Numerical investigations of tool life in thixoforging in consideration of the workpiece in the semi-solid state

The present contribution deals with numerical investigations of a challenging forming process called thixoforming. This technology takes advantage of the semi-solid material state in order to produce geometrically complex parts. The reliability of numerical investigations of such processes depends on the accuracy of the used input data. Current commercial FEA software packages provide material databases with a great amount of material properties for diverse materials. However, material properties valid for temperature ranges exceeding the solidus temperatures are often insufficiently described or at times not available at all. Since thixoforming of steel requires the aforesaid temperature ranges a material model, consisting of two sections, has been developed for the numerical description of thixoforming processes. The first one describes the material behaviour below the solidus temperature and comprises an approach from structure mechanics, whereas the second section model describes the thixotropic behaviour above the solidus temperature based on the Ostwald-de Waele power law. An appropriate material description enables the investigation of the life cycle of tools by calculating local thermal and mechanical stresses more accurately. In this context numerical calculations of the thermo-mechanical tool loads during thixoforging were carried out. The Sehitoglu’s fatigue model has been implemented in the commercial FE software Simufact.forming by means of user-defined subroutines. In addition low-cycle fatigue as well as thermo-mechanical fatigue tests were performed in order to calibrate the model.