Predicting microstructure evolution for friction stir extrusion using a cellular automaton method

Friction stir extrusion (FSE) offers a solid-phase synthesis method consolidating discrete metal chips or powders into bulk material form. In this study, an FSE machine tool with a central hole is driven at high rotational speed into the metal chips contained in a chamber, mechanically stirs and consolidates the work material. The softened consolidated material is extruded through the center hole of the tool, during which material microstructure undergoes significant transformation due to the intensive thermomechanical loadings. Discontinuous dynamic recrystallization is found to have played as the primary mechanism for microstructure evolution of pure magnesium chips during the FSE process. The complex thermomechanical loading during the process drives the microstructure evolution. A three-dimensional finite element process model is developed using commercial software DEFORM 11.0 to predict the thermal field, mechanical deformation and material flow during the FSE process. Using the simulated thermomechanical loadings as input, a cellular automaton model is developed to simulate the dynamic evolution of the material grain microstructure. The predicted grain size is in good agreement with the experimentally measured grain size. This numerical study provides a powerful analysis tool to simulate the microstructure transformation for friction stir-based processes.