Thermoelastoplasticity applied to T-shaped tube hydroforming optimization

International audienceHydroforming simulation of T-shaped tube is performed using an thermoelastoplastic model implemented in Forge-2005© (fully implicit FEM code). A thick cylindrical AA-6060-T4 aluminium alloy tube is inflated in a T-shaped die. A third tool limits the material flow in the central branch during loading. An iterative scheme is used to obtain the load path. A three dimensional case of T-shaped hydroforming is proposed. This work studies the sensitivity of the process regarding to crucial parameters like, die/tube friction, tools kinematic, die radius, inner pressure law and material hardening. Impact on thickness repartition and plastic strain in the finale part is studied. An improved load path is proposed regarding to those aspects. In the same time the position and size of potentially dangerous zones regarding to plastic instability or damage are checked. Looking to the gradients of plastic strain and triaxiality [FORMULA] through the thickness, the interest of a 3D continuum damage tool to predict formability is enlightened

Sur les formulations élastoplastiques non locales en gradient d'endommagement

Il est bien établie que les résultats d analyse éléments finis de structures impliquant des comportements mécaniques adoucissants, souffrent d une dépendance à la discrétisation spatio-temporelle. Cet ouvrage à travers l étude de plusieurs formulations non locales propose d aider au choix de modèles aptes à assurer l indépendance vis-à-vis des aspects discrétisationels. Après une analyse des différentes formes de dépendance au maillage, les solutions existantes sont présentées. En se limitant volontairement aux cas des modèles élastoplastiques endommageables, plusieurs propositions sont spécifiquement étudiées. Les formulations sont présentées puis étudiées pour des cas de sollicitations simples de traction plane. Les aspects numériques et l implémentation dans l environnement du logiciel ABAQUS-Standard sont particulièrement détaillés. Plusieurs de ces formulations font appel à un élément non local spécifique dont la structure et l implémentation sont présentées. Pour chaque formulation étudiée, la capacité à contrôler la taille de la zone de localisation ainsi que la dépendance de la dissipation globale vis-à-vis du maillage sont étudiées. Enfin les limites théoriques et numériques de chaque formulation sont misent en lumière. Cette étude apporte des éléments scientifiques et pratiques pour le choix d une formulation élastoplastique endommageable assurant l indépendance des réponses éléments finis à la discrétisationFEM results of softening materials are well known to show pathological mesh dependency. The main goal of the present work, studying different types of formulation, is to give some elements to choose the better models able to avoid mesh sensitivity. A general study of the different sources of mesh and time dependency is carried out, and some existing solutions are presented. The case of elastoplastic damaging models with destination to industrial applications is specifically studied in the rest of the thesis. Formulations are presented and studied for a simple tension test, with various spatial discretisation. Numerical aspects and implementation in ABAQUS-standard environment is discussed. A new non local element is used for some of the formulations. The structure and details of this element are presented. For a given set of meshes, the ability of each formulation presented to control the size of the necking zone is studied. In the same time the independence of the global dissipation to the mesh size is checked. Practical and theoretical limits are highlighted. This study, gives some scientific and practical elements for the choice of a non local elastoplastic damaging model able to avoid discretisation sensitivity. For the end, the principal conclusions and the better developments in prospect are givenTROYES-SCD-UTT (103872102) / SudocSudocFranceF

COLD FORMING OF OXIDE DISPERSION STRENGTHENED STEELS

International audienceOxides Dispersed strengthened (ODS) stainless steels are foreseen for fuel cladding tubes in the coming generation of fission sodium cooled nuclear reactors. To achieve better creep performances their body-centered matrix is reinforced by a very fine oxides dispersion. Those grades are currently obtained by Powder Metallurgy (PM). After mechanical alloying with the oxide and the hot extrusion as a rough tube, the material is commonly cold formed (CF) to obtain a cladding tube. Many cold forming techniques has been proposed and among them, the industrial Mannesmann tube pilgering technique is one of the most promising. Using this technique, this study prospects the behavior of the ODS steel during various sequences of cold work and recovery Heat Treatments (HT). The ferritic/martensitic (9wtpercent Cr) structure of the grade induces phase transformation during high temperature heat treatments. Therefore, the way to take advantage of phase transformation to recover better microstructural and mechanical properties is under prospect. The present study focuses on the crystallographic texture evolution during various forming sequences. The microstructure is prospected by Scanning Electronic Microscopy and X-Ray Diffraction including synchrotron facilities. The Mannesmann tube pilgering process gives access to highly (up to 100percent) cold work tubes. Unexpectedly, a texture memory effect is observed for those tubes after heat treatment what was not reported previously. Using synchrotron facilities, in-situ heat treatments reveal the contribution of the process parameters on the occurrence and the intensity of this texture memory effect. Those data are of crucial interest for the design of further industrial cold forming sequences of ODS ferritic/martensitic steel

Texture evolution in Oxide Dispersion Strengthened (ODS) steel tubes during pilgering process

International audienceOxide Dispersion Strengthened (ODS) steels are foreseen as fuel cladding material in the coming generation of Sodium Fast Reactors (SFR). Cladding tubes are manufactured by hot extrusion and subsequent cold forming steps. In this study, a 9 wt% Cr ODS steel exhibiting α-γ phase transformation at high temperature is cold formed under industrial conditions with a large section reduction in two pilgering steps. The influence of pilgering process parameters and intermediate heat treatment on the microstructure evolution is studied experimentally using Electron Backscattering Diffraction (EBSD) and X-ray Diffraction (XRD) methods. Pilgered samples show elongated grains and a high texture formation with a preferential orientation along the rolling direction. During the heat treatment, grain morphology is recovered from elongated grains to almost equiaxed ones, while the well-known α-fiber texture presents an unexpected increase in intensity. The remarkable temperature stability of this fiber is attributed to a crystallographic structure memory effect during phase transformations

Numerical assessment of large hexagonal seamless steel tube extrusion feasibility

International audienceThe study assesses feasibility of hot extrusion of a large seamless hexagonal 9%Cr-1%Mo steel tube. The manufacturing chain starts from a pierced cylindrical billet, hot extruded and to be further cold drawn in several passes. Preliminary industrial tests have shown thickness reduction in extrusion limited by a huge initial force peak (+25 %) reaching the press capacity. To understand this force peak, thermomechanical numerical simulation (ForgeNxt®) of the furnace-press transfer and extrusion stages is carried out. Constitutive model at high temperature, high strain and strain rate has been selected from literature. Surface properties, namely Heat Transfer Coefficient (HTC) and friction coefficient, have been made space- and time-dependent to represent glass lubrication. Numerical results are qualitatively compared to industrial experimental values to evaluate the prediction of the model. It suggests that the difficult start of the glass melting and flow along a cooled die affects the force peak. Practical improvements are suggested on this basis, together with possible refinements of the simulation for more precision and insight into extruded tube quality

Numerical Simulation of the Crystallographic Texture Evolution during Hot Extrusion of Oxides Dispersed Strengthened Steels

International audienceOxides Dispersed Strengthened (ODS) stainless steels are foreseen for fuel cladding tubes in the coming generation of fission nuclear reactors. In spite of a bcc matrix, those steels present a convenient creep behavior thanks to very fine oxides dispersion. Those grades are currently obtained by Powder Metallurgy (PM). After mechanical alloying with the oxide, the powder is commonly consolidated as seamless tube. On CEA facilities, new ferritic ODS stainless steels are produced by Hot Extrusion (HE). The control of the microstructure after extrusion is a key issue for this grade regarding service conditions. In order to explain the microstructure induced by hot processing, the thermo-mechanical history applied to the material must be taken into account. In this study, the strain and thermal histories are obtained from Finite Element Method simulation. Thus, crystallographic texture development during hot extrusion of ODS ferritic steels is simulated using a Visco-Plastic Self-Consistent (VPSC) model. By comparing the texture predictions with the experimental observations, it is shown that self-consistent model reproduces the extrusion texture, α-fiber, very well in the case of monotonic loading. However, for complexes strain path observed during HE, VPSC results differ from the experimental deformation texture

Plastic instabilities analysis during T-shaped tubes hydro-forming process

International audienceDuring T-shaped or Y-shaped tubes hydroforming the tube wall is flatten on the external die. This "in die" forming involves an important compressive stress along the outer normal direction directly related to the inflating pressure. In this particular case plastic instabilities and damage evolution are strongly dependent on the sign of hydrostatic stress. Hydroforming simulations of T-shaped and Y-shaped thick tubes are performed using an elastoplastic model implemented in Forge-2005 (fully implicit Finite Element Method code). A three dimensional analysis is ran using tetrahedral solid elements. An augmented Modified Maximum Force Criterion coupled to an undamage behavior law is expressed in the sheet tangent plan basis. This criterion is compared to a fully coupled Lemaitre damage model. Due to low triaxiality state, damage only growth in specifical regions. This study highlights the interest of continuum damage modeling for the formability and the final part load carrying capacity prediction for "in die" hydroforming

High temperature EBSD experiment applied to in-situ observation of phase transformation in steels

The mechanical properties of steels strongly depend on the high temperature austenite microstructure and the subsequent phase transformation events occurring during cooling that control the final microstructure. Thus, the analysis of the austenite parent phase is key to improve our understanding of the inherited microstructures. Indirect parent-phase reconstruction has been introduced as a possibility to calculate the parent grain orientations from the orientation data of the low temperature phase, based on the orientation relation between the two phases [1]. Even if this approach often gives powerful information, it only works on displacive transformation microstructures and may fail for some specific configurations (ex. reconstruction of twin boundaries or parent grains transformed with strong variant selection).Nowadays, the development of compact hot stages compatible with EBSD working conditions as well as the emergence of high-speed EBSD camera open a promising way to track direct microstructure evolutions induced by phase transformations by high temperature EBSD analysis [2].In this work we have performed state of the art in-situ HT EBSD experiments on ferritic stainless steels (10 to 12%Cr) with and without ODS (Oxides Dispersed Strengthening). Our purpose is to monitor the ferrite to austenite transformation during heating to (1) identify the preferential nucleation sites of austenite according to the initial microstructure, (2) the presence of non-transformed ferrite and (3) characterize the HT austenite microstructure expected to be nanosized [3]. We aim to give a feedback on advantages and drawbacks of in- situ HT EBSD analysis applied to phase transformation and its complement to indirect crystallographic reconstruction.[1]Germain, L., N. Gey, R. Mercier, P. Blaineau, et M. Humbert. « An advanced approach to reconstructing parent orientation maps in the case of approximate orientation relations: Application to steels ». Acta Materialia 60, no 11 (2012): 4551‑62. https://doi.org/10.1016/j.actamat.2012.04.034.[2]Ubhi, H. S., J. Parsons, N. Othen, S. Campbell, R. Poole, et A. Gholinia. « In-situ EBSD phase transformation and recrystallisation ». Journal of Physics: Conference Series 522, no 1 (2014). https://doi.org/10.1088/1742- 6596/522/1/012011.[3]Durand, Anthony, Denis Sornin, Yann de Carlan, Gabriel Spartacus, François Brisset, Ludovic Delbes, Benoît Baptiste, Thierry Baudin, et Roland Logé. « Characterization of Untransformed Ferrite in 10Cr and 12Cr ODS Steels ». Materialia 16 (mai 2021): 101066. https://doi.org/10.1016/j.mtla.2021.101066