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This diploma thesis is focused on description of the properties of a layer of Inconel 718 applied on austenitic steel AISI 304 by the Cold Spray and subsequently remelted by electron beam. The first part presents the Cold Spray with its properties, advantages and disadvantages, and also describes the principle of electron beam remelting and other possible uses of electron beam, for example welding, drilling, heat treatment etc. The second part describes the material and the methods used for the preparation and evaluation of the samples. There are evaluated the porosity, microstructure and microhardness of the layers applied by the Cold Spray and these properties are further compared with the properties of the same layers remelted by electron beam. In conclusion, the results of the porosity of the layers applied by the Cold Spray are discussed with the literature and the results of electron beam remelting are only partially described here, because it was not possible to find literature about this topic. There are also suggestions for further research of the properties of this layers, which is necessary to know before implementing this method of producing layers for commercial production.

Additive Manufacturing (AM) is a method of producing three-dimensional objects using additive processes. It allows great flexibility in the processes and reduces the design-to-production time. Laser Metal Deposition (LMD) is one of AM methods under development and is based on the deposition technology. LMD has advantages in grain growth control, material functional grading, lower material storage requirement and more spatial freedom. Considering the outstanding features, it is important to study the characteristics of LMD products, which in this study is Alloy 718 for aerospace applications. Single-wall Laser LMD samples are built with varied process parameters using gas-atomized Alloy 718 powders. Two experiments were carried out with focuses on 1) evaluations and comparisons of the microstructural characteristics, porosity and hardness of the samples are performed; 2) The effect of heat treatments including solution treatment and aging on the microstructure as well as the hardness. The results of the experiments revealed directional solidification features and typical phases such as γ matrix, Laves phase and carbide. 0.06% average porosity and a majority of < 20 µm size are measured from the LMD samples. Only spherical gaseous pores are found while no lack-of-fusion pore is found. A hardness Vickers of 246 in average hardness is measured from the LMD samples. In the heat treated samples, δ phases were found; By direct-aging at 750 ℃ for 10 to 15 hours, the samples reach a maximum hardness of around 382 HV. The same hardness was reached by 1 hour of solution treatment at 950 ℃ followed by 5 hours aging at 750 ℃. The effects of processing parameters on the characteristics of LMD processed Alloy 718 are compared and discussed. A 2-dimentional map of porosity distribution along the length of the sample is made and the patterns are investigated along both the length and the height of the sample. It is found in the sample that the starting part of the deposit is higher in number of pores while the finishing part is larger in pore size. It is also found that the top layer of the deposit has the highest porosity level, pore number and pore size. Moreover, the hardness gradient along the build-up direction is evaluated and discussed. No significant hardness gradient was found. The precipitation hardening effect of LMD process and possible improvements are also discussed.

Increasing tool life is a significant objective in production. Achieving this objective in a machining process poses a significant challenge, especially during cutting hard-to-cut materials such as superalloys, due to the severe tool chipping/failure at the beginning of the cut. Although numerous attempts have been carried out to improve tool performance and prolong tool life during the machining of difficult-to-cut materials over the past several years, researchers have not obtained sufficient control over sudden tool failure/chipping. The focus of this study is to prolong tool life and control tool chipping by developing an ultra-soft deposited layer on the cutting tool that can protect it during the machining of difficult-to-cut materials such as Inconel 718. In the current study, an ultra-soft layer of material is deposited on the tool through two different techniques; a typical physical vapor deposition (PVD) technique and a novel developed method called “pre-machining”. In the PVD method, the soft layer is deposited under a high vacuum environment using a PVD coater. In the novel pre-machining method, the soft layer is deposited through a very short machining process involving Al-Si. It should be mentioned that soft coatings have never been used before for machining applications of difficult-to-cut materials including Inconel 718. This study shows that in contrast to what is expected, depositing an ultra-soft layer on the cutting tool significantly improves tool performance, by reducing chipping, and improving the machined surface integrity during cutting of Inconel 718. The obtained results show up to a 500% ± 10% improvement in tool life and around a 150% ± 10% reduction in cutting forces. Significant reductions in work hardening, residual stress, and surface roughness on the machined surface were other main achievements of the current study. / Thesis / Doctor of Philosophy (PhD) / Inconel 718 is considered to be a difficult-to-cut material due to its poor machinability. Significant tool failure at the early stage of cutting is the main challenge of machining this material and is the most significant contributing factor to its high manufacturing costs. Studies show that the common methods used to tackle this issue have not been completely successful. The goal of the present study is to tackle the machining challenges of Inconel 718 by developing tool coatings that meet the specific needs of the material to eliminate tool failure and thereby improve overall machining performance. For this purpose, a new tool coating material and a novel deposition technique that can be used as an alternative for commonly used coatings were developed in this study to improve the tool performance during the machining of Inconel 718. In addition, thorough studies have been carried out to gain a better understanding of the dominant wear phenomena and tool surface treatments that result in an improvement in the machinability of Inconel 718.

Nickel super alloy Inconel 718 was tested and compared to Haynes 230 using Gleeble and Varestraint mechanical test methods. Hot cracking susceptibility was examined in either alloy using a sub-scale Varestraint test method at 5 augmented strain levels: 0.25%, 05.%, 1%, 2%, and 4%. Maximum crack length, total crack length, and number of cracks were measured for each strain level. Gleeble hot ductility on-heating and on-cooling tests were performed on both alloys. Inconel 718 was tested on-heating at target temperatures of 1600˚F, 2000˚F, 2100˚F, 2200˚F, and on cooling at 1600˚F, 1700˚F, 1800˚F, 1900˚F, and 2100˚F. Haynes 230 was tested on-heating at target temperatures of 2050 ˚F, 2200 ˚F, 2240 ˚F, 2330 ˚F, and on-cooling at 1800 ˚F, 1900 ˚F, 1990 ˚F, 2040 ˚F, 2090 ˚F, 2100 ˚F, 2140 ˚F, and 2190 ˚F. Ductility in Gleeble samples was measured in a reduction of surface area. A nil-strength temperature was established for either alloy. The nil-strength temperature was 2251˚F and 2411˚F, for Inconel 718 and Haynes 230, respectively. The nil ductility temperature <5% R/A) was 2188˚F for Inconel 718 and 2341˚F for Haynes 230. Ductility recovery temperature occurred at 1924˚F for Inconel 718 and 2147˚F for Haynes 230. The brittle temperature range was determined to be 326˚F for Inconel 718 and 228˚F for Haynes 230. Varestraint testing revealed that Inconel 718 had a lower threshold strain for crack initiation than Haynes 230 (0.5% vs 1%), and a higher number of cracks, as well as a larger maximum crack length, at every strain level. These results show a greater tendency for liquation cracks to form in Inconel 718 than in Haynes 230.

De nombreux alliages de structure doivent leurs propriétés mécaniques à la présence de précipités inter ou intragranulaires. Ainsi les superalliages à base nickel, de matrice austénique γ, sont souvent renforcés par des précipités de phases intermétalliques ordonnées. Au sein de l’alliage Inconel 718, outre la phase γ’ de structure L12 (cubique simple), on trouve des précipités de Ni3Nb sous la forme métastable γ" (D022 - tétragonal centré) ou sous la forme stable δ (D0a - orthorhombique). Le rôle des précipités γ’, γ" et δ sur les propriétés macroscopiques de l'alliage est connu et largement utilisé en contexte industriel. Cependant les mécanismes de précipitation et de transformation de ces précipités ne sont toujours pas complètement élucidés, ce qui a motivé ce travail. La microstructure de l’alliage a été caractérisée par microscopie optique (MO) et électronique (à balayage et en transmission ; MEB et MET) dans l’état de livraison et après des traitements thermiques isothermes et anisothermes. Les essais d’analyse thermique différentielle (ATD) nous ont permis de préciser les domaines de température de précipitation et de dissolution des différentes phases présentes (γ’, γ" et δ). Différents états de précipitation ont été obtenus à l’aide de traitements thermiques isothermes basés sur les diagrammes temps-températuretransformation (T.T.T.) disponibles dans la littérature. Les observations en MET « à haute résolution » des précipités des phases δ et γ’’ ont permis de caractériser certains des défauts structuraux présents dans ces précipités. Nous montrons ainsi que les défauts d’empilement au sein de la phase γ’’ peuvent servir de germes pour la précipitation de . Alors que la structure des interfaces δ/γ ainsi que les défauts d’orientation au sein des lamelles de δ suggèrent que la croissance de la phase δ a lieu directement à partir de la matrice . / Many structural alloys are strengthened by the presence of precipitates in the grains or at grain boundaries. Nickel based superalloys often present an austenitic γ matrix in which ordered intermetallic phases precipitate. In the alloy Inconel 718, one can find γ’ L12 cubic ordered precipitates together with the compound Ni3Nb in its metastable form γ" (D022 - tetragonal) or the stable phase δ (D0a - orthorhombic). The incidence of those precipitates on macroscopic properties of the alloy 718 is well known and widely used in industrial applications. However the mechanisms responsible for the precipitation and transformation of these phases are not fully understood, which motivated the present study. The alloy microstructure has been observed by optical microscopy (OM) and electron microscopy (scanning and transmission, SEM and TEM) in the as received state as well as after heat treatment (isothermal and anisothermal). Differential thermal analysis (DTA) was used to determine the precipitation and dissolution temperatures of the phases γ', γ" and δ. Various precipitation microstructures were obtained by heat treatments based on available TTT diagrams. Some of the structural defects present in γ" and δ precipitates have been characterised by lattice imaging TEM observations. It is shown that stacking faults in γ’’ phase can act as a seed for the germination of . The structure of the δ/γ interface and the orientation defects in δ lamellae suggest that the growth of δ phase occurs directly from the matrix (and not by transformation of the γ’’ phase).

The aim of this thesis was to investigate if the robotized welding method FSW (Friction Stir Welding) could be applied for joining a rotating structure in an aero engine at Volvo Aero Corporation. FSW is expected to introduce less defects than today’s welding methods and could therefore be suitable for critical aero components. The material is the nickel based alloy Inconel 718, however a material experimentation is outside the scope of this report.The main goal of this study is to verify if the ESAB ROSIO robot based FSW-system has a suitable work space to be able to weld the rotating structure, and if the welding tool has accessibility to the joints. The FSW-process needs a rigid fixture, and a number of fix-ture concepts are presented based on a proposed weld sequence. A final fixture design is proposed, which requires a new design of the structure.The accessibility studies were performed in the robot simulations software Robot Stu-dio. This showed that the robot was unable to weld the proposed model in all areas in the original design. If the robot and the rotating structure can be redesigned the access will be achieved as shown in Robot Studio simulations.In order to be implemented in real production a number of further actions need to be taken and the result of this study can be a basis for these.

L'optimisation de la durée de vie des disques de turbines conçus en superalliage à base de nickel, tel que l'Inconel 718 DA, constitue une vraie problématique industrielle. En effet, lors d'essais de fatigue oligocyclique à chaud sur des éprouvettes prélevées sur ces disques, il a été constaté que la durée de vie dépendait de la zone de prélèvement testée. Cette dispersion inattendue des résultats s'avère être un facteur très important d'un point de vue industriel car le dimensionnement des pièces étant conservatif, il est établi à partir des valeurs de durée de vie les plus faibles ce qui conduit à une pénalisation forte en termes de performance du point de vue de la conception. Le but de cette étude est donc d'expliciter la relation entre les paramètres de forgeage, les propriétés microstructurales et donc les durées de vie afin de proposer des pistes pour réduire la dispersion des résultats de fatigue pour optimiser ainsi le dimensionnement des structures. Dans un premier temps, l'étude de l'effet des paramètres de forgeage a été réalisée grâce au logiciel Forge2®. Cette étape a permis de mettre en évidence les paramètres de forgeage influant sur la microstructure et de définir des gammes de forgeage sur des lopins expérimentaux. Les mécanismes d'initiation et les modes d'endommagement ont été identifiés pour chaque microstructure de l'alliage. Il s'avère que, dans le cas des matériaux testés, deux mécanismes d'amorçage sont observés : un amorçage sur particule interne avec formation de « fish-eye » et un amorçage sur grain sub-surfacique. Une corrélation directe avec la microstructure et le domaine de durée de vie a pu être établie. L'effet du rapport entre la queue de distribution de taille de grains et la taille des particules a pu être mis en évidence, ainsi que la nocivité de la présence de joints Σ3 dans des gros grains. / Optimization of turbine discs in superalloy Nickel based durability is a real industrial challenge. The different regions of a wrought Inconel 718 DA superalloy turbine disk may present a wide range of behavior in fatigue life due to the variability of the microstructure. To improve the fatigue dimensioning, it is necessary to understand the relation between the forging parameters, the microstructure and the fatigue behavior. Firstly, the effect of the forging parameters on the microstructure was realized thanks to simulation calculation via Forge2® and experimental forging were realized. Secondly, fatigue tests under strain control conditions were performed at 450°C for all microstructures. Initiation mechanisms and fracture behavior are identified for each microstructure of this superalloy. Grain size and phases distributions were characterized in the specimens and related to fatigue failure initiation modes. The 3D distribution of the Titanium nitrides was also investigated using synchrotron laminography both in terms of size and spatial distribution. 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. A relation between the microstructure heterogeneities and the durability is established. Indeed, the initiation mechanism, which depends on the grain size distribution compare to the particles size, impacts the durability. The presence of Σ3 boundaries in large grains proves to be harmful.

L’utilisation de différents alliages à base nickel est largement répandue dans les domaines industriels où les sollicitations mécaniques ou environnementales sont particulièrement sévères. Dans le cas des applications nucléaires, ceux-ci sont utilisés pour leur excellente résistance aux phénomènes de Corrosion Généralisée et de Corrosion Sous Contrainte. Les présents travaux s’intéressent tout particulièrement à la nuance 718 utilisée pour faire les composants fortement sollicités (ressorts, vis, …) présents au sein des assemblages combustibles. Ce sont ainsi plus de 20 millions de pièces qui sont actuellement en service dans les réacteurs français. Les enjeux industriels de sûreté des composants conduisent à des recherches permanentes en vue d’accroître encore davantage leur fiabilité. Des travaux sont ainsi menés depuis plus de quinze ans en étroite collaboration entre Areva et le Cirimat, dans cet objectif. Ces travaux ont en particulier permis la découverte d’un traitement thermique permettant de désensibiliser totalement l’alliage 718 vis à vis des phénomènes d’endommagement assisté par l’environnement. Ce phénomène a été associé à une évolution tant de l’état métallurgique que de la chimie du matériau. Les travaux présentés dans ce manuscrit s’inscrivent dans la droite lignée de ceux entrepris précédemment. Les paramètres clefs du procédé développé alors ont été identifiés afin d’être reproduits. Des premiers essais conduits en laboratoire ont permis de mettre en évidence une diminution de la sensibilité à la fissuration assistée par l’environnement des matériaux ainsi traités. L’efficacité se révèle néanmoins partielle dans ces conditions. Des essais menés en parallèle dans une enceinte industrielle ont permis d’aboutir à des résultats similaires. Ces résultats se révèlent partiellement satisfaisant au regard de ceux obtenus dans les études précédentes. Aucune différence entre l’état métallurgique des matériaux obtenus dans le cadre de cette étude et ceux issus des travaux antérieurs n’a pu être mise en évidence. En revanche, une différence peut être constatée dans l’évolution de la chimie du matériau, notamment concernant la teneur en éléments interstitiels. Une analyse approfondie des atmosphères de traitement thermique a donc été entreprise afin d’identifier les mécanismes entrant en jeu dans le procédé de désensibilisation et permettant de conduire à une diminution de la teneur en élément interstitiel, en particulier le carbone. Il est ainsi apparu qu’une réaction directe entre l’atmosphère et les éléments interstitiels mis en jeu ne peut avoir lieu. D’autres mécanismes ont alors été explorés afin de conduire à une décarburation de l’alliage 718. Il a alors été mis en évidence que plusieurs alliages modèles Ni-Cr et Ni-Cr-Fe pouvaient être décarburés dans des conditions compatibles avec celles du traitement thermique de désensibilisation, alors qu’aucun échantillon issu du matériau d’étude ne conduit à un résultat similaire. Ceci a été attribué au fait que la majeure partie du carbone contenu dans ce matériau se présente sous la forme de carbures de niobium ou de titane. Cette solution, trop difficile et incertaine à développer d’un point de vue industriel n’a pas été poursuivie. Une voie alternative au traitement thermique seul a été également initiée afin d’atteindre l’objectif industriel initial : réduire fortement la sensibilité de l’alliage 718 à la fissuration assistée par l’environnement. Celle-ci consiste en une adaptation de la gamme thermomécanique de fabrication des produits d’études. Des résultats prometteurs sont présentés dans ce manuscrit mais doivent nécessairement être étoffés avant de pouvoir envisager sereinement une mise en application industrielle de ce procédé alternatif. / Ni-based alloys are widely used in various industrial environments when high mechanical properties and high resistance to uniform corrosion and stress corrosion cracking are required. The present work deals with components made of alloy 718 that are used in fuel assemblies of Pressurized Water Reactors (springs, bolds, screws, …). These are highly solicited and there are more than 20 million in services in core of PWR power plant in France, at present time. A major concern for nuclear industry on based on safety during operations and permanent research works are carried out in order to improve reliability of these components. For more than fifteen years, Areva and Cirimat have performed various studies to complete this goal. Few years ago, this collaboration was successful with the discovery of a particular heat treatment process that allows the desensitization of alloy 718 to environment assisted cracking. This phenomenon was associated to an evolution of both metallurgical state and material chemistry. Results presented in this document are the right continuation of this previous work. Key process parameters have been identified thanks to previous results in order to be reproduced. Experiments were conducted in laboratory equipment, at a first time. They revealed a slight decrease of material susceptibility to environment assisted cracking. Nevertheless, process efficiency appears not as satisfying as observed in previous results. Other tests have been conducted at the same time in an industrial facility and reveal similar results. Atmosphere analyses have been carried out in both laboratory and industrial equipment in order to identify desensitization mechanism, particularly the decrease in carbon content. It appears that a direct reaction between atmosphere and interstitial elements could not be considered as a slight surface oxidation may appear during heat treatment cycle. Other decarburization mechanisms have been explored using both alloy 718 and other Ni-Cr(-Fe) model materials. Decarburizing conditions have been determined on all model materials. Nevertheless, none of them allow the decarburization of studied alloy 718. This was attributed to the fact that most of the carbon in this alloys is precipitated under niobium and titanium carbides. This solution could have been continued but appeared too difficult and highly uncertain regarding industrial scale application. An alternative processing to a sole heat treatment was initiated to reach our main goal: significantly decrease susceptibility of alloy 718 to environment assisted cracking. It is based on an optimization of thermomechanical process route. Some promising results are presented in this document. They must be developed before considering an application to an industrial scale.

Additive Manufacturing (AM) is an uprising manufacturing process for parts with complex geometries and low production quantities. Within the layer-wise building process, less additional processes are needed, to produce the parts. This allows a building of parts within a reasonable time- and costs-range. Especially industries, such as aerospace industry, can profit from AM. Electron beam – Powder bed fusion (EB-PBF) is a common technique, within AM, to produce metallic parts out of special alloys such as Alloy 718. This superalloy is a Nickle-Iron based alloy that has high mechanical properties, even in elevated temperatures (up to 650ºC). The combination of such material properties with high geometrical freedom creates new opportunities for the industry. However, it must be noted that a significant drawback of AM-techniques is the need for post-processing because of surface roughness- and microstructural characteristics. Commonly, the produced parts utilize mechanical post process such as milling to provide good surface roughness and dimensional accuracy. To reduce the surface roughness in the contour region, and therefore reduce the amount of mechanical post-processing, the present survey elaborates the effect of relevant parameters on contours such as the number of contours, scanning speed, focus offset and beam current. By using Design of Experiments (DOE), two batches were carried out: one screening batch and a two-level-full factorial design. In those batches, 15×15×15 mm cubes were printed with various parameters and, after that, analysed by using White light interferometry (WLI), Optical microscopy (OM) and Scanning electron microscopy (SEM). Furthermore, the program ImageJ was used to perform porosity and melt pool measurements. It had been observed that the number of contours had quite a considerable impact on the final surface roughness and the number of defects. Samples with two contours, instead of only one, tend to have a lower surface roughness. Nevertheless, the parameters and their interaction were found to have fundamental effects on the resulting surface roughness and microstructure.