Research On Influence Of Sheet Anisotropy On Springback Prediction And Compensation In Multi-Point Forming

Multi-point forming is a flexible forming method,which discretizes the overall profile of the traditional mold into regularly arranged and height-adjustable multipoint punches.With the assistance of the numerical control system,the mold profile can be quickly adjusted.Therefore,a set of multi-point dies can form the parts with different shapes,and is suitable for the forming of curved parts produced in small batches.In recent years,multi-point forming has been used in more and more fields such as vehicles,building curtain walls,high-speed rail,aerospace,etc.,and the processed materials are becoming more and more diverse,such as steel sheets,aluminum alloy plates,titanium alloy sheets,laminated sheets etc.These materials have strong anisotropy due to rolling or extrusion in the production process.Springback of the sheet occurs after unloading,which reduces the shape accuracy of the part.Although the multi-point die has the characteristics of rapid adjustment,the mold can be adjusted appropriately according to the size of the springback to improve the accuracy of the formed part,but due to the complex impact of the anisotropy of material on the springback,it is difficult to predict the springback after forming.The traditional isotropic springback prediction and compensation method cannot provide an accurate mold surface,so that the adjustment and correction of multi-point dies are still relatively blind.Therefore,for the anisotropy sheet,it is necessary to establish a method that can accurately describe the anisotropy of the material to predict and compensate the springback.Aiming at the influence of the anisotropy of material on springback and the characteristics of loading in multi-point forming,a method that can predict and compensate the springback of the sheet metal in multi-point forming was proposed using multi-scale and partition hybrid modeling technology.To discuss the applicability of the method,08 Al steel sheet,SUS430/AA1050/TA1 laminated sheet and 7B04-T7451 Al plate were studied as examples.The main research contents are as follows:(1)Research on anisotropic mechanical properties based on multi-scale modelingThe macroscopic anisotropy is greatly affected by the microscopic grain size and orientation,a mechanical property prediction method based on multi-scale modeling is proposed.Based on the evolution of dislocation density with strain,the hardening equations for slip and twinning systems are derived,and a rate-dependent constitutive model of crystal plasticity is developed.Using the tangent modulus update method,the implicit integration algorithm of the single crystal model is derived to calculate the rotation of the grain,the shear strength of the slip system and the stress and strain during the deformation.Using the grain size and aspect ratio of EBSD statistics,the traditional random Voronoi tessellation method is modified to establish a three-dimensional representative volume element considering the grain size distribution function,and the grain orientation is mapped to the grain element through the Bunge Euler angle.The element is coupled with the crystal plastic constitutive model,which fully describes the inter-grain interaction of the material during deformation at the micro-scale,and correlates the micro-grain and texture with the macro-mechanical properties.After verifying the crystal finite element model using single crystal tensile test and in-situ tensile test,it was applied to 08 Al steel sheet,7B04-T7451 Al plate and SUS430/AA1050/TA1 laminated sheet and the corresponding parameters were set for scale effect research.The results compared with the tensile tests of the three materials show that the established crystal finite element model can reflect the influence of the microstructure on the macroscopic mechanical properties,and can be used to predict the mechanical properties under complex strain paths such as biaxial tension and pure shearing.(2)Finite element analysis of springback prediction based on partition hybrid modeling methodThere is inhomogeneity in the macroscopic mechanical properties of the material,and the calculation cost of the crystal finite element analysis is huge.This paper proposes a partition hybrid modeling technology to describe it.The method divides the sheet into a plurality of regions with approximately uniform properties according to the inhomogeneity of the material during modeling.The normalized stress component is used to characterize the stress state in each region,and the phenomenological yield criterion is identified by using the optimization algorithm and neural network model,so that it can accurately fit the normalized yield stress and r-value predicted by the crystal finite element under different strain paths.The yield criteria were implanted into ABAQUS by a VUMAT subroutine,and the stress was updated and solved through the cutting plane method.Taking 7B04-T7451 Al plate and SUS430/AA1050/TA1 laminated sheet with thickness inhomogeneity as examples,the thickness direction is divided into several thickness layers by the partition hybrid modeling technology.Using Hill’s 48 and Yld20004-18 p to describe the plane anisotropy of BCC and FCC materials,CB2004 to describe the tensioncompression asymmetry of HCP materials and apply it to the multi-point forming process simulation.Multi-point forming experiments were carried out to verify the results.The results show that the model established using the partition hybrid modeling technique predicts springback more accurately than the isotropic model.The influence of anisotropy on springback prediction is clarified.The identification strategy of introducing anisotropy parameters into springback prediction finite element are proposed to provide effective guidance for production.(3)Calculation algorithm of springback in multi-point forming considering anisotropyBased on the anisotropic mechanical properties predicted by multi-scale modeling and the idea of hybrid modeling,combined with the fact that the stress state of the sheet metal during the multi-point forming process is approximately pure bending and the strain path is complex,a calculation method of double-curvature bending springback of sheet metal considering anisotropy is proposed.The neutral surface is decomposed into many tiny surfaces by means of structural discretization.Combining the yield criterion applicable for each thickness layer with the classical elastic-plastic theory,the distribution law of stress relative to the neutral plane is deduced.Using the boundary strain conditions,the formula of the thickness direction stress is deduced and introduced into the calculation of the elastic-plastic interface layer.The calculation formula of anisotropic bending moment based on the stress invariant is established and the bending moments of different thickness layers are superimposed,and the total bending moment in the calculation formula of the curvature of the neutral layer after springback can be obtained.The proposed calculation method is verified by finite element simulation and experiment of doublecurvature multi-point bending.The results show that the proposed theoretical calculation model,after substituting the corresponding material parameters,can be used for different material types of sheets in multi-point forming with higher accuracy than isotropic models.(4)Springback compensation considering sheet anisotropyBased on results of the springback predicted by finite element method,and using the springback calculation formula to inversely calculate the compensated curvature,the calculation method of springback compensation factor considering anisotropy is established.In the process of compensating the surface,the compensation factor considering the anisotropy is coupled with the direct-geometric curvature algorithm(DCA)to correct the curvature of each microplate element after the surface is discretized.According to the feature points of the inversely calculated unit,the compensated continuous and smooth mold surface is obtained by the Bezier surface splicing method and imported into the multi-point CAD software.The basic body is adjusted according to the compensation surface.The experiments and simulations results show that the compensation method considering the anisotropy have better convergence and higher accuracy than the isotropic compensation method.The influence of anisotropy on the springback compensation accuracy of sheets with different shapes and thicknesses is discussed.A compensation strategy and its parameter identification method for sheets with uneven properties are proposed,which is applied to springback compensation in multi-point CAD software.