Two aerospace superalloys, Inconel 718 and IMI 318, were machined with different grades of PVD (KC730 and KC732) and CVD (KC950) coated tools in order to evaluate their performance under various cutting conditions and to further investigate the effect of the machining conditions on surface finish and surface integrity of the work materials. A self-propelled rotary tool was also developed and used for machining under the finishing conditions. Tool wear, component forces and surface roughness were recorded and analysed during the machining trials. Study of the surface integrity involved physical as well as metallographic examination and analysis of the machined surfaces. The results of the machining trials show that the multi-layer (TiN/TiCN/TiN) PVD coated KC732 tools gave the best overall performance when machining both Inconel 718 and IMI 318, especially at lower feed conditions. Flank wear, excessive chipping, flaking of tool materials close to the cutting edge or on the rake face were the dominant failure modes when machining with the PVD coated tools while flank wear and notching were dominant when cutting with the CVD coated tools. These failure modes are associated with attrition, abrasion, diffusion and plastic deformation wear mechanisms acting individually or in combination during machining. The statistical regression analysis of the tool life data shows that wear of the PVD and CVD coated tools used for machining Inconel 718 was mainly affected by cutting speeds employed while cutting speed and feed rate exhibited similar influence on tool performance when machining IMI 318 with PVD coated tools. Tool life equations for each of the three coated grades when machining both superalloys under the cutting conditions investigated were derived. Severe plastic deformation and hardening of the machined surfaces occurred after machining both materials due to a combined action of increased component forces, thus increased stresses, and high temperature. Softening of the top surface layer when machining IMI 318 can be attributed to overaging of the titanium as a result of highly localised surface heating during machining. Tearing of the machined surfaces occurred when machining IMI 318 with the PVD coated tools, particularly with KC732 tools as a result of irregular flank wear and excessive chipping of KC732 tools. The self-propelled rotary tool (SPRT) incorporating K68 straight grade carbide exhibited superior wear-resistance when machining IMI 318 due to the absence of thermally related wear mechanisms caused by reduced temperature and the use of the entire edge of a round insert during rotary cutting. The minimal subsurface alterations (such as plastic deformation and hardness) when machining Inconel 718 and IMI 318 with the SPRT can also be attributed to lower cutting temperature with rotary action.

The thesis work discuss about how the materials mechanical properties are influenced by the microstructure. The most common way of altering the microstructure of the material is by heat treatment.The mechanical properties that are of interest are strength, toughness, ductility, creep and fatigue. The material under consideration iswidely used superalloy In718. Two different sets of specimens areheat treated at different temperatures and influence of heat treatmenton the grain size is analyzed. In order to get better understanding ofthe grain size on mechanical properties, microstructural investigation was done using SEM. Efforts are made to understand the influence of different elements on the overall characteristic of the material. The tensile, creep and stress relaxation tests were conducted and the results were discussed. / Master Thesis

The aim of this work is to characterize the microstructural and mechanical behaviors of Additive Manufactured (AM) Inconel 718 and investigate the feasibility of using this material to produce thermal management devices. This was done in two parts: a literature review of AM phenomena that effect heat transfer rates and impede or benefit production using these methods, and a study that characterized Laser Engineered Net Shaping (LENS) Inconel 718 while investigating and manipulating the thermal history. The literature review provides evidence that while there are still many unsolved issues, it’s quickly becoming possible to use AM to produce thermal management devices that will exceed current limitations. The study showed that although LENS Inconel 718 exhibits nonstandard material behaviors, evidence indicate that it’s possible to control these behaviors to influence desired results. Overall, it is believed that the use of AM in producing thermal management devices will be inevitable and beneficial.

Inconel 718 is a nickel-based superalloy with a series of superior properties, such as high strength, creep-resistance, and corrosion-resistance. Additive manufacturing (AM) is appealing to Inconel 718 because of its near-net-shape production capability to circumvent poor machinability. However, AM parts are prone to detrimental porosity, reducing their fatigue resistance. Thus, further understanding of AM fatigue behavior is required before widespread industrial use. The microstructural and fatigue properties of heat treated AM Inconel 718, produced using Laser Engineered Net Shaping (LENSTM), are evaluated at room and elevated temperatures. Fully reversed, strain-controlled fatigue tests were performed on cylindrical specimens at strain amplitudes of 0.001 to 0.01 mm/mm. Fracture surfaces were inspected using a scanning electron microscope (SEM). Heat treatment caused initial dendritic microstructure to mostly reorm into an equiaxed grain structure. AM specimens experienced reduced fatigue life in testing as compared to wrought material due to inclusions or pores near the surface

Cette étude financée par SNECMA et le CNES a pour objectif de mieux comprendre et caractériser le résultats de la solidification de l'inconel 718. Cet objectif est motivé par le besoin grandissant de fiabilité sur les pièces brutes de fonderies utilisées dans l'assemblage des moteurs de fusée. En effet, lors de la solidification, la ségrégation chimique provoque la formation de phase fragile. C'est notamment le cas des phases de Laves. Ainsi, un four de fonderie sous vide et un moule instrumenté ont été mise au point afin de couler au laboratoire des plaques de géométrie variable. Le but étant d'obtenir plusieurs échantillons obtenu sous différente conditions de solidifications. Une méthode d'analyse quantitative au MEB a par la suite été mise au point pour analyser les 4 coulées exploitables. Elle a permis de quantifier la quantité d'intermétalliques en fonction de la vitesse de refroidissement. Mais aussi leur répartition vis à vis des joints de grains d'une part et l'impact de la désorientation entre chaque grain d'autre part. La ségrégation chimique a aussi pu être quantifiée selon ces mêmes paramètres.De travaux numériques ont été entrepris en parallèles avec tout d'abord la simulation macroscopique des coulées avec le logiciel Thercast. Ces simulations ont permis de connaître les conditions de solidification de l'ensemble des échantillons coulés et ont fourni des données d'entré pour les autres aspects numériques. Par exemple, avec un modèle élément fini et automate cellulaire, la formation de la structure granulaire a pu être modélisée. Enfin, la simulation de la microségrégation a permis de valider notre compréhension des phénomènes prédominant lors de la solidification de l'inconel 718. Ces simulations numériques apportent à la fois un éclairage sur les résultats expérimentaux mais aussi démontrent leurs possibilités dans l'optique de mettre en place un modèle global de solidification recouvrant ces trois échelles pour des pièces industrielles / The metallic alloy “Inconel 718” is frequently found in aerospace and aeronautic components. In this study, it is used in the case of the main engine turbopump of the Ariane 5 launcher. This case is obtained by investment casting. Thereby, a great focus is turned on as cast defect like detrimental interdendritic phase. For example, the Laves phase, about 1%, has poor mechanicals characteristics. Several samples was cast under vacuum at MINES ParisTech Cemef. Various superheat and cooling rate are tested. After specific preparation, the microstructure was observed by scanning electron microscopy (SEM). On a surface representative of solidification phenomenon, the NbC phase, the Laves phase and the Delta phase were quantified and chemical segregation measurements were performed. The results give a tendency of the detrimental phases to form at the grain boundaries. This phenomenon is stronger as the cooling speed is lower. Furthermore, the grains boundaries disorientations as an impact. This experimental information is used to model the sample solidification at three different scales. At the macroscopic scale with Thercast software, at an intermediate scale whit the simulation of the grain structure. These simulations were performed whit a model developed at Mines Paristech, CEMEF. The last scale is the simulation of the microsegregation. This model takes into account thermodynamic equilibrium, cooling rate and microstructure geometries. This numerical work improved the comprehension of the solidification phenomenon. A global solidification simulation of industrial piece is the aim of Cnes (French space agency) and Snecma who support this work.

L’Inconel 718 est un superalliage base nickel, élaboré dans les années 60, utilisé dans la fabrication de pièces pour les parties chaudes des moteurs d’avion. Il acquiert ses propriétés mécaniques et sa microstructure finale au cours du procédé de mise en forme appelé forgeage à chaud. La maîtrise de ce procédé nécessite de comprendre l’interaction entre les phénomènes d’écrouissage et de recristallisation dynamique tout en intégrant l’influence de diverses conditions thermomécaniques. Cette étude s’est focalisée, expérimentalement, sur les phénomènes liés au forgeage à chaud en mise en forme unipasses et multipasses super-δ-solvus (1050 °C). Afin de les caractériser, des essais de compression ont été réalisés à l’échelle de pions. Des trempes à l’hélium, après déformation, ont permis de figer les microstructures dans le but de comprendre leur évolution en fonction des paramètres thermomécaniques (ε, et T). Des observations ont ensuite été réalisées expérimentalement : microscopie optique et à balayage, EBSD et diffraction des neutrons. Une attention particulière a été portée sur l’évolution de la phase δ, influençant indirectement les propriétés mécaniques de l’alliage, au cours de traitements thermiques puis thermomécaniques. La quantification ainsi que la détermination des cinétiques d’évolution statiques et dynamiques de cette phase a permis de mieux comprendre son influence au cours du forgeage sub-δ-solvus (980 °C). Un chaînage séquentiel a été développé entre deux modèles, l’un de plasticité cristalline implémenté dans un code éléments finis (CPFEM) et l’autre de recristallisation, implémenté dans un code automates cellulaires. Ce chaînage séquentiel permet de décrire les évolutions de champs mécaniques et de microstructures au cours du forgeage à chaud et a été validé par une comparaison avec les résultats expérimentaux. MOTS CLÉS : forgeage à chaud, Inconel 718, recristallisation dynamique, mise en forme multipasses, phase δ, plasticité cristalline (CPFEM), automates cellulaires, chaînage séquentiel, agrégats polycristallins 3D. / Developed in the 60’s, the nickel-base superalloy, Inconel 718, is widely used for hot parts of aircraft engines. The hot forging process confers to the alloy its final microstructure and its mechanical properties. The control of the process requires a deep knowledge of the interactions between the hardening phenomena and the dynamic recrystallization for the various thermomechanical conditions which are used. The present study mainly focuses on the experimental characterization of the phenomena linked to hot forging in the super-δ-solvus domain (1050 °C), as well for a single pass process as for a multipass one. Hot compression tests are used to simulate forging. After deformation, samples are helium quenched in order to freeze the microstructure that allows understanding its evolution as a function of the thermomechanical parameters (ε, and T). Microstructure analyses have then been performed using optical and scanning microscopy, EBSD, and neutron diffraction. A specific attention is paid on the study of the -phase evolution as it has a direct influence on the mechanical properties of the alloy. Its evolution is followed along thermal and thermomechanical treatments. The measure of the static and dynamic precipitation kinetics has led to a better understanding of the -phase role during hot forging at temperatures below solvus (980 °C). A sequential coupling is developed, based on two models; the first one is a crystal plasticity model implemented in a finite element code (CPFEM), the second one being a modeling of recrystallization using a cellular automata approach. The coupling allows the evolutions of the mechanical fields and the microstructure to be simulated during hot forging. The numerical results fit correctly most of the experimental data, mechanical and structural.

Introduction Annealed Inconel 718 alloy was chosen for the beam window at the Los Alamos Neutron Science Center (LANSCE) Isotope Production Facility (IPF) [1]. The window was replaced after 5 years of operation. Mechanical properties and microstructure changes were measured to assess its expected lifetime. Material and Methods A cutting plan was developed based on the IPF rasterred beam profile (FIG. 1). 3-mm OD samples were cut out from the window and thinned to 0.25-mm thick. Shear punch tests were per-formed at 25 °C on 21 samples to quantify shear yield, ultimate shear stress, and ductility. From 1-mm OD, 0.25-mm thick shear punched out disks, 4 TEM specimens of ~30×10×2 μm were obtained using standard FIB lift-out techniques. TEM was performed on an FEI Tecnai TF30-FEG operating at 300 kV. Results and Conclusions TABLE 1 shows MCNPX tally results of accumulated dpa, He and H content from both protons and neutrons fluences and ANSYS steady-state irradiation temperature for the 3-mm OD samples [2]. These peak values are at the peak density of Typically increases in shear yield and shear maximum stress occur with increasing dose. In this case, highest shear yield and ultimate stress was on the lowest dose samples at the outer edge (FIG. 2). Optical microscopy images of the fracture surfaces on the shear punched out disks show no significant change in the fracture mode or reduction in ductility in the un-irradiated, high and low dose irradiated samples. One un-irradiated and 4 irradiated samples (5, E, 16 and 19) were selected for TEM analysis. Figure 3 shows bright field TEM images of an un-irradiated, high and low dose irradiated samples. Un-irradiated sample shows some dislocations and some large precipitates. The high dose sample #5 (~11 dpa, 122 oC) shows small loops and dislocations (left and center images) and no γ\' or γ\'\' precipitates in SAD from z = [011] (right image). Low dose sample #19 (~0.7 dpa, 40 oC) shows a high density of dislocation loops (left image), high density of H/He bubbles (center image) and presence of γ\'\' precipitates in SAD from z = [011] (right image). Radiation induced-hardening is highest at the low dose region in the outer most edge. The hardening from γ\'\' precipitates is determined to be more pronounced than that from trapped bubbles. The lack of significant hardening in the highest dose region is attributed to a lower dis-location density and no γ” precipitates or bubbles [3]. Identification of H or He bubbles and the higher accumulation of these bubbles in the low dose region (no direct beam hitting) warrant further studies. Despite the evidence of irradiation-induced hardening, this spent beam window appears to retain useful ductility after 5 years in service. At the conclusion of 2013 run cycle, the current in-service beam window had reached the same dpa as of the spent window. We plan to extend the service of the current in-service window until it reaches its intended design threshold limit of ~20 dpa (in the highest dose region). Additional measurements at higher dpa values will enable better decision-making in managing risks of the window failure.

L'alliage 718 est un superalliage à base de nickel très utilisé par l'industrie dans la mesure où il offre de nombreuses latitudes en termes d'optimisation de propriétés en regard des conditions d'utilisation. L'industrie aéronautique, plus particulièrement les motoristes comme SNECMA, l'utilise pour réaliser les disques de turbine des turboréacteurs. Ces disques sont forgés puis subissent un traitement thermique dit « Direct Aged » qui confère à l'alliage les propriétés mécaniques requises pour cette application. Cependant, lorsque l'on dépasse 550°C, l'alliage 718 présente, en fonction de son histoire de fabrication, une sensibilité plus ou moins importante à la fissuration assistée par l'oxydation. Cette étude se propose donc d'aborder la problématique de l'amorçage de fissures dans diverses conditions de chargement pour la microstructure DA. Pour mener ce travail, une géométrie inhabituelle d'éprouvette est utilisée : l'éprouvette en V. Celle-ci permet de localiser la déformation en surface dans des conditions de triaxialité des contraintes proches de celles rencontrées dans certains composants. Afin de découpler l'action de l'oxydation de celle liée à la sollicitation, un dispositif expérimental de traction sous air et sous vide a été employé. Une analyse systématique par microscopie électronique à balayage des surfaces sollicitées a été menée pour localiser et quantifier l'amorçage. Celui-ci se produit sur deux types de site, les carbures/carbonitrures et les joints de grains. Grâce à une approche quantitative, les conditions expérimentales conduisant à un mode d'amorçage privilégié ont pu être déterminées. Les résultats obtenus montrent l'intérêt de la géométrie d'éprouvette en V pour favoriser et quantifier l'amorçage des fissures. Ces résultats semblent mettre en évidence une inhibition de l'amorçage de fissures intergranulaires par l'oxydation dans l'alliage 718 DA, ce qui correspond à un comportement original pour un alliage 718. La microstructure particulière des joints de grains de l'alliage 718 DA semble être à l'origine du comportement spécifique observé. / Alloy 718 is a nickel-based superalloy widely used in the industry insofar as it provides many options in terms of properties optimization in view of using conditions. Aeronautics industry, more specially aircraft engine manufacturer like SNECMA, uses this alloy to manufacture disks of turbojet engine. These disks are forged then aged by a specific thermal treatment called ?Direct Aged? (DA) in order to achieve needed mechanical properties for this application. However, above 550°C, the alloy 718 is known to be relatively sensitive to oxidation assisted intergranular cracking, depending upon its fabrication history. This study therefore proposes to investigate crack initiation problems in various loading conditions for DA alloy 718. To carry out this work, uncommon samples are used: V-shape specimens. This geometry allows localizing strain at the sample surface under conditions of stress triaxiality similar to those endured in real components. Tests were both conducted under air or under vacuum to understand the effect of oxidation on crack initiation. A systematic analysis of the stressed samples, using scanning electron microscopy, was performed to observe and quantify crack initiation. Cracks initiate on two types of sites, carbides/carbonitrides and grain boundaries. Thanks to a quantitative approach, experimental parameters which determine crack initiation mode have been identified. Experimental results have shown the advantage of the V-shape specimen geometry to promote and quantify crack initiation. In addition, these results highlight that intergranular crack initiation seems inhibited by oxidation in DA alloy 718, which is corresponding to an unusual behavior for an alloy 718. The microstructure near grain boundaries appears to cause this specific behavior.

Introduction Annealed Inconel 718 alloy was chosen for the beam window at the Los Alamos Neutron Science Center (LANSCE) Isotope Production Facility (IPF) [1]. The window was replaced after 5 years of operation. Mechanical properties and microstructure changes were measured to assess its expected lifetime. Material and Methods A cutting plan was developed based on the IPF rasterred beam profile (FIG. 1). 3-mm OD samples were cut out from the window and thinned to 0.25-mm thick. Shear punch tests were per-formed at 25 °C on 21 samples to quantify shear yield, ultimate shear stress, and ductility. From 1-mm OD, 0.25-mm thick shear punched out disks, 4 TEM specimens of ~30×10×2 μm were obtained using standard FIB lift-out techniques. TEM was performed on an FEI Tecnai TF30-FEG operating at 300 kV. Results and Conclusions TABLE 1 shows MCNPX tally results of accumulated dpa, He and H content from both protons and neutrons fluences and ANSYS steady-state irradiation temperature for the 3-mm OD samples [2]. These peak values are at the peak density of Typically increases in shear yield and shear maximum stress occur with increasing dose. In this case, highest shear yield and ultimate stress was on the lowest dose samples at the outer edge (FIG. 2). Optical microscopy images of the fracture surfaces on the shear punched out disks show no significant change in the fracture mode or reduction in ductility in the un-irradiated, high and low dose irradiated samples. One un-irradiated and 4 irradiated samples (5, E, 16 and 19) were selected for TEM analysis. Figure 3 shows bright field TEM images of an un-irradiated, high and low dose irradiated samples. Un-irradiated sample shows some dislocations and some large precipitates. The high dose sample #5 (~11 dpa, 122 oC) shows small loops and dislocations (left and center images) and no γ\' or γ\'\' precipitates in SAD from z = [011] (right image). Low dose sample #19 (~0.7 dpa, 40 oC) shows a high density of dislocation loops (left image), high density of H/He bubbles (center image) and presence of γ\'\' precipitates in SAD from z = [011] (right image). Radiation induced-hardening is highest at the low dose region in the outer most edge. The hardening from γ\'\' precipitates is determined to be more pronounced than that from trapped bubbles. The lack of significant hardening in the highest dose region is attributed to a lower dis-location density and no γ” precipitates or bubbles [3]. Identification of H or He bubbles and the higher accumulation of these bubbles in the low dose region (no direct beam hitting) warrant further studies. Despite the evidence of irradiation-induced hardening, this spent beam window appears to retain useful ductility after 5 years in service. At the conclusion of 2013 run cycle, the current in-service beam window had reached the same dpa as of the spent window. We plan to extend the service of the current in-service window until it reaches its intended design threshold limit of ~20 dpa (in the highest dose region). Additional measurements at higher dpa values will enable better decision-making in managing risks of the window failure.

Shear spinning of Haynes 282, Alloy 718, Alloy 600 and AISI 316L was done using several tool feeds and mandrel clearances. Multi passing of the materials was limited due to strain hardening and circumferential cracking except for AISI 316L. The effect of the tool feed and the mandrel clearance on the successful forming of the materials was established. The successfully spun samples were solution heat treated at varying temperatures and holding times to establish a range of grain sizes and hardness levels. An aging heat treatment process was performed for Haynes 282 and Alloy 718 to achieve precipitation strengthening. The micro hardness measurements were conducted for the materials prior to spinning and after spinning. The same was also done after the various heat treatment processes. Grain size mapping was conducted by the use of lineal intercept methods. Comparison of the results in terms of grain sizes and hardness values was done. The temperature ranges suitable for full recrystallization of the materials after the shear spinning were identified and the effect of the holding times on the grain growth established. Comparison with unspun samples showed that the heat treatment times required to achieve comparative hardness and grain sizes were distinctively different.