Fast in-situ annealing stage coupled with EBSD: A suitable tool to observe quick recrystallization mechanisms

International audienceA heating stage has been developed to perform in-situ annealing in a SEM equipped with an EBSD system in order to study recrystallization mechanisms. High temperature treatments could then be performed inside the SEM, up to 1180 °C and with high heating and cooling rates (~ 100 °C s− 1). Samples were cooled down to room temperature to perform EBSD orientation mapping in between successive short-duration heat treatments. Microstructure evolution snapshots obtained this way allow gaining an insight into recrystallization mechanisms. The interest of such experiments is shown for two examples: static recrystallization of cold deformed pure tantalum and post‐dynamic evolution of hot-deformed Zircaloy4

Evolution of microstructure and twin density during thermomechanical processing in a γ-γ' nickel-based superalloy

International audienceMicrostructure evolution has been studied in the nickel-based superalloy PER®72 subjected to hot torsion, to annealing below the primary γ' solvus temperature and to annealing at a supersolvus temperature, with a special emphasis on grain size and twin content. Dynamic abnormal grain growth occurs before the onset of dynamic recrystallization. The resulting bimodal grain size distribution affects the grain-coarsening kinetics at the supersolvus temperature, so that the final microstructures depend on the former straining stages. As a consequence, the twin content does not follow a univocal relationship with the average grain size. The grain boundary velocity history before reaching the final grain size is a contributing factor, and this is notably related to the initial grain size distribution width. Dynamically recrystallized microstructures are by nature more homogeneous and thus give rise to lower rates in supersolvus grain coarsening, and accordingly lead to relatively lower twin densities

3D FEM simulation of the flow forming process using Lagrangian and ALE methods

Reprinted with permission from AIP Conf. Proc.May 17, 2007 -- Volume 908, pp. 257-262 MATERIALS PROCESSING AND DESIGN; Modeling, Simulation and Applications; NUMIFORM '07; Proceedings of the 9th International Conference on Numerical Methods in Industrial Forming Processes; doi:10.1063/1.2740821 Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of PhysicsInternational audienceThe process of flow forming is numerically modeled using finite element codes based on the Forge2005® software. Two numerical approaches are considered. The first one uses an updated Lagrangian formulation. The problem is solved with help of a self-contact management algorithm. The second approach consists in using an ALE formulation that permits to optimize meshing with an adaptive method based on the Zienkiewicz-Zhu error estimation. The ALE method is well adapted to incremental forming processes such as flow forming and allows dealing with difficulties generated by the contact between the work piece and tools. Both formulations are coupled with complex tool kinematics. The Lagrangian formulation gives realistic results. The ALE formulation is promising with regard to computational time, and simulations on simple configurations show fairly good agreements with Lagrangian results

Finite element model of primary recrystallization in polycrystalline aggregates using a level set framework

International audienceThe paper describes a robust finite element model of interface motion in media with multiple domains and junctions, as is the case in polycrystalline materials. The adopted level set framework describes each domain (grain) with a single level set function, while avoiding the creation of overlap or vacuum between these domains. The finite element mesh provides information on stored energies, calculated from a previous deformation step. Nucleation and growth of new grains are modelled by inserting additional level set functions around chosen nodes of the mesh. The kinetics and topological evolutions induced by primary recrystallization are discussed from simple test cases to more complex configurations and compared with the Johnson-Mehl-Avrami-Kolmogorov theory

Modélisation numérique du laminage à pas de pèlerin de tubes ODS en vu de limiter les risques d'endommagement

National audiencePour les Réacteurs à Neutrons Rapides au sodium, les matériaux de gainage de référence pour les très forts taux de combustion sont les nuances ferritiques/martensitiques ODS. Ces matériaux présentent en effet des bonnes propriétés en fluage, en résilience et en résistance à l'oxydation. Toutefois la présence des oxydes ODS font de ces nuances des matériaux très difficiles à mettre en forme. Classiquement le tube de gainage est mis en forme à froid à partir d'une ébauche tubulaire par une succession de passes de laminage à pas de pèlerin et de traitements thermiques. Dans le cadre de cette étude la modélisation numérique du procédé de laminage dans une configuration de type HPTR a été entreprise. Le modèle prend en compte toute la complexité des phénomènes physiques, mécaniques ainsi que le modèle de comportement du matériau pour simuler les déformations élastoplastiques cycliques qui apparaissent au cours de la mise forme des tubes minces. La modélisation de la cinématique du procédé a déjà été réalisée. L'utilisation de capteurs numériques pour suivre le chemin de déformation de la matière lors du procédé permet d'estimer la nature et l'amplitude des déformations cycliques subies

Strain induced abnormal grain growth in nickel base superalloys

International audienceUnder certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate

Numerical life prediction of mechanical fatigue for hot forging tools

Issu de : ESAFORM 2009 - 12th ESAFORM Conference on material forming, Enschede, THE NETHERLANDS, 27–29 April 2009International audienceIn the forging industry, tools represent an important part in term of production and costs. Enhancing their life cycle is then a challenging issue. Several mechanical and thermal mechanisms are responsible for hot forging tools damage such as wear, thermal and mechanical fatigue. This work will be focused only on the mechanical fatigue life prediction for hot forging tools. Both experimental data analysis and numerical simulation will be discussed in this paper. The aim is to perform qualitative and quantitative indicators of mechanical fatigue. First, experimental data of fatigue tests are used to identify both plastic strain-based Manson Coffin and stress-based Basquin life laws for 2 tool steel grades. These laws are quite classical for fatigue prediction [1-4]. The half-life strain or stress amplitudes are usually used for their identification but these amplitudes are very expensive to obtain from a numerical point of view since it is well known that hot work martensitic steels present a continuous cyclic softening from the first cycle till the rupture. Therefore an important number of cycles have to be simulated to reach these mechanical parameters at half-life. For all theses reasons, an alternative methodology is used [4]. The fatigue life curves are established using the mechanical parameters that are identified from the first hysteresis loops of fatigue experiments. Comparisons are performed with the fatigue laws coming from more classical identification procedure performed at half life cycle. Good agreement is shown between experimental data and the new laws. A lower scattering is even observed in experimental results in comparison to the traditional fatigue laws. Then these new laws are introduced in the commercial software Forge® and are then applied to different industrial cases. A pretty good agreement is observed between predicted tool life and industrial value

Multiaxial fatigue criterion accounting for anisotropy in forged components

International audienceNumerical modelling of fatigue behavior for anisotropic structures has become critical for design applications. This is particularly true for forged components due to the intrinsic anisotropy of the material resulting from the process. The aim of this study is to relate the microstructure scale to the process scale, i.e. the engineer scale. Anisotropy induced by the forming process and the most relevant feature which results from forging, is the preferential orientation of structural defects and grains in the direction of the deformation. Grain flow is modelled using a fiber vector at the level of the representative elementary volume. It can then be used to improve and refine the Papadopoulos fatigue criterion by taking into account fatigue limits for each direction of anisotropy. In practice, it is very tedious to determine precisely these fatigue limits and impossible to obtain experimentally all of them for each direction of uniaxial loading. To circumvent this difficulty, we simulate the problem at the microstructure scale by considering fiber vector as the result of the inclusion and grain orientation. Microstructures are then precisely modelled using DIGIMICRO software. A representative elementary volume including inclusions is meshed and high cycle fatigue simulation is performed. The results can be used in order to optimize the preform of the component before simulation

EBSD coupled to SEM in situ annealing for assessing recrystallization and grain growth mechanisms in pure tantalum

International audienceAn in situ annealing stage has been developed in-house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high-purity tantalum deformed sample has been annealed at variable temperature in the range 750°C-1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single-phase material

Effect of grain orientation on the development of dislocation substructures during cold-deformation of pure Tantalum. Link with static recrystallization and recovery.

International audienceRecrystallization and recovery of pure polycrystalline tantalum are highly influenced by the intragranular dislocation structure developed during plastic deformation. A tantalum oligocrystal has been deformed by compression at room temperature. The resulting intragranular dislocation structures have been characterized using a FEG scanning electron microscope coupled with an EBSD system. Based on these experimental observations and crystal plasticity simulations, the development of dislocation substructures is related to the crystallographic stability of grain orientations