Inconel 718 (IN 718) is widely used in applications, such as aircraft and power turbine components. Recently, fiber laser welding has become an attractive joining technique in industry for fabrication and repair of service-damaged components. However, a major limitation in the laser welding of IN 718 is that liquation cracking occurs. In the present work, autogenous fiber laser welding of IN 718 was used to study the effects of welding parameters and different pre-weld heat treatments on liquation cracking. Contrary to previous studies, a dual effect of heat input on cracking is observed. A rarely reported effect of heat input is attributed to process instability. Liquation cracking increases with pre-weld heat treatment temperatures that increase grain size and/or, possibly, intregranular boron segregation. The study shows that pre-weld heat treatment at 950oC can be used for repair welding of IN 718 without significant loss in cracking resistance.

The Ni-base superalloy, IN-718, has been coated with alumina sols. Coated surfaces, carrying alumina layers having thicknesses between 0.6 μm and 3.6 μm show a significantly reduced oxidation rate when compared with uncoated reference surfaces, even if heating temperature is increased up to 900 °C and heating time is extended to 800 h. Alumina layers were prepared via sol-gel processing using a modified Yoldas procedure to obtain alumina sols. No change in rheological sol behavior was observed for more than 1 year of aging under static conditions at room temperature. Depending on pH value, modified Yoldas sols contain a manifold of Al species, among them Al13 polycations. Thermal evolution of sol derived alumina powders depends on Al speciation of parent sols. Depending on sol composition, both gamma-Al2O3 and eta-Al2O3 occur as intermediate transition aluminas. Phase composition and gas phase velocity influence oxygen permeability of thin layers prepared with modified Yoldas sols.

Das Ziel der Arbeit war die Weiterführung der Substrukturanalyse von plastisch verformten metallischen Werkstoffen mit kubischer Struktur auf der Basis des Disklinationskonzeptes unter Berücksichtigung unterschiedlicher homologer Temperaturen und Stapelfehlerenergien. Die Untersuchungen wurden an Stauchproben von Wolfram, Aluminium und der Legierung Inconel 718 mit Hilfe der TEM, der REM (EBSD) und der röntgenografischen Profilanalyse durchgeführt. Bei allen drei Werkstoffen konnten im Temperatur- und Umformbereich T/Ts<=0,3, Deformation>=0,3 auf der Basis von lokalen TEM-Desorientierungsmessungen Partialdisklinationen identifiziert und ihre Frankvektoren bzw. Defektstärken bestimmt werden. Die Ergebnisse der EBSD-Untersuchungen und der Röntgendiffraktometrie sind mit den TEM-Beobachtungen kompatibel und zeigen, dass die Kombination der drei Methoden für die systematische Substrukturanalyse an stark verformten Werkstoffen gut geeignet ist.

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<div>Defects are unavoidable, life-limiting and dominant sites of damage and subsequent failure in a material. Ni-based superalloys are commonly used in high temperature applications and inevitably found to have defects in the form of inclusions, voids and microscopic cracks which are below the resolution of standard inspection techniques. A mechanistic understanding of the role of defects in such industrially relevant bulk polycrystalline material is essential for philosophies of design and durability to follow and ensure structural integrity of components in the inevitable presence of such defects. The current understanding of defect-mediated damage, in bulk Ni superalloys, is limited by experimental techniques that can capture the local micromechanical state of the material surrounding the defect. In this work, we combine mechanical testing with in-situ, non-destructive 3-D X-ray characterization techniques to obtain rich multi-modal datasets at the microscale to interrogate complex defect-microstructure interactions and elucidate the mechanisms of failure around defects. The attenuated X-ray beam, after passage through the material, is utilized through computed micro-tomography to characterize the defects owing to its sensitivity to density differences in the material. The diffracted X-ray beam, after illuminating the material, is employed through high energy diffraction microscopy in various modes to interrogate the evolving micromechanical state around the discovered defects.</div><div>Three case studies are performed with specimens made of a Ni-based superalloy specially designed and fabricated to have internal defects in the form of: (i) an inclusion, (ii) a microscopic crack, and (iii) voids. In each case, the grain scale information is investigated to reveal heterogeneity in the local micromechanical state of the material as a precursor for the onset of failure. Models and simulations based on finite element or crystal plasticity are utilized, wherever necessary, to assess the factors essential to the underlying mechanism of failure. In the first case study, the detrimental effects of an inclusion in initiating a crack upon cyclic loading is interrogated and the state of bonding, residual stresses, and geometrical stress concentrations around the inclusion are demonstrated to be of utmost importance. In the second case study, the propagation of a short fatigue crack through the microstructure is examined to reveal the crystallographic nature of crack growth through the (i) alignment of the crack plane with the most active slip system, (ii) the correlation between the crack growth rate and the maximum resolved shear stresses, and (iii) the dependence of the crack growth direction on microplasticity within grains ahead of the crack front. In the third case study, the role of voids in ductile failure under tensile loading is explored to illuminate the activation and operation of distinct mechanisms of inter-void shear and necking under the control of the local state of stress triaxiality and the local plasticity within the grains at critical sites of fracture.</div><div>In summary, a grain scale description of the micromechanical state has been unambiguously determined through experiments to examine the heterogeneity around defects in the material. It has enabled us to identify and isolate the nature of factors essential to the activation of specific mechanisms at the onset failure. The grain scale thus provides an ideal physical basis to understand the fundamentals of defect mediated damage and failure instilling trust in the predictive capabilities of models that incorporate the response of the grain structure. The generated datasets can be used to instantiate and calibrate such models at the grain level for higher fidelity. </div>

Les composants des industries de l'aéronautique, du spatial ou de l'énergie sont souvent en fonctionnement dans des environnements sévères où les températures et les pressions peuvent atteindre des valeurs importantes. Les matériaux traditionnels sont alors remplacés par des superalliages dits réfractaires, l'Inconel 718 est un de ces matériaux. Ils sont en particulier caractérisés par la conservation de leurs propriétés mécaniques à hautes températures. L'Inconel 718 est un alliage à base nickel, réputé comme difficile à usiner. Cette difficulté de mise en forme, liée à ses caractéristiques physiques et mécaniques, impose généralement une utilisation très importante de fluides de coupe ; ceux-ci représentent une fraction significative du coût de la pièce usinée. L'objectif est alors de migrer vers l'usinage à sec et grande vitesse. Cependant, du fait des caractéristiques mécaniques et du caractère réfractaire de l'Inconel 718, les surfaces générées peuvent être affectées thermiquement durant l'opération d'usinage avec la mise en place de contraintes résiduelles en traction néfastes pour la tenue en service des pièces usinées. Au travers des travaux présentés, nous nous proposons de montrer la faisabilité de l'usinage à sec de l'Inconel 718 et ses conséquences sur l'intégrité des surfaces réalisées. Des essais de tournage sous lubrification et à sec ont été réalisés dans des conditions de semi-finition avec un outil en carbure de tungstène revêtu. Les surfaces et sous surfaces ont été observées et qualifiées par un ensemble de moyens : microscope à interférométrie, microscope électronique à balayage, microduromètre, goniomètre rayons X pour l'analyse des contraintes résiduelles. L'analyse de l'intégrité des surfaces a permis de mettre en évidence les principaux facteurs influents. La qualité des surfaces produites dépendant de la maîtrise de la température de coupe, une technique de mesure des températures a alors été développée pour une opération de fraisage ; elle est basée sur la technologie des thermocouples. Les essais menés ont permis de déterminer la température en surface et en sous surface ; ces informations sont des éléments importants de validation d'un modèle thermique de la coupe. Enfin, à la suite d'essais de fraisage périphérique et de la mesure des efforts pour différentes conditions de coupe, nous présentons une comparaison entre ces résultats expérimentaux et une modélisation du procédé. La modélisation correspond à une approche thermomécanique de la coupe. En termes d'efforts, la comparaison est tout à fait satisfaisante ; un prolongement de cette approche pour valider l'aspect thermique, est toutefois envisagé.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / The nickel-based superalloys have a high mechanical strength which remains at elevated temperature, high creep and fatigue resistances and excellent oxidation resistance. This makes these alloys highly recommended for use in high temperature working environments such as mechanical components for the aerospace industry. However, these characteristics are major problems when machining them, as it promotes high heat generation in the flow zone, resulting in the development of high wear rates on the cutting tools. In order to reduce the problems caused by the high temperatures generated, the application of a cutting fluid, when possible, is essential to reduce friction at the chip-tool-workpiece interfaces and lower the temperatures in the cutting zone. Seeking to further increase in the efficiency of cutting fluids during machining of nickel alloys, this work presents a study of the influence of solid lubricants, graphite and molybdenum disulphide (MoS2) mixed to a vegetal based cutting fluid, applied by conventional method (flooding) and minimal quantity of fluid - MQF in turning of Inconel 718 with carbide and ceramic (mixed, whisker and SiAlON) tools. When turning with cemented carbide tools the addition of graphite to the cutting fluid provided the best results, while with ceramic tools MoS2 presented the best performance. The life of the carbide tool had an average increase above 200% in conventional flooding application of cutting fluid when compared to MQF, regardless the addition of solid lubricant. The addition of solid lubricants promoted an increase in the life of whisker and SiAlON tools. The flank wear was dominant for cemented carbide tools and SiAlON ceramics, while for whisker and mixed ceramics the notch wear was predominant. Attrition and diffusion wear mechanism were observed in all evaluated conditions. The addition of solid lubricant to the cutting fluid provided significant improvements in the surface roughness values for most of the evaluated conditions. However, there were no significant changes in the machining forces and cutting temperature. The residual stress was tensile and compression, depending on the fluid application method. Overall, the addition of solid lubricant showed no significant differences. / As superligas à base de níquel apresentam alta resistência mecânica que se mantém em elevadas temperaturas, altas resistência à fluência e à fadiga e excelente resistência a oxidação. Isso torna estas ligas altamente recomendadas para utilização em ambientes que trabalham a altas temperaturas, como por exemplo na fabricação de componentes mecânicos para a indústria aeroespacial. Entretanto, esta característica representa um grande problema quando elas são usinadas, pois promove elevada geração de calor na zona de fluxo, implicando no desenvolvimento de altas taxas de desgaste da ferramenta de corte. A fim de reduzir os problemas causados pelas altas temperaturas geradas, a aplicação de um fluido de corte, quando possível, é essencial, proporcionando redução do atrito na interface cavaco-ferramenta-peça e menores temperaturas na zona de corte. Na busca de aumentar ainda mais a eficiência dos fluidos de corte na complexa usinagem das ligas de níquel, este trabalho apresenta um estudo da influência dos lubrificantes sólidos grafite e bissulfeto de molibdênio (MoS2) misturado a um fluido de corte de base vegetal, aplicados pelo método convencional (jorro) e mínima quantidade de fluído MQF, no torneamento do Inconel 718, com ferramentas de metal duro e cerâmicas (mista, whisker e SiAlON). No torneamento com ferramentas de metal duro a adição de grafite ao fluido de corte proporcionou os melhores resultados, enquanto que no torneamento com ferramentas cerâmicas, foi o MoS2 que apresentou melhor desempenho. A vida da ferramenta de metal duro teve um incremento acima de 200% na usinagem convencional (jorro) quando comparado com a usinagem por MQF, sem considerar a adição do lubrificante sólido. A adição de lubrificantes sólidos promoveu um incremento na vida das ferramentas whisker e SiAlON. O desgaste de flanco foi predominante para as ferramentas de metal duro e cerâmica SiAlON, enquanto que nas cerâmicas whisker e mista o desgaste de entalhe foi predominante. Os mecanismo de desgaste de attrition e difusão foram observados em todas as condições avaliadas. A adição de lubrificante sólido ao fluido de corte proporcionou melhorias significativas nos valores da rugosidade para a maioria das condições avaliadas. No entanto, não se observou mudanças significativas nas forças e temperatura de usinagem. As tensões residuais foram de tração e compressão, dependendo do método de aplicação do fluido. No geral, a adição do lubrificante sólido não apresentou diferenças significativas. / Doutor em Engenharia Mecânica

The aerospace industry is constantly striving to becoming more economical and environmentally friendly. One of many efforts to achieve this is the Lightcam project which in this case is evaluating the use of additive manufacturing in the form of electron beam melting in conjunction with the nickel-based superalloy, Alloy 718. This combination is not fully explored and examined. For this purpose, a demonstrator vane was produced and it was subsequently evaluated in this thesis. The evaluation was performed in as-built condition and was divided in non-destructive testing, evaluation of these methods and metallographic review to confirm the results, and potentially revealing more properties. The non-destructive testing was performed using conventional radiography and computed tomography. Both methods struggled to deliver complete and reliable results, for varying reasons. Radiography could deliver results of the whole vane, but these were impossible to evaluate due to the rough surface created by the electron beam melting process. The computed tomography on the other hand was not affected by the rough surface and produced usable, though not complete, results of the vane. The reason for the computed tomography’s inability to deliver complete results was the material, varying thickness and complex geometry of the vane. As a complement and to verify the results from the non-destructive testing, a metallographic examination was conducted. These tests were conducted with the aim of answering the following three questions:  What non-destructive testing methods are suitable to evaluate Alloy 718 components manufactured with electron beam melting? - Neither radiography nor computed tomography are suitable as a sole evaluation method, for various reasons. All surface dependent methods were deemed unsuitable without testing due to the rough surface. What types of defects and in what quantity can they be found in the produced vane? - Defects found are: Porosity and lack of fusion, both found as internal and partially external and in varying sizes. Where are the defects located? - Pores are mainly found in the center of sections modeled to a 3mm thickness. Lack of fusion was found between build layers in all thicknesses. Apart from these results, hardness was found to vary depending on build height, increasing from the bottom towards the top. Microstructure was also found to vary with the build height, but always consisting of either equiaxed or columnar grains. / Lightcam