L’influence de traitements thermiques de revenu sur la corrosion localisée ainsi que sur la résistance mécanique en traction de l’alliage d’aluminium 2024 a été étudiée. Cet alliage est utilisé en construction aéronautique, à l’état T351 (mise en solution, trempe, écrouissage et maturation) grâce aux ses propriétés mécaniques élevées ; à l’état T6 (mise en solution, trempe, revenu au pic de dureté) il présente un bon compromis résistance/ductilité. Par contre, la présence d’hétérogénéités microstructurales le rend sensible à la corrosion localisée. Le traitement T7 (mise en solution, trempe, revenu après pic de dureté) peut améliorer la tenue à la corrosion au détriment des propriétés mécaniques. Dans un premier temps, il s’est agit de déterminer les conditions d’obtention des états métallurgiques T6 et T7 à étudier, à partir des courbes de dureté Vickers en fonction du temps de traitement. Les états métallurgiques obtenus, la microstructure et les propriétés mécaniques à l’échelle macroscopique et locale, ont été analysés. Un point important de cette étude a consisté en la recherche d’une méthode optimale pour la détermination de la distribution des particules intermétalliques grossières, paramètre de premier ordre pour les couplages galvaniques mis en jeu lors de la dissolution des particules. Dans un second temps, le comportement en corrosion, à différentes échelles a été suivi. Des techniques électrochimiques tel que le tracé de courbes de polarisation potentiocinétique et le suivi de potentiel libre, ont permis d’évaluer l’influence du traitement de revenu sur comportement globale en corrosion. A l’échelle locale, la dissolution des particules intermétalliques grossières a été suivie par l’évolution du potentiel de surface en fonction des différents traitements thermiques. Cela a pu être déterminé par microscopie à force atomique, couplée à l’analyse chimique des phases obtenue par spectroscopie à sélection d’énergie. Le taux de corrosion ainsi que l’abattement des propriétés mécaniques à différentes échelles, ont clairement montré l’importance du traitement T7 par rapport au traitement T6. L’ensemble des résultats présente un intérêt industriel dans la mesure où la résistance à la corrosion associée aux propriétés mécaniques peuvent indiquer la probabilité de l’endommagement du matériau. / Nr

In order to characterize mechanical components used in high responsibility applications, the knowledge of chemical composition and results from regular experiments such as traction, impact and hardness tests is important, but not sufficient. They cannot supply the necessary information that permit anticipating, in a reliable way, the components behavior in actual working conditions. As an example, there are engines cylinder head submitted to mechanical and thermal tensions that are relatively high during the in service use, and very high in same very demanding condition. During long run times and any failure in the cooling and/or lubrication the temperature may reach 300ºC. This temperature variation causes thermal shocks which may generate cracks and/or a wide ranging of plastic deformation in regions close to the pistons. Even not considering the thermal shock effects caused by failure, even so, a short number of start-up and shutdown cycles of engine, are considered the main cause of small cracks. This indicates that the generation of cracks in cylinder head may be considered as low cycle thermomechanical fatigue problem. Another problem is the microstructure heterogeneity in the component due to the casting process, leading to different physical and mechanical properties in the same piece. Besides the presence of porosity generated by gas bubbles and voids of solidification, which may be as great as short crack, reducing the nucleation life and changing the problems focus for the fatigue crack growth. The purpose of this study was to determine the isothermal and thermomechanical fatigue property through low cycle fatigue, as well as the fatigue crack growth, relaxation, microestrutural characterization and modeling of mechanical behavior by finite element for the aluminum alloy employed in the manufacture of engine cylinder head by the national automotive industry. Isothermal fatigue experiments were carried out at temperature of 120ºC and 280ºC and the thermomechanical performed in phase between the temperatures of 120ºC and 280ºC. The relaxation experiments were performed at some temperatures with trapezoidal wave loading, whereas the experiments of fatigue crack growth carried out at temperatures of 120ºC, 200ºC and 280ºC for sine and trapezoidal wave loading in displacement and load control. The microstructure analysis was also made in the specimens after the fatigue test by optical microscopy and scanning electron microscopy SEM. The results of these experiments showed that the casting defects and materials inhomogeneities, coupled long run times in high temperatures and loads, are a critical factor in the component performance. These results help us to establish accurate models for life prediction of the engine cylinder head. / Para caracterizar componentes usados em aplicações de alta responsabilidade não basta apenas conhecer a composição química e os resultados de ensaios de tração, impacto e dureza, pois estes podem não fornecer os subsídios necessários que permitam prever, de maneira confiável, o comportamento dos componentes nas condições reais de trabalho. Exemplo disto são os cabeçotes de motor automotivos, submetidos a tensões térmicas e mecânicas relativamente altas durante seu uso normal e altíssimo em condições extremas. Durante longos tempos de funcionamento e eventuais falhas na refrigeração e ou lubrificação a temperatura pode chegar a valores próximos de 300ºC. Esta variação de temperaturas provoca choques térmicos que podem gerar trincas e/ou uma grande quantidade de deformação plástica em regiões próximas aos pistões. Desconsiderando a presença de choques térmicos provocados por falhas, ainda assim, uma pequena quantidade de ciclos de acionamento e parada do motor, é considerada como os principais causadores de pequenas trincas. Isso indica que o surgimento de trincas em cabeçotes de motor deve ser considerado um problema de fadiga termomecânica de baixo ciclo. Outro problema é a heterogeneidade microestrutural no componente devido ao processo de fundição, levando a propriedades mecânicas e físicas diferentes em uma mesma peça. Além da presença de porosidade gerada por bolhas de gás e vazios de solidificação, que podem adquirir tamanho tal que se aproximem de pequenas trincas, diminuindo a vida para a nucleação e assim mudando o foco do problema para o de propagação de trinca por fadiga. A proposta deste trabalho foi a de determinar as propriedades de fadiga isotérmica e termomecânica através de ensaios de fadiga de baixo ciclo, bem como as propriedades de propagação de trinca por fadiga, relaxação, caracterização microestrutural e modelagem do comportamento mecânico por elementos finitos para a liga de alumínio utilizada na fabricação de cabeçotes de motores automotivos pela indústria nacional. Os ensaios de fadiga isotérmica foram realizados nas temperaturas de 120ºC e 280ºC e os ensaios termomecânicos foram realizados em fase entre as temperaturas de 120ºC e 280ºC. Os ensaios de relaxação foram realizados em várias temperaturas com carregamento de onda trapezoidal, enquanto que os ensaios de propagação de trinca por fadiga foram realizados nas temperaturas de 120ºC, 200ºC e 280ºC para carregamentos de onda senoidal e trapezoidal em controle de carga e de deslocamento. Foi feita também análise microestrutural nos corpos de prova, após os ensaios de fadiga, por microscopia óptica e por microscopia eletrônica de varredura MEV. Os resultados destes ensaios mostraram que os defeitos de fundição e a falta de homogeneidade no material, aliados a longos tempos de exposição a carregamentos e em altas temperaturas, constituem um fator crítico no desempenho do componente. Estes resultados ajudarão a estabelecer modelos precisos de previsão de vida para os cabeçotes de motor.

Titanium and aluminium alloys are among the most used construct materials due to their physical and mechanical properties except steels. The joining of these alloys can improve properties of whole construction but it is still difficult task. Especially welding of titanium and aluminium alloys is difficult cause formation of undesirable intermetalic phases in the weld. This thesis focuses on influences of electron beam welding parameters especially focusing and deflection of beam and preheating of base material to quality of heterogeneous join of titanium alloy Ti6Al4V and aluminium alloy EN AW-6061 – T651. There is described preparation of welded joins and brazed joins in the thesis, which are evaluated by light microscopy, scanning electron microscopy and EDS analysis of chemical composition. There was evaluated presence and chemical composition of formated intermetalic phases in the welded joins and quality and defects in the brazed joins.

This master‘s thesis deals with the suggestion of suitable tools for milling aluminum alloys profiles for the construction of aperture fillings and their aplication to a CNC machining center for increasing the efficiency of the cutting process. The first part of the thesis is focused on the description of the company and the workplace of production of aluminum alloys constructions. The following pages contain description of standardized products, which are used to identify the test profile that is part of the experimental verification. Next part of this thesis is characteristic of aluminum alloys and tool materials suitable for their machining. Second part of this work contains an analysis of the current state, suggestion of new cutting tools and their application to the production proces. At the end of this thesis there is the technical-economic evaluation that deals with the time and economic savings of the applied design.

<p>Commercial sheets, strips and wires are currently produced from aluminum alloys by multi-step deformation processing involving rolling and drawing. These processes typically require 10 to 20 steps of deformation, since the plastic strain or reduction that can be imposed in a single step is limited by material workability and process mechanics. In this work, a fundamentally different, single-step approach is demonstrated for producing these aluminum products using machining-based deformation that also enables higher material workability in the formed product. Two process routes are proposed: 1) chip formation by Free Machining (FM), and 2) constrained chip formation by Hybrid Cutting Extrusion (HCE).</p><p>Using the very soft and highly ductile commercially pure aluminum alloys as representative systems, various material flow transitions in response to the concentrated shear deformation are observed in FM including plastic instabilities. The flow instabilities usually manifested as folds of varying amplitudes on the unconstrained surface of the chips, are features that limit the desirability of the chip and potential use for strip applications. To suppress these instabilities, two strategies both involving deformation geometry design are outlined: 1) By using large positive rake angle, the flow can be transformed to be more laminar and thus reduces to a substantial amount, the flow instabilities. This also makes it possible for light rolling/drawing reductions to be adopted to smoothen the residual surface folds to improve the strip finish. 2) By using a constraining tool coupled with the cutting tool in what is referred to as HCE, the initial instability that leads to plastic buckling of the material is suppressed, thereby making the flow laminar and thus improve the quality of the strips.</p><p>Key property attributes of the chips produced by the shear-based deformation processes such as improved mechanical properties and in the case of HCE, superior surface finish compared to conventional processes of rolling/drawing are highlighted. Implications for commercial manufacture of sheet, strip and wire products are discussed.</p>

Au cours des dernières décennies, des efforts importants ont été menés dans la modélisation des processus de rupture ductile entraînant des progrès substantiels. Cependant, la compréhension complète des mécanismes de rupture ductile dans des états de contraintes spécifiques demeure une question ouverte. Ceci est dû au manque de bases des données expérimentales et à la non validation des modèles pour ces conditions de chargement. Dans ce travail, les acquisitions de données sont principalement obtenues en utilisant la laminographie, ce qui rend possible l'imagerie de régions d'intérêt d'échantillons plats. L'utilisation d'éprouvettes larges (et minces) permet de générer différents états de contraintes et des conditions aux limites pertinentes pour l'ingénierie, qui ne pouvaient pas être évaluées jusqu'à présent en trois dimensions et en essais in-situ à des échelles micrométriques. La corrélation d'images volumiques (DVC) est utilisée pour mesurer les champs de déplacement à l'intérieur des échantillons en acquérant des images de laminographie 3D. Deux classes de matériaux représentatives de deux modes génériques de rupture ductile ont été examinées, à savoir les alliages d'aluminium (rupture par instabilité) et la fonte à graphite sphéroïdal (rupture par croissance de vide et coalescence).L'observation de la microstructure et les interactions déformations-endommagement pour différentes géométries d'échantillons et pour différents niveaux de triaxialité des contraintes associés ont été étudiées pour des alliages d'aluminium à une résolution micrométrique. De plus, un cadre combiné numérique-expérimental (DVC-FE) est introduit pour valider les simulations numériques à l'échelle microscopique pour la fonte à graphite sphéroïdal. Les simulations par éléments finis (FE), qui représentent la microstructure des matériaux étudiés, sont conduites avec des conditions aux limites de Dirichlet extraites des mesures DVC. Enfin, le cadre DVC-FE a été amélioré et utilisé comme une procédure d'identification intégrée pour l'étude du comportement élasto-plastique de la matrice ferritique de la fonte, non seulement en termes de champs cinématiques induits par la microstructure aléatoire, mais aussi avec les niveaux de charge globaux. / In the last few decades significant efforts have been made in modeling ductile failure processes resulting in substantial progress. However, the full understanding of ductile failure mechanisms under specific stress states still remains an open question. This is partly due to missing experimental data and validation of models for such loading conditions.In this work, data acquisitions are mainly obtained by using laminography, which makes the imaging of regions of interest in flat samples possible. The use of large (and thin) specimens allows various stress states and engineering-relevant boundary conditions to be generated, which could not be assessed in three dimensions and in-situ at micrometer scales before. Digital Volume Correlation (DVC) is used for measuring displacement fields in the bulk of samples by registering 3D laminography images. Two material classes that are representative of two generic modes of ductile failure have been examined, namely, Al-alloys (failure by instability) and cast iron (failure by void growth and coalescence). The observation of microstructure and strain-damage interactions at micrometer resolution for various specimen geometries and associated levels of stress triaxiality are studied for Al-alloys. Additionally, a combined computational-experimental (DVC-FE) framework is introduced to validate numerical simulations at the microscopic scale for nodular graphite cast iron. Finite Element (FE) simulations, which account for the studied material microstructure, are driven by Dirichlet boundary conditions extracted from DVC measurements.Last, the DVC-FE framework is upgraded to an integrated identification procedure to probe elasto-plastic constitutive law of the cast iron ferritic matrix not only in terms of kinematic fields induced by the random microstructure but also by overall load levels.

No description available.

No description available.

Fiber-metal laminates (FML) are the advanced materials that are developed to improve the high performance of lightweight structures that are rapidly becoming a superior substitute for metal structures. The reasons behind their emerging usage are the mechanical properties without a compromise in weight other than the traditional metals. The bond remains a concern. This thesis reviews the effect of pre-treatments, say heat, P2 etch and laser treatments on the substrate which modifies the surface composition/roughness to impact the bond strength. The constituents that make up the FMLs in our present study are the Aluminum 2024 alloy as the substrate and the carbon fiber prepregs are the fibers. These composite samples are manufactured in a compression molding process after each pre-treatment and are then subjected to different tests to investigate its properties in tension, compression, flexural and lap shear strength. The results indicate that heat treatment adversely affects properties of the metal and the joint while laser treatments provide the best bond and joint strength.

Mechanical properties of structural materials are highly correlated to their microstructure. The relationship between microstructure and mechanical properties can be established experimentally. The growing need for structural materials in industry promotes the study of microstructural evolution of materials by design using computational approaches. This thesis presents the microstructural evolution of two different structural materials. The first uses a genetic algorithm approach to study the microstructural evolution of a high-temperature nickel-based oxide-dispersion-strengthened (ODS) alloy. The chosen Ni-20Cr ODS system has nano Y2O3 particles for dispersion strengthening and submicron Al2O3 for composite strengthening. Synergistic effects through the interaction of small dispersoids and large reinforcements improved high-temperature strength. Optimization considered different weight factors on low temperature strength, ductility, and high temperature strength. Simulation revealed optimal size and volume fraction of dispersoids and reinforced particles. Ni-20Cr-based alloys were developed via mechanical alloying for computational optimization and validation. The Ni-20Cr-1.2Y2O3-5Al2O3 alloy exhibited significant reduction in the minimum creep rate (on the order of 10-9 s-1) at 800oC and 100 MPa. The second considers the microstructural evolution of AA 7050 alloy during friction stir welding (FSW). Modeling the FSW process includes thermal, material flow, microstructural and strength modeling. Three-dimensional material flow and heat transfer model was developed for friction stir welding process of AA 7050 alloy to predict thermal histories and extent of deformation. Peak temperature decreases with the decrease in traverse speed at constant advance per revolution, while the increase in tool rotation rate enhances peak temperature. Shear strain is higher than the longitudinal and transverse strain for lower traverse speed and tool rotation rate; whereas for higher traverse speed and tool rotation rate, shear and normal strain acquire similar values. Precipitation distribution simulation using TC-PRISMA predicts the presence of η' and η in the as-received AA 7050-T7451 alloy and mostly η in the friction stir welded AA7050 alloy, which results in the lower predicted strength of friction stir welded alloy. Further, development of modeling assists in process optimization and innovation, and enhances the progression rate. Accelerating the development process requires coupling experimental methods with predictive modeling. The overall purpose of this work was to develop an integrated computational model with predictive capabilities. In the present work, an application tool to predict thermal histories during FSW of AA7050 was developed using COMSOL software.