Application of X-Ray Microtomography to Quantify the Liquid Fraction of M2 Steel for Semi-Solid Forming Process

Abstract

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 at 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 percentage and distribution of liquid fraction at the semi-solid state with high energy 3D X-ray microtomography was investigated on M2 steel grade as a ‘model’ alloy. The obtained results have been compared to 2D observations using EDS technique in SEM on heated and quenched specimens. They showed a good correlation making both approaches very efficient for the study of the liquid zones at the semi-solid state.

Microstructure observation and quantification of the liquid fraction of M2 steel grade in the semi-solid state, combining confocal laser scanning microscopy and X-ray microtomography

“…As stated in Ref. [15], considering the resolution of different techniques, the good agreement between 2-D image analysis (resolution: 1 nm) and 3-D X-ray microtomography (1.2 lm) for the measurement of the liquid fraction provides an opportunity to quantify the liquid fraction when the microstructure at high temperature can be preserved by quenching. It means that it will then be possible to investigate the liquid fraction and the liquid flow in each zone of complex thixoforged parts made in M2 steel after quenching.…”

“…9, corresponding to newly formed carbides as seen with CLSM experiments. Moreover, these new carbides formed after quenching are located at the grain boundaries where the liquid is supposed to appear first at high temperature [15]. A 1.2 lm resolution was used in this study: it was the best achieved so far with this technique.…”