Rozprawy doktorskie na temat „Al SiC MMCs”

The corrosion of SiCp / 6061 A1 in 3% NaCl solution, at pH values of 3.8, 7.2 and 10.2, has been studied using SEM and combined scanning Auger electron/ X-ray microscopy. The MMC, fabricated by powder metallurgy, contains approximately 15 percent (volume) SiC as < 10 mum particles. Scatter diagrams are applied to analyze elemental maps created by scanning Auger microscopy. The experiments reveal that some impurity elements such as Fe, Ti etc., introduced during fabrication, form intermetallic compounds and that these are the dominant factors causing micro-galvanic corrosion. The Auger maps show, by inspection, that oxygen is enriched after exposure around these intermetallics. This has been confirmed by using scatter diagrams to analyze the data contained within the map. The phenomenon is probably caused by the dissolution of aluminium, resulting in the deposition of porous corrosion products such as Al(OH)3 and AlOOH. Because of a low overpotential for oxygen reduction, the intermetallic compounds are expected to act as cathodes. This is confirmed by the finding of magnesium hydroxide on the surface of the intermetallics after exposure of the specimen in MgCl2 solution. It is shown that the onset of this deposition can be used to estimate the cathodic current density at the cathode. Current density at the SiC particles, estimated by this method is negligible, suggesting that the principal cause of corrosion is found in the interaction between the aluminium alloy and the intermetallics. The same method is used for SiC, / 6061 A1 MMC. Intermetallic particles are found much more likely to be located at a fibre / matrix interface, and they are still a sensitive factor for localized corrosion. There is some Cr in the Fe-containing intermetallics in this matrix, this type of intermetallic is not very susceptible to micro-galvanic corrosion. Another different aspect is that the reinforcing material in SiCf / 6061 A1 MMC is directly associated with the micro-corrosion cell. The reinforcing SiC fibre belongs to the SCS series which consists of three different layers. The SiC is grown on a carbon core by chemical vapour deposition and a protective coating is applied on the outside of the fibre. This coating is mainly composed of carbon which is slightly enriched in silicon at the surface. In this investigation we found that carbon core and carbon coating act as cathodes and form a galvanic cell with surrounding aluminium alloy. This is confirmed by the magnesium decoration method. Electrochemical techniques are also used. Several kinds of SiC fibres and graphite fibres are cathodically polarized in 3% NaCl solution. By comparison with the anodic polarization curve of A1 alloy, the galvanic cells between A1 alloy and carbon core or protective coating are confirmed. There is no galvanic effect between A1 and pure SiC. Al3Fe is extracted from intermetallic enriched A1 alloy, and the electrochemical performance of this intermetallic is investigated.

This thesis presents an investigation of excimer laser surface melting (LSM) on AA6061-T6 alloy and SiCp/AA6061-T6 composite in terms of microstructure and corrosion behaviour. Hardness and wear resistance of the melted layer for both materials were also evaluated and compared with the untreated specimen to understand if the improvement of the corrosion resistance could be achieved without sacrificing the wear resistance. The intermetallic particles within the aluminium matrix are believed to initiate of both galvanic and pitting corrosion for both materials. To effectively dissolve these intermetallic particles, laser fluence from 1 to 8.5 J/cm2 with a number of pulses from 10, 25 to 50 were selected to achieve an optimisation of the LSM condition. It was found that the increase of laser fluence increased the melt depth, but also promoted the formation of defects such as micro-cracks and pores. For AA6061-T6 alloy, under the best laser condition (3 J/cm2 with 50 pulses), the amount of large intermetallic particles (2-10 µm), such as AlFeSi, AlFeMnSi and Mg2Si, were significantly reduced resulting in the formation of a relatively homogeneous and defect-free melt layer with only some small randomly distributed of intermetallic precipitates. For the SiCp/AA6061-T6 composite, under the best laser condition (6 J/cm2 with 25 pulses), decomposition of SiC particles was achieved as well as the dissolution of large AlMgSiCr and AlFeSi intermetallic particles in the melt layer. The melt layer had a relatively complex microstructure consisting of three different regions. MgO was found at the bottom of the melted layer which was discontinuous along the interface between the melted region and bulk material or in some places, at the bottom of the melted layer. The corrosion behaviour of both materials before and after LSM was evaluated using electrochemical measurements and immersion test in deaerated 0.6 M NaCl solution. After LSM the AA6061-T6 with and without SiC showed a better corrosion resistance compared with untreated alloys. The pitting potential of the LSM for both materials was shifted to a more positive value with a significant reduction of the passivation current density. In addition, an intergranular corrosion test based on the standard ASTM G110-92 showed that the LSM had significantly improved the corrosion resistance of both materials due to dissolution of intermetallic particles as well as the removal of the SiC particles in the composite material within the melted regions. In addition, the wear resistance of as-received SiCp/AA6061-T6 composites was found to be much higher than that of the LSM specimen. This is attributed to the decomposition of the majority of the SiCp in the melted region since the contribution to the hardness from the SiC particle in the untreated specimen is much greater that the Si-rich region in the melt layer after LSM.

L’attività sperimentale presentata in questo elaborato è volta a conoscere le proprietà meccaniche di una lega di magnesio rinforzata con particelle ceramiche (Carburo di silicio SiC). Scopo del lavoro sarà pertanto il confronto fra le proprietà meccaniche del materiale base ZM21 rispetto al materiale prodotto in Israele con rinforzo di carburo di silicio, con particolare riferimento alle proprietà a trazione, a compressione e alla distribuzione di particelle nella matrice (analisi metallografica).

Materiais compósitos são projetados e fabricados para várias aplicações de alto desempenho, incluindo componentes para os segmentos automobilístico, aeroespacial, aeronáutico, naval, de defesa, de óleo e gás, energia eólica e até equipamentos esportivos. Porém, a união por soldagem de Compósitos de Matriz Metálica de Alumínio (Al-CMM) ainda é um grande obstáculo para a maior disseminação desta classe de materiais estruturais. As mudanças microestruturais decorrentes do ciclo de soldagem e/ou do tratamento térmico afetam sensivelmente as propriedades mecânicas e físico-químicas finais da junta e do metal base nas proximidades de mesma, daí a importância de se estudar a evolução microestrutural que prospera nestas etapas. O presente trabalho caracterizou a microestrutura do compósito liga-A356/SiCp soldado por laser de fibra de Itérbio, empregando-se nessa tarefa técnicas de microscopia óptica, radiografia e microscopia eletrônica de varredura, assim como difração de raios-X e de elétrons retroespalhados, ensaio instrumentado de dureza e microtomografia computadorizada. O foco das análises realizadas restringiu-se à geometria dos cordões de solda, à expulsão de SiC particulado da zona soldada, à volatilização de elementos químicos da poça de soldagem, à formação de precipitados fragilizantes de Al4SiC4 em formato de agulhas no cordão de solda e à determinação das regiões com concentração de poros, todos estes fenômenos tendo efeitos nocivos, em maior ou menor extensão, no desempenho global da junta do Al-CMM soldada a laser, notadamente em suas propriedades mecânicas e eletroquímicas.
Microstructural materials composites are designed and manufactured for various high performance applications, including components for different industries like automobile, aerospace, aeronautical, naval, defense, oil and gas, wind energy and even sports equipment. However, the junction by welding of Aluminum Metal Matrix Composite (Al-MMC) is still a major obstacle to the further spread of this class of structural materials. The microstructural changes resulting from the welding cycle and / or post-weld heat treatment significantly affect the final mechanical and physicochemical properties of the joint and the base metal near it, hence the importance of studying the microstructural evolution that thrives in these steps. This study aims to characterize the microstructure of the composite alloy-A356 / SiCp welding by Ytterbium fiber laser, using optical microscopy, X-ray, scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, instrumented test hardness and computed microtomography. The focus of the analyzes was restricted to the geometry of the weld bead, the particulate SiC expulsion of the welded zone, volatilization of chemical elements from the welding zone, formation of precipitates embrittlement of Al4SiC4 in needle shape in the weld bead and determining the concentration of regions with pores, all these phenomena have harmful effects to a greater or lesser extent, the overall performance of the joining Al-CMM laser welded, mainly in their mechanical and electrochemical properties.

Une étude sur la réduction/suppression de l'inductance de limitation di/dt pour IGCTs et du clamp RCD en utilisant des diodes rapides en silicium (Si) et des diodes en carbure de silicium (SiC) dans les convertisseurs multiniveaux modulaires (MMC). Cette thèse contient :- Analyse des sous-modules de MMC HVDC existants.- Évaluation de l'intérêt des IGCTs dans les sous-modules MMC HVDC et comparaison des pertes avec les IGBT, en utilisant des facteurs de mérite spécifiques aux MMC créés dans cette thèse.- Test de double pulse avec diode à récupération rapide dans un module plastique pour tenter de réduire et supprimer l'inductance limitant le di/dt.- Packaging de puces de diodes SiC PiN à haute tension et courant élevé, test avec IGCT dans le même montage, pour tenter de réduire et supprimer l'inductance limite di/dt, et analyser les spécificités de la diode SiC dans ce montage
A study on Integrated gate-commutated thyristors (IGCT) di/dt limiting inductance and RCD-clamp reduction/suppression using plastic module silicon (Si) fast recovery diodes and silicon carbide (SiC) diodes, in Modular Multilevel Converters (MMC). This PhD contains:- Analysis of existing HVDC MMC Submodules.- Assessment of the interest of the IGCT in HVDC MMC Submodules and losses comparison with IGBTs, using MMC-specific figures-of-merit created in this thesis.- Double pulse test with fast recovery diode in plastic module to attempt to reduce and suppress the limiting di/dt inductor.- Packaging of High-Voltage High-Current SiC PiN diode dies, test with IGCT in the same setup to attempt to reduce and suppress the limiting di/dt inductor and analyze the specificities of the SiC diode in this setup

Le courant alternatif (AC) se prêtant bien à la majorité des problématiques de production, de transport et de distribution de l'électricité, on comprend qu'il soit massivement utilisé. Cependant, depuis plus d'un siècle, les bénéfices du courant continu haute tension (HVDC, pour High Voltage Direct Current) pour les longues distances sont bien connus. Aux interfaces, des convertisseurs AC/DC sont requis, leur composition évoluant au fil des avancées technologiques. Après avoir présenté les spécificités du HVDC et les contraintes qu'il introduit sur les convertisseurs AC/DC, ce manuscrit se focalise sur trois topologies : Modular Multilevel Converter (MMC), Alternate Arm Converter (AAC) et Series Bridge Converter (SBC). Elles sont présentées, dimensionnées et analysées en détail, puis comparées de façon quantitative en utilisant des indicateurs de performance originaux. Il en ressort que le MMC et le SBC sont particulièrement intéressants. La méthode de commande conventionnelle du MMC est ensuite présentée et ses propriétés structurelles sont mises en évidence. Une première loi de commande originale est présentée, avec des performances similaires mais une complexité inférieure à l'état de l'art. La seconde est non linéaire, basée sur la théorie de la platitude différentielle, et permet un suivi de puissance très rapide tout en assurant la stabilité exponentielle globale du système. Ces lois de commande sont évaluées en simulation, avec un modèle moyen et un modèle détaillé intégrant 180 sous-modules par bras. La dernière partie concerne le SBC. Après l'avoir modélisé, des résultats concernant une analyse structurelle de la topologie sont présentés ainsi qu'une loi de commande originale. Le rôle fondamental du transformateur pour les convertisseurs à structure série comme le SBC est souligné. Enfin, les performances de la loi de commande proposée sont testées en simulation
As Alternating Current (AC) is well suited for most of the production, transmission, and distribution applications, its massive use is easy to understand. However, for over a century, the benefits of High Voltage Direct Current (HVDC) for long-distance energy transmission are well known. To connect both, AC/DC converters are mandatory, whose nature evolves with technological progress. After the problematic induced by HVDC on AC/DC converters is presented, this manuscript is focused on three topologies: Modular Multilevel Converter (MMC), Alternate Arm Converter (AAC) and Series Bridge Converter (SBC). They are presented, sized, analyzed thoroughly, and compared in quantitative terms, using original key performance indicators. It appears that MMC and SBC are particularly promising. The conventional control method of the MMC is then presented, and its structural properties are highlighted. A first original control law is presented, with similar performances but less complexity than the state-of-the-art. A second control law, non-linear and based on differential flatness theory, is introduced. It allows a very fast power tracking response while ensuring the global exponential stability of the system. These control laws are tested in simulation, using an average model and a detailed model with 180 sub-modules per arm. The last part is dedicated to the SBC. After a modeling step, some results regarding its structural analysis are presented, and an original control law is introduced. The essential role of the transformer for series converters like the SBC is highlighted. Finally, the performance of the proposed control law is assessed in simulation

The modified mapping-collocation (MMC) method is applied to analyze a circumferential crack in an isotropic curved beam. Based on the method a MATLAB code was developed to obtain the stress field. Incorporating the stress correlation technique, the opening and sliding fracture mode stress intensity factors (SIF)s of the crack for the beam under pure bending moment load case are calculated. The MMC method aims to solve two-dimensional problems of linear elastic fracture mechanics (LEFM) by combining the power of analytic tools of complex analysis (Muskhelishvili formulation, conformal mapping, and extension arguments) with simplicity of applying the boundary collocation method as a numerical solution approach. Qualitatively, a good agreement between the computed stress contours and the fringe shapes obtained from the photoelastic experiment on a plexiglass specimen is observed. Quantitatively, the results are compared with that of ANSYS finite element analysis software. The effect of crack size, crack position and beam thickness variation on SIF values and mode mixity is investigated.

碩士
中山醫學大學
職業安全衛生學系碩士班
102
The metal matrix composite (MMC) was widely used in automobile and aerospace. However it is difficult to machining and welding, the components was thus made to net shape or near-net-shape. To expand the application of MMC, the traditional fusion welding process was used to weld the components of MMC. However, it has several potential hazardous factors, high temperature, fume, toxic gases, and radiations. Those hazards will affect the labor’s health and safety. Furthermore, the density of reinforcement is lighter than that of metal matrix, thus a segregation defect would form when the traditional fusion welding process was applied to join the metal matrix composite. In this study, the solid state diffusion bonding process, hot pressing method was used to join Cu/(SiC)p metal matrix composite. To compare the different interlayers for bonding quality, aluminum foil and copper foil with a thickness 50 μm was selected as a bonding layer, and a direct bonding of Cu/(SiC)p MMC without using interlayer. The major bonding parameters of solid state diffusion bonding, bonding durations, bonding temperature, and fraction of reinforcement were investigated. The bonding strength of MMCs was evaluated using a shear test, the observation and compositions identified on the fracture surfaces were using optical microscopy (OM) and scanning electron microscope (SEM). The phase identified was using Electron Probe X-ray Microanalyzer (EPMA). Hardness measurements were conducted using a 100g load with a testing time of 15s. The experiment result showed that the Cu/(SiC)p MMCs bonded successfully using the hot pressing process. With increasing bonding temperature and bonding durations , the relative density increased and the shear strength increased. An elevated bonding temperature enhances the atomic interdiffusion between Cu/(SiC)p base metal and the Cu interlayer. However, at high fractional SiC of Cu/(SiC)p MMCs, its relative density and the shear strength were decreased because the SiC particles clusters at bonding interface to obstruct the atomic interdiffusion, and the delamination was formed at bonding interlayer. With increasing bonding durations, the atom has enough time to conduct atomic interdiffusion and to improve the bonded quality. A dissimilar interlayer to bond Cu/(SiC)p MMCs using the hot pressing process conducted, the crack was formed due to different of coefficient thermal expansion (CTE) between Cu/(SiC)p and Al interlayer. For Cu/(SiC)p MMCs bonded without interlayer, its bonding quality would be degreed when a high fractional SiC reinforcement in Cu/(SiC)p MMCs, because SiC particles at bonding interface would obstruct atomic interdiffusion from a good bonding strength. Therefore, a good bonding quality can be achieved for Cu/(SiC)p MMC bonding with the Cu interlayer.