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

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.

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.

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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.