Research On Numerical Simulation Of Multi-Stage Metal Forming Based On Combined Hardening Model

Sheet metal forming is a very important plastic processing technology,which has been widely used in automotive,electrical,marine,aerospace manufacture and other fields.For some parts with complex structural shapes that cannot be formed by one stamping stage,they should be manufactured by multi-stage forming method.The traditional sheet metal forming process relies mainly on past experience and try and error method.Advanced materials such as high-strength steel have been used more and more by the excellent mechanical properties,but some defects are more prone to occur,such as wrinkles,cracking,and springback,which may greatly increase the difficulty of quality control of parts.With the development of finite element simulation technology for sheet metal forming,the numerical calculation method can accurately describe the sheet forming process,predict the forming defects,and shorten the development cycle,which has received more and more attention.Due to the complicated forming process of sheet metal,most of the research work on the numerical simulation of sheet metal forming is concentrated in single stage forming.The mechanism of multi-stage metal forming is more complicated,as the geometry and material properties of the sheet are possible to change in each stage.In this paper,the multi-stage sheet metal forming process has been studied with numerical simulation technology based on actual production process,and some progress has been made.Firstly,the material constitutive model used in sheet metal forming simulation is studied,in order to model the Bauschinger effect which is a key factor influence the accuracy of high-strength steel,an improved combined model based on the Yoshida-Uemori(Y-U)hardening model especially designed for multi-stage sheet metal forming is proposed.The initial yield surface with anisotropic parameters of this model is defined according to the Barlat-Lian yield criterion,and the anisotropic axis can be automatically adjusted in different stage.It assumes that there exists different coefficient on equivalent back stress of boundary surface between stages.Associated material parameters can be determined by employing available tension-compression test data.A special designed multi-axial tension and compression experiment was performed to verify the proposed material model.With the improved experimental device,the uniaxial and multiaxial tension and compression experiments of high strength steel DP600 and aluminum alloy AA5182 were carried out.One element finite element model with different constitutive model is used to compare the prediction accuracy.The results shows that,in comparison with other constitutive models,the multi-stage combined hardening model can achieve a higher accuracy in predicting springback in the numerical simulation of the multi-stage sheet metal forming.Secondly,an automatic positioning method in multi-stage sheet forming simulation is proposed.In order to get a more accurate initial position between sheet and the tools,the real forming result of the previous stage is utilized to replace the initial sheet to adjust the tool position before current stage is submitted to the solver.As compared with other positioning methods for multi-stage simulation,the proposed method is of low time cost,good stability and high position accuracy.Thirdly,the compensation coefficient in the springback compensation of multi-stage sheet metal forming is studied.In each iterative of springback compensation,the compensation coefficient is corrected according to the relationship between the expected compensation value and the actual compensation value.A multi-stage springback compensation method based on adaptive compensation coefficient is proposed.Work with the multi-stage combined hardening model and the automatic positioning method,it is proved by an actual example that the method can significantly reduces the iteration times,and provides a theoretical reference for the improvement of the tool design.Finally,based on the above research,an Auto Setup module for multi-stage numerical simulation designed by object oriented method was developed for the actual production process,which includes gravity,forming,trimming,flanging,and springback.By the experiment and numerical simulation of the multi-stage manufacturing process of an A-pillar,it is proved that,the Auto Setup model is steady,reliable and user friendly,and it can accurately predict the springback.