Numerical Simulation Of Aluminum Alloy Sheet Forming Process

Sheet metal stamping technology is one of the indispensable plastic processing technologies in the modern industrial manufacturing process,and has been widely used in many production fields such as aerospace,automotive,medical equipment,home appliance manufacturing,etc.It has the advantages of fast forming speed and high material utilization.However,due to the different kinds of materials,process parameters and conditions,processing defects such as wrinkling,cracking and spring back are very easy to appear when metal sheets are subject to large plastic strains,they will greatly affecting the normal operation of production operations.Using the finite element numerical analysis method to accurately predict defects by simulating the stamping process,and then optimize the process parameters and process trajectory curve,is an effective way to solve the above problems.It has great significance for improve product quality,save production costs,and shorten product development cycle.A complete material constitutive model is the basis for obtaining accurate prediction results,which deeply affects the reliability of simulation analysis.The mesoscopic damage model studies the evolution of defects in mesoscopic scale during the material deformation process,it also reveals the causes of microcracks generation,propagation,and fracture.Changes in stress and strain are all derived by this theory.Therefore,the prediction of ductile fracture which is calculate by the mesoscopic damage model is more accurate.At present,the most widely used mesoscopic damage model is the Gurson-Tvergaard-Needleman(GTN)porous plastic damage model.This model is based on the isotropic mechanics theory,it did not consider the anisotropy of the stamped sheet and ignored the effect of shearing on the evolution of micro-cavities.It is not suitable for failure modes dominated by shear loads.Based on the GTN damage model,considering the anisotropy of the sheet and the effects of shear,a damage model that is more in line with the characteristics of stamping processing is established and applied to the study of the defect formation mechanism of AA5052 aluminum alloy sheet stamping.The main research contents of the paper are as follows:1.The research background and significance of this topic,as well as the research work and application of mesoscopic damage models by experts and scholars at home and abroad are introduced in detail.Based on the introduction of two isolated microporous models,the Gurson porous metal plasticity model and the optimization of Tvergaard and Needleman are introduced.Then the advantages and disadvantages of the GTN damage model are described.Analyze the shear modification of GTN model by Xue,Nahshon,and Zhou;2.The phenomenological method is used to integrate the equivalent stress described by Barlat'91 anisotropic yield criterion and flow stress described by Voce hardening equation into the potential equation of the GTN damage model,and the shear correction parameters proposed by Zhou in his model are applied to describe the shear damage of the material.The constitutive equation of the improved model and its numerical implementation method are derived in detail.The backward Eular constitutive integration algorithm is used to compile it into an explicit user-defined material subroutine VUMAT,which is applied to the finite element simulation software ABAQUS;3.According to the actual needs of finite element simulations and experiments,through uniaxial tensile tests and numerical simulation methods,the material mechanical parameters and Barlat'91 anisotropy parameters of AA5052 aluminum alloy sheet were measured and calculated.And its real stress-strain curve was fitted.Based on the previous work,combined with the finite element inverse calculation,the GTN damage parameters of the material and various parameters related to shear were obtained;4.By performing one element tensile and shear simulations in ABAQUS,and comparing the calculation results with ABAQUS's own porous metal plastic model to verifies the correctness of the VUMAT.The equivalent plastic strain,equivalent stress and mesopore volume fraction calculated by the two models in the element tensile simulation are almost same,but the results calculated in the element shear simulation are quite different.That is not only verifying the correctness of the model in VUMAT,but also reveals that the original GTN model is not suitable for mechanical simulations dominated by shear loads;5.In order to explore the specific effects of different process parameters on the sheet forming process and accurately predict the cracking defects of the sheet,the VUMAT was used in ABAQUS to perform multiple finite element simulations of stamping process of the AA5052 aluminum alloy sheet.By adjusting the blank holder pressure and the stamping speed,etc.,the damage parameter cloud diagram and the curves of maximum thickening rate and maximum thinning rate are used as the criteria to judge the final forming quality.The results show that for the stamping die used,the boundary between the bottom fillet and its side is the most prone to cracking defects,and with the increase of stamping speed,blank holder force and material friction coefficient,the maximum damage value and the maximum thinning rate of the plate have an upward trend.Finally,combined with the simulation results,a set of optimal processing technology parameters was selected,and the pressure holding stage in the classic embossing processing curve was premised on the part where the blank is prone to cracks,so that the metal can fully flow and form.