The reduction of fuel consumption and carbon dioxide emissions are currently a key factor for the aviation industry due to major concerns about climate change and more restrictive environmental laws. One way to reduce both fuel consumption and CO2 emissions is by significantly decreasing the vehicle’s weight while increasing engine's efficiency. In order to meet these requirements, the European aero engine industry is continuously focusing on alternative manufacturing methods for load carrying structures in advanced materials, such as titanium and nickel-based superalloys. Alternatively to traditional large-scale single castings, new manufacturing methods involve sheet metal parts, small castings and forgings assembled by welding. These new manufacturing methods allow more flexible designs in which each part is made of the most suitable material state, leading to several advantages such as reduction of product cost and weight while increasing engine's efficiency. Nickel-based superalloys are widely used in the aero engine industry, typically constituting up to 50% of the total weight of the aircraft engine. Due to their excellent material properties at high temperatures in severe corrosive environments, these superalloys are employed most extensively in the hot sections of gas turbine engines for both military and civil aircrafts with running temperatures up to 650°C. In this thesis, a manufacturing process chain including forming and welding in the nickelbased superalloy 718 is studied. The main focus in the work lies on determining the thermomechanical properties, modelling and simulation of cold forming, study forming limits based on Nakazima tests for forming limit curves (FLC) and applying a damage and failure criterion. The work also comprises a brief study on hot forming. Finally, modelling of a subsequent welding procedure is included where residual stresses from the forming simulation are used to predict shape distortions due to the welding procedure. The results are compared with experimental observations. The cold forming procedure of a double-curved component made of alloy 718 is studied using FE-analyses and forming tests. The same geometry was used to produce a hot forming tool. During forming tests at room temperature, micro cracks and open cracks were observed in the draw bead regions, not indicated when formability is assessed using a forming limit curve (FLC). Standard material models such as von Mises or Barlat Yld2000-2D were not capable of accurately predict the behaviour of the material after the point of diffuse necking, making the prediction of damage and failure during forming a challenge. The GISSMO damage model was therefore calibrated and used to predict material failure in forming of alloy 718. Tensile, plane strain, shear and biaxial tests at room temperature are performed up to fracture and continuously evaluated using Digital Image Correlation (DIC) by ARAMIS™. In this work, the GISSMO damage model is coupled with the anisotropic Barlat Yld2000-2D material model for forming simulations in alloy 718 at room temperature using LS-DYNA. Numerical predictions are able to accurately predict failure on the same regions as observed during the experimental forming tests. Comparisons of the distribution of damage on one of the draw beads between simulations and damage measurements by acoustic emission indicate that higher damage values correspond to bigger micro cracks. Numerical FE-predictions of the cold forming and subsequent welding procedure shows that the welding procedure further increases the shape distortions. This was found to agree with experimental observations. / Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator

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).

There is an upsurge of research interest on Alloy 718 additively manufactured (AM) by electron beam powder bed fusion (EB-PBF) technique in aero and land-based gas turbine engines. However, the surface quality of the manufactured components has always been a major challenge. Several factors, including powder particle size, layer thickness, beam parameters, scanning strategies, and inclination angle of the build, govern the surface characteristics. Along with surface roughness resulted from partially melted powder particles, surface defects such as balls, satellites, microcracks as well as up-skin and down-skin surfaces can enhance the vulnerability of the manufactured parts to corrosion. When the surface is unable to withstand the exposed environment adequately, corrosion can be triggered. The surface-induced corrosion failures are increasingly becoming more challenging as the AM components often have complex geometries that render them even more difficult to finish. So, the relatively poor surface finish is the barrier to the full exploitation of the AM industry. In the present study, to achieve the desired surface quality, hence an improved high temperature corrosion performance, shot peening was implemented on Alloy 718 parts manufactured by EB-PBF. The high temperature corrosion behavior of the parts was investigated in an ambient air environment at 650 and 800 °C for up to 336 h. The underlying physical and chemical factors at play of the parts exposed to the corrosive environment were investigated too. The effect of topographical features (e.g., surface roughness) and microstructural characteristics (e.g., grain structure, phases, and defects) on high temperature corrosion behavior were analyzed by 3D surface profilometry, hardness test, optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy disperse spectroscopy (EDS), X-ray diffractometry (XRD) and electron backscatter diffraction (EBSD). The surface roughness and high temperature corrosion rate of the parts was significantly reduced after shot peening.

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.

This thesis focuses on experimental characterization, understanding and modelling of nickel-based alloy 718, for a large range of loading conditions. Alloy 718 is the most widely used nickel-based superalloy, due to its high strength, high corrosion resistance and excellent mechanical properties at high temperatures. In this work, the mechanical behavior and microstructure evolution of this alloy during high strain rate deformation is investigated. Compression tests using a Split-Hopkinson pressure bar (SHPB) device were performed and the microstructure of the deformed sample was observed using optical microscope (OM) and scanning-electron microscope (SEM) coupled with electron back-scattered diffraction (EBSD) technique. The microstructural evolution according to the deformation conditions was characterized. For high deformation temperatures (1000 C and above), recrystallisation is identifed as the main deformation mechanism. A physics-based model was employed to simulate the deformation behavior of alloy 718. This type of models accounts for the microstructural mechanisms taking place during deformation. Knowledge about the deformation mechanisms of alloy 718, acquired experimentally and from literature, enables to formulate mathematically the microstructural phenomena governing the deformation behavior of the alloy. The proposed model includes the effects of strain hardening, grain boundary strengthening (Hall-Petch), solid solution strengthening, phonon and electron drag and recovery by dislocation glide and cross-slip. It is calibrated and validated using data obtained from mechanical tests, as well as values captured by the microstructural analysis. / H2020-MSCA-ITN-2017 grant agreement Nº764979 - ENABLE project

Laser-Directed Energy Deposition (L-DED), an Additive Manufacturing (AM) processused for the fabrication of parts in a layer-wise approach has displayed an immense potential over the last decade. The aerospace industry stands as the primary beneficiary due to the L-DED process capability to build near-net-shape components with minimal tooling and thereby producing minimum wastage because of reduced machining. The widespread use of Alloy 718 in the aero-engine application has prompted huge research interest in the development of L-DED processing of this superalloy. AM processes are hindered by low build rates and high cycle times which directly affects the process costs. To overcome these issues, the present work focusses on obtaining high deposition rates through a high material feed. Studying the influence of process parameters during the L-DED process is of prime importance as they determine the performance of in-service structures. In the present work, process parameters such as laser power, scanning speed, feed rate and stand-offdistances are varied and their influence on geometry and microstructure of Alloy 718 single-track deposits are analyzed. The geometry of deposits is measured in terms of height, width and depth; and the powder capture efficiency is determined by measuring areas of deposition and dilution. The microstructure of the deposits shows a column ardendritic structure in the middle and bottom region of the deposits and equiaxed grains in the top region. Nb-rich segregation involving laves and NbC phases, typical of Alloy718 is found in the interdendritic regions and grain boundaries. The segregation increases along the height of the deposit with the bottom region having the least and the top region showing the highest concentration of Nb-rich phases due to the variation in cooling rates. A high laser power (1600 W – 2000 W) and a high scanning speed (1100 mm/min) are found to be the preferable processing conditions for minimizing segregation. Another approach to minimize segregation is by performing post-build heat treatments. The solution treatment (954 °C/1 hr) and double aging (718 °C/8 hr + 621 °C/ 8 hr) standardized for the wrought form of Alloy 718 is applied to as-built deposits which showed a reduction in segregation due to the dissolution of Nb-rich phases. Upon solution treatment, this reduction is accompanied by precipitation of the delta phase, found predominantly in top and bottom regions and sparsely in the middle region of the deposit.

The purpose of this report is to evaluate the hardness and microstructure in bent and welded samples of Alloy 718. The results will be used by GKN aerospace to evaluate the simulated values of the production process of vines in a jet engine. In total, eleven samples from three different production chains are evaluated. All samples are bent and go through different stages within the production including bending, solution treatment and welding. The samples are cut and mounted in bakelite then polished and etched in order for the microstructure to be seen and evaluated. Hardness tests were made on the mounts to evaluate how bending, solution treatment and welding affects the hardness. The bent samples without the solution treatment became harder depending on the amount of cold deformation. The amount of cold deformation controls how fast the material recrystallizes during subsequent solution treatment. During the solution treatment, the δ-phase (Ni3Nb) is precipitated in the grain boundaries which prevents a coarser grain size and therefore promotes a smaller grain size.  The laser weld creates a small HAZ (heat affected zone) that becomes softer because the heat dissolves the δ-phase which therefore triggers the grains to grow. The weld consists of eutectic γ-dendrites with interdendritic pools of alloying elements. After the solution treatment, many needle shaped δ-phases arise from the pools and HAZ. The hardness measurements were tested on a manual machine which makes potential human error important to consider when the measurements are evaluated. When the grain size measurements are done on the solution treated samples, the grain boundaries can be difficult to determine because the δ-phases and twins create wide and incorrect boundaries. The results will be used by GKN Aerospace in order to verify their simulations. The heterogenic material after solution treatment will probably be studied further. / Syftet med projektet är att undersöka hårdhet och mikrostruktur i bockade och svetsade prover tillverkade av Alloy 718. Totalt undersöks elva stycken prover som är tillverkade på olika sätt enligt tre tillverkningskedjor. Alla provbitar är bockade och har sedan genomgått olika många steg i tillverkningen som består utav bockning, upplösningsbehandling och svets. Genom att kapa upp provbitarna i mindre bitar, baka in de i bakelit och etsa de studerades mikrostrukturen och hur den påverkas av bockning, upplösningsbehandling och svets. Vidare gjordes hårdhetsmätningar över olika delar av proven för att undersöka hur mycket hårdare materialet blir vid kalldeformation och hur upplösningsbehandling och svets påverkar hårdheten i Alloy 718. Endast bockade prov blir hårdare beroende på hur mycket kalldeformation provbiten har utsatts för. Hur mycket provet har deformerats styr hur snabbt rekristallisationen sker vid en efterföljande upplösningsbehandling. Under upplösningsbehandlingen utskiljs även δ-fas (Ni3Nb) i korngränser vilket främjar en mindre kornstorlek. Lasersvetsen ger ett litet värmepåverkat område men värmen från svetsen bidrar till att δ-fasen löses upp och större korn bildas. Svetsen består av långa eutektiska γ-dendriter med interdendritiska poler av mycket legeringsämnen. Vid upplösningsbehandling efter utförd svets bildas det δ-fas i de värmepåverkade och i svetsen. Hårdhetsmätningarna har utförts på en manuell hårdhetsmaskin vilket innebär att felmarginalen blir större då den mänskliga felfaktorn spelar en stor roll. Vid beräkning av kornstorlek har det varit svårt att utskilja vad som är en korngräns i upplösningsbehandlade prov då mängden δ-fas efter upplösningsbehandling ger tjocka korngränser.  Resultaten kommer användas av GKN Aerospace för att verifiera sina simulationer. Det heterogena materialet efter upplösningsbehandlingen kommer troligtvis studeras vidare.

There is a strong industrial driving force to find alternative production technologies in order to make the production of aero engine components of superalloys even more efficient than it is today. Introducing new and nonconventional machining technologies allows taking a giant leap to increase the material removal rate and thereby drastically increase the productivity. However, the end result is to meet the requirements set for today's machined surfaces.The present work has been dedicated to improving the knowledge of how the non-conventional machining methods Abrasive Water Jet Machining, AWJM, Laser Beam Machining, LBM, and Electrical Discharge Machining, EDM, affect the surface integrity. The aim has been to understand how the surface integrity could be altered to an acceptable level. The results of this work have shown that both EDM and AWJM are two possible candidates but EDM is the better alternative; mainly due to the method's ability to machine complex geometries. It has further been shown that both methods require post processing in order to clean the surface and to improve the topography and for the case of EDM ageneration of compressive residual stresses are also needed.Three cold working post processes have been evaluated in order to attain this: shot peening, grit blasting and high pressure water jet cleaning, HPWJC. There sults showed that a combination of two post processes is required in order to reach the specified level of surface integrity in terms of cleaning and generating compressive residual stresses and low surface roughness. The method of high pressure water jet cleaning was the most effective method for removing the EDM wire residuals, and shot peening generated the highest compressive residual stresses as well as improved the surface topography.To summarise: the most promising production flow alternative using nonconventional machining would be EDM followed by post processing using HPWJC and shot peening.

Heat generated in a machining process is a common and critical obstacle faced in today's machining industries. The heat generated in the cutting zone has a direct negative influence on the tool life which, in turn contributes to increase the manufacturing costs. Especially, in machining of Heat Resistant Super Alloys, HRSA this is a very limiting factor. HRSA are capable of retaining their mechanical strength and hardness at elevated temperatures. This property is advantageous in the application in e.g. aero-engines but also a disadvantage, since it also lowers the machinability significantly. This work is an attempt to improve the heat transfer from the cutting zone, which would lead to an increase in the tool life. To achieve this goal, the cutting tool has been modified to create an improved interface between the coolant and tool in the high-temperature areas. Two generations of inserts have been designed and investigated. Firstly, an insert with surface texture features has been created with the purpose of increasing the available surface area for heat dissipation: First generation, Gen I. Secondly, a GenII was designed as a further improvement of Gen I. Here, several channel features on the rake face were added, reaching out from the contact zone to the near proximity of the cutting edge. This with the purpose of improving access of the coolant closer to the cutting edge. The experiments were conducted in facing operations of Alloy 718 with uncoated round carbide inserts. All experiments were carried out with high-pressure coolant assistance, with a pressure of 16 MPa on the rake face and 8 MPa on the flankface, respectively.The two generations of inserts, Gen I and Gen II, were experimentally evaluated by tool wear analysis in comparison with a regular insert. The results shows that the tool life increased significantly for the Gen I insert, compared to a catastrophic failure of the regular insert at the same conditions. Regarding the Gen II insert,an increase in tool life by approximately 30 to 40 percent compared to Gen I insert was observed.

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