Greger Bergman

Numerical implementation of a constitutive model for simulation of hot stamping

In order to increase the accuracy of numerical simulations of the hot stamping process, an accurate and robust constitutive model is crucial. During the process, a hot blank is inserted into a tool where it is continuously formed and cooled. For the steel grades often used for this purpose, the initially austenitized blank will decompose into different product phases depending on the cooling and mechanical history. As a consequence, the phase proportions change will affect both the thermal and mechanical properties of the continuously formed and cooled blank. A thermo-elastic–plastic constitutive model based on the von Mises yield criterion with associated plastic flow is implemented into the LS-Dyna finite element code. Models accounting for the austenite decomposition and transformation induced plasticity are included in the constitutive model.The implemented model results are compared with experimental dilatation results with and without externally applied forces. Further, the calculated isothermal mechanical response during the formation of a new phase is compared with the corresponding experimental response for two different temperatures.

Experimental and Numerical Evaluation of the Heat Transfer Coefficient in Press Hardening

When producing thin ultra high strength steel components with the press hardening process, it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components where it is of great importance to accurately predict the final component properties early in the product development process. The transfer of heat is a key process that affects the evolution of the mechanical properties in the product and it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulations. In this study an experimental setup is developed combined with an elementary inverse simulation approach to predict the interfacial heat transfer coefficient (IHTC) when the hot blank and cold tool are in mechanical contact. Different process conditions such as contact pressure and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation, a thermo-mechanical coupled simulation model is used with a thermo-elastic-plastic constitutive model including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the pressure dependence for the two materials is different and the heat transfer coefficient is varying during quenching. This information together with further testing will be used as a base in a future model of the heat transfer coefficient influence at different conditions in press hardening process.

VERIFICATION OF THERMOMECHANICAL MATERIAL MODELS BY THIN-PLATE QUENCHING SIMULATIONS

A computational model for quenching simulations of thin plates has been developed. The model is examined by comparisons with experiments with one-sided water spray cooling. With this experiment, th ...

Estimation of material parameters at elevated temperatures by inverse modelling of a Gleeble experiment

A method for estimation of inelastic material parameters from Gleeble experiments is presented. Material parameters at elevated temperatures are estimated by inverse modelling of a Gleeble compression test during continuous cooling. The direct problem is solved using the implicit finite element program NIKE2D. The material behaviour is described by a rate independent thermo-elastic-plastic model with a nonlinear isotropic hardening law. The objective function is formed based on the discrepancy in force-displacement data between the numerical model prediction and the experiment. Minimisation of the objective function with respect to the material parameters is performed using an in-house optimisation software shell which is built on the subplex method. A Gleeble experiment with different deformation and temperature histories has been used for validation of the computed parameters. The validation experiment is simulated using both the material parameters achieved through inverse modelling and traditional estimation methods. The analysis showed better agreement when using the parameters estimated by the numerical method presented in this paper.

Integration of a product design system and nonlinear finite element codes via a relational database

A database for finite element models and related data is developed and incorporated into a prototype system for integration of non‐linear finite element codes with a product design system. In the prototype system, the database is used as a link for integrating commercial, public domain as well as in‐house codes. In the present system, the public domain finite element codes NIKE2D, NIKE3D, DYNA2D, DYNA3D and TOPAZ2D are integrated with the CIM–system I–DEAS. The prototype system is primarily intended as a platform in research projects for development of integrated environments tuned for simulations of specific manufacturing processes such as quenching, welding, hot rolling, metal powder compaction and hot isostatic pressing.

Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process

When producing thin ultra high strength steel components with the hot stamping process it is essential that the final component achieves desirable material properties. This applies in particular to ...