Study On Anisotropic Yield Criterion And Forming Limit Of AA6061 Sheet At Cryogenic Condition

As a new manufacturing process,cryogenic forming has a broad application prospect in the field of aluminum alloy forming.The excellent strength,ductility,and hardening ability of aluminum alloys at low temperatures are the foundation of the cryogenic forming process of aluminum alloys.The accuracy of the constitutive equation is of great significance to the design and optimization of the forming process.In this paper,AA6061 was taken as the research object.Based on the self-designed and built test device,the anisotropic yield criterion and forming limit of AA6061 at room and cryogenic temperatures were studied.The main research contents and conclusions are as follows:(1)Three cryogenic test devices were designed and built,including cryogenic uniaxial tension,biaxial tension and stamping devices.By soaking in liquid nitrogen,the specimen can reach-170±5℃ and remain constant in the designed devices,thereby achieving data acquisition of mechanical properties and formability of aluminum alloy plates under cryogenic temperature.(2)The basic mechanical properties of 6061 aluminum alloy at room and cryogenic temperatures were studied by uniaxial tension tests with three directions(0°,45° and 90°),and the modified Voce hardening was constructed.The uniaxial yield stresses and anisotropy coefficients were considered,and the room temperature Yld2000-2D anisotropic yield criterion was obtained by an inverse identification method.Based on one single biaxial tension test,the cryogenic Yld2000-2D yield criterion was constructed.Deep drawing tests and finite element simulations at room and cryogenic temperatures were carried out,and the accuracy of yield criterion parameters were verified by comparing experimental and simulated results.(3)Uniaxial tensile and biaxial tensile tests were conducted at room and cryogenic temperatures to construct the necking forming limit diagram of 6061 aluminum alloy.Using EBSD and TEM analysis techniques,the microstructure of 6061 aluminum alloy in biaxial strain state was characterized,and the reason for improving the formability of 6061 aluminum alloy at cryogenic temperature was explained from the microscopic level.(4)Construction and accuracy verification of GTN(Gurson-Tvergaard and Needleman,GTN)damage models for AA6061 at room and cryogenic temperatures.The GTN damage model parameters of AA6061 at different temperatures were obtained by inverse identification method.The parameters of modified Voce hardening models,Yld2000-2D yield criterion,and GTN damage models at room and cryogenic temperatures are written into the VUMAT subroutine to predict the forming limit,which are determined based on uniaxial tensile and biaxial tensile tests.The accuracy of the damage model was verified by comparing the experimental and simulation results.