INCONEL 718 SINGLE AND MULTIPASS MODELLING OF HOT FORGING

10International audienceA better understanding of the competition between several mechanisms (dynamic recovery, dynamic recrystallization and plasticity hardening) is crucial for aircraft engine manufacturers. The aim of this paper is to improve the microstructure and therefore the mechanical properties of a nickel based superalloy used for rotating forged pieces. A nickel superalloy microstructure is the result of several successive hot forging processes: multipass processes, with intermediate dwell time and quenching. In this paper, an original three dimensional approach able to simulate these processes is proposed. The specific role of the different steps of the processes is analysed. In this approach, several forging thermo-mechanical parameters are taken into account: the working temperature, the strain rate, the final strain, the interpass time, etc. At high forging temperature, the studied INCONEL 718 presents an austenitic matrix γ (face centred cubic) assumed to be in a single phase. This approach proposes a sequential coupling of two models, one devoted to deformation and the other to recrystallization. Such a coupling enables the estimation of the effect of deformation and of different recrystallization types on mechanical behaviour and on micro-structural evolution. The approach is performed at the grain scale and takes into account the whole thermo-mechanical cycle with a focus on the dynamic behaviour. The first polycrystalline model is based on the plasticity mechanisms at the grains scale. The framework corresponds to finite transformations (large lattice rotations and small elastic strains). The model is implemented in ABAQUS and CAST3M finite element codes. The second model is based on the recrystallization theory and uses a 3D cellular automaton. It describes dynamic recrystallization phenomena such as nucleation-growth and static or post-dynamic recrystallization. Such recrystallization mechanisms were observed during interpass time or during the successive heatings depending on the thermo-mechanical paths used in multipass forging. Dislocation densities are the internal variables common to the two models. The simulations are performed on a 3D Representative Elementary Volume (aggregate) obtained from Electron Back Scattering mapping. Numerical results are compared to experimental microstructures

Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play a critical role in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is then employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models

Material characterization and finite element modelling of cyclic plasticity behavior for 304 stainless steel using a crystal plasticity model

Low cycle fatigue tests were carried out for a 304 stainless steel at room temperature. A series of experimental characterisations, including SEM, TEM, and XRD were conducted for the 304 stainless steel to facilitate the understanding of the mechanical responses and microstructural behaviour of the material under cyclic loading including nanostructure, crystal structure and the fractured surface. The crystal plasticity finite element method (CPFEM) is a powerful tool for studying the microstructure influence on the cyclic plasticity behaviour. This method was incorporated into the commercially available software ABAQUS by coding a UMAT user subroutine. Based on the results of fatigue tests and material characterisation, the full set of material constants for the crystal plasticity model was determined. The CPFEM framework used in this paper can be used to predict the crack initiation sites based on the local accumulated plastic deformation and local plastic dissipation energy criterion, but with limitation in predicting the crack initiation caused by precipitates

A new method to extend the stress response of triboluminescent crystals by using hydrogels

Polyacrylamide hydrogel entrapment of EuD4TEA or Cu(NCS)(py)2(PPh3) radically extends the emission time of the triboluminescent (TL) crystalline particles by a factor of 103, optimized when matching the hydrophilic/hydrophobic characteristics of the TL/gel components. Triboluminescence intensity improves with hydration of the TL/hydrogel composite. The composites may be used in impact-related sensor applications

Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single-crystal tungsten strength

Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of ?111?{110} and 111 {112} slip systems in the homologous temperature range from 0.08Tm to 0.33Tm, where Tm =3680 K is the melting point in tungsten.</p

Modeling of deformation and rotation bands and of deformation induced grain boundaries in IF steel aggregate during large plane strain compression.

A computation using crystal plasticity modeling of an actual IF steel aggregate plane strain compression deformation, underlines the formation of different deformation bands morphologies and grain splitting occurrence, already experimentally observed by different authors. The model based on dislocation densities as internal variables, developed in the framework of finite deformation and implemented in the Finite Element Method, is able to capture the main characteristics of different inhomogeneities and to analyze their formation and further development with strain, from the determination of the active and latent slip systems, and also from the quantification of their dislocation densities and corresponding glide rates evolutions. The respective boundary conditions and material properties effects are discussed. (C) 2003 Elsevier Ltd. All rights reserved

Etude expérimentale et analyse numérique de l'influence des hétérogénéités induites par la déformation à froid sur la recristallisation primaire d'un acier IF-Ti

Ce travail se situe dans la thématique de l'influence des procédés de mise en forme sur le comportement des matériaux métalliques. La thèse a consisté à étudier le développement d'hétérogénéités induites par la déformation au cours du laminage, ainsi que leur effet sur la germination et la croissance de nouveaux grains. La caractérisation du comportement mécanique et de recuit a permis d'identifier les lois de comportement et de recuit. L'étape suivante a consisté, à l'aide du maillage 3D d'agrégats cristallins et d'une loi de comportement du monocristal intégrée dans la méthode des éléments finis, à étudier l'influence de l'interaction intergranulaire 3D, en compression plane, sur le développement des hétérogénéités. Cet effet est faible, d'où la possibilité d'effectuer des calculs 2D représentatifs. Pour simuler la recristallisation, un intérêt a été porté à l'estimation de l'énergie stockée. Il est montré que le travail plastique ne représente pas cette énergie, au contraire de la densité de dislocations. Différents types d'hétérogénéités ont ensuite été étudiés, à l'aide de calculs éléments finis, et ont été comparés à des résultats de la littérature. Il a été montré qu'une forte déformation locale n'est pas toujours associée à une forte énergie stockée, ou à un fort gradient d'orientation. L'effet résultant en recristallisation est discuté. Ensuite, une comparaison du champ d'orientation a été effectuée entre les déformations expérimentale et numérique d'un agrégat, et un très bon accord est trouvé. Le recuit expérimental de l'agrégat est effectué, et les microstructure et texture résultantes sont comparées aux résultats d'un calcul Monte Carlo prenant les résultats éléments finis comme données initiales. Un accord qualitatif est obtenu, et permet de dégager l'importance du gradient d'énergie, d'orientation et du voisinage sur la germination et la croissance des grains, et sur le développement des texture et microstructure recristallisées.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Finite element simulation and experimental validation of the local texture of an IF steel aggregate submitted to channel die compression

An experimental deformation analysis of a grain aggregate is presented as well as a finite element method (FEM) which enables to compute some particular quantities which cannot be easily obtained experimentally, like the stress state and the stored energy of cold work. Comparisons are made between OIM measurements after deformation and FEM results performed on a mesh that matches exactly the original grain shapes and orientations before deformation. A very good agreement is found, especially intragranular microstructures are well predicted