Dissertations / Theses on the topic 'AA 2024'

Thesis (M.S.)--University of Cincinnati, 2005.
Title from electronic thesis title page (viewed Apr. 21, 2006). Includes abstract. Keywords: Polyaniline, Coatings, Corrosion. Includes bibliographical references.

In this study investigations were made into the effects of Ultrasonic Impact Treatment (UIT) on pre-exfoliated AA 2024 T351. Electrochemical tests were conducted to determine any changes in electrochemical behaviour of the alloys due to UIT condition. Uniaxial monotonic, cyclic and fatigue crack propagation (FCG) tests were conducted on material in As-Received (AR) condition and UIT condition in air and in a corrosive environment by pre-exfoliating the test specimens at pre-defined set of time periods and temperature range (20 °C). It was clear that the fatigue performance was severely reduced by the introduction of the corrosion environment for AR specimens. SEM analysis suggests that UIT conditioned AA 2024-T351 exhibits resistance to exfoliation corrosion at ambient and temperatures ranging between 20°C to 40°C when compared to AR specimens. However, these results are not supported by potentiodynamic polarisation curves which show a decrease in corrosion resistance of UIT specimens. Also it is understood that there is a considerable amount of Cu refinement and enrichment near the surface when the AA 2024-T351 is subjected to exfoliation corrosion tests. Whilst hydrogen is in an atomic state, it can be adsorbed onto the metal surface and consequently diffuses into the matrix and can have serious detrimental effects. A reference line for minimal pre-existing hydrogen in the alloy is identified and the magnitude of hydrogen is found to be 180 Arbitrary Hydrogen Units (AHU). It is also found that in corrosion environment, the hydrogen ingress and further charging is prominent at ambient temperatures for AR samples, and showed damage over the full width of cross-section. The following conclusions were drawn: 1. UIT surface treatments, followed by exfoliation corrosion have shown increased resistance to a reduction in mechanical properties, notably tensile and yield strength. Fractographic analysis further supported this finding by showing smaller average brittle failure depths for UIT specimens when compared to AR specimens. SEM analysis of AR samples (without exfoliation) showed a crystallographic contribution to the mode of failure where high density slip bands are formed and the initial failure exhibits a step format. 2. It is observed that during exfoliation, hydrogen ingress and adsorption is more prominent at ambient temperatures for AR samples. 3. A slight improvement In Low Cycle Fatigue (LCF) life is observed for pre-exfoliated and UIT conditioned sample but not for UIT conditioned samples only. Little effect is observed for either treatment in the High Cycle Fatigue (HCF) region. 4. Fatigue crack initiation occurred from the edges for all samples. 5. Fatigue crack propagation of exfoliated specimens exhibited faster crack propagation than As-Received specimens. UIT caused retardation in crack propagation rate in AR samples but not in corroded samples. Failure of pre-exfoliated, UIT treated samples failed within the treated strip. It is also noted that crack deviation can occur when the crack tip reaches a secondary phase particle. 6. Nanocrystallisation generates uniformity of the surface which refines the secondary phase particles and helps mitigate crack initiation sites.

The objective of this study was to investigate the low velocity impact behavior of both monolithic and laminated aluminum alloy plates. For this purpose, a drop-weight test unit was used. The test unit included the free fall and impact of an 8 kg hammer with an 8 mm punching rod from 0.5 m to 4 m. The relationship between the change in static mechanical properties (hardness, ultimate tensile strength, yield strength, strain hardening rate) and low velocity impact behavior of monolithic aluminum plates were investigated. Tested material was AA 2024, heat treatable aluminum alloy, which was artificially aged to obtain a wide range of mechanical properties. In the second stage of the study, the relationship between the low velocity impact behavior of laminated plates was compared with that of monolithic aluminum plates at identical areal densities. For this purpose, a series of AA 2024 thin plates were combined with different types of adhesives (epoxy, polyurethane or tape). Finally, fracture surface of the samples and microstructure at the deformation zone were examined with both scanning electron microscope and optical microscope. It is found that the ballistic limit velocities of AA 2024 plates increase with increase in hardness, yield strength and ultimate tensile strength. It is also found that a linear relation exists between the ballistic limit velocity and strain hardening rate or hardness. When the low velocity impact behaviors of laminated and monolithic targets were compared, it was seen that monolithic targets have a higher ballistic limit velocity values for from the 2.5 to 10 mm thick targets. It was also observed that adhesives are not so effective to strengthen the low velocity impact performance. On the other hand, with increasing Charpy impact energy, penetration and perforation behaviors are getting worse in 10 to 30 joules energy range. Different types of failure mechanisms involving, plugging, dishing, stretching and bending were determined. For high strength and thick plates plugging type deformation was leaded. In contrast, for thinner and weaker targets bending, stretching and dishing type failures were dominating. For laminated targets also dishing type failure was determined.

The present study investigates the effect of fluorine species during anodizing of aluminium and AA2024-T3 alloy in sulphuric acid and tartaric-sulphuric acid (TSA) electrolytes. The investigation comprises four main parts; (i) Effects of fluoride on barrier film formation on aluminium. (ii) Effects of fluoride and fluorozirconic acid (FZ) on porous film growth on aluminium in sulphuric acid. (iii) Effects of FZ on porous film growth on aluminium and AA 2024-T3 alloy in sulphuric acid and TSA. (iv) Effects on anodizing of other fluoroacids (fluoroboric (FB), fluorosilicic (FS) and fluorotitanic acid (FT)). The anodic films were examined by analytical scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, Rutherford backscattering spectroscopy, nuclear reaction analysis and glow discharge optical emission spectroscopy. The behaviour of fluoride ions during the growth of barrier-type films on aluminium was investigated in ammonium pentaborate solution with added sodium fluoride. Additions of up to 3.5 x 10-3 M sodium fluoride had a negligible influence on the film growth. In contrast, 3.5 x 10-2 M sodium fluoride reduced the efficiency to 60% as fluoride ions promoted field-assisted ejection of Al3+ ions from the film. Incorporated fluoride ions migrated inwards at a rate about twice that of O2- ions, forming a fluoride-rich layer at the film base. The study of the influence of FZ on formation of porous anodic films in sulphuric acid and TSA employed a range of anodizing voltages, electrolyte temperatures and anodizing times. Fluoroacid increased the growth rate, with a reducing influence as the temperature increased. The films contained fluoride and sulphate ions, zirconium was not detected. The fluoride concentration decreased with increasing temperature, whereas the sulphate concentration was unaffected. Anodizing aluminium and AA 2024-T3 alloy in other fluoroacids resulted in similar influences on the anodizing behaviour as FZ. The differences in growth rate, film composition and film morphology were comparatively small and did not show a systematic dependence on the type of fluoroacid employed. Boron, silicon and titanium were not detected in the films.

O processo de estiramento consiste na conformação de chapas, barras e seções laminadas ou extrudadas, pelo "esticamento" do material sobre uma ferramenta (bloco) que contém a forma (linha de sistema) desejada. Este processo é utilizado principalmente na fabricação de peças da indústria aeroespacial. Os materiais empregados são ligas de alumínio, titânio, aço e níquel. O estiramento de chapas de ligas de alumínio, com relação à têmpera do material, pode ser realizado em três condições: com o material recozido (condição "O"), com o material na condição final de têmpera (condição "TXXX"), ou após a solubilização do material (condição "W"). A utilização de uma ou outra condição de estiramento está intimamente ligada à complexidade da geometria da peça, à qualidade da ferramenta de estiramento e aos valores de alongamento da liga. O estiramento com o material recozido (condição "O") apresenta os melhores resultados com relação ao retorno elástico (spring back), mas a conformação total da peça resulta em valores elevados de deformação e um ou todos os problemas podem acontecer: ruptura do material, aparecimento de "estrias" (bandas de Lüders) e cascas de laranja (orange peel). A situação ideal, em termos de custos de fabricação e tempo de execução, seria realizar o estiramento da peça na condição final de têmpera do material (TXXX). Entretanto, quando comparadas com os aços, as ligas de alumínio apresentam valores de alongamento inferiores, o que faz com que este material suporte níveis menores de deformação. Com níveis menores de deformação, o retorno elástico será muito grande e a peça não atenderá aos requisitos de forma (linha de sistema) necessários para uma montagem adequada. Como uma alternativa para os problemas encontrados no estiramento de chapas nas condições "O" e "TXXX", desenvolveu-se uma condição intermediária, chamada de "W". Esta condição é metaestável e é obtida após a solubilização da liga de alumínio. O estiramento das chapas de alumínio, nessa condição metaestável, é uma boa solução para as situações citadas, ou seja, aparecimento de defeitos superficiais (estrias e cascas de laranja) ou elevado retorno elástico ou efeito mola (spring back). Faz-se necessário, entretanto, um conhecimento detalhado do processo de estiramento, para garantir uma faixa de dureza na qual se obtém um resultado adequado. Deve-se estirar a chapa para valores de dureza em que não se obtenham defeitos superficiais ou elevados valores de retorno elástico. Este trabalho indica que o estiramento da liga AA 2024 após a solubilização (condição "W") apresenta valores de retorno elástico inferiores aos apresentados pelas outras condições avaliadas ("O" e "T3"), sem aumento significativo da rugosidade superficial, o que indica menor susceptibilidade ao aparecimento de defeitos superficiais, como a casca de laranja (orange peel).

Composants principaux des couches de conversion chimique sur le marché, les composés à base de chrome hexavalent vont être interdits par la réglementation européenne REACh à partir de l'année 2024. Depuis plusieurs années, les industriels du secteur aéronautique développent donc des solutions de substitution basées sur le chrome trivalent, moins toxique et moins néfaste pour l'environnement que le chrome hexavalent, tout en conférant des propriétés anticorrosion prometteuses aux alliages d'aluminium. Le déploiement des solutions de conversion chimique à base de CrIII à l'échelle industrielle a mis en évidence une sensibilité forte de ces procédés aux propriétés de surface des alliages d'aluminium. Ce projet de thèse vise à améliorer la robustesse du procédé de conversion au chrome trivalent en identifiant les paramètres de premier ordre contrôlant les performances anticorrosion des couches. Pour cela, des éléments de compréhension concernant les modifications de surface causées par la préparation de surface et les mécanismes de croissance de la couche ont été apportés. Les travaux de thèse ont été réalisés sur l'alliage d'aluminium 2024 largement employé dans le domaine aéronautique. Différents lots d'alliage, états de surface et états métallurgiques ont été considérés afin de déterminer l'impact de la métallurgie de l'alliage sur les propriétés de surface après préparation de surface ainsi que sur les propriétés anticorrosion de la couche formée. L'étude de la réactivité de l'alliage dans le bain de conversion chimique a ensuite permis de mettre en évidence les mécanismes de nucléation et croissance de la couche de conversion. Un modèle de formation de la couche en surface de l'alliage 2024-T3 a ainsi été proposé
Hexavalent chromium, one of the main components of the chemical conversion coatings on the market, will be banned by European REACh regulations from 2024. From several years, manufacturers in the aeronautical sector have been developing substitution solutions based on trivalent chromium. These solutions with promising anti-corrosion properties as compared to hexavalent chromium are less toxic and less harmful to the environment than hexavalent chromium. However, the deployment of the CrIII conversion coatings to the industrial scale highlighted a higher sensitivity of these processes, as compared to the CrVI solutions, to the surface properties of the aluminium alloys. This work aims at improving the robustness of the trivalent chromium conversion process of the aluminium alloys by identifying the first order parameters controlling the anticorrosion performances of the conversion coatings. For that purpose, elements of understanding concerning the surface modifications caused by the surface preparation as well as data on the mechanisms of growth of the coating were brought. The thesis work was carried out on aluminium alloy 2024, which is widely used in the aeronautical field. Various alloy batches, surface states and metallurgical states were considered in order to determine the impact of alloy metallurgy on the surface properties after surface preparation as well as on the anticorrosive properties of the coating. The study of the reactivity of the alloy in the chemical conversion bath gave relevant data on the mechanisms of nucleation and growth of the conversion layer. As a result, a model for the formation of the conversion coating for 2024-T3 alloy was proposed

The aircraft industry has long recognised the importance of climate protection and the benefits of reducing weight for the production of cost effective and fuel efficient aircraft structures. Fibre laser welding provides advantages over conventional riveting, mainly in terms of weight reduction and time saving. However, significant changes in microstructure, metallurgical state and associated mechanical properties occur in welded joints. Such changes can result in residual stresses, distortions and defects formation in the welded structure, thus significantly influencing the performance and service life. In order to maintain structural integrity of welded structures, the relationship between welding process and performance of the structure needs to be fully assessed. In this thesis, comprehensive relationships between materials, welding process, microstructure and mechanical properties of welded joints were established. Welding parameters including power density, laser power, welding speed, focal position, filler metal feed rate and shielding gas composition were optimised to produce high quality full penetration welds. Solidification cracking was found to be a critical issue in AA 2024-T3 when welding without filler metal. The addition of filler metal reduced its crack sensitivity but it was also necessary to provide the optimum feed rate to avoid welding defects and keyhole instability. Sufficiently high laser power and low welding speed were required for full penetration and also to minimise welding defects. Both argon and helium shielding gases were found to be effective since only weakly ionised laser induced vapour plume was formed rather than strongly ionised plasma. Softening in AA204-T3 deteriorated the plastic straining capacity of the weld due to confined plasticity development within the weld. A poor weld quality resulted in a mixed mode of brittle and ductile failure and contained micro porosities and hot cracks, whereas, a good weld quality led to a ductile mode with significantly less welding defects. In the case of Ti-6Al-4V, the strength was the greatest in the weld as a result of martensitic microstructure formed during fast cooling rates. Local plastic deformation was the lowest in the weld and therefore, failed in the parent material but at the cost of reduced ductility relative to the unwelded parent tensile specimens. The residual stresses and distortions due to time dependent and localised heating imposed during fibre laser welding were numerically simulated with thermal and mechanical boundary conditions integrated in the finite element models including post weld heat treatment, mechanical stress relieving treatment and various clamping arrangements. Mechanical boundary conditions had relatively small influence on residual stresses in thin sheets of butt welded specimens, whereas, greater restraints led to higher residual stresses and lower restraints led to lower residual stresses in T-joint specimens. Non-isothermal diffusional and diffusionless phase transformations in Ti-6Al-4V were modelled and their influence on residual stresses and distortions was examined. Phase transformations only had a small influence on the magnitude and distribution of residual stresses and distortions because the level of internal stresses due to phase transformation remained low unlike other materials which exhibit greater differences in the specific volumes between phases. Post weld heat treatment (PWHT) induced diffusional phase transformations via decomposition of martensite into α. It also decreased the magnitude of y stresses to the yield strength of Ti-6Al-4V at the treatment temperature by releasing the locked-in stresses. Mechanical stress relieving was also studied for reducing residual stresses and distortions, by means of plastic deformation applied during as well as after welding. When the load reached more than 50% of its yield strength, the stresses became compressive. Residual stresses were experimentally measured using X-ray and neutron diffraction techniques They were found to be dependent on the crystallographic hkl plane due to the presence of microscopic stresses. In the case of Ti-6Al-4V, the reflections were weak and only few times larger than the background due its highly incoherent cross-section. In addition, texture in Ti-6Al-4V weld also contributed to lower intensity counts observed during measurements. As a result, only certain peaks were detected in certain orientations. The Y residual stresses in the welding direction were very high but not as high as the yield strength of the material at room temperature for both AA 2024-T3 and Ti-6Al-4V. They were largely tensile in nature only within the weld and tended to be weakly compressive in the rest of the specimen. Comparative analyses between experimental and numerical results showed good agreements, proving the robustness of the finite element models.

A redução de peso é uma questão tecnológica fundamental para a indústria aeroespacial, uma vez que diminui o consumo de combustível, resultando em redução de custos e contribuindo para a redução da emissão de gases de efeito estufa. Devido à relação favorável entre resistência mecânica e peso, as ligas de alumínio de elevada resistência mecânica contribuem favoravelmente para este aspecto. Entretanto, como estas ligas são dificilmente soldáveis pelos processos tradicionais envolvendo fusão, o procedimento de junção utilizado em aeronaves é a rebitagem, resultando em ganho de peso. O processo de soldagem por fricção (friction stir welding -FSW), desenvolvido no início dos anos noventa pelo The Welding Institute (TWI) do Reino Unido, se constituiu em um grande avanço para a soldagem das ligas de alumínio utilizadas na indústria aeroespacial, pois permite a produção de soldas mais confiáveis e virtualmente livres de defeitos. Entretanto, o aquecimento das peças e a deformação mecânica durante a FSW geram zonas com diferentes características microestruturais que, de acordo com a literatura, apresentam resistências à corrosão diferentes. Por oferecerem elevada resolução lateral, as técnicas eletroquímicas locais são úteis para elucidar as diferenças de reatividade local de eletrodos heterogêneos, como no caso de metais soldados. No presente trabalho técnicas eletroquímicas locais foram empregadas para caracterização da resistência à corrosão em meio contendo cloreto das diferentes zonas geradas pela soldagem de topo da liga de alumínio 2024-T3 por FSW, comparando-a com a exibida pelo metal base. O estudo foi complementado com a caracterização microestrutural destas regiões e também por ensaios macroscópicos de corrosão. Os resultados dos procedimentos de caracterização microestrutural confirmaram que a FSW provoca modificações na microestrutura das regiões afetadas pelo processo, principalmente no que concerne à distribuição das nanopartículas precipitadas durante o envelhecimento natural da liga. Por sua vez, os resultados dos ensaios macroscópicos de corrosão e eletroquímicos locais mostraram-se concordantes na determinação da região mais sensível à corrosão, que foi verificada como sendo as zonas termicamente afetada (Heat Affected Zone - HAZ) e termomecanicamente afetada (Thermomechanically Affected Zone - TMAZ) do lado do avanço da ferramenta de soldagem, mostrando também que as regiões afetadas pelo processo de soldagem apresentam resistência à corrosão inferior à do metal base. Através do uso da espectroscopia de impedância eletroquímica local (Local Electrochemical Impedance Spectroscopy - LEIS) foi evidenciado que o acoplamento galvânico entre as diferentes zonas geradas durante o processo de soldagem não desempenha um papel relevante na aceleração do processo corrosivo, o que está em desacordo com os resultados publicados em diversos estudos realizados com esta liga soldada por FSW. O trabalho apresenta ainda uma contribuição teórica original demonstrando que medidas de ângulo de contato e de espectroscopia de impedância eletroquímica em uma gota séssil podem ser usadas simultaneamente para a determinação da capacitância da dupla camada elétrica. As previsões do modelo teórico foram confirmadas tanto através de resultados obtidos com um sistema modelo como também em determinações realizadas nas diferentes regiões geradas pela soldagem por FSW da liga 2024-T3.
Weight reduction is a fundamental technological issue for the aerospace industry, as it decreases the fuel consumption, resulting in reduced both costs and greenhouse gases emission. Due to the favorable relation between strength and weight, high strength aluminum alloys favorably contribute to this aspect, but they remain difficult to weld by conventional processes involving fusion, and, therefore, the junction procedure used in aircraft is riveting, resulting in weight gain. The friction stir welding (FSW) process, developed in the early nineties by the \"The Welding Institute\" (TWI), United Kingdom, is a major breakthrough for the welding of aluminum alloys as it allows the production of more reliable and virtually defect-free welds. However, the heating of the parts and the mechanical deformation during FSW generate zones with different microstructures with different corrosion resistances. As they offer high lateral resolution, local electrochemical techniques are useful for elucidating differences in local reactivity of heterogeneous electrodes, as the case of welded metals. In the present work, local electrochemical techniques were employed to characterize the corrosion resistance in chloride environment of the different zones generated by butt welding the 2024-T3 aluminum alloy by FSW, and to compare this response with that displayed by the base metal. The study was complemented with the microstructural characterization of these regions and also by macroscopic corrosion tests. The results of the microstructural characterization confirmed that FSW causes changes in the microstructure of the regions affected by the process, especially with regard to the distribution of the precipitated nanoparticles during the natural aging of the alloy. The results of the macroscopic corrosion and of the local electrochemical tests showed good agreement in the determination of the most sensitive regions to corrosion, which were found to be the heat affected (HAZ) and the thermomechanically affected (TMAZ) zones of the advancing side of the weld tool. They also showed that the regions affected by the welding procedure have a lower corrosion resistance than the base metal. By using Local Electrochemical Impedance Spectroscopy (LEIS), it was shown that the galvanic coupling between the different areas generated during the welding process does not need to be taken into account in the description of the corrosion process, which is at odds with the results published in several studies of this alloy welded by FSW. The work also present an original theoretical contribution, demonstrating that contact-angle measurements and electrochemical impedance spectroscopy in a sessile drop can be used simultaneously to determine the capacity of the interface. The theoretical model predictions were confirmed by the experimental results obtained both with a model system and in the different regions generated by FSW of aluminum alloy 2024-T3.

A aplicação de camada de conversão de cromato é comumente realizada na superfície de ligas de alumínio para proteger estes materiais da corrosão. Apesar de sua inigualável eficiência e do desejável efeito autorreparador, este tratamento utiliza Cr6+, um agente carcinogênico e que pode acarretar graves consequências ambientais durante a destinação dos resíduos tóxicos e poluidores gerados durante o processo. Neste trabalho, avaliou-se um filme sol-gel de silício-zircônio contendo microemulsão de cério (SGMECe) como alternativa ao tratamento por cromação, optando-se por evitar o uso de compostos orgânicos voláteis (tóxicos), minimizar a geração de resíduos poluidores e, ainda, incorporar aditivos inibidores de corrosão em nanorreservatórios. O filme de silício-zircônio foi preparado pelo método sol-gel a partir de tetraetoxissilano e acetato dea zircônio; o elemento inibidor de corrosão foi introduzido no sol na forma de nanogotículas dispersas em um sistema de microemulsão água-em-óleo. Para o preparo da microemulsão utilizou-se óleo de Pinhão-manso como fase orgânica, uma solução aquosa de cério e tensoativos não-iônicos biodegradáveis, tendo sido a formulação e avaliação da estabilidade deste sistema propostas neste trabalho. Análise por microscopia eletrônica de varredura mostrou que os filmes SGMECe foram homogeneamente depositados e aderidos na superfície de ligas AA2024-T3, sem a necessidade de pré-tratamento por decapagem. Apesar de filmes sol-gel inorgânicos apresentarem baixa flexibilidade, induzindo a formação de trincas durante a evaporação de solvente, a incorporação de microemulsão parece ter minimizado a formação destes defeitos, provavelmente por diminuição da tensão capilar devido à presença do tensoativo. Ensaios eletroquímicos mostraram que o filme SGMECe atua como barreira passiva, inibindo cineticamente o processo de corrosão da liga de alumínio. Além disso, a adição de quantidades crescentes (de 3 a 7% em massa) de microemulsão de cério ao sol-gel resultou em aumento da resistência do filme e da resistência à transferência de carga, corroborando com a conclusão de que o filme SGMECe atua inibindo a corrosão da liga AA2024, sendo o cério o elemento ativo deste efeito inibidor.

In this work, the nucleation and growth of pores in anodic films formed on aluminium in chromic acid and the effect of low levels of sulphate impurity in the anodizing bath on the formation of the films on aluminium and AA 2024 alloy are investigated. The sulphate concentrations considered include levels within specified limits for industrial processing. The anodizing is carried out either potentiostatically or by stepping the voltage. The films are examined by scanning electron microscopy, transmission electron microscopy and atomic force microscopy to determine the pore spacing, pore population densities, pore diameters and film thicknesses. Film compositions were determined using energy-dispersive X-ray spectroscopy, Rutherford backscattered microscopy and nuclear reaction analysis. In order to investigate the mechanism of pore formation, two tracer methods are employed. In one method, anodic films are formed first in an arsenate electrolyte in the second method, a tungsten tracer band deposited by magnetron sputtering. The behaviours of arsenic and the tungsten are investigated during the subsequent anodizing in chromic acid. The results suggest that the initiation and growth of pores in occurred as a result of electric field assisted chemical dissolution. The effect of sulphate impurity in the chromic acid is investigated using electrolytes with different sulphate content. In the initial stages of anodizing aluminium at 100 V, sulphate impurity at a level of 38 ppm in the chromic acid is shown to lead to significant incorporation of sulphate ions into the anodic film, a lower current density, a smaller cell size and less feathering of the pore walls. In addition, the efficiency of film formation is increased. In later stages of anodizing, the growth of larger pores and cells, leads to a duplex film morphology, with finer pores in the outer region. The change in pore size correlates with a reduction in the incorporation of sulphate into the film. From the results of sequential anodizing experiments, it is suggested that incorporated sulphate ions generate a space charge layer, which has an important role in determining the current density. The effects of higher sulphate concentrations up to 3000 ppm are investigated, which are shown to significantly affect the current density and the pore diameter. Anodizing of aluminium and AA 2024 alloy was also carried out according to industrial practice. The results show that there is significant effect of sulphur impurity on the film thickness. Corrosion tests in 3.5 % NaCl solution for the alloy after anodizing in low (smaller or equal to 1.5 ppm) and high (~38 ppm) sulphate-containing chromic acid electrolytes demonstrate a better corrosion resistance with films formed in the latter electrolyte.

A liga de alumínio AA2024-T3, amplamente utilizada na indústria aeroespacial, apresenta elevada resistência mecânica conferida pela adição de elementos de liga, principalmente o Cu, que formam precipitados finamente distribuídos e coerentes com a matriz, durante o processo de envelhecimento controlado. No entanto, durante este processo, também se formam intermetálicos (IMs) maiores com composição e atividade eletroquímica diferentes da matriz, tornando a liga altamente suscetível a processos de corrosão localizada. A literatura apresenta diversos trabalhos sobre o estudo da corrosão da AA2024-T3 associada à presença dos IMs, porém, o tema ainda é bastante controverso, e muitos aspectos precisam ser estudados com mais detalhes ou mesmo ainda desvendados. Nos últimos anos, técnicas de microscopia de alta resolução, como a microscopia eletrônica de transmissão (TEM) e microscopia eletrônica de varredura (MEV), associadas à espectroscopia por energia dispersiva de raios X (EDS) e a microscopia de força atômica (AFM) bem como técnicas eletroquímicas convencionais e localizadas têm auxiliado nestas pesquisas. Na presente tese serão apresentados os resultados dos estudos realizados sobre a corrosão da liga AA 2024-T3 em soluções de cloreto de baixa concentração. A investigação foi realizada aliando as técnicas de caracterização anteriormente descritas com técnicas eletroquímicas tradicionais: acompanhamento do potencial de circuito aberto (OCP) com o tempo, curvas de polarização (CP) e espectroscopia de impedância eletroquímica (EIS) e técnicas com elevado poder de resolução espacial: como a técnica do eletrodo vibratório de varredura (SVET). A caracterização microestrutural confirmou a heterogeneidade dos IMs, e como contribuições principais, evidenciou a existência de fases secundárias (cristalinas ou não) no interior dos próprios IMs e que a matriz da liga nas proximidades dos IMs apresenta microestrutura variável, todos estes aspectos podendo contribuir de maneira sinérgica para o complexo comportamento de corrosão exibido pelos IMs. Além do mais, mostrou-se que na corrosão os IMs estão submetidos a ataque eletroquímico e químico, este último ocasionado por aumento local do pH. Os resultados dos testes eletroquímicos convencionais mostraram que nas condições empregadas no presente estudo a corrosão da liga é dominada pelo comportamento de corrosão dos IMs, e que, dentro deste contexto, o transporte de oxigênio até os sítios catódicos formados pelos IMs, ao invés da concentração de cloretos, parece ser a etapa controladora da velocidade de corrosão. Por outro lado o tratamento da região em altas freqüências dos diagramas de EIS se mostrou um importante aliado na compreensão dos processos que ocorrem na interface quando da corrosão da liga. No que se refere às técnicas localizadas, a viabilidade do uso da técnica do eletrodo vibratório de varredura (SVET) para estudo do início da corrosão na AA2024-T3 foi avaliada e um sistema para aquisição de dados experimentais por espectroscopia de impedância localizada (LEIS) foi montado e validado. A SVET confirmou que a atividade eletroquímica da liga AA2024-T3 se inicia desde os primeiros instantes de imersão do material no eletrólito teste.
The AA2024-T3 aluminium alloy is largely used in the aerospace industry. It has high mechanical properties due to the presence of alloying elements, mainly copper, that form finely distributed precipitates coherent with the matrix, during controlled aging treatment. However, during this process, large intermetallics (IMs) are also formed with composition and electrochemical activity different from the matrix, making the alloy highly susceptible to localized corrosion. Many works have been published in literature on the corrosion behavior of the AA2024-T3 alloy associated to the presence of IMs, however, the subject is still controversial and many aspects need further detailed investigation. In the last years, high resolution microscopy techniques, such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) associated to X-ray energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM) simultaneously with conventional and localized electrochemical techniques have largely contributed to these studies. In the present work the results of studies carried out on the corrosion behavior of the AA 2024-T3 alloy in chloride solutions of low concentration are presented. The investigation was undertaken by combining the above characterization techniques with conventional electrochemical techniques: open circuit potential (OCP) measurements as a function of immersion time, polarization curves (PC) and electrochemical impedance spectroscopy (EIS) and also techniques of high spatial resolution such as scanning vibrating electrode technique (SVET). The microstructural characterization of the AA 2024-T3 alloy corroborated the heterogeneous nature of the IMs, and as the main contributions, evidenced the presence of secondary phases (either crystalline or amorphous) inside the IMs and the heterogenous microstructure of the matrix at the vicinity of the IMs. All these properties might sinergically contribute to the complex corrosion behavior showed by the IMs. Besides, it was also found that the IMs suffer electrochemical and chemical attack, this last type being caused by local pH increase. The results of the conventional electrochemical techniques showed that under the conditions adopted in the present study, the corrosion of the AA2024-T3 is ruled by the corrosion behavior of the IMs. In this perspective, the oxygen transport to the cathodic sites generated by the IMs rather than the chloride concentration, seems to be the controlling step of the corrosion rate. On the other hand, the analysis of the EIS high frequency data allowed to elucidate the the interfacial processes occurring during the corrosion of the aluminium alloy investigated. The viability of the use of SVET to study the corrosion initiation in the AA2024-T3 was evaluated and a system for experimental data acquisition by localized electrochemical impedance spectroscopy was installed and validated. The SVET showed that the electrochemical activity of the AA2024-T3 takes place since the first periods of immersion in the test electrolyte.

L’alliage d’aluminium 2024 T3 est utilisé dans l’aéronautique pour sa légèreté et ses bonnes propriétés mécaniques. Il présente cependant une microstructure hétérogène le rendant particulièrement sensible à la corrosion. La stratégie de protection actuelle consiste à incorporer des composés à base de CrVI dans les revêtements ; à cause de leur toxicité, il devient cependant de plus en plus urgent de les remplacer. C’est dans ce contexte que s’inscrit cette thèse qui a pour objectif d’étudier l’évolution des matériaux et les phénomènes de corrosion en combinant une technique d’analyse de surface (l’ellipsométrie in situ) et la spectroscopie d’impédance électrochimique. Ce couplage étant une avancée significative dans la compréhension des relations entre le mode de synthèse, la microstructure, la durabilité et les propriétés anticorrosion, permet l’élaboration de revêtements plus performants. Une partie de l’étude a été consacrée à la synthèse de revêtements denses hydrophobes par voie sol gel pour améliorer les propriétés barrières. D’autre part, des films mésostructurés, susceptible d’accueillir et de faciliter la diffusion d’inhibiteurs de corrosion ont été étudiés. Le but final étant de mettre au point un système bicouches pour optimiser les propriétés anticorrosion
Aluminum alloy 2024 T3 is widely used in the aeronautical field due to its high weight-to-strength ratio. However, such material is particularly sensitive to corrosion because of its heterogeneous structure. To protect these alloys, the current strategy is to use a three layer stack doped with CrVI based corrosion inhibitors. Nevertheless, those substances are highly toxic. Therefore, their replacement has become a critical issue for the aircraft industry. In this context, synthesis and characterization of new protective coatings are particularly important. The objective of this work is thus to study materials evolution and corrosion phenomena using in situ ellipsometry and electrochemical impedance spectroscopy. Coupling those tow techniques provides new insights to understand the relationship between coating synthesis, microstructure, durability and anticorrosion properties. On one hand, a study has been done on hybrid hydrophobic layers to enhance barrier properties. On the other hand, mesostructured layers, potential matrix for anticorrosion inhibitors have been studied. The final goal is to couple those two materials in a multi-layer system in order to optimize anticorrosion properties

A numerical simulation is conducted using LS-DYNA to simulate hard projectile impact on a friction stir welded (FSW) plate. As the hard projectile has a wide range of velocity, mass and shape, when referring to AMC 25.963(e) of CS-25, ―Fuel Tank Access Cover‖, the hard projectile can be defined as 9.5 mm cubic-shaped steel engine debris with an initial impact velocity of 213.4 m/s (700 ft/s). This preliminary study was to evaluate whether the fuel tank adjacent skin panel joined by FSW would pass the regulation. First, the geometry and Johnson-Cook material model of the FSW joint were developed based on previous experimental research and validated by comparison with the tensile test on the FSW specimen. Then the impact on an Aluminium Alloy 2024 (AA 2024) plate without FSW was modelled. The minimum thickness of a homogeneous AA 2024 plate which could withstand the impact from engine debris is 3 mm. Finally the impact on 3 mm thick AA 2024 FSW plate was simulated. The welding induced residual stress was implemented in the plate model. The impact centre was changed from the nugget zone to the thermo-mechanically affected zone, heat-affected zone and base material zone of the FSW joint. Penetration only occurred in the model with impact centre on the nugget zone. Additional simulation indicated that increasing the thickness of the FSW plate to 3.6 mm could prevent the penetration.

Orientador: Eugênio José Zoqui
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: A tixoconformação é uma técnica de processamento de materiais no estado semi-sólido que permite a produção de peças com geometrias near net shape com melhores propriedades mecânicas e com razoável redução no custo final de produção, se comparados aos métodos de fundição e conformação plástica tradicionais. Dentro das várias possibilidades de materiais passíveis de trabalho por esta via, as ligas de alumínio são as mais utilizadas, servindo a diversos segmentos industriais. Contudo, o fornecimento de matéria prima próprias ao tixo-processamento está restrito a alguns produtores, justificando a necessidade de aprofundamento e ampliação de conhecimento com respeito às ligas já produzidas por fundição convencional. Este trabalho apresenta um exame da viabilidade de tixoconformação de duas ligas comerciais: AA 2011 e AA 2014. Esta avaliação foi executada em três etapas distintas: 1) análise dos seus constituintes, das temperaturas de fusão e de solidificação, dos microprecipitados existentes e dos tipos de porosidade; 2) reaquecimento de amostras para observar a resposta à globularização em duas temperaturas distintas (duas frações sólidas) e em quatro tempos de espera diferentes, com a posterior análise quantitativa por meio da metalografia; 3) ensaios de viscosidade nas mesmas condições, com posterior análise microestrutural via MEV e EDS das amostras que apresentaram valores de viscosidade muito altos. Os resultados destas avaliações mostraram que estas duas ligas são tixoconformáveis até a temperatura de 610°C, pois apresentaram valores de viscosidade abaixo de 1x106 Pa.s e tensão máxima abaixo de conformação de 1,0 MPa, conforme escolhas adequadas entre tempos de espera e taxa de aquecimento. Porém, também foi constatada a formação de alumina nos poros de algumas amostras após o reaquecimento necessário à tixoconformação, fornecendo um valor mais alto de viscosidade para estas amostras, se em comparação aos valores encontrados em outros ensaios com pouca ou nenhuma formação de alumina. A eliminação de porosidade na matéria prima com a conseqüente minimização do surgimento de alumina viabilizará a tixoconformação de ambas as ligas
Abstract: Thixoforming is a technique for processing materials in the semi-solid state which allows the production of parts near-net-shape geometries, with better mechanical properties and reasonable reduction in the final production cost, compared to traditional casting and plastic forming. Among the various possibilities of materials that can work in this way, aluminium alloys are the most used. However, the world provision of raw material for thixoforming is restricted to few producers, justifying the need for deepening and widening of knowledge with respect to alloys already produced by the conventional casting industries. This dissertation presents an examination of the feasibility of thixoforming of two commercial alloys: AA 2011 and AA 2014. This evaluation was performed in three steps: 1) analysis of their constituents, analysis of melting and solidification temperatures, analysis of the existing micro precipitates and quantity and types of porosity, 2) reheating samples to observe the globularization response to two different temperatures (two solid fractions) and four holding times, with the subsequent quantitative analysis by metallography, 3) tests of the viscosity under the same conditions, with subsequent microstructural analysis by SEM and EDS of the samples that showed very high viscosity values. The results of these evaluations showed that these two alloys are thixoformable up to temperatures of 610°C, since their viscosity values were achieved below than 106 Pa.s, and consequently maximum conformation stress of 1.0 MPa, as appropriate choices between waiting times and heating rate were achieved. However, the formation of alumina in the pores of some samples after the necessary reheating to previous to thixoforming, provide a higher value of viscosity, when compared to values found in other trials with little or no alumina. The elimination of porosity in the raw material, with the consequent minimizing the appearance of alumina will make possible the thixoforming of both alloys
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

Les couches de conversion actuellement utilisées dans l'industrie aéronautique, pour protéger le métal de la corrosion et favoriser l'adhérence de la peinture, contiennent du chrome hexavalent, composé toxique et cancérigène dont l'utilisation va prochainement être interdite par la réglementation européenne REACh. L'une des pistes envisagée pour remplacer ces couches chromatées est l'utilisation de couches de conversion TCP (Trivalent Chromium Protection). Ces travaux portent sur l'étude des couches de TCP et s'attachent à caractériser chaque étape du traitement de surface industriel, incluant les étapes de prétraitement et de post-traitement. Des techniques d'analyse de surface (XPS, ToF-SIMS, AFM, MEB et PM-IRRAS) ont été utilisées pour analyser la composition chimique et la morphologie de la surface après chaque étape du traitement. Cette étude a été réalisée sur un alliage d'aluminium AA 2024-T3, très utilisé en aéronautique pour ses propriétés mécanique, mais présentant une faible résistance à la corrosion. Les résultats obtenus ont notamment mis en évidence que la couche de TCP se forme sur toute la surface de l'alliage (composés intermétalliques et cavités), et ont permis de comprendre comment le post-traitement permet d'améliorer la résistance à la corrosion de la couche de TCP. Ces travaux s'intéressent également à des couches de conversion sans chrome, à base de zirconium, étudiées ici pour servir de point de départ au développement d'une conversion sans chrome qui respecterait les exigences de résistance à la corrosion
Conversion coatings are used in aerospace industry to protect the metal from corrosion and to promote paint adhesion. Currently, chromate conversion coatings are used, but chromate is toxic and carcinogenic and its use will be forbidden by the European REACh regulation. TCP (Trivalent Chromium Protection) conversion coatings, are considered as a promising alternative to replace chromate conversion coating. This work focuses on the characterisation of the TCP layer and considers each step of the industrial surface treatment, including pre-treatment and post-treatment steps. Surface analytical techniques (XPS, ToF-SIMS, AFM, SEM and PM-IRRAS) were used to analyse the chemical composition and morphology of the surface after each step in the process. This work was done on an aluminium alloy AA 2024-T3, commonly used in the aerospace industry for its good mechanical properties, but poorly resistant to corrosion. The results obtained demonstrate, among other things, that the TCP layer totally covers the surface (intermetallic compounds and cavities) and enable to understand how the post-treatment can improve the corrosion resistance of the TCP coating. This work also focuses on conversion coating based on zirconium, which are studied here to be used as a starting point to develop a new conversion coating without chromium, meeting the corrosion resistance requirement

In recent times, application of aluminum alloys is favored in the transportation sectors such as the aerospace and automobile industries where reduced fuel consumption and greenhouse gas emission are major priorities. In these applications, these alloys can be exposed to dynamic shock loading conditions as in the case of car crash and birds’ collision during aircraft’s take-off or landing. This study therefore focused on the deformation and damage mechanisms in AA 2017, AA 2024 and AA 2624 aluminum alloys under both quasi-static and dynamic impact loading conditions. Cylindrical specimens of the selected aluminum alloys were investigated under both quasi-static loading at 3.2 x10-3 s-1 using an Instron R5500 mechanical testing machine and dynamic impact loading using the split Hopkinson pressure bar at strain rates ranging between 2000 and 8000 s-1. The effects of strain rate and temper condition on the microstructural evolution in the alloys during mechanical loading were studied. The electron backscatter diffraction (EBSD) technique was used to investigate the texture of the naturally-aged AA 2017 and AA 2624 alloys before and after dynamic shock loading. The contributions of the major alloying elements such as copper, magnesium and silicon to the microstructural evolution and deformation behavior of the alloys under the dynamic shock loading condition were also studied using the energy dispersive spectroscopy (EDS) technique. Results showed that the morphology and atomic distribution of particles in the investigated alloys are functions of the temper condition. The hardness of all the three alloys was higher in the age-hardened conditions than the annealed ones. Although deformation of the alloy under quasi-static compressive loading was dominated by strain hardening, flow softening leading to strain localization and formation of shear bands occurred once certain critical strain values were reached. Under both quasi-static and dynamic loading, the alloys with low Cu:Mg ratio (AA 2024 and AA 2624) showed higher mechanical strength in age-hardened condition than that with high Cu:Mg ratio (AA 2017). All the alloys in the annealed condition exhibited an enhanced plasticity and formability. Intense strain localization leading to formation of adiabatic shear bands (ASBs) was the principal contributor to failure in the alloys under dynamic impact loading. Both deformed and transformed bands were observed, with cracking occurring mainly along the transformed shear bands. The tendency for formation of adiabatic shear bands is observed to be a function of the alloy composition, temper condition, strain, strain rate and strain hardening rate. In the natural aging condition, AA 2024 showed the highest susceptibility to formation of ASBs followed by AA 2624 and AA 2017 in that order. On the other hand, AA 2024 has the least susceptibility in the artificially-aged condition. Occurrence of bifurcation of transformed bands in dynamic impacted specimens is dependent on temper condition, strain and strain rate. The mechanism of fracture of the precipitation hardened samples is typical of ductile fracture occurring sequentially by nucleation, growth, and coalescence of micro-voids processes within transformed band. Elongated grains were observed to arrest propagating shear band depending on the angle the band makes with elongated grains. The higher the angle of inclination of a shear band to the grain on its path, the higher the tendency of the grain to stop its propagation. Texture analysis of the impacted specimens of AA 2017-T451 and AA 2624-T351 shows that the former has a higher tendency for the evolution of ultra-fine DRX grains within the transformed shear band. High strain rate led to the development of CD//<111> orientations at the expense of CD//<110> orientations. Schmid factor calculations performed on few different orientations in the starting microstructure shows that CD//<110> is less susceptible to slip deformation and consequently underwent rotation to CD//<111>.

Friction Stir Processing (FSP) is emerging as one of the most competent Severe Plastic Deformation (SPD) methods for producing bulk ultra-fine grained materials with improved properties. The significant advantage of FSP is that it can be used for localized microstructural modification which is not possible with the other common SPD techniques such as Equal Channel Angular Processing (ECAP), High Pressure Torsion (HPT), and Accumulative Roll Bonding (ARB). The process is derived from the basic principles of Friction Stir Welding (FSW), a solid state welding technique developed for the high strength aluminium alloys used in structural applications. In FSP, a non-consumable rotating tool with a shoulder and a pin is traversed along the region on the work-piece which is to be modified. In the present investigation on FSP, two heat treatable aluminium alloys with different hot deformation behaviour, 2024 (Al-Cu-Mg) and 2219 (Al-Cu) and a strain hardenable alloy 5086 (Al-Mg) has been considered. FSP involves complex thermo-mechanical interactions and hence the optimization of process parameters is an important aspect to be considered for a successful processing. The number of process parameters involved is more in FSP and the three most important parameters are tool rotational speed, tool plunge depth (normal load on the work piece) and the tool traverse speed. These parameters are varied for a fixed tool geometry and tool tilt angle in a custom-built FSW/FSP machine. A parametric study has been carried out in order to have a clearer picture on the relative importance of various parameters by using tools of different pin lengths. The tool plunge depth and tool rotational speed are also varied in the parametric study. It has been observed that tool plunge depth is the most important parameter for the FSP of high strength aluminium alloys and the first parameter to be optimized. As per the inputs obtained from this parametric study, a systematic experimental procedure has been developed (a bottom-up approach) for optimizing the most important process parameters of FSP. The optimal process parameters obtained from the experimental bottom-up approach has helped in achieving bulk tensile strength higher than the starting material strength for the strain hardenable alloy 5086-O. In heat treatable alloys, due to the presence of a weaker heat affected zone the achievable strength in a single pass FSP were 93% and 80-85% of the starting material strength in the alloys 2024 and 2219 respectively. Micro-tensile testing of the samples taken from the nugget zone of the alloy 2024 indicated an ultimate tensile strength of 1.3 times the starting material strength. This strength increase is attributed to the combined effects of grain size strengthening and precipitation hardening. FSP has been perceived as a grain refinement technique and hence the most important region in any processed sample is the nugget zone. Due to the continuous stirring of the tool pin at high rotation rates, it is possible that different regions in the nugget zone can develop varied microstructure and crystallographic texture. The nugget zone of the optimally processed samples are characterized in detail using the advanced characterization techniques such as Scanning Electron Microscopy (SEM), Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analyzer (EPMA) in order to understand the underlying micro-mechanisms of microstructure and texture evolution. Micro-texture studies on the alloys revealed gradients in textures across the thickness with the dominance of shear texture components. The bulk texture is weaker in all the three alloys. Bulk texture measurements revealed that the texture development during FSP is an alloy independent phenomenon. The dominant texture component observed is different in heat treatable and strain hardenable alloys. The dominant component of texture is identical in both the heat treatable alloys irrespective of the differences in optimal process parameters and the thickness of the plates used. Microstructural evolution during FSP is more of an alloy dependent phenomenon. Particle Stimulated Nucleation (PSN) and Strain Induced Boundary Migration (SIBM) are observed as the dominant nucleation mechanisms of Dynamic Recrystallization (DRX) in the heat treatable and strain hardenable alloys respectively. Normal grain growth through the Burke and Turnbull mechanism is observed with the presence of few larger grains in the microstructure caused by geometrical coalescence. DRX has been observed to occur through separate nucleation and grain growth stage in all the three aluminium alloys and hence indicative of a discontinuous process. Bulk texture development during FSP has been correlated to the microstructure evolution with the mechanisms of PSN and SIBM both weakening the textures in all the alloys. In order to expand the understanding as a commercially viable technique and studying the stability of FSP microstructure and texture, multiple processing routes have been employed. In the Multi-Pass FSP (MP-FSP), the processing is carried out at the same location and the objective is to study the stability of the processed samples under extreme conditions of strain and temperature. In Multi-Track FSP (MT-FSP), an overlap ratio of 0.33 is selected for the successive passes which will allow partial nugget zone penetration. MT-FSP can be used for producing large volume of fine grained materials. It is observed that the microstructure and crystallographic texture is stable under the mild and extreme conditions of strain and temperature. Subsequent heat treatment studies after FSP in the alloy 2024 confirmed that the processed microstructure is stable up to temperatures as high as 723K (450°C). These results are indicative of the advantage of FSP as a successful materials processing technique in which the retained lower strain energies leading to the development of a stable microstructure and texture. Compressive residual stresses are observed at different regions in the nugget zone of all the alloys after FSP. This is attributed to the combined effects of a solid state processing route and the optimal selection of process parameters.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006.
Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3776. Adviser: Robert B. Gennis. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.