Numerical Modeling of Zirconium Alloys Hot Extrusion

Abstract

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.

Simulation of Continuous Dynamic Recrystallization Using a Level-Set Method

“…They are representative of the conditions experienced by a billet during the hot extrusion process. Additional details about the manufacture of zirconium alloys can be found in reference [ 37 ]. The four test cases differ with the GB properties and the restrictions set for formation of subgrains.…”

Towards Multi-Scale Modeling of Zirconium Alloys Recrystallization and Application to Thermo-Mechanical Processes Optimization

“…Indeed, microstructure evolution is a function of the thermo-mechanical history of the material, namely strain, strain rate and temperature during extrusion (dynamic recrystallization) and during the cooling of the tube or further heat-treatment (post-dynamic recrystallization). The finite element model developed with FORGE® NxT software and presented in detail in [9] can provide such data, as the one presented in Fig. 5.…”

“…Industrial samples. Tubes were extruded with the industrial standard extrusion parameters which are an extrusion ratio of about 10 and an initial temperature of 600°C .Mode details about this hot extrusion process and associated parameters can be found in [9]. Microstructure was characterized at mid-thickness, at 4 different positions from the front end of an extruded tube (0, 60, 150 and 300 mm) with optical microscopy, and at 2 positions (60 and 300 mm) with EBSD.…”

Characterization and modeling of the influence of initial microstructure on recrystallization of zircaloy-4 during hot forming

“…The particular case of hot extrusion used to produce cladding tubes illustrates how this is critical. Indeed, a low recrystallized fraction after hot forming is susceptible to induce crack formation during the subsequent steps of cold pilgering [6,7].…”

“…The present article intends to take advantage of the recent advances made in full-field simulations in order to improve our understanding of CDRX of Zy-4. It summarizes a complete and extensive study of Zy-4 recrystallization during and after hot forming in conditions representative of industrial processes [6]. First, it describes how microstructure evolutions have been characterized and what are the main takeaways about recrystallization mechanisms.…”