Anisotropic Behavior For Cryogenic Forming Of 2219 Aluminum Alloy Spherical Shell

In order to solve the problem that it is difficult to form large-sized integral thin shell of aluminum alloys at room temperature,a novel technology for cryogenic forming has been developed,which can significantly improve the formability of aluminum alloys.At present,the research on cryogenic forming of aluminum alloys has not considered material anisotropy,and the anisotropy of aluminum alloys under cryogenic temperature and its effect on the deformation of various area are still unclear.For this reason,this paper takes spherical shell as the research object.The mechanical propeies and anisotropy of aluminum alloy sheets at different cryogenic temperatures were systematically studied,a cryogenic anisotropic constitutive model based on the Hill90 yield criterion was established,the corresponding VUMAT subprogram was developed,and the drawing deformation law of spherical shell at cryogenic temperature was clarified through numerical simulation and experiments.The mechanical propeies and anisotropy parameters of 2219 aluminum alloy sheet at different temperatures were tested by the uniaxial tensile test,and the cryogenic temperature constitutive model of 2219 aluminum alloy was established.With decreasing temperature,the strength,plasticity and hardening ability of 2219 aluminum alloy were significantly improved,and reached the maximum at-196°C.In general,the r andΔr values of 2219 aluminum alloy decrease with decreasing temperature.Among them,r₀,r₄₅ and r₉₀ at-196℃decreased from0.50,1.14,0.42 at room temperature to 0.43,0.90 and 0.39,respectively.The|Δr|at-196℃decreased from 0.68 at room temperature to 0.49,with a decrease range of 38.3%.Research shows that 2219 aluminum alloy has better formability and deformation uniformity at cryogenic temperature.The VUMAT subroutines for cryogenic anisotropic thermal-mechanical coupling was developed,and the influence of different sheet sizes on specimen wrinkling,earing and thickness distribution at cryogenic temperature was studied by numerical simulations.With the decrease of the sheet size,the 0°and 90°directions of the specimen tend to produce finer and deeper wrinkles,while the 45°direction produces shallower and wider wrinkles,and the smaller the slab size,the more difficult it is to suppress wrinkling.The smaller the sheet diameter,the smaller the earing ratio of the drawing specimen and the smaller the thickness reduction.Considering the wrinkling,earing and thinning of the specimen,the optimal sheet diameter is 255mm.The effects of different temperature fields on the earing behavior,equivalent strain and thickness distribution of the specimens were studied by numerical simulations.The simulation results of the uniform temperature field show that,as the temperature decreases,the earing rate is smaller,and the thickness distributions in the three directions are more similar,and the thickness reduction is also significantly reduced.The simulation results of the gradient temperature field show that,with the decrease of flange temperature,the smaller earing rate of the specimen,and the thickness distributions in three directions are more similar,and the more serious thickness reduction is.The forming ability,earing behavior and thickness distribution of the specimen at different temperatures were studied by the cryogenic deep drawing experiment,and the accuracy of the numerical simulation was verified.Spherical shell cannot be formed at room temperature,the drawing height is 55mm,and it can be formed at gradient temperature,and the drawing height is 85mm,which is 54.5%higher than room temperature.The lower the flange temperature,the smaller earing rate of the specimen,but the more serious thickness reduction.Therefore,the cryogenic forming should reasonably match the cryogenic temperature to achieve uniform forming of the overall structure of the aluminum alloy.