| The transportation industry is driving the growing demand for high-strength,light-quality and high-formability parts.Aluminum alloys with poor forming ability at room temperature can improve its plastic deformation ability when formed at high temperatures,thereby obtaining formed parts with complex shapes.However,the hardening and damage mechanisms of materials under thermal deformation are complex,and the prediction of ductile fracture under thermal deformation is the key to control product's quality.In this paper,taking 2124 aluminum alloy as the research object,a damage-coupled viscoplastic constitutive model was established to describe the hardening,damage and fracture behavior of the material under thermal deformation,and the model was applied to the fracture prediction of isothermal bulging process and process analysis of a double-C part under hot forming via material user subroutine VUMAT.The main work is as follows:Hot tensile testing of 2124 aluminum alloy were carried out at a temperature range of350-450°C and a strain rate range of 0.001-0.1s-1,and the effects of temperature and strain rate on peak stress and elongation were analyzed.The metallographic testing were conducted on the specimens near the fracture position of hot tensile,and the evolution law of material's microstructure at different temperatures and strain rates was analyzed.A dislocation-based viscoplastic model was established to describe the hardening behavior of 2124 aluminum alloy under hot deformation.In addition,based on power-law nonlinear damage evolution law,a high temperature damage model was proposed considering the effects of plastic strain,stress triaxiality,temperature and strain rate,and the damage variable was coupled with yield criterion to describe material softening.The material parameters of the viscoplastic model were obtained by genetic algorithm,and a set of parameter equations between material parameters and temperature were constructed to improve the accuracy of the model.In addition,in order to obtain the parameters of damage model related to stress triaxiality,notched tensile testing were carried out at high and low temperatures,and the evolution relationships of stress triaxiality and equivalent plastic strain were obtained.The user-defined material subroutine VUMAT in ABAQUS software was developed.The finite element model of thermal tensile was established,and the damage model parameters related to evolution characteristics were reversed based on thermal tensile test data.The damage evolution behavior of thermal tensile process at a temperature of450?and a strain rate of 0.1s-1 was analyzed.Taking hot forming of the double-C part and isothermal bulging as research object,the forming was simulated,and analyzed by the established constitutive model.Firstly,the damage evolution behavior of isothermal bulging process at the temperature of 400°C was analyzed.The isothermal bulging experiment under this condition was carried out,and the rupture position and thickness distribution after rupture of experimental and simulated specimens were compared to verify the reliability of the proposed damage model for predicting the fracture behavior of 2124 aluminum alloy under thermal formation.In addition,based on orthogonal experiment,the influence of different process parameters(temperature,speed,blank holder forces and friction coefficient)on damage evolution of the double-C part under hot forming process was studied,and the best process parameter condition was obtained with range analysis method.Corresponding thermoforming experiment of the double-C part was carried out to verify the validity of process analysis,and the finite element analysis of hot forming process under the best process condition was carried out to clarify the deformation characteristics. |
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