Numerical calculation of thermal load and tool life in thixoforming of steel

Thixoforming is an innovative forming process based on the thixotropic properties of metals. To set the thixotropic state, the material is usually inductively heated so that the temperature of the blank to be processed lies between the liquidus and solidus lines. Thus, the material is formed in a partially liquid state, which is characterised by higher flowability. This enables the production of components with a high degree of geometric complexity, while the mechanical properties are better compared to casting. Thixoforming thus combines advantages of forming with those of casting. Challenging in thixoforming of steel alloys are the high occurring temperatures, which lead to a high and complex tool loading.

Therefore, the aim of the research project is the development of a numerical model for the prediction of the tool life considering the cyclic thermomechanical loading resulting from the thixoforming process of a steel alloy. The determination of the tool life requires a realistic description of the flow behaviour of the partially liquid metals in order to determine the occurring tool loads. First, a suitable two-phase model for the representation of the material behaviour in both the solid and liquid state is developed and implemented in a commercial FE application by means of a user-defined subroutine. For parameterisation, cylinder upset tests are performed in a wide temperature range up to above the solidus temperature. Subsequently, the behaviour of the tool material under cyclic thermomechanical loading is tested in LCF and TMF tests considering process-relevant boundary conditions. Based on the results, the damage model according to Sehitoglu is parameterised and implemented by means of a subroutine. Finally, the numerically predicted tool life is validated by experimental thixoforming tests. For this purpose, a suitable process is first designed numerically and then realised experimentally.