Study On The Springback Of Uhss Stamping And The Effects Of Die Elastic Deformation And Temperature Rise

Ultra high strength steel(UHSS)has been used in automotive structural components to further reduce the weight of the auto body and improve the crashworthiness performance.However,UHSS sheets demonstrate more obvious kinematic hardening,which results in severe springback.In the present work,the kinematic hardening of UHSS has been tracked by using a NSK-Swift model in order to simulate the loading process more accurately.For unloading process,inertia relief is adopted as a new control approach in springback calculation.Morever,springback compensation based on the NSK Swift model is done.Also the die structure of the stamping process was regarded as elastic body instead of rigid definition in finite element(FE)simulation.Thermo-mechanical analysis for stamping process is also done to study the relationship between the temperature and forming parameters.The main contents of this research are shown as follows:1.The material constants of the NSK Swift model is determined with tension data and the NSK-Swift model is used to simulate the loading process.Then LS-Dyna software is used to conduct the springback prediction.The experiment results show that the NSK Swift model improves the springback prediction accuracy greatly compared with that predicted by isotropic hardening model,showing great significance in further industrial application.2.In springback calculation stage,two control options in LS-Dyna can be used to eliminate rigid body motion.One method is to define constraints at three nodes(CTN)and the other is inertia relief(IR).Thecomparison of the two control methods shows that the inertia relief method is as accurate as the method defining constraints and improves the springback computational efficiency by 40%.Moreover,the inertia relief approach doesn't need to define constrains on parts and introduces no artificial deformation,which better reflects the actual industrial production.3.The NSK Swift model is proved much effective than isotropic hardening model in springback prediction,so the NSK Swift model is employed in forming stage of the iterative compensation flow.As the NSK Swift model is not programmed with the solver for springback and compensation,the isotropic hardening model is still used in springback and compensation stage to improve calculation efficiency,which will not influence the accuracy as kinematic hardening doesn't occur in this two stages.Springback compensation based on the NSK Swift model can reach acceptable tolerance ±0.5mm through the iterative compensation method in LS-Dyna.4.The die structure of the stamping process was regarded as elastic body instead of rigid definition in finite element(FE)simulation.Discussions and conclusions about the effect of elastic die structures definition in springback are made.5.Thermo-mechanical analysis for cold stamping process is done and the effect of yield strength,friction coefficient and pitch time to the temperature of die and blank is studied by orthogonal design.