Research On Solid Granular Medium Drawing And Reverse Drawing Combine Forming Technology

Thin-walled shell structures are widely used in aerospace and automobile industries.Soft die forming has gradually become the main forming method for complex section shell structure because of its strong applicability,wide application range and high forming quality.In order to obtain thin-walled shell structure parts with uniform wall thickness distribution and high forming accuracy,the forward and backward composite deep drawing process of solid particle medium was proposed in this paper.The positive and negative drawing process of solid particle medium is to replace the gas or liquid in the existing soft die forming process with solid particles,and the positive drawing and reverse drawing are completed in the same time.In order to obtain a simulation model that can accurately describe the pressure transfer characteristics of particles,three classical finite element constitutive models(Mohr-Coulomb model,Drucker-Prager model,Drucker-Prager Cap)and DEM discrete element models for analyzing granular materials such as particles and rock and soil are established respectively.Through triaxial compression test of solid particle medium,the material characteristic parameters required by each constitutive model in simulation are obtained accurately.A particle pressure transfer test is established,and the results are compared with those of three finite element constitutive models and DEM discrete element model.Among them,the error of simulation results of DP-CAP constitutive model is small and the trend is consistent with that of particle pressure transfer test,which can accurately describe the particle pressure transfer characteristics.In the process of positive and negative drawing of granular medium,slab goes through the process of bending,unloading,reverse bending and unloading again,which has obvious cyclic and repeated loading characteristics.Based on Hill48 anisotropic yield criterion,a nonlinear hybrid hardening model considering anisotropy and Bauschinger effect of thin slab was established.A set of transparent anti-buckling fixture was designed by ourselves,and the material characteristic parameters required by the model were obtained accurately with the optical strain measurement system.The anisotropic mixed hardening constitutive model of thin slab is verified by cyclic tension and compression tests and continuous bending tests.The average deviation between the simulation results and the experimental results is only 2.1%,which can accurately describe the cyclic loading deformation behavior of thin slab.In the process of forming parabolic bottom cylindrical parts,there is a risk of fracture at the side wall of the deep drawing part due to excessive radial tensile stress,and there is also a risk of inner wrinkle at the hanging side wall at the bottom of the parabolic part due to excessive circumferential compressive stress.In order to comprehensively determine the two main failure behaviors,the forming limit diagram established by the bidirectional tensile test and the wrinkling limit diagram established by the finite element method and the main critical strain analysis model is used to establish the fracture and wrinkling limit diagram which comprehensively determine the occurrence conditions of wrinkling and fracture on the side wall of deep drawing parts.The nonlinear mixed hardening model based on Hill48 anisotropic yield criterion and the pressure transfer model based on Drucker-Prager Cap criterion for solid particle medium were established.The finite element simulation model of positive and negative composite deep drawing for solid particle medium was established based on the fracture and crepe forming limit diagram as the basis of failure determination.The finite element model can accurately predict the contour and wall thickness distribution of the forming parts and the locations of breakage and wrinkling,and determine the optimal forming process parameters by analyzing the finite element model.It provides a reliable simulation research method for solid particle medium forward and backward deep drawing and related forming process research,and has certain practical application value.The forward and backward drawing tests of solid particle medium were established.The flat-bottom,hemispherical bottom and conical bottom tubular parts with the maximum thinning rate less than 10.6% were successfully manufactured by the optimal forming parameters.The contour shape and wall thickness distribution of each forming part meet the target requirements,the surface quality is high and the forming precision is high,which proves the feasibility and practicability of this process.A high quality forming process with high forming precision and good forming quality is provided for the normal temperature forming of thin-walled shell structural parts.