Application of X-ray Microtomography to Quantify the Liquid Fraction of M2 Steel for Semi-solid Forming process

Thixoforging, one variant of semi-solid metal processing in which the metallic alloys are processed at low liquid fraction (0.1< Fl < 0.3), is used to produce complex parts with high mechanical properties. Steel thixoforging faces more challenges as compared to that of low melting point materials due to high processing temperature and lack of understanding of the thermomechanical behavior of materials in the given conditions. It is crucial to study the microstructure in the semi-solid state to improve the understanding of the thixoforging process since the material behavior strongly depends on main parameters: the liquid fraction, its distribution as well as the coherence of the solid skeleton. The microstructure has a great influence on the viscosity of the material, on the flows and finally on the final shape and mechanical properties of the thixoforged parts. Here, the characterization of the volume fraction and distribution of liquid in the semi-solid state with high energy 3D X-ray microtomography is investigated on M2 steel grade as a ‘model’ alloy. The obtained results are compared to 2D observations using EDS technique in SEM on heated and quenched specimens. They show a good correlation making both approaches very efficient for the study of the liquid zones in the semi-solid state.