Numerical service life calculation of forming tools during thixoforging using the Sehitoglu model

The approach of the most common industry-specific FE-systems for the service life evaluation of forming tools holds some disadvantages. For example, within the decoupled tool analysis, load situations are typically analysed at room temperature and thus, the transient temperature development in the forging die is not considered in the lifetime evaluation. An elastic material behaviour is assumed for the tool component to be analysed. In this way, both deformation and fatigue behaviour are only approximately simulated. However, plastic deformation can occur in critical areas of the tool due to the operationally high tool loads, which can increase during operation and lead to hardening or softening processes in the material. For this reason, neglecting the temperature distribution in the tool makes it impossible to reproduce thermally induced damage processes. Therefore, Sehitoglu's life prediction approach, which is able to take into account thermo-mechanical loads in the tool during thixoforging and to calculate tool life, is presented in this publication. Since, particularly in thixoforming, where the temperatures exceed those of hot forming, the consideration of thermally induced damage plays a decisive role in order to predict the possibly early failure of the tools. Thus, an appropriate numerical design is possible. For this purpose, experimental tests such as low cycle and thermo-mechanical fatigue tests were performed for the parameterisation of the model. By means of optimisation routines, the model's parameters were determined. Subsequently, FE-simulations of a cup backward extrusion process were carried out to calculate tool fatigue along with different damage proportions. Finally, validation of the numerical calculations took place by a comparison with experimental results.