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Hot extrusion is used within the forming route of cladding tubes to deform the billets into tube hollows, which are then cold-rolled to produce the final tubes with the suitable properties for in-reactor use. The hot extrusion goals are to give the appropriate geometry for cold pilgering (diameter and wall thickness suitable for cold-rolling mills, good concentricity) without surface defects and microstructural heterogeneities, which are detrimental for subsequent rolling. To ensure a good quality of the tube hollows, extrusion parameters (e.g., temperature of the billet, ram speed, extrusion ratio, lubrication) and extrusion tools have to be chosen as a function of the press capacity and mechanical and microstructural properties of the extruded alloy. For this purpose, finite-element and analytical models are used in addition to experimental tests. These models, particularly the former, can take into account the thermomechanical coupling that occurs in the tube and the tools during extrusion and provides a good prediction of the extrusion pressure and thermomechanical history of the extruded product. Together with a microstructural evolution model identified with small-scale extrusion tests, torsion tests, and heat treatments, this last result can also be used to calculate the fragmentation of the microstructure in the die and the meta-dynamic recrystallization after extrusion. Once validated against experimental data, numerical models were used to better understand and master the extrusion loads, the microstructure evolutions at the front end of the extruded tubes, and the temperature and stresses applied to the tools.

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Dynamic recrystallization is one of the main phenomena responsible for microstructure evolution during hot forming. Consequently, obtaining a better understanding of dynamic recrystallization mechanisms and being able to predict them is crucial. This paper proposes a full-field numerical framework to predict the evolution of subgrain structures upon grain growth, continuous dynamic recrystallization, and post-dynamic recrystallization. To be able to consider a subgrain structure, two strategies are proposed. One relies on a two-step tessellation algorithm to generate a fully substructured microstructure. The second strategy enables for the simulation of the formation of new subgrains during hot deformation. Using these tools, the grain growth of a fully substructured microstructure is modeled. The influence of microstructure topology, subgrain parameters, and some remaining stored energy due to plastic deformation is discussed. The results highlight that the selective growth of a limited number of subgrains is observed only when mobility is a sigmoidal function of disorientation. The recrystallization kinetics predicted with different criteria for discrimination of recrystallized grains are quantitatively compared. Finally, the ability of the framework to model continuous dynamic and post-dynamic recrystallization is assessed upon a case study representative of the hot extrusion of a zircaloy-4 billet (T=650 °C;ε˙=1.0s−1;εf=1.35). The influence of grain boundary properties and nucleation rules are quantified to evaluate the model sensitivity and suitability. Application of these numerical tools to other thermomechanical conditions and microstructures will be presented in an upcoming article.

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