Deformation Behavior And Formability Of 2219 Aluminum Alloy Sheets At Cryogenic Temperature

Aluminum alloy thin-walled curved components are critical to aerospace equipments,which directly affects the reliability and carrying capability of aircrafts.Considering the disadvantages of poor formability at room temperature（RT）,microstructure damage and surface defects from hot forming,a novel deep drawing method for high-strength aluminum alloy thin-walled curved components was proposed at cryogenic temperatures.Further,the hardening behavior and formability of a 2219 aluminum alloy sheet were studied under complex stress states at cryogenic temperatures.The macro and micro deformation mechanisms were revealed with regard to the double enhancements of hardening exponent and elongation at ultralow cryogenic temperatures.Moreover,the mechanism of defect formation was also discussed during the deep drawing process,which provides a theoretical guidance for the forming of integral thin-walled curved components at low cryogenic temperatures.Hardening behaviors of 2219 aluminum alloy were revealed during cryogenic deformations.The effects of heat treatment and cryogenic temperatures on the strength,elongation and hardening exponent were analyzed,respectively.The―double enhancement effect‖of the 2219 aluminum alloy was discovered during cryogenic deformations and the critical temperature was determined as-150℃.Compared to those at RT,the elongation and hardening exponent of the alloys are 29.9%and 0.351 at the critical temperature of-150℃,which respectively increases by 24.6%and 28.6%.Further,the-196℃increases by 64.3%and 46.9%,indicating a higher strain strengthening and resistance to local instability.Microstucture evolutions regarding average misorientation,dislocation density,stored strain energy and slip distance were quantitatively elucidated by cryogenic quasi-in situ tensile tests.Compared with the plane slip at RT,the hardening during cryogenic deformations is mainly caused by multiple slip of dislocations.Slip bands appear slightly with low propagation at cryogenic temperatures,which is the result of higher resistance of dislocation motion.The localized distribution of dislocations occurs along grain boundaries at RT.However,the dislocations of cryogenic deformation are uniformly distributed in grain interiors,which contributes to the formation of numerous networks and cells of dislocations.Furthermore,the avalanche effect of dislocation substructures is effectively suppressed.Hence,the initiation and propagation of cracks along grain boundaries are macroscopically retarded during cryogenic deformations.A novel in-situ apparatus was developed for testing the sheet cryogenic formability under complex stress conditions.Cryogenic deformation behaviors of solution-treated 2219 aluminum alloy sheets under biaxial stress were elucidated via this apparatus.Moreover,cryogenic forming limits of 2219 aluminum alloy sheets were obtained,and a dome geometric model was established for the rigid punch bulging process.Furthermore,flow stress curves of the solution-treated2219 aluminum alloy sheet were obtained under cryogenic and biaxial stress conditions.The effect of cryogenic temperatures on the deformation uniformity and bulging limit was revealed.Compared to RT,the limiting dome height（LDH）increases by 47.8%at-160℃and the average deviation ratio of thickness is reduced by 37.0%at the same bulging height.The limit major strain is 0.34 along the central hole of the bulged dome,and the limit expansion ratio reaches 42.9%at-196℃.It is concluded that the strain strengthening,deformation uniformity and resistance to local thinning of the alloy sheets are obviously enhanced under biaxial stress states at cryogenic temperatures.Forming limit diagram（FLD）and deep drawability of 2219 aluminum alloy sheets under complex stress states were experimentally studied by using the cryogenic digital image correlation（DIC）testing device.Based on the FLD and forming limit stress diagram（FLSD）damage models,the influences of cryogenic temperatures were studied on the limiting drawing ratio（LDR）,drawing height,thickness distribution,damage factor,stress and strain path s of flat-bottomed cups.Compared to RT,FLD₀s at-160℃and-196℃increase by 26.1%and39.1%,respectively.The limit deep drawing height and LDR at-160℃are increased by 21.3%and 6.8%,respectively.The distributions of thickness and damage factor at the round corner of the deep-drawn cups become more uniform at-160℃.Deep drawing experiments of an ellipsoidal curved component were first conducted under cryogenic conditions.The cooling curves of the sheet and die were obtained,and an optimal temperature wind ow was determined for cooling the sheet.The influence of various cryogenic temperatures,binder force and thickness-diameter ratios on the wrinkling and splitting was analyzed,respectively.The ability of resistance to splitting of aluminum alloy sheets could be improved by the cryogenic deep drawing process,and the process window was obviously broadened under low cryogenic conditions.Moreover,based on the established process window for cryogenic deep drawing,the 2219 aluminum alloy ellipsoidal components were formed with a thickness-diameter ratio of3.1‰.After artificial aging,the mean tensile strength and elongation of cryogenic deep-drawn components are 436 MPa and 6.1%,respectively.From above results,a 3 m integral dome of launch vehicles was successfully fabricated via cryogenic forming method.