Optimization of Laser Metal Deposition Process for 2205 Duplex Stainless Steel

This work aims to optimize the process parameters for laser based DED manufacturing of a water jet impeller with critical requirements to material and mechanical properties. The concerned material is 2205 duplex stainless steel with balanced ferrite-austenite microstructure. The optimization approach of the processing parameters relies on the assessment of their effect on microstructure and mechanical properties versus the requirements set by the maritime industry. The work aims to achieve an as-built microstructure with the required ferrite-austenite balance and mechanical properties, without the need for post-processing heat treatment. The work particularly focuses on the influence of the deposition speed of the DED process. The results show that duplex stainless steel with a 50-50 ferrite-austenite balance can be achieved directly from the DED process. A high deposition speed produced fine-grained microstructure resulting in a high tensile strength and toughness, well above the set requirements. However, it reduced the ductility, represented by tensile elongation due to the formation of welding defects. Reducing the deposition speed by 20% eliminated the welding defects but resulted in the formation of a distinct microstructure with coarse grains, elongated in the deposited layer. This microstructure improved the tensile elongation, but strongly reduced the toughness, represented by Charpy V impact energy values. The coarse grains in the deposited layer facilitated a fast fracture propagation initiated by the placement of the Charpy V notch. However, the presented results demonstrate the great potential for manufacturing duplex stainless steels by DED, where a suitable microstructure for optimal mechanical performance can be obtained by narrowing the optimization windows on the process parameters.publishedVersio

Optimization approach of DED process to fulfil the requirements on material properties and component performance of waterjet impeller

Directed energy deposition (DED) processes have a great potential for ship building industry to reduce the lead time of ship part supply. However, ship parts such as propellers and impellers have critical requirements on material's properties and component's performance. To ensure the exploitation of DED manufacturing for critical parts, an optimization approach of DED process was developed and demonstrated on waterjet impeller made of duplex stainless steel. The developed approach is based on the understanding of the effects of DED process and heat treatment on the material microstructure, and the effects of the microstructure on the material properties, all optimized to fulfil the requirements on the component. The manufacturing and qualification testing were planned based on the requirements in accordance with the current standard guidance. The assessments by standard testing methods, including tensile, Charpy impact, and bending tests demonstrates that the DED material with an optimized heat treatment fulfil the standard requirements. Corrosion and cavitation tests were also performed and demonstrates the high resistances of DED material to cavitation erosion.publishedVersio

Modèle physique de la plasticité d'un cristal métallique CFC soumis à des chargements alternés :<br />Contribution à la définition d'une modélisation multiéchelles de la mise en forme des métaux.

This work is realised in the perspective to contribute to the development of physical based models of plastic deformation that are intended to be used in numerical simulations of FCC metals forming operations. In this purpose, we propose a micromechanical modelling making use of micro structural variables such as dislocation densities and their distributions on each slip system. Physical models of isotropic and kinematics work hardening are proposed in order to simulate alternated (cyclic) loads. In addition to define particular model of metal behaviour, this work presents various steps that lead to use it in a macroscopic numerical simulation. This consists in the definition of experimental procedure which provides necessary data to validate the model. This experimental procedure makes use of Digital Image Correlation (DIC) and Infrared (IR) images processing to measure simultaneously heterogeneous displacements and temperature fields at the samples surface. Physical based model is then implemented in a commercial finite element code “ABAQUS” in order to simulate complex boundary conditions which can be found in metal forming operation. In spite of the fact that the modelling of such boundary conditions is not realised in this work, this model presents high potential to make it.Les opérations de mise en forme (particulièrement l'emboutissage) font subir au métal des chargements alternés avec de grandes amplitudes de déformation, le plus souvent suivis de changements de trajets. Une simulation numérique de ces opérations nécessite l'utilisation d'un modèle du comportement qui puisse rendre compte efficacement des trajets de chargements complexes. Cette thèse est une contribution à l'établissement d'un modèle de comportement plastique destiné à être utilisé pour la simulation numérique des opérations de mise en forme. Le parti a été pris de recourir à un modèle micromécanique dont les variables sont des paramètres micro-structuraux intrinsèques comme les densités de dislocations et leurs distributions sur chaque système de glissement. Le modèle traite des chargements alternés dans un cadre physique unifié défini pour l'ensemble des équations constitutives décrivant l'écrouissage. Au-delà de l'intérêt de définir un modèle particulier, ce travail apporte une contribution à la mise en œuvre des différentes étapes qui doivent conduire à son utilisation dans une simulation numérique macroscopique. Il s'agit de définir un protocole reproductible, plutôt indépendant du modèle considéré et exploitable pour d'autres modélisations. En particulier, des protocoles expérimentaux sont définis pour produire des données dans des conditions reproductibles. Des moyens de caractérisation récents comme l'analyse d'images dans les domaines visible et infrarouge sont utilisés pour étudier la réponse expérimentale thermomécanique de différents échantillons à caractère fortement cristallin. Ensuite, l'implantation du modèle dans un code éléments finis commercial permet d'envisager des simulations complexes qui, si elles ne s'adressent pas actuellement à des opérations d'emboutissage, comportent toutefois les ingrédients pour les traiter. Ce travail, à l'interface de nombreux champs disciplinaires, apporte donc une contribution au développement des modèles de comportement plastique des matériaux métalliques qui porte sur l'ensemble des briques nécessaires au développement de simulations numériques utilisant des modèles à caractère physique

Assessment of advanced Taylor models, the Taylor factor and yield-surface exponent for FCC metals

High-resolution crystal plasticity-finite element method (CPFEM) simulations are performed to provide new reference values of the Taylor factor M and the isotropic yield surface exponent for high stacking fault energy face-centred-cubic (FCC) polycrystalline metals with random orientations. The visco-plastic Taylor factor with strain rate sensitivity is introduced and linearly extrapolated to its zero strain rate sensitivity limit to give the new reference value of M. The linear extrapolation technique is also employed to define the new reference value of . The obtained new reference values are 2.7 and 6.9, for M and , respectively, which are much smaller than the reference values currently used for FCC materials based on full constraint (FC) Taylor model calculations, i.e. 3.07 for M and 8 for a. Other state-of-the-art Taylor-type models, e.g. ALamel, ALamel with the type III relaxation (ALamel-T3) and the visco-plastic self-consistent (VPSC) models, can also give values for M and a much smaller than the FC-Taylor calculations. The performance of the CPFEM and these state-of-art Taylor-type models in terms of resolving deformation and stress fields within the aggregate can only be assessed in a statistical manner since all are statistical aggregate models. Selected statistical distributions are analysed for all models, by means of local deviations of the velocity gradient tensor, of the plastic deformation-rate tensor and of the stress tensor etc., for uniaxial tensile deformation. The ALamel models are found to provide similar statistics as CPFEM, whereas the VPSC model results are qualitatively different. The intra-grain analysis for CPFEM demonstrates that the intra-grain interactions are as much important as the local interactions at the grain boundaries.acceptedVersio

Modèle physique de la plasticité d'un cristal métallique CFC soumis à des chargements alternés (contribution à la définition d'une modélisation multiéchelles de la mise en forme des matériaux)

CHAMBERY -BU Bourget (730512101) / SudocSudocFranceF

A fundamental model of aluminum single crystal behavior with physical description of kinematic work hardening

International audienc

Experimental and numerical study of the thermo-mechanical behavior of Al bi-crystal in tension using full field measurements and micromechanical modeling

International audienceValidation and identification of micromechanical models are very exacting and sensitive tasks. The experimental identification process must provide the necessary data to verify that the modeled global behavior of the material and the corresponding changes in its microstructure are correct. This work describes an experimental procedure to validate micromechanical models focusing on their global and local thermo-mechanical responses. The procedure is then applied to evaluate the performance of a micromechanical model used in a macroscopic tensile testing simulation of a flat Al bi-crystal sample. Digital Image Correlation (DIC) and Infrared (IR) image processing are simultaneously used to measure heterogeneous displacement and temperature fields on the sample surface. The experimental data are used to validate a finite element simulation in which a dislocation-based model of FCC single crystal behavior is implemented. In addition to the verification of modeled macroscopic behavior through a global loading curve, experimental displacement and temperature fields are used to verify the ability of the model to match the local heterogeneous experimental response in each grain. Mechanical dissipated powers estimated from temperature fields are compared to the dissipated powers computed from local mechanical variables (shear stress, shear strain on activated slip systems, and dislocation density). Considering that the simulated strain fields are directly validated by comparison with results provided by DIC, the dissipated powers are used to validate indirectly the computed stress fields in each grain of the sample

Assessment of advanced Taylor models, the Taylor factor and yield-surface exponent for FCC metals

High-resolution crystal plasticity-finite element method (CPFEM) simulations are performed to provide new reference values of the Taylor factor M and the isotropic yield surface exponent for high stacking fault energy face-centred-cubic (FCC) polycrystalline metals with random orientations. The visco-plastic Taylor factor with strain rate sensitivity is introduced and linearly extrapolated to its zero strain rate sensitivity limit to give the new reference value of M. The linear extrapolation technique is also employed to define the new reference value of . The obtained new reference values are 2.7 and 6.9, for M and , respectively, which are much smaller than the reference values currently used for FCC materials based on full constraint (FC) Taylor model calculations, i.e. 3.07 for M and 8 for a. Other state-of-the-art Taylor-type models, e.g. ALamel, ALamel with the type III relaxation (ALamel-T3) and the visco-plastic self-consistent (VPSC) models, can also give values for M and a much smaller than the FC-Taylor calculations. The performance of the CPFEM and these state-of-art Taylor-type models in terms of resolving deformation and stress fields within the aggregate can only be assessed in a statistical manner since all are statistical aggregate models. Selected statistical distributions are analysed for all models, by means of local deviations of the velocity gradient tensor, of the plastic deformation-rate tensor and of the stress tensor etc., for uniaxial tensile deformation. The ALamel models are found to provide similar statistics as CPFEM, whereas the VPSC model results are qualitatively different. The intra-grain analysis for CPFEM demonstrates that the intra-grain interactions are as much important as the local interactions at the grain boundaries