Modélisation de la fabrication directe de pièces par projection laser : application au Ti-6Al-4V

International audienceLa projection laser permet de fabriquer de façon relativement simple des pièces complexes, dont les dimensions sont proches des cotes finales. Le procédé s'accompagne néanmoins d'importantes variations locales de température, à l'origine d'une microstructure hétérogène, et de contraintes résiduelles difficiles à maîtriser. Une bonne manière d'aider au développement du procédé est de mettre en place sa simulation numérique, afin de prévoir la microstructure et les contraintes résiduelles. Des conditions laser adaptées et une stratégie de balayage optimisée doivent permettre d'obtenir la microstructure désirée et des contraintes résiduelles minimisées

Couplage plasticités/endommagement fondé sur la décomposition en modes de Kelvin pour les superalliages monocristallins

Le comportement anisotrope des superalliages monocristallins CFC doit être pris en compte lors de l'étude de la durée de vie des aubes de turbine. La prise en compte des symétries matérielles de la famille d'alliage considérée permet de proposer à la fois un critère de plasticité macroscopique et une contrainte effective en utilisant la décomposition spectrale en modes de Kelvin du tenseur d'élasticité de Hooke. Une loi d'endommagement incrémentale anisotrope peut alors être couplée à différents modèles de plasticité. Elle permet le dimensionnement en fatigue-fluage

High temperature tensile properties of β-γ-γ\u27-MCrAlY and β-Ni(Al,Pt) bond-coatings and interdiffusion zone with Ni-based single crystal superalloys

MCrAlY overlay coatings and Pt-modified aluminide diffusion coatings are commonly used in thermal barrier coating (TBC) systems for turbine blade and vane applications. Purposely designed for oxidation and corrosion protection, MCrAlY and aluminide coatings have a ductile-to-brittle transition temperature (DBTT) of about 600 to 800°C, i.e. in the temperature range of service conditions. Therefore, these coatings can be a source of premature crack initiation under thermomechanical loading at low/intermediate temperature. They also creep at high temperature. This drastic change in local mechanical properties significantly impair the structural integrity of such multi-layered materials. Current damage-tolerant design of TBC systems preferentially deals with DBTT than with the effective temperature- and time-dependent mechanical properties of the individual layers constituting the TBC systems. Data on high temperature properties of the bond-coatings and the interdiffusion zone with the substrate are lacking. Indeed, these local properties are particularly difficult to assess up to 1100°C, both using freestanding-layer[1-4] or multi-layer specimen approaches[5]. Improvements in the prediction of the mechanical behavior and the lifetime of TBC systems require the understanding and the quantification of such local mechanical properties. Please click Additional Files below to see the full abstract

Fatigue life and initiation mechanisms in wrought Inconel 718 DA for different microstructures

International audienceWrought Inconel 718 DA superalloy disk zones present a wide range of behavior in fatigue life due to the variability of the microstructure. In order to link the effect of the forging conditions and achieved microstructure to the fatigue life, two microstructures have been tested in fatigue. Fatigue tests under strain control were performed at 450°C. Grain size distributions and phase distributions were characterized in the specimens and related to fatigue failure initiation modes. Fatigue crack initiation was seen to occur on large grains in stage I for the larger grain material whereas for the material with slightly smaller grains initiation from internal nitrides caused failure via so-called fish-eye cracks. The different steps of these failure modes are discussed using data from the literature to gather the ingredients for a quantitative assessment of the fatigue lifetime using fracture mechanics

High temperature micromechanical behavior of a Pt-modified nickel aluminide bond-coating and of its interdiffusion zone with the superalloy substrate

The micromechanical properties of a b-(Ni,Pt)Al bondcoating was investigated between 700°C and 1000°C using ultrathin freestanding bond-coating specimens. Its brittle-to-ductile transition temperature was close to 750°C, with a significant ductility above 800°C (up to 23 pct at 1000°C). The tensile strength decreased from 450 MPa at 750°C down to 50 MPa at 1000°C. Fractographic observations evidenced the material brittleness at intermediate temperature with large cleaved grains and its ductility above 750°C with important necking of individual grains

Essais biaxiaux de comportement sur Inco718DA pilotés avec CIN

Pour mieux caractériser et dimensionner les matériaux métalliques pour disques Snecma développe des modèles de comportement, des lois d'endommagement et des critères de rupture et de fatigue mieux adaptés aux sollicitations multiaxiales que rencontrent les pièces en service. Suivant cette démarche une étude a été lancé ayant comme but le développement d'un modèle de plasticité adapté au comportement de l'Inco718DA, un alliage utilisé largement dans la fabrication des disques de turbines, et de le valider sous conditions multiaxiales proches du domaine de fonctionnement du moteur. Ce travail se focalise sur le développement des essais de comportement cycliques faits sur des éprouvettes cruciformes en utilisant la Corrélation d'Images Numériques pour piloter la machine

Simulation des effets d'échelle et de Kt sur les conditions d'arrêt d'une fissure de fatigue et sur la limite d'endurance apparente

Un critère de non-propagation étendu aux fissures courtes a été développé dans une précédente étude. Le travail actuel utilise ce critère pour prédire les évolutions du rapport d'ellipticité d'une fissure se propageant dans un gradient de contrainte et de son seuil de non-propagation à partir de calculs éléments finis. Les résultats montrent, en plus des effets de gradients, un effet d'échelle en accord avec les résultats expérimentaux. Cette étude pourra être étendue aux cas de chargements complexes présentant des surcharges

Multiaxial fatigue analysis of a high performance Nickel-based superalloy

Abstract—Over the past four decades, the fatigue behavior of nickel-based alloys has been widely studied. However, in recent years, significant advances in the fabrication process leading to grain size reduction have been made in order to improve fatigue properties of aircraft turbine discs. Indeed, a change in particle size affects the initiation mode of fatigue cracks as well as the fatigue life of the material. The present study aims to investigate the fatigue behavior of a newly developed nickel-based superalloy under biaxial-planar loading. Low Cycle Fatigue (LCF) tests are performed at different stress ratios so as to study the influence of the multiaxial stress state on the fatigue life of the material. Full-field displacement and strain measurements as well as crack initiation detection are obtained using Digital Image Correlation (DIC) techniques. The aim of this presentation is first to provide an in-depth description of both the experimental set-up and protocol: the multiaxial testing machine, the specific design of the cruciform specimen and performances of the DIC code are introduced. Second, results for sixteen specimens related to different load ratios are presented. Crack detection, strain amplitude and number of cycles to crack initiation vs. triaxial stress ratio for each loading case are given. Third, from fractographic investigations by scanning electron microscopy it is found that the mechanism of fatigue crack initiation does not depend on the triaxial stress ratio and that most fatigue cracks initiate from subsurface carbides

Low cycle biaxial fatigue behavior of direct aged Nickel-based 718 superalloy

In recent years, significant advances in the fabrication process of nickel-base supperalloy leading to grain size reduction have been made in order to improve fatigue properties of aircraft turbine discs. Indeed, a change in particle size affects the initiation mode of fatigue cracks as well as the fatigue life of the material. The present study aims to investigate the fatigue behavior of a newly developed nickel-based superalloy under biaxial-planar loading. Low Cycle Fatigue (LCF) tests are performed at different stress ratios to study the influence of the multiaxial stress state on the fatigue life of the material. Full-field displacement and strain measurements as well as crack initiation detection are obtained using Digital Image Correlation (DIC) techniques. Results related to different load ratios are presented and an appropriate biaxial lifetime prediction is given. Crack detection, strain amplitude and number of cycles to crack initiation vs. triaxial stress ratio for each loading case are mentionned. From fractographic investigations by scanning electron microscopy it is found that the mechanism of fatigue crack initiation does not depend on the triaxial stress ratio and that most fatigue cracks initiate from subsurface carbides