode of liquid pressure on the uniform deformation of curved shells were studied.The pre-bulging shape was optimized by response surface methodology,the influence of pre-bulging shape on the strain and thickness distributions were discussed.Applying the optimized bulging shape,the strain value of curved surface shell can be improve about 75%,at the same time,and uniformity can be increased about 45%.The influence of double sided hydroforming on strain and thickness were obtained.The deformation can be improved about 65% and 25% with increased liquid pressure and pressure ratio.After the solution-SHP-artificial aging process,the strength of 2219 curved surface shell was improved obviously,the elongation was decreased.The tensile strength is 440 MPa which is 13% higher than that in T6 state.