Factors such as environmental requirements and fuel efficiency have pushed aerospace industry to develop reduced-weight engine designs and thereby light-weight and thin-walled components. As component wall thickness gets thinner and the mechanical structures weaker, the structure becomes more sensitive for vibrations during milling operations. Demands on cost efficiency increase and new ways of improving milling operations must follow. Historically, there have been two “schools” explaining vibrations in milling. One states that the entry angle in which the cutting insert hits the work piece is of greater importance than the exit angle. The other states that the way the cutter leaves the work piece is of greater importance than the cutter entry. In an effort to shed some light over this issue, a substantial amount of experiments were conducted. Evaluations were carried out using different tools, different tool-to-workpiece offset positions, and varying workpiece wall overhang. The resultant force, the force components, and system vibrations have been analyzed. The first part of this work shows the differences in force behavior for three tool-to-workpiece geometries while varying the wall overhang of the workpiece. The second part studies the force behavior during the exit phase for five different tool-to-workpiece offset positions while the overhang is held constant. The workpiece alloy throughout this work is Inconel 718. As a result of the project a spread sheet milling stability prediction model is developed and presented. It is based on available research in chatter theory and predicts the stability for a given set of variable input parameters. / <p>QC 20121206</p>

Orientador: Marcos Valério Ribeiro / Resumo: Apesar de ser amplamente utilizado em componentes aeroespaciais, o Inconel 718 apresenta algumas características que dificultam a sua usinagem: dureza elevada, resistência em altas temperaturas, forte afinidade para reagir com materiais de ferramentas e baixa condutividade térmica. Além do mais, esta liga possui tendência para a formação da aresta postiça de corte, endurecimento por deformação, assim como efeito abrasivo de carbonetos e fases intermetálicas, que resultam em tensões mecânicas e térmicas elevadas na aresta de corte. A qualidade de acabamento exigido pela indústria, para este material, é de 1,6 µm de rugosidade média (Ra) e 6 µm de rugosidade total (Rt). Sabendo da importância do Inconel 718, assim como da necessidade de conciliar os desafios de usinagem com a qualidade exigida, o objetivo deste trabalho foi encontrar a condição experimental que resulte em melhores resultados de usinagem. Para encontrar a condição ótima, a liga foi usinada utilizando duas ferramentas: experimental de cerâmica – Al2O3 + MgO (perfil S) e comercial de metal duro revestido (perfil C). Com a metodologia de Taguchi foram planejadas duas matrizes experimentais. Para a ferramenta cerâmica, a usinagem ocorreu a seco e nas seguintes condições: avanços de 0,10–0,20–0,30 mm/rev; velocidades de corte de 300–400–500 m/min; profundidades de usinagem de 0,20–0,35–0,50 mm. A ferramenta de metal duro revestido foi utilizada em profundidade fixa de 0,5 mm, nas seguintes condições experimentais: av... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Despite being widely used in aerospace components, the Inconel 718 presents some characteristics that make difficult its machining: high hardness, resistance at high temperatures, strong affinity to react with tools materials and low thermal conductivity. Moreover, this alloy has a tendency to form the built up edge, hardening by deformation, as well as the abrasive effect of carbides and intermetallic phases, which result in high mechanical and thermal tensions in the cutting edge. The surface finishing quality required by the industry, for this material, is 1.6 μm of average roughness (Ra) and 6 μm of total roughness (Rt). Knowing the importance of Inconel 718, as well as the need to combine the machining challenges with the required quality, the objective of this work was to find the experimental condition that results in better machining results. To find the optimal condition, the alloy was machined using two tools: experimental ceramic – Al2O3 + MgO (profile S) and commercial coated tungsten carbide (profile C). With Taguchi methodology, two experimental matrices were planned. For the ceramic tool, the machining occurred in the dry and under the following conditions: feed rates of 0.10–0.20–0.30 mm/rev; cutting speeds of 300–400–500 m/min; machining depths of 0.20–0.35–0.50 mm. The coated carbide tool was employed at a fixed machining depth of 0.5 mm, under the following experimental conditions: feed rates of 0.10–0.15–0.20–0.25 mm/rev; cutting speeds of 55–70–85–100 m/m... (Complete abstract click electronic access below) / Doutor

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.

The diploma thesis deals with the development of process parameters of SLM technology for the material IN718. The main goal is an experimental development of a set of parameters for the production of thin-walled parts with regard to material density, surface roughness and tightness. The essence of the development of parameters is an experimental explanation of the influence of laser power and scanning speed on the morphology of single tracks, which are used for the production of a thin wall. Together with walls of larger widths and volume samples, it is possible to create an intersection of parameters by which is possible to create components formed by a combination of thin-walled and volume geometry. The performed research created a material set, where the parameters of thin walls are used for the area of contours of bulk samples. We managed to produce a wall with an average width of 0.15 mm and roughness of 6 m, which meets the requirement for the tightness. The meander scanning pattern achieved a relative material density of 99.92%, which is more than with the supplier's parameters. Based on the acquired knowledge, it was possible to apply a set of parameters to components combining both geometries.

The aim of this thesis was to modify microstructure and coating-substrate interface of CoNiCrAlY coatings deposited by HVOF and cold spray on Inconel 718 substrates. Electron beam remelting and annealing in a protective atmosphere were used to modify the coatings. Microstructure, chemical and phase composition were analyzed. The effect of beam current, transversal velocity and beam defocus on remelted depth was evaluated. As-sprayed microstructure and chemical composition of coatings were analyzed and compared with remelted samples. The effect of annealing of the as-sprayed and remelted samples was evaluated. Remelted layers exhibited dendritic structure. Chemical composition changed only after remelting of interface and part of a substrate. When only the coating was remelted, chemical composition remained the same. Phases coarsened after the annealing. Chemical composition changed after annealing due to the diffusion.

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In order to develop a better understanding of the property, microstructure evolution and thermal history of FSW Inconel 718's, the strain, strain rate and thermal cycles need to be determined. In order to estimate the strain field of a deformed body, a displacement function needs to be determined. A 3D deformation model was developed to determine the displacement coefficients. A rectangular box created in this model deforms following a linear displacement function. Three orthogonal planes cut this deformed box, which leads to three deformed planes. The shape parameters (L, H, θ¹ and θ²) on the three orthogonal planes can be expressed as the functions of displacement coefficients. Although the displacement coefficients can not be expressed in the forms of the shape parameters symbolically, a numerical solution can be found using numerical optimization methods. The shape parameters were obtained by assuming the displacement coefficients (three cases). Then, the numerical optimization was carried out to determine the displacement coefficients. The solved displacement coefficients are the same as the assumed ones, which shows that this inverse problem can be solved, and this model is robust to determine the displacement function numerically. This model was used to estimate the strain and strain rate at the boundary of the nugget zone of Friction Stir Welding (FSW) Inconel 718. A numerical/experimental methodology was developed to estimate the thermal history in the stir zone of FSW Inconel 718.The thermocouple experiment was conducted to measure the thermal cycles in Heat Affected Zone (HAZ). Using the measured temperature in HAZ and a numerical model, the peak temperature (1039 ºC) and cooling rate (58.18 ºC/s) were determined. The microstructure in different regions was characterized and co-related with the thermal cycles. In order to understand the microstructure evolution in the stir zone, the strain rate (12.612 s-1) was estimated using the mathematical model as mentioned above. According to the estimated thermal history and strain rate, the assumption that the dynamic recrystallizaiton occurred during FSW was made. The grain size in the nugget zone affects the hardness. The relationship among the microstructure, mechanical properties, and thermal cycles was discussed.

This thesis aims to investigate the manufacturability of nickel-based superalloys, IN625 and IN718, using the laser powder bed fusion (LPBF) process. The study provides a better understanding of the process-structure-property of nickel-based superalloys, their fatigue life, and subsequent post-processing. First, the process-structure-property was investigated by selecting a wide range of process parameters to print coupons for IN625 and IN718. Next, a subset of process parameters was defined that would produce high relative density (>99%), low surface roughness (~2 μm), and a low tensile RS. Second, a multi-scale finite element model was constructed to predict the temperature gradients, cooling rates, and their effect on RS. At constant energy density, RS is affected by scan speed, laser power, and hatch spacing, respectively. Third, the optimum set of parameters was used to manufacture and test as-built and shot-peened samples to investigate the fatigue life without costly heat treatment processes. It was found that shot peening resulted in a fatigue life comparable to wrought heat-treated unnotched specimen. Additionally, IN625 had a better fatigue life compared to IN718 due to higher dislocations density as well as the absence of γ´ and γ´´ in IN718 due to the rapid cooling in LPBF. Finally, the effect of post-processing on dimensional accuracy and surface integrity was investigated. A new approach using low-frequency vibration-assisted drilling (VAD) proved feasible by enhancing the as-built hole accuracy while inducing compressive in-depth RS compared to laser peening, which only affects the RS. These favorable findings contributed to the scientific knowledge of LPBF of nickel-based superalloys by determining the process parameters optimum window and reducing the post-processes to obtain a high fatigue life, a better dimensional accuracy, and improved surface integrity. / Thesis / Doctor of Philosophy (PhD)

This study develops a method to simulate a full-scale inertia friction weld with a sub-scale specimen and modifies a direct drive friction stir welder to perform the welding process. A torque meter is fabricated for the FSW machine to measure weld torque. Machine controls are modified to enable a force control during the IFW process. An equation is created to measure weld upset due to deflection of the FSW machine. Data obtained from a full-scale inertia friction weld are altered to account for the geometrical differences between the sub-scale and full-scale specimens. The IFW are simulated with the sub-scale specimen while controlling spindle RPM and matching weld power or weld RPM. The force used to perform friction welding is scaled to different values accounting for specimen size to determine the effects on output parameters including: HAZ, upset, RPM, torque, power and energy of the weld. Increasing force has positive effects to upset, torque, power and energy of the welds, while reducing the size of the HAZ.