Tesi sul tema "AA6061"

Magnesium (Mg) is a light-weight metal that has extraordinary physical and chemical properties for many potential applications in automobile, military, and electronics. Aluminum alloys, because of its light-weight, high strength and corrosion resistance have a wide range of commercial applications. Given these two, sometime competing, alloy systems, there are now many applications where the metallurgical compatibility of Mg- and Al-alloys are required for engineering applications. One such case is the development of diffusion barrier for U-Mo metallic fuel in Al-alloy cladding, where Mg, with its complete immiscibility with U and Mo is being considered as the diffusion barrier. While negligible diffusional interaction between Mg and U-Mo alloys have been reported, diffusional interaction between the Mg and Al-alloy cladding has not been investigated. In this study, solid-to-solid diffusion couples were assembled using discs of pure Mg (99.999 %) and AA6061 Al-alloy. After preparation, Mg was diffusion bonded to AA6061 in sealed quartz capsule at 300°, 350°, and 400°C for 720, 360, and 240 hours, respectively. Scanning electron microscopy was used to inspect the interdiffusion zone, while phase identification was performed using X-ray energy dispersive spectroscopy. One specific phase that exists in the binary Mg-Al system, labeled "?" was observed and characterized by transmission electron microscopy. From the preceding data, the growth rates as well as interdiffusion coefficients of the intermetallic phases were extracted and compared to previous investigations using pure Mg and Al.
M.S.M.S.E.
Masters
Materials Science Engineering
Engineering and Computer Science
Materials Science and Engineering

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.

Includes bibliographical references.
Surface roughening is an undesirable feature in materials. This work investigates the cause of surface roughening in four plates of AA6061 with different thicknesses (Le. 6 mm, 9 mm and two 12 mm). Production of these plates requires thermomechanical processing of the cast slab via a two stage process, namely the initial rolling stage on a hot roughing mill (HRM) and final rolling on the hot finishing mill (HFM). After rolling. the plates are levelled by a tensile stretching process and it is during this process that surface roughening of the plates can arise. Three of these plates exhibited severe roughening and the other 12 mm thick plate exhibited mild surface roughening during the stretch levelling process. Investigations looked closely at how the cause of surface roughening is related to the thermomechanical processing. The four products studied exhibited different degrees of surface roughening during uniaxial tensile deformation. Detailed investigation of the microstructure of these four products was carried out. It was found that the plates that exhibited severe roughening had large flat and elongated grains at the surface. It is the rotation of these surface grains during tensile deformation that was thought to be inducing the roughness in the material. The microstructure evolution was explained relative to the thermomechanical history of the plates. Plane strain compression (PSC) tests were used to simulate the thermomechanical processing cycles on the HFM. Large strain rates used in the industry were compensated with an adjustment in temperature so that an equivalent Zener-Holloman parameter (Z) could be achieved. The microstructure of the PSC specimen was characterised using the electron back scattered technique. It was found that the grain size increases with a decrease in the value of Z.

Includes bibliographical references.
Effects of rolling variables - strain (?), strain rate (?) and temperature (T) - on grain structure and hardness evolution in AA6061 plates were investigated by simulating the rolling process using plane strain compression (PSG). Particular emphasis was exerted on understanding the impact of the variables on the overall mean grain size and hardness, as well as how they influence the grain size and hardness profiles through the thickness of the rolled plates. Prior to PSG investigations, the grain size and hardness profiles of the supplied material (transfer and finished plates) were determined using a Reichert MeF3A optical microscope and Vickers hardness tester. No significant grain size variations were observed through the thickness of transfer plate. On the contrary, finished plates exhibited grain size gradients, with grain size progressively decreasing from the surface to the centre of the plate thickness.

A indústria automotiva tem mostrado um crescente interesse pelas ligas de alumínio, em especial as ligas de alumínio da série 6xxx. Esta classe é amplamente empregada nas áreas de construção e de transporte, devido a sua boa resistência mecânica e excelente resistência à corrosão. No setor automobilístico sempre houve a necessidade de aprimoramento dos estudos do comportamento em fadiga, pois os componentes estruturais são submetidos a carregamentos vibratórios, esforços e tensões cíclicas e, como consequência, podem trincar e finalmente fraturar. A presença de um entalhe geralmente diminui a vida em fadiga, criando regiões de altas tensões triaxiais localizadas que restringem a deformação plástica, fragilizando o material. A resposta mecânica depende das solicitações, microestrutura, componentes da liga e propriedades do material. Neste trabalho foi realizado o estudo do comportamento em fadiga de alto ciclo e da sensibilidade ao entalhe de três ligas de alumínio da série 6xxx destinadas à fabricação de componentes de carroçarias para caminhões e ônibus: AA 6005, AA 6063 e AA 6351, todas na condição T6. As curvas S/N foram obtidas por meio de ensaios de fadiga em flexão rotativa (R = -1). Ensaios com peças entalhadas (Kt ? 3,0) permitiram determinar e comparar o fator de concentração de tensão em fadiga e a sensibilidade ao entalhe das ligas estudadas. Também foi estudada a influência da microestrutura e das partículas intermetálicas sobre as propriedades de fadiga. As superfícies das peças fraturadas foram observadas ao MEV e verificou-se que a maioria dos sítios de nucleação de trincas por fadiga ocorreram próximos a partículas de segunda fase que atuam como concentradores de tensão.
The automotive industry has shown a growing interest in aluminum alloys, particularly the AA 6xxx series. This class of alloys is widely used in construction and transportation because of their good mechanical strength, easy fabrication and excellent corrosion resistance. In the automotive sector there was always a need for improved studies of the fatigue behavior, because the structural components are subjected to vibratory loads and cyclic stresses and, as a consequence, may eventually crack and fracture. The presence of a notch generally decreases the fatigue life, creating regions of high triaxial stresses which restrain plastic deformation, weakening the material. The response depends on the mechanical solicitations, microstructure, the alloy components and the material properties. In this work it was conducted a study of the high cycle fatigue behavior and the notch sensitivity of three aluminum alloys used in components of truck and bus bodies: AA 6005, AA 6063 and AA 6351, all provided in T6 condition. The S/N curves were obtained by tests in rotating bending fatigue (R = -1). Tests with notched samples (Kt?3.0) allowed to determine and compare the fatigue concentration factor and the notch sensitivity of the studied alloys. It was also studied the influence of microstructure and the intermetallic particles on the fatigue properties. The surfaces of the fractured samples were observed via SEM and showed that most of the nucleation sites of fatigue cracking occurred near the second phase particles, which act as stress concentrators.

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Solda a Ponto por Fricção (Friction Spot Welding – FSpW) é um processo relativamente novo de soldagem desenvolvido e patenteado pelo GKSS na Alemanha, que visa complementar e/ou substituir tecnologias de uniões de soldagem convencionais. Essa nova tecnologia de soldagem se dá pela união no estado sólido, onde duas ou mais chapas são unidas através da transferência de energia térmica e mecânica que é gerada pela rotação da ferramenta. O presente trabalho visou avaliar a aplicabilidade da união da liga de alumínio 6061-T6 pela técnica de Solda a Ponto por Fricção. Essas soldas foram produzidas em configuração de sobreposição utilizando condições de soldagens variadas, por meio de diferentes combinações de velocidade de rotação, profundidade de penetração e velocidade de penetração; os quais foram estabelecidos através de uma análise estatística, utilizando como ferramenta o Box-Behnken. A integridade da junta soldada foi avaliada através dos efeitos dos parâmetros de processo, na geometria e na microestrutura da junta, e também no desempenho mecânico. Realizou-se também a otimização do processo de soldagem e um estudo sobre o comportamento de fratura das soldas submetidas a um esforço de cisalhamento. Os resultados mostraram que a melhor combinação de parâmetros foi encontrada na condição de soldagem: 130 rpm, 4,0 mm/s e 1,4 mm, com um valor médio de resistência ao cisalhamento de 6243,29 N, com boa reprodutibilidade. O botão da solda é constituído por três elementos geométricos que se mostraram inerentes ao processo: cunha, união principal e união secundária. Estes elementos mostraram possuir forte influência sobre o desempenho mecânico.
Friction Spot Welding (FSpW) is a relatively new welding process patented by GKSS in Germany. This new welding technology is given by the joining in a solid state, in which two or more plates are joined by thermal and mechanical energy transfer that is generated by the rotation of the welding tool. The aim of this work is to study the integrity of the welds of AA6061-T6, by the FSpW process. These welds were produced in an overlapping configuration using different welding conditions, through different combinations of rotational speed, depth of penetration and penetration speed, which were set through a statistical approach using the Box-Behnken. The integrity of the welded joint was evaluated by the effects of process parameters on geometry and microstructure of the joint, and also in the mechanical performance. It was also performed the optimization of the welding process and a brief study on the fracture behavior of the welds subjected to a shear stress. The results showed that the best combination of parameters was found in the welding condition: 130 rpm 4.0 mm / s and 1.4 mm with an average value of the shear strength of 6243.29 N, with good reproducibility. The metallurgical investigation revealed three geometric elements that are inherent to the process: primary union and secondary union and hook. These elements were shown to have strong influence on the mechanical performance.

O uso das ligas de alumínio em aplicações estruturais tem crescido consideravelmente nas últimas décadas. Nos transportes, a baixa massa específica do alumínio resulta em uma alta razão resistência/ peso, favorecendo a fabricação de aviões, trens e automóveis. Com a crescente preocupação em reduzir a emissão de gases poluentes, vem tornando-se promissora a alternativa de reduzir o peso dos veículos substituindo peças convencionalmente produzidas com outros materiais por peças de alumínio. As ligas tratáveis termicamente da série 6xxx são frequentemente escolhidas para estas aplicações. Assim, para que o emprego dessas ligas seja otimizado, é importante um estudo mais detalhado de suas propriedades mecânicas, principalmente sob solicitações cíclicas. Neste trabalho foram estudadas ligas de Al-Si-Mg que são amplamente utilizadas nas indústrias automotivas, em especial na fabricação de componentes de carroçarias para caminhões e ônibus. Foi realizado o estudo do comportamento em fadiga de baixo ciclo e fadiga multiaxial das ligas AA6005 T6, AA6063 T6 e AA6351 T6, fornecidas pela CBA (Companhia Brasileira de Alumínio) visando caracterizar e comparar essas ligas em sua microestrutura, propriedades de tração e fadiga. As propriedades básicas de fadiga foram determinadas por meio do método ε-N (fadiga de baixo ciclo) e os ensaios foram realizados com controle de deformação total, onda triangular e taxa de deformação 0,005 seg-1. As análises dos laços de histerese elasto-plástica permitiram inferir sobre aspectos microestruturais relacionados às propriedades mecânicas das ligas estudadas. O comportamento em fadiga multiaxial foi avaliado por meio de carregamentos combinados axial-torcional em fase e fora de fase. Para ajustar os dados experimentais obtidos, foram testados alguns dos modelos encontrados na literatura. Os cálculos baseados no modelo de plano crítico, proposto por Fatemi e Socie, apresentaram resultados satisfatórios. Também foram realizadas análises microestruturais e fractográficas para as três ligas. As superfícies de fratura dos ensaios de fadiga multiaxial mostraram resultados diferentes de acordo com o carregamento adotado. A avaliação comparativa das três ligas estudadas fornece subsídios para fundamentar a seleção de materiais para a fabricação de componentes estruturais para o setor automotivo.
The use of aluminum alloys in structural applications has grown considerably in recent decades. In transportation, the low density of aluminum results in a high strength-to weight ratio, proving attractive for production of aircrafts, trains and automobiles. With a growing concern for the reduction of pollutant gas emissions, aluminum alloys are becoming a promising alternative to diminish vehicle weight through the replacement of conventionally produced parts made from other heavier materials for aluminum parts. The heat treatable alloys from the 6xxx series are often chosen for these applications. Therefore, to optimize the employment of these alloys, a detailed study of their mechanical properties, primary under cyclic solicitations is necessary For the present study Al-Mg-Si alloys were chosen, which are widely used in automotive industries, particularly in the manufacturing of components for trucks and bus bodies. The low-cycle fatigue behavior and multiaxial fatigue of the three following aluminum alloys: AA6005 T6, AA6063 T6 and AA6351 T6, provided by CBA (Brazilian Aluminum Company), were assessed, with the aim of characterizing and comparing these alloys in their microstructure, tensile properties and fatigue. The basic properties of fatigue were studied by ε-N method (low cycle fatigue) and the experiments were performed with total strain control, triangular waveform and with a constant deformation rate of 5.0x10-3 s-1. The analyses of hysteresis loops elasto-plastic provided insight about microstructural aspects, related to mechanical properties of the studied alloys. Multiaxial fatigue behavior was assessed in combined axial-torsion loading in phase and out of phase. To adjust the experimental data, some models found in the literature were tested. Calculations based on critical plane model, proposed by Fatemi Socie, presented satisfactory results. Furthermore, microstructure analyses and fractography were performed for these three alloys. The fracture surface of multiaxial fatigue assays demonstrated different results according to the adopted loading. Comparative evaluation of the three studied alloys provides support for the selection of materials for manufacturing structural components of the automotive sector.

Orientadores: Maria Helena Robert, Elisa Maria Ruiz Navas
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: Este trabalho estuda o processo de fabricação de compósitos de matriz de alumínio AA6061 reforçado com TiCN por metalurgia do pó, envolvendo as etapas de mistura de pós, compactação uniaxial e sinterização com fase líquida. Para efeitos de comparação foram produzidos e caracterizados compactados da liga AA6061 sem adição de reforços. Foram investigados os parâmetros de processo: teores de reforço (5% e 10% massa), teor de aditivos Pb e Sn (0,1, 0,15, 0,2 e 0,4% massa), pressão de compactação (400, 600 e 800 MPa), tempos (15, 30, 45 e 60 min) e temperatura de sinterização (590, 600, 610 e 620 ºC). Em cada etapa do processo foram caracterizados os produtos (mistura de pós e compactados); o produto final obtido, após sinterização, foi caracterizado com relação à sua microestrutura, propriedades físicas (densificação e variação dimensional) e mecânicas (resistência à flexão e dureza). Os resultados obtidos mostraram uma grande eficiência do processo na obtenção de compósitos; a adição do teor de reforço de 5%TiCN foi eficiente na promoção de rupturas das camadas de óxidos do pó da liga de alumínio compactado à pressão de 400 MPa, auxiliando a sinterização por difusão da fase líquida formada a partir da fusão de Al+Mg2Si, melhorando a densificação e diminuindo a variação dimensional dos produtos sinterizados. Do ponto de vista metalúrgico, os materiais compósitos obtidos apresentaram microestruturas homogêneas, com uma boa distribuição dos reforços na matriz e relativa diminuição de poros. A adição de Pb e Sn promovem maior eficiência de ativação de mecanismos de sinterização; para compactados produzidos à pressão de 800 MPa, a adição de 0,1% desses elementos já apresentou significativa influência na sinterização. Com relação às propriedades mecânicas e físicas observou-se que a adição de TiCN aumentou quase no dobro de seus valores obtidos quando são comparados com a liga AA6061
Abstract: This work investigates the process of production of composites of the alloy AA6061 reinforced with TiCN particles, by powder metallurgy involving the steps: conventional mixture of powders, compaction by uniaxial cold pressing and sintering with formation of a liquid phase. For comparative analysis it was also produced sintered AA6061 without addition of reinforcements. The following processing parameters were studied: reinforcing particles content (5 and 10 wt%); content of trace elements Pb and Sn (0.1, 0.15, 0.2 0.4 wt%); compaction pressure (from 400, 600 and 800 MPa); time and temperature of sintering (15, 30, 45, 60 min and 590, 600, 610, 620 oC). In each step products were characterized (powder mixture and green compacts); the final sintered product was characterized related to microstructure, physical (densification and dimensional changes) and mechanical (hardness and bending strength) properties. Obtained results showed high efficiency of the applied process to produce reliable composite products; the addition of 5 wt% TiCN was efficient to promote fracture of the oxide layer in the aluminum particles surface during pressing. At sintering temperatures liquid phase is formed by Al+Mg2Si melting and is distributed among particles through the fractures of the oxide layer, improving the material densification and its mechanical properties. Microstructures obtained showed homogeneous distribution of TiCN and reduced porosity, whereas AA6061 alloy microstructure showed higher porosity. Addition of Pb and Sn promoted higher efficiency of sintering mechanisms in compacts submitted to high pressures, leading to enhanced physical and mechanical properties in those materials.
Doutorado
Materiais e Processos de Fabricação
Doutor em Engenharia Mecânica

O desenvolvimento de ligas de alumínio de propriedades mecânicas intermediárias possibilitou a extrusão de alumínio a participar de grandes mercados como a construção civil, transportes, automobilística, estruturas, entre outras. O conhecimento do processo de extrusão propicia o aumento de produtividade com melhor desempenho e precisão dimensional do produto acabado. Neste trabalho, procurou-se analisar as características de conformação de perfis sólidos das ligas de alumínio AA6063 e AA6082 com razão de extrusão (RE) variando de 2,0 a 27,8 e tubular com RE de 8,7. A partir da extrusão realizada em uma prensa industrial de 3300 toneladas de força, compararam-se os resultados obtidos da tensão média de escoamento (\"sigma\") em função da temperatura de deformação e do parâmetro de Zener-Hollomon (Z) e este último em função do tamanho de subgrão observado no produto extrudado. Foram obtidos, em condições industriais, para estas ligas os valores da constante \"alpha\" que relaciona a RE com a pressão de fim de extrusão. Foram realizadas observações metalográficas e constatadas regiões de recristalização e de não recristalização diferentes para as ligas estudadas, bem como as relações do parâmetro Z e o tamanho do subgrão do produto sólido e tubular na liga AA6082. Em termos de textura foi possível obter as figuras de pólo e do tipo FDO no perfil extrudado ilustrando a presença de fibras do tipo \"beta\" (característica de produto conformado) e cubo (característica de produto recristalizado/recuperado). Os resultados confirmam valores próximos àqueles encontrados na literatura.
The development of medium strength aluminum alloys enabled aluminum extrusions to take part of the large world markets, associated with the building, transport and automotive industries, amongst several applications. The knowledge of the extrusion process details enables the productivity increase with better performance and higher dimensional accuracy of the finished extrusion product. In the present study the deformation characteristics of solid extrusion profiles in the AA6063 and AA6082 alloys with extrusion ratios (ER) in the range of 2.0 to 27.8 and for solid and tubular of 8.7 sections were studied. Extrusions were carried out in an industrial 3300 ton press. Results are compared in terms of average flow stress as a function of extrusion temperature and of the Zener-Hollomon (Z) parameter; the latter one also as a function of the observed subgrain size in the final product. Under industrial conditions, for the studies alloys, the values of the \"alpha\"-constant (which relates the end pressure to the extrusion ratio), has been evaluated. Metallographic observations have been conducted to study the recrystallised and unrecrystallised regions in the studied alloys, as well as the relationship between the Z parameter and the subgrain size of the end extrusion, both for the solid and tubular products, mainly for the AA6082 alloy. In terms of texture components it has been observed that the pole figures and the ODF´s of the extruded product presented the \"beta\" (characteristic of rolled/worked products) and cube (characteristic of recrystallised/recovered products). The results obtained confirmed those reported in the literature.

Dissertação (mestrado)—Universidade de Brasília, Faculdade UnB Gama, Programa de Pós-Graduação em Integridade de Materiais da Engenharia, 2016.
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O principal objetivo deste trabalho é a obtenção de materiais compósitos de matriz metálica reforçado com subproduto da produção de ferro gusa frente à crescente preocupação com a preservação ambiental e a sustentabilidade. O compósito de matriz de liga de alumínio AA6061 reforçado com escória de alto forno a carvão coque foi obtido pela metalurgia do pó com variações volumétricas de partículas de reforço em frações de 5, 10 e 15%. Os pós foram misturados e compactados uniaxialmente em molde metálico com diferentes pressões (160, 320, 480, 640, 800 e 960 MPa). As amostras foram sinterizadas a 550 °C em forno tubular e em atmosfera inerte. Amostras da matriz sem reforço foi produzida com o objetivo de verificar, por comparação, a influência da adição de diferentes teores de reforço. O compósito obtido foi caracterizado fisicamente via densidade, mecanicamente através dos ensaios de dureza e flexão e microestruturalmente por meio de microscopia eletrônica de varredura. As medições de densidade e os ensaios mecânicos foram realizados antes e depois da sinterização. A influência da pressão de compactação na densificação do material também foi analisada antes e após a sinterização. Como resultado, os compósitos apresentaram uma distribuição homogênea das partículas de reforço e também uma melhoria progressiva nas propriedades mecânicas (resistência a flexão e dureza) com o incremento do teor de escória adicionado comparado ao material sem reforço obtido pelo mesmo processo, garantindo a viabilidade da aplicação do subproduto do alto forno no setor industrial.
The main objective of this work is to obtain composite materials of reinforced metal matrix with by-product of pig iron works face of growing concern with environmental preservation and sustainability. The AA6061 aluminum alloy matrix composite reinforced with coke oven blast furnace slag was obtained by powder metallurgy with volumetric variations of reinforcing particles in fractions of 5, 10 and 15%. The powders were mixed and uniaxially packed in a metal mold at different pressures (160, 320, 480, 640, 800 and 960 MPa). The samples were sintered at 550 ° C in a tubular oven and in an inert atmosphere. Samples of the non-reinforced matrix were produced in order to verify, by comparison, the influence of the addition of different reinforcement contents. The obtained composite was physically characterized by density, mechanically through the hardness and flexure tests and microstructurally by means of scanning electron microscopy. Density measurements and mechanical tests were performed before and after sintering. The influence of the compacting pressure on the densification of the material was also analyzed before and after sintering. As a result, the composites presented a homogeneous distribution of the reinforcing particles and also a progressive improvement in the mechanical properties (flexural strength and hardness) with the increase of the added slag content compared to non-reinforced matrix by the same process, ensuring the viability of by-product of pig iron application in industrial sector.

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

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O presente trabalho tem como escopo processar e fabricar compósitos de matriz metálica da liga de Alumínio AA6061 com reforço particulado de nitreto de silício (Si3N4) e nitreto de alumínio (AlN) através de técnicas de Metalurgia do Pó (MP) com o propósito de melhorar as propriedades mecânicas deste material. O estágio atual da indústria exige materiais com propriedades especificas que não podem ser atendidas pelo uso isolado das ligas metálicas, dos materiais cerâmicos e dos materiais poliméricos. É o caso particular das indústrias aeronáutica, aeroespacial e de transporte, que demandam materiais estruturais com baixa densidade, fortes, rígidos, resistentes à abrasão, ao impacto e a corrosão. Essa exigência conduz o pesquisador a investigar os materiais compósitos. A distribuição homogênea da fase de reforço na matriz é um requisito primordial para melhorar as propriedades mecânicas. Nesse contexto, sabe-se que a Metalurgia do Pó (MP) promove uma melhor distribuição do reforço na fase matriz em relação ao processo convencional de fundição, obtendo-se então materiais com melhores propriedades mecânicas notadamente dureza e resistência ao desgaste. Este trabalho investiga especificamente o uso de moagem de alta energia (MAE), seguida de compactação uniaxial a frio e sinterização a vácuo na obtenção de compósitos de matriz metálica da liga de alumínio AA6061 reforçado com nitreto de silício (Si3N4) e nitreto de alumínio (AlN). A Metalurgia do Pó (MP) é uma técnica de fabricação de produtos metálicos obtidos por meio de conformação de pó metálico seguido de tratamento de sinterização conferindo propriedades físicas e mecânicas de acordo com as normas MPIF, ASTM, SAE e demais correlacionadas. A moagem de alta energia possibilita a produção de mistura homogênea e pós finos promovendo melhoria na sinterabilidade dos compactados, isso devido ao fato desta moagem promover deformações plásticas nas partículas metálicas e fraturas, levando a um refinamento contínuo dos pós envolvido na moagem. Os resultados de dureza apresentaram-se linearmente crescentes com o aumento da fração de reforço na matriz sendo que o compósito reforçado com nitreto de silício apresentou, no geral, melhor desempenho que o reforçado com nitreto de alumínio.

Orientador: Itamar Ferreira
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
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Resumo: Mostra-se fundamental para qualquer projeto de engenharia a necessidade de se conhecer em grande profundidade o comportamento mecânico dos materiais. Não menos importantes são as relações existentes entre as diferentes propriedades mecânicas, que serão detenninantes quanto à viabilidade e ao desempenho de um componente mecânico. Tendo em vista que a utilização das ligas de alumínio se toma cada vez mais importante e presente no dia-a-dia da indústria, este trabalho tem por objetivo analisar alguns métodos de estimativa das propriedades mecânicas cíclicas a partir das propriedades mecânicas monotônicas e comparar os valores estimados com os resultados experimentais, obtidos em ensaios de fadiga de baixo ciclo, verificando a viabilidade de seu uso nas ligas de alumínio AA6261-T6 e AA6351-T6. Foram analisadas as propriedades mecânicas monotônicas (módulo de elasticidade, limite de escoamento, limite de resistência à tração, tensão de fratura verdadeira, alongamento total em 25mm, redução de área, defonnação de fratura verdadeira, coeficiente de resistência e coeficiente de encruamento) e cíclicas (coeficiente de resistência à fadiga, expoente de resistência à fadiga, coeficiente de ductilidade à fadiga, expoente de ductilidade à fadiga, coeficiente de resistência cíclico e coeficiente de encruamento cíclico). Os resultados obtidos nos ensaios de tração e de fadiga de baixo ciclo apresentaram uma grande dispersão quando comparados aos valores nominais da literatura, porém com desvios-padrão muito pequenos, mostrando-se confiáveis. Nenhuma das relações encontradas na literatura com o objetivo de prever a resposta do material quando solicitado ciclicamente foram confinnadas. Utilizando os seis métodos encontrados na literatura, os valores dos parâmetros de fadiga foram estimados a partir das propriedades mecânicas obtidas nos ensaios de tração, apresentando uma variação significativa e tendo no método de Baümel a melhor aproximação aoS' valores obtidos experimentalmente. Assim, a previsão da vida em fadiga destas ligas de alumínio (AA6261-T6 e AA6351-T6) depende do conhecimento de suas propriedades cíclicas obtidas em ensaios de fadiga controlados por amplitude de defonnação
Abstract: It is very . important for any engineering design the need to know better the mechanical behavior of the materiaIs. Not less important are the existing relations between different mechanical properties that will be determinate regarding viability and the performance of a mechanical component. Having in mind that the aluminum alloys utilization becomes every time more important and present in the everyday of the industry, this dissertation has for goal analyze some methods for predicting the fatigue properties from the monotonics mechanical properties and to compare the estimate values with the experimental results, obtained in low cycle fatigue tests, and verifying the viability for the aluminum alloys AA6261-T6 and AA6351-T6. The monotonic properties (elasticity module, yield strength, tensile strength, true fracture stress, total elongation in 25mm, reduction of area, true fracture deformation, resistance coefficient and strain hardening coefficient) and cyclic properties (fatigue strength coefficient and exponent, fatigue ductility coefficient and exponent, cyclic strength coefficient and cyclic strain hardening coefficient) have been determined. The obtained results from tencion and low cycle fatigue tests presented a very small standard deviation but a reasonable difference when comparing to the literature values. Six methods for predicting the fatigue properties from the monotonic properties were used. In general, these methods do not agree to the experimental results and in the Bãumel method there is the small difference between experimental and predicted values. As a result ofthat, for these aluminum alloys (AA6261-T6 and AA6351T6) the six forecast methods of the fatigue properties obtained from literature are not suitable
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

This thesis systematically investigates the effects of a recently developed modified ageing procedure of aluminium alloys, termed the T6I6 temper, on the microstructural development and mechanical properties of the Al ??? Mg ??? Si - Cu alloy 6061. For the T6I6 temper, a conventional single stage T6 temper is interrupted by an ageing period at a reduced temperature (65??C) to facilitate secondary precipitation, before resuming the final ageing at the temperature of the initial T6 treatment. The T6I6 temper was found to cause simultaneous increases in tensile properties, hardness, and toughness as compared with 6061 T6. Al ??? Mg ??? Si ??? Cu alloys are medium strength alloys widely used in the automotive industry and their further improvement is underpinned by stringent demands for weight reduction placed on the transportation industry in recent years. The potential for further improvement of the mechanical properties was found in the control of secondary precipitation that may take place even in some fully aged alloys when exposed to reduced temperatures. The overall improvement in the mechanical properties of 6061 T6I6 was attributed to the formation of finer and more densely dispersed precipitates in the final microstructure. The refinement of precipitates was facilitated by control of the precipitation processes and gradual evolution of the microstructure throughout each stage of the T6I6 treatment. The results indicated that the concentration and the chemical environment of the vacancies controlled the precipitation processes in this alloy. Findings also show that the proportion of the different precipitate phases present in the final microstructure, as well as the amount of the solute in these precipitates, can be controlled and modified utilizing secondary precipitation. A number of analytical techniques were used in this study. The evolution of the microstructure was studied using Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Three Dimensional Atom Probe (3DAP). Vacancy-solute interactions were studied using Positron Annihilation Lifetime Spectroscopy (PALS) and 3DAP. The distribution of the solute was studied using 3DAP and Nuclear Magnetic Resonance (NMR). Differential Scanning Calorimetry (DSC) was used to identify precipitation reactions and to determine the stability of vacancy-associated aggregates.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2012.
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Ligas de alumínio da série 6000 (Al-Mg-Si) são conhecidas por apresentarem boas propriedades mecânicas. Estas propriedades são incrementadas através de tratamentos térmicos específicos que precipitam Mg2Si, conferindo uma maior dureza ao material. Com frequência estas ligas são expostas a trabalhos que envolvem desgaste, porém devido principalmente à sua baixa dureza estes materiais possuem uma baixa resistência ao desgaste. Para incrementar a resistência ao desgaste microabrasivo com frequência são depositados revestimentos duros sobre a superfície do material. Neste trabalho foi estudado o desempenho tribológico das ligas AA6101 T4 e AA6351 TF revestidas com TiN (Ts =100°C) pela técnica de deposição por feixe de elétrons. Resultados indicam um incremento de aproximadamente 40% na resistência ao desgaste microabrasivo para as amostras revestidas com Nitreto de Titânio (TiN) para ambas as composições. Na caracterização foram realizados ensaios de desgaste microabrasivo em um tribômetro CALOWEAR e as amostras foram analisadas através de microscopia eletrônica de varredura (MEV), interferometria ótica, microscopia de força atômica (AFM) e difração de raios X. ______________________________________________________________________________ ABSTRACT
Aluminium alloys of 6000 series (Al-Mg-Si) are known to have good mechanical properties. These properties are improved with heat treatments to precipitated Mg2Si, giving an increased hardness to the material. Often these alloys are exposed to work involving wear, but mainly due to its low hardness of these materials low wear resistence are found. To increase the microabrasive wear resistance often hard coatings are deposited on the surface of the material. In this work, the tribological performance of AA6101 T4 and AA6351 TF alloys coated with TiN (Ts = 100°C) by electron beam technique deposition. The results indicate an increase of approximately 40% on microabrasive wear resistance for the samples coated with titanium nitride (TiN). The characterization tests were analysed by scanning electron microscopy (SEM), optical interferometry, atomic force microscopy (AFM) and ray X diffraction (XRD).

Les besoins de réduction du poids se sont concrètement traduits par l'introduction de nouvelles nuances plus légères dans les structures automobiles. Ainsi, des alliages d'aluminium ont commencé à être intégrés dans les pièces de structure de plusieurs véhicules. La faible masse volumique des alliages d'aluminium (2,7g/cm 3 ) permet d'alléger le poids du véhicule qui entraîne une diminution de la consommation de carburant et, donc, des émissions de gaz à effet de serre. La striction et la rupture sont les principaux modes de défaillance qui entrainent le rebut systématique des pièces. C'est pourquoi, améliorer la prédiction d'apparition de ces défauts lors de la simulation va dans le sens d'une meilleure maitrise du procédé. Dans le cadre de ce travail doctoral, deux modèles sont développés pour simuler le comportement à grandes déformations d'alliages d'aluminium : un modèle polycristallin de type Taylor et un modèle à un ou plusieurs éléments finis par grain.Les diagrammes limites de formage (DLF) pour les deux alliages d'aluminium AA5754 et AA6063 ont été simulés numériquement en utilisant une formulation par éléments finis pour les polycristaux basée sur l'hypothèse de Taylor.Les DLF conventionnels et de l'hydroformage ont été traces. L'effet des chemins de déformation sur la formabilité des alliages d'aluminium a aussi été étudié. Finalement, des simulations numériques avec les données de diffraction des électrons rétrodiffusés (EBSD) pour l'alliage d'aluminium AA5754 ont été effectuées en utilisant le modèle à un ou plusieurs éléments par grain. Ces simulations sont exécutées avec différents modèles du durcissement (Asaro, Bassani et puissance).

Fletcher Aluminium is a New Zealand company that manufactures aluminium extrusions for the building market. Their extrusion process involves using a hydraulic ram to force heated raw supplied material, in the form of large diameter cylindrical logs, through a die land that is the desired product shape. The final quality of the extruded product is influenced by the extrusion temperature, ram pressure and extrusion speed. The speed of extrusion at Fletcher Aluminium is limited by the extrusion ram pressure because the extrusion machine is operating at the pressure limit of the compressor. Currently the company requires an increase in the extrusion speed, while maintaining quality requirements where these improvements are desired without upgrading the compressor. The press pressure is required to overcome friction and the flow stress of the billet, which is dependent on the precipitates and solutes conditions and their strengthening effects in the billet. However, the preheating temperature and extrusion speeds need to be sufficiently low enough to avoid hot tearing. This research starts to increase the ram speed by decreasing the Mg2Si content for precipitation strengthening. Heat treatment may be used to dissolve Mg2Si content by billet preheat to a temperature at or exceeds solvus temperature (TSolvus). However, a higher (than TSolvus) billet temperature may have two conflicting effects. First, a higher temperature in general reduces the flow stress thus lowering the required pressure. On the other hand, at temperature higher than Tsolvus, Mg2Si should completely dissolve, resulting in solid solution strengthening thus increasing the flow stress. The objective of the study is to quantify the strengthening contributions from solute atoms (Mg / Si) and precipitates (Mg2Si) on the as-received and heat treated test samples using room temperature mechanical testing and phase diagram analysis. The selected heat treatment conditions fully enclose the preheating temperature and time range currently used in Fletcher Aluminium production. Simulations of the extrusion were conducted by hot compression testing (Gleeble test), to measure the combination of strengthening effects during deformation at elevated temperatures. The room temperature tests have shown that hardness, yield strength (YS) and ultimate tensile strength (UTS) are minimised at approximately 405°C. The minimum point is shown to be caused by the combination of strengthening and softening due to solid solution and precipitation strengthening. The higher values of hardness, YS and UTS at lower and higher preheating temperatures are mainly due to precipitation and solution strengthening respectively. The findings show that, the solution of Mg2Si gradually increases as the temperature increases at temperatures exceeding 405°C. Hardness, YS and UTS were compared in order to determine the correlation between each mechanical property. A correlation exists, but it is not simple to translate between each mechanical property. The relationship between hardness and stress observed for 8% permanent strain was also investigated; there was no improvement in correlation. The elevated temperature tests show that initial flow stress and peak flow stress reduces linearly with temperature increases, which means higher testing temperature results lower precipitation and solid solution strengthening effects. Additionally, the constants of the Gleeble – Holomon equation were numerically calculated and are similar to published values. The Gleeble – Holomon equation was combined with Felthams equation in order to provide a relationship between extrusion velocity and extrusion temperature.

The influence of recycling on aluminium alloys and subsequent influence on the microstructure and corrosion performances have been investigated. The investigation was commenced by taking two block cast, recycled aluminium alloys (AA5050 and AA6011) and rolling them into 1mm gauge plate. In the case of AA6011, the plate was subjected to subsequent solution heat treatment and artificial aging steps, in order to attain certain temper specifications. To replicate the automotive paint bake industrial practice, a sample was subjected to a 2% tensile stretch followed by heat treatment for 30 minutes at 180˚C. Microstructural observations revealed Al-Fe-Mn-Si intermetallics to be the dominant secondary phase in both alloys. The size, distribution and composition of these were unaffected by artificial aging. Mg2Si was found in a coarse, localised form in both alloys also, albeit in much less amounts in AA5050. The presence of this phase was likely due to poor homogenisation during thermomechanical processing. HR-TEM of AA6011 revealed needle/rod shaped precipitates, aligning in the [001]Al lattice direction. This is consistent with β''/β' hardening precipitates consisting of magnesium and silicon. Circumstantial evidence was found for the copper-containing Q phase precipitate also. An additional, unidentified precipitate was observed, nucleating on the {111} habit plane of the aluminium matrix. The high iron content of AA6011 retarded the precipitation hardening response by capturing elements associated with hardening precipitates in the Al-Fe-Mn-Si intermetallics. Electrochemical corrosion experiments revealed the materials had a high susceptibility to localised corrosions, with the open circuit potential and breakdown potential possessing similar values. Atmospheric corrosion experiments showed that artificial aging had a large influence on the preferred corrosion mechanism. Non-heat treated samples showed susceptibility for pitting corrosion. This was particularly true for the -T4P temper, which showed large scale pitting. Heat treated samples saw an introduced susceptibility to intergranular corrosion. This was attributed to precipitation at grain boundaries, which would then form a microgalvanic couple with adjacent depleted zones. In the case of the -T8P temper, tensile stretching introduced defects into the sub-grain microstructure. This resulted in intergranular corrosion fronts of increased width, where grains with higher stored energy undergo preferential dissolution alongside the grain boundary attack. Overall, the detrimental effects of high iron content need to be overcome before AA5050 and AA6011 can be seriously considered for use in the automotive industry. However, the corrosion performance of AA6011-T8P is encouraging.

The present work investigates the evolution of microstructure during the annealing of an industrial precipitation hardened aluminum alloy AA6111. Special emphasis is placed on understanding the interaction between recovery, recrystallization and precipitation through a combination of experimental and modelling approaches. Experimentally, extensive heat treatments were carried out to study the effect of varying initial precipitate conditions on the isothermal annealing behaviour of the alloy in the temperature range of 250-445°C. A total of four prior aging conditions were considered: naturally aged (T4), peak aged (PA), overaged (OA) and severely overaged (SOA). It was found that recrystallization was severely retarded at the annealing temperature of 325°C irrespective of prior precipitate conditions. Microscopic evidence confirmed that the growth of recrystallizing grains is directly related to the non-uniform spatial distribution of precipitates. Subsequently, a new microstructure model was developed to link the changes in the microstructure to the mechanical properties of the alloys. The model which was developed based on the internal state variable approach is capable of translating quantitatively the interaction between recovery, subgrain growth, recrystallization and precipitation into a yield stress vs. time relationship. The effect of non-uniform distribution of precipitates was considered explicitly by using a simple rule of mixtures. The validity of the model was verified by comparing the model calculations to the experimental data obtained from overaged 40% cold rolled AA6111.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Welding and corrosion will play a key role in the further increased use of aluminium alloys in industrial applications. This is especially true as the oil and gas industry is moving into the arctic region in search for new oil and gas reservoirs. The effect on corrosion of the low temperatures that will be encountered in this region must be understood. In this study, six extruded and welded AA6082-T6 flat bars with thicknesses ranging from 10 to 30 mm have been subjected to material characterization, an accelerated intergranular corrosion (IGC) test, recording of polarization curves in artificial seawater at high and low temperatures and measurements of open circuit potential (OCP) at high and low temperatures.The results from this study show that the extent and weakening of the heat affected zone (HAZ) will not change as the thickness is increased up to 30 mm. Moreover, the HAZ in the examined profiles will not be more susceptible to IGC and pitting corrosion than the base material. It is therefore recommended that the standards on aluminium structures, like EN 1999-1-1[1], include welded aluminium profiles up to 30 mm, as compared to the maximum thickness of 15 mm at present time. Results also show that the corrosion behaviour at low ocean temperatures will be superior compared to higher temperatures. This is shown by the raising of the pitting potential, corrosion potential and OCP at low temperatures, in addition to a lowering of the cathodic limiting current.

It is well known that the finishing temperature of 6xxx alloys determines the resultant texture, which clearly has a strong effect on formability. Specifically, the texture is determined by whether the finishing temperature is above or below the ß transus. This study was initiated in order to directly determine the mechanisms that take place during the hot deformation of AA6111 to directly explain these observations. Accordingly, the effect of inter-pass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield Thermomechanical Compression (TMC) facility, using the hit-hold-hit-quench approach at temperatures of 320°C, a strain rate of 85 s'1 to an initial strain of 0.5, unloaded and held for delay times of 0,6,60,600 and 6000 seconds, followed by the second deformation. Hardening of the alloy was observed, the extent of which was dependent on the hold time. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. TEM identified precipitation, predominantly ß and Q phases, on dislocation lines, the size of which was a function of the hold time. In contrast to conventional ageing studies, the Q phase was found to be the majority precipitate phase. The coarsening rate of the Q during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. Texture analysis through EBSD revealed that the texture was mainly composed of the α and β fibres, indicating a classic hot deformation texture with no evidence of a recrystallisation texture. This, the EBSD maps themselves, optical and transmission electron microscopy indicated that no recrystallisation had occurred for any inter-pass delay time. Small changes in deformation texture were observed with changes in inter-pass delay time, however, such differences were believed to be small. Deformation substructure increased in size (e.g. microband width) with increased inter-pass time, as expected, which with the increase in precipitate size led to the softening observed at 600 and 6000 seconds. The implications of these observations are discussed.

Le présent travail a été réalisé dans le cadre d’un projet de recherche entre le groupe de mécanique des solides du Département de génie civil de l’Université de Sherbrooke et l’entreprise ALCAN. Son objectif principal était d’étudier expérimentalement le comportement de l’aluminium AA6111 en grandes déformations plastiques. Pour cette étude, nous avons conçu et réalisé un dispositif adaptable sur une presse universelle afin de transformer l’effort de tension en effort de cisaillement, ainsi qu’un échantillon pour l’essai de cisaillement constitué d’une tôle rectangulaire entaillée au milieu. Nous avons effectué des essais de tension simple, de tension plane et de cisaillement simple avec des échantillons appropriés qui nous ont permis de décrire le comportement du matériau sous ces types de sollicitations. Les données ainsi obtenues ont été introduites dans un logiciel de simulation par éléments finis afin de décrire la distribution des contraintes et des déformations à travers les échantillons. Ces résultats pourront ensuite être utilisés pour connaître la réponse de l’aluminium AA6111 à différentes sollicitations subies lors des opérations de formage.

Among the bulk metal forming processes hot forging is often the only option if large reductions of the forging load are required or if the material formability needs to be significantly increased or even if specific thermally-induced microstructural changes are needed to take place during the deformation process. Moreover it still retains at some extent the positive features of the cold forging processes: high production rates, complex final shapes attainable and little to no material waste. Hot forging is therefore used for the production of large parts, with complex shapes and especially when using materials characterized by low formability and high toughness at room temperature or if particular microstructural characteristics are required. It is worth to be underlined that these are often the features of the innovative metal alloys that have been increasingly being used in the last decade and namely: Mg alloys, Al alloys, Ti alloys and superalloys. Finite Element metal forming numerical simulation has become an increasingly important process optimization tool, due to the growing computational power available at reduced costs, which spread it in the industrial world. Its use allows reducing process design time and prototyping costs as well as long and expensive plant downtimes for process variable tuning. For all these reasons hot forging has become a strategic process and its accurate numerical simulation is encountering great industrial interest. One of its main targets is the determination of the maximum strain that the material can undergo during the deformation process, since it is strictly related with both the final shape and the surface quality of the manufactured part. In this sense hot formability modelling provides a meaningful example of a topic which is both of great scientific and industrial interest. From the scientific point of view ductile damage modelling has been developed at first for cold processes, for which crack formation is usually a major issue. Last decade research efforts focused on the development of more advanced fracture criteria, keeping into account a complete characterization of the stress state dependency of formability. However the complex analytical formulation of these criteria and their expensive experimental calibration kept them de facto away from practical industrial use. On the other hand hot formability modelling was traditionally addressed by direct application of the conventional cold fracture criteria, under the implicit assumption of isothermal conditions. This approach has evident limits since it does not account for temperature influence on material formability and it does not even provide any physical insight on the different fracture mechanisms that can develop as temperature changes. In recent years some research efforts have been performed on a deeper investigation upon this point: experimental formability campaigns on different metal alloys were carried out in order to assess the temperature and strain rate influence, while some analytical models were proposed in order to describe the damage evolution at high temperature and in particular the onset of the so-called “hot shortness”. However these models are still quite simple and can describe only a limited variety of formability trends, losing in accuracy once complex microstructural phenomena take place. Moreover their validation has been done with simple laboratory tests and not with real processes industrial trials in which non-uniform thermo-mechanical conditions are present and the material can evolve through fracture mode-changing regimes. The objective of this work is the elaboration of a novel approach to hot ductile fracture modelling, capable to represent accurately the formability evolution of a metal alloy as a function of both the main thermo-mechanical variables and its physical and microstructural characteristics, yet providing a tool simple enough to be of industrial utility. To this aim the hot cross wedge rolling of a precipitation hardened aluminium alloy was taken as industrial reference case, since it is an innovative and non-standard forging process which entails variable and non-homogeneous thermo-mechanical conditions. The study case is of remarkable interest since it has a narrow process temperature window, being limited at the top by Mannesmann-type axial cracking onset and at the bottom by unwanted grain coarsening. Moreover the metal alloy used, the AA6082-T6, has the microstructural features, namely the intermetallic precipitates, that make it a good example of a metal alloy of wide industrial use, that during forming can undergo complex microstructural changes. A hot tensile test campaign was performed on a wide range of thermo-mechanical conditions and the results highlighted an unexpected negative strain rate influence on formability. Fractographic and micro-chemical analysis were then performed in order to assess the microstructural reasons of this behaviour and finally two approaches to the material formability modelling were proposed and calibrated extending to hot conditions the classic Oyane-Sato fracture criterion. The first one consists on the empirical calibration of the criterion by means of a bi-linear interpolation of the experimental data, while the second one entails a physically-based analytical formulation of the material fracture locus, which has also the advantage of being of easier calibration. These models were then validated on the cross wedge rolling process simulation by comparison with the industrial trials results and the outcomes were critically assessed.
Tra i processi di formatura massivi di materiali metallici, la forgiatura a caldo è spesso l'unica opzione nei casi in cui siano richieste forti riduzioni del carico di forgiatura o se la formabilità del materiale deve essere notevolmente aumentata o anche se risulta necessario ottenere durante il processo di deformazione determinate modifiche microstrutturali indotte termicamente. Inoltre essa conserva ancora in certa misura le caratteristiche positive dei processi di forgiatura a freddo: alta produttività, possibilità di realizzare forme finali complesse e scarto di materiale ridotto o inesistente. La forgiatura a caldo viene quindi utilizzata per la produzione di pezzi di grandi dimensioni, di forma complessa e soprattutto quando si utilizzano materiali caratterizzati da bassa formabilità ed elevata tenacità o se particolari caratteristiche microstrutturali sono richieste. Vale la pena di sottolineare che queste sono spesso le caratteristiche delle leghe metalliche innovative che sono state sempre più utilizzate negli ultimi dieci anni e precisamente: leghe di magnesio, alluminio, titanio e superleghe. La simulazione numerica agli Elementi Finiti di processi di formatura di materiali metallici è diventata nell’ultimo decennio uno strumento sempre più importante per l’ottimizzazione di processo, grazie alla maggiore potenza di calcolo disponibile a costi ridotti, che ha permesso la sua diffusione nel mondo industriale. Il suo utilizzo permette di ridurre i tempi di progettazione del processo ed i costi di prototipazione ed anche lunghi e costosi tempi di fermo impianto per la taratura delle variabili di processo. Per tutti questi motivi la forgiatura a caldo è diventata un processo strategico e la sua accurata simulazione numerica incontra grande interesse industriale. Uno dei suoi obiettivi principali è la determinazione della massima deformazione che il materiale può subire durante il processo deformativo, dal momento che essa è strettamente legata sia con la forma finale che con la qualità superficiale del componente prodotto. In questo senso la modellazione della formabilità a caldo fornisce un esempio significativo di un argomento che è al tempo stesso di grande interesse scientifico e industriale. Dal punto di vista scientifico la modellazione del danneggiamento duttile dei materiali è stata originariamente sviluppata per le lavorazioni a freddo, per cui la possibile formazione di cricche è un problema di maggior rilievo. Gli sforzi di ricerca nell'ultimo decennio si sono concentrati sullo sviluppo di criteri di frattura più avanzati, che hanno permesso la caratterizzazione completa dell’influenza dello stato tensionale sulla formabilità. Tuttavia la complessa formulazione analitica di questi criteri e la loro costosa calibrazione sperimentale ne hanno di fatto impedito la diffusione in ambito industriale. D'altra parte la modellazione della formabilità a caldo è stata tradizionalmente affrontata tramite la diretta applicazione dei convenzionali criteri di frattura a freddo, sotto l'ipotesi implicita di condizioni isoterme. Questo approccio presenta evidenti limiti, in quanto non tiene conto dell'influenza della temperatura sulla formabilità del materiale, né permette di intuire il senso fisico dei diversi meccanismi di frattura che possono svilupparsi al variare della temperatura. Negli ultimi anni alcuni sforzi si sono fatti per approfondire quest’ultimo punto: campagne sperimentali di formabilità su diverse leghe metalliche sono state eseguite per valutare l'influenza della temperatura e della velocità di deformazione, mentre alcuni modelli analitici sono stati proposti per descrivere l’evoluzione del danneggiamento ad alte temperature ed in particolare l’insorgere della "fragilità a caldo". Tuttavia questi modelli sono ancora abbastanza semplici e possono descrivere solo una varietà limitata di comportamenti del materiale, perdendo in precisione nel caso avvengano fenomeni microstrutturali complessi. Inoltre la loro validazione è stata effettuata con semplici test di laboratorio e non su reali processi industriali in cui si sviluppano condizioni termo-meccaniche non uniformi e il materiale può evolvere attraverso regimi in cui i meccanismi di frattura sono variabili. L'obiettivo di questo lavoro è l'elaborazione di un nuovo approccio alla modellazione della frattura duttile a caldo, in grado di rappresentare accuratamente l'evoluzione della formabilità di una lega metallica come funzione sia delle principali variabili termo-meccaniche che delle sue caratteristiche fisiche e microstrutturali, restando al contempo uno strumento sufficientemente semplice da essere di utilità industriale. Per questo scopo è stata presa come caso di riferimento industriale la rullatura trasversale a caldo di una lega di alluminio indurita per precipitazione, dal momento che si tratta di un processo di forgiatura non convenzionale ed innovativo e che comporta condizioni termo-meccaniche variabili e non omogenee. Il caso di studio è di notevole interesse poiché è caratterizzato da una stretta finestra di temperatura di processo, limitata superiormente dall’insorgenza di criccatura assiale per effetto Mannesmann ed inferiormente da un indesiderato ingrossamento della grana cristallina. Inoltre, la lega metallica utilizzata, l'AA6082-T6, ha delle caratteristiche microstrutturali, vale a dire i precipitati intermetallici, che lo rendono un buon esempio di una lega metallica di largo uso industriale, che durante la formatura può subire variazioni microstrutturali complesse. Una campagna di prove di trazione a caldo è stata eseguita su un ampio spettro di condizioni termo-meccaniche ed i risultati hanno evidenziato un’inattesa influenza negativa della velocità di deformazione sulla formabilità. Analisi frattografiche e micro-chimiche sono quindi state eseguite al fine di valutare le ragioni microstrutturali di questo comportamento ed infine due approcci alla modellazione della formabilità della lega sono stati proposti e calibrati estendendo alle alte temperature il classico criterio di frattura di Oyane-Sato. Il primo consiste nella calibrazione empirica del criterio mediante interpolazione bi-lineare dei dati sperimentali, mentre il secondo si basa su una formulazione analitica physically-based del fracture locus del materiale, che ha anche il vantaggio di essere di più facile calibrazione. I modelli sono stati poi validati sulla simulazione del processo di rullatura trasversale comparandone i risultati con quelli delle prove industriali e valutandoli in modo critico.

A liga de alumínio 6082 apresenta de média para alta resistência mecânica e é de grande importância no mercado de perfis extrudados. O processo de jateamento com granalha de aço em componentes automotivos e aeronáuticos, visando induzir uma condição otimizada de tensão residual de compressão na superfície, tem se mostrado eficaz na melhoria das propriedades mecânicas em fadiga desses componentes. Neste sentido, este trabalho tem como objetivo estudar os efeitos do jateamento com granalhas de aço na tensão residual e no comportamento mecânico em fadiga da liga de alumínio 6082 solubilizada e envelhecida artificialmente. Inicialmente foram feitos tratamentos térmicos de solubilização e envelhecimento artificial visando a determinação da melhor condição em termos de resistência mecânica da liga de alumínio 6082. A seguir foram realizados experimentos com diversos tipos de granalhas esféricas de aço e em condições variadas de jateamento (velocidade de rotação e tempo), definindo-se a melhor condição de tensão residual medida por difração de raios x. Ensaios de fadiga em corpos de prova jateados com granalha de aço na melhor condição de tensão residual foram executados em máquina flexo-rotativa e comparados com os resultados do material sem jateamento. A melhor condição de tensão residual ocorreu em decorrência do jateamento com o uso da granalha de aço S170PEE, rotação da turbina à 2500 rpm e tempo de exposição de 1 minuto. A curva de tensão residual em profundidade para a condição escolhida apresentou resultados coerentes somente até 76 μm devido à presença de grãos grandes em profundidades superiores a esta. As curvas S-N mostraram que houve um ganho na vida em fadiga em tensões elevadas e baixo número de ciclos para romper devido jateamento com granalha de aço. Ao contrário, ocorreu um efeito deletério na vida em fadiga em tensões baixas e número de ciclos elevados para romper, devido à influência do aumento da rugosidade em ensaios de fadiga mais longos.
Aluminum alloy 6082 exhibits medium to high strength and is widely used as extruded products. Shot peening of automobile and aircraft components, carried out to induce optimum residual compressive stress on the surface, improves fatigue properties of these components. In this context, the aim of this investigation was to study the effects of shot peening on the residual stress and fatigue behavior of solution treated and artificially aged 6082 aluminum alloy. Initially, solution treatment and artificial ageing of this alloy were carried out to determine the best conditions in terms of alloy strength. Subsequently, experiments were carried out with different types of spherical steel shots, under a variety of peening conditions (rotation speed and duration), to define the best set of conditions to increase residual stress that was measured using x-ray diffraction. Fatigue tests were carried out using a rotating-bending machine on shot peened specimens that had the highest residual stress. These results were compared with those of specimens that were not shot peened. The highest residual stress was observed when peening was done with S170PEE steel shots, at 2500 rpm for 1 minute. The variation in residual stress as a function of depth, for the above-mentioned peening conditions, was coherent only up to a depth of 76 μm and attributable to the presence of large grains at depths higher than this. The S-N curves revealed an increase in fatigue life at high stresses and at low cycles to failure and attributed to shot peening. Contrary to this, a deleterious effect in terms of reduction in fatigue life was observed at low stresses and at high cycles to failure, due to increase in surface roughness of the specimens during the extended fatigue tests.

Dans le cadre d'un programme de recherche visant à l'allègement de la structure des véhicules, l'origine de lignage dans des tôles en aluminium AA6016 a été étudiée. Ce phénomène, qui peut apparaître à la suite d'une déformation plastique, est apparenté à de la rugosité de surface alignée dans la direction de laminage (DL). Sa présence est néfaste à une bonne finition de surface, et son intensité est appréciée visuellement par les fabricants.Une méthode de quantification rationnelle a été développée. La caractérisation de la distribution morphologique des motifs de rugosité a été rendue possible par l'utilisation de fonctions fréquentielles telle la densité de puissance spectrale. La note globale, construite à partir de la quantification individuelle des composantes de lignage pur et de rugosité globulaire, s'est montrée en bon accord avec les estimations visuelles, et notamment avec le niveau de lignage intermédiaire regroupant plusieurs aspects de surface différents.La microstructure des matériaux à l'état T4 a été expérimentalement mesurée couche de grains par couche de grain à l'aide d'un couplage entre polissage contrôle et acquisition par EBSD. Les 4 à 5 premières couches sous la surface (-120μm) semblent jouer un rôle mécanique prépondérant dans la formation du lignage car elles offrent à la fois une grande taille de grains moyenne, une importante ségrégation d'orientations cristallines, et une forte similitude de longueurs d'onde entre la rugosité de surface et les motifs de la microtexture.Des simulations numériques ont permis de vérifier que les couples de texture identifiés (Cube/Goss, Cube/Aléatoire et Cube/CT18DN) possédaient des différences d'amincissements hors-plans suffisantes pour générer l'ondulation d'une couche d'éléments. En revanche, l'influence mécanique de cette même couche décroit très rapidement avec son enfouissement dans la profondeur et devient négligeable sous plus de 4 couches d'éléments.

Environmental issues and regulations are pushing the automotive industry to produce more efficient and environmentally friendly vehicles. To address these issues, reducing the weight of the vehicles by use of aluminum alloys is increasing. AA6022-T43 is a new sheet aluminum alloy designed specifically for automobile enclosure panels. Because this alloy is new, little data exists on its mechanical properties in the open literature. AA6022-T43 is received by the automotive industry in 1-mm thick sheet stock and subsequently stamped into the desired component. The design and manufacturing processes of the component are guided by the materials mechanical behavior. This study characterizes the mechanical and microstructural properties of uniaxially strained AA6022-T43.

A soldagem a laser em ligas de alumínio para aplicações aeronáuticas vem sendo extensamente estudada. Algumas aeronaves certificadas como os modelos Airbus A318, A319, A320 e A380 já empregam o processo em regiões de fuselagem e na junção de revestimentos com reforçadores longitudinais. A qualidade das juntas é uma característica chave para a aplicação bem sucedida do processo e neste aspecto, técnicas de ensaio não destrutivo são fundamentais para a garantia da qualidade das soldas. Neste trabalho, as técnicas de ultrassom e raios-X aliadas a metalografia, foram empregadas na caracterização de soldas autógenas em chapas de alumínio da liga AA6013. Chapas com 1,6mm de espessura foram soldadas com laser a fibra de itérbio com potências entre 1 e 2kW e diversas velocidades, com e sem o uso de gases de proteção. A qualidade das soldas foi avaliada com base na presença de defeitos típicos como porosidade, trincas, e outras imperfeições, e sua correlação com os parâmetros do processo de soldagem. Os resultados obtidos mostraram que ambas as técnicas de inspeção não destrutivas foram satisfatórias para a detecção de defeitos típicos nas soldas. Pela técnica de raios-X descontinuidades com dimensões acima de 32?m foram detectadas e posteriormente confirmadas por análise metalográfica. Já a técnica de ultrassom mostrou-se bastante satisfatória na detecção de porosidades agrupadas (tipo de descontinuidade mais comum nas amostras) através da redução da amplitude do sinal das ondas ultrasônicas refletidas na superfície oposta, e conseqüentemente ao transdutor do equipamento. A utilização de transdutores com freqüência acima de 5MHz mostrou-se mais eficiente para a detecção dos poros neste trabalho, sobretudo através da técnica da utilização de sensores de múltiplos elementos (phased array). Entretanto não foi possível estabelecer se esta eficiência depende do maior número de sensores ou da maior freqüência de operação em comparação ao ultrassom convencional.

This thesis is concerned with the mechanical properties and microstructure evolution during the novel solution Heat treatment Forming cold die Quenching (HFQ) process. HFQ is a hot sheet forming technology which incorporates the forming and quenching stages to produce high strength and high precision Al-alloy sheet parts. The work in the thesis divided into three main sections: Firstly, viscoplastic behaviour of AA6082 at different deformation temperatures and strain rates was identified through analysis of a programme of hot tensile tests. Based on the results from the hot tensile tests, a set of unified viscoplastic-damage constitutive equations was developed and determined for AA6082, providing a good agreement with the experimental results. SEM tests were carried out to investigate the damage nucleation and failure features of the AA6082 during hot forming process and the results are discussed. Secondly, the viscoplastic-damage constitutive equations were implemented into the commercial software ABAQUS via the user defined subroutine VUMAT for the forming process simulation. An experimental programme was designed and testing facilities were established for the validation of the FE process modelling results. A fairly good agreement between the process simulation and the experimental results was achieved. This confirms that the established FE process simulation model can be used for hot stamping of AA6082 panel parts. Further process modelling work was carried out to identify the optimal forming parameters for a simplified representation of a panel part. Finally, a precipitation hardening model was developed to predict the post-ageing strength of AA6082 panel parts, having varying amounts of forming-induced plastic strain. The model was tested against results of experiments which were carried out to investigate the effect of pre-deformation on the ageing kinetics of AA6082. The model is shown to fit and can be used to explain changes in the strength of the material. This set of equations was implemented in the VUMAT, in combination with the viscoplastic damage constitutive equation set, to model the whole HFQ process. The FE model was tested with experimental ageing and hardness results providing good agreements, which are discussed in light of the future development of the HFQ process.

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The use of aluminum becomes increasing because of the lightness of this metal and its high corrosion resistance. The anodization of the aluminum is now a well known and is widely used to increase the durability of the metal. This electrochemical technique forces the growth of oxide layer. The anodized layer has the peculiarity of having the nanotubes which allows the insertion of pigments and other compounds within these. The anodizing process, industrially used followed by coloration, according to the literature has been applied a current of 50 mA/cm2, dye concentration approximately 2-5 g / L, 15-18% sulfuric acid and temperature 40C. For these different factors, there is no a rigid control, therefore, there must be an optimization study of the process because the use of many reagents on an industrial scale can lead to an undesirable environmental impact, beyond the gas emission due to concentration of the acid used, even high energy expenditure. In this study it was used an organic dye to be deposited in the aluminum alloy AA6351 electrochemically anodized and studied, using a factorial design in the process to minimize the costs and to improve the metal protection. The experimental techniques used in this study were: chemometrics, anodizing, coloring by immersion, open circuit potential, anodic potentiostatic polarization, charge transfer resistance, electrochemical impedance spectroscopy, optical microscopy, scanning electron microscopy, microanalysis and Raman spectroscopy. The parameters for the experimental design, using chemometrics, were taken from the literature, as follows: current density, time and electrolyte concentration for the anodization, and dye concentration for the coloring. Measurements of charge transfer resistance (RCT) have demonstrated which tests would offer the greater protection. Two of the experimental tests, showed an RCT around by 2.85 x 108.cm2. These tests showed two situations: (1st) when anodization current density is high, less anodization time and dye are needed; (2nd) when anodization current density is low, much time and dye are needed. The polarization curves showed a current density of the samples anodized and colored are very small when compared with aluminum only polished. The electrochemical impedance spectroscopy also showed greater resistance of the layer developed on the colored pieces. The scanning electron microscopy showed that the diameter of the nanopores of the aluminum anodized, in first case, are around by 11.7 nm, so, therefore, less dye is needed to fill the nanoporos layer. In second case, the nanopores diameters are smaller than the first case; it is around by 7.6 nm, requiring higher dye concentration. In optical microscopy it was observed that the parameter also influence the tone of the chosen color. The energy dispersive system and the microanalysis showed have no heavy metals on the surface of aluminum neither in the dye composition. Raman spectroscopy proved that compound is on surface and did not change in the coloring process.
A utilização do alumínio torna-se cada vez maior, devido à leveza do metal e sua elevada resistência a corrosão. A anodização do alumínio é uma técnica bem conhecida e está sendo muito utilizada para o aumento da durabilidade do metal. Esta técnica força eletroquimicamente o crescimento da camada de óxido. A camada anodizada tem a peculiaridade de possuir nanotubos o que permite a inserção de pigmentos e outros compostos no interior destes. O processo de anodização, utilizado industrialmente seguido da coloração, de acordo com a literatura, tem sido aplicado uma corrente de 50 mA/cm2, concentração de corante da ordem de 2-5 g/L, 15-18% de ácido sulfúrico e temperatura de 40C. Nota-se que não há um controle industrial desses diversos fatores que existem no processo, com isso, é preciso que haja um estudo de otimização do processo, pois a utilização de muitos reagentes em escala industrial pode levar a um impacto ambiental indesejável, além da emissão de gases devido a concentração do ácido utilizada e até gasto elevado de energia. Neste trabalho foi utilizado um corante orgânico para ser depositado na liga de alumínio AA6351 anodizada e estudado eletroquimicamente utilizando-se o planejamento fatorial para minimizar custos do processo, e melhorar a proteção do metal. As técnicas experimentais utilizadas neste trabalho foram: quimiometria, anodização, coloração por imersão, potencial de circuito aberto, polarização potenciostática anódica, resistência de transferência de carga, espectroscopia de impedância eletroquímica, microscopia óptica, microscopia eletrônica de varredura, microanálise e espectroscopia Raman. Os parâmetros para o planejamento experimental, utilizando-se da quimiometria, foram retirados da literatura, sendo eles: densidade de corrente, tempo e concentração do eletrólito para a anodização; e concentração do corante para a coloração. As medidas de resistência de transferência de carga (RTC) demonstraram a possibilidade de identificar quais dos ensaios ofereceriam maior proteção. Dois dos ensaios do planejamento experimental mostraram uma RTC por volta de 2,85 x 108 .cm2. Estes ensaios mostraram duas situações: (1º) quando a densidade de corrente de anodização é alta, menos tempo de anodização e corante são necessários; (2) quando a densidade de corrente de anodização é baixa, mais tempo de anodização e corante são necessários. As curvas de polarização mostraram uma densidade de corrente, das amostras anodizadas e coloridas, com valores muito menores quando comparado com o alumínio somente polido. A espectroscopia de impedância eletroquímica também mostrou uma resistência maior da camada desenvolvida nas peças coloridas. A microscopia eletrônica de varredura mostrou que o diâmetro dos nanoporos do óxido de alumínio do ensaio na primeira situação são maiores, da ordem de 11,7 nm, e por isso é necessário menos corante para preencher a camada de nanoporos, enquanto na segunda os nanoporos eram menores, da ordem de 7,6 nm exigindo maior concentração do corante. Na microscopia óptica foi possível observar que os parâmetros também influenciam na tonalidade da coloração escolhida. Os ensaios de sistema de energia dispersiva e de microanálise não apresentaram metais pesados na superfície do alumínio nem na composição do corante. A espectroscopia Raman comprovou que o composto está na superfície e que não sofreu alteração no processo de coloração.

The effect of different aging treatments on fatigue crack growth behavior of AA6013 aluminum alloy was investigated. C(T) (Compact Tension) specimens were prepared in L-T and T-L direction for fatigue crack growth tests. Samples were in T651 as received, T42 which is solution heat treated at 538 º
C for 90 minutes, water quenched and aged in room temperature for 96 hours, and one group of samples were overaged at 245 º
C for 12 hours after T42 condition was achieved. Hardness and conductivity measurements were achieved for all conditions after the heat treatments. Fatigue crack growth tests were performed at as received condition T651, T42 and 245 º
C aged samples in laboratory air with sinusoidal loading of stress ratio R=0.1 and at a frequency of 1 Hz. The highest fatigue crack growth resistance is observed for T651 T-L and 245 º
C overaged L-T condition.

This research project was conducted to increase weld gap tolerance in Friction Stir Welding (FSW) of 8 mm thick aluminum alloy 6082 – T6. Investigation was done on I-STIR PDS platform and a Johnford milling machine. The research project involved tool-pin design with varying re-entrant features and varying parameters as a method of weld gap closing to produce successful welds. Direction of spindle rotation and dwell time were established as part of a preliminary study. Clockwise spindle rotation with 20 seconds dwell time allows sufficient plasticity and improved material flow which assisted in achieving welds with prior 30 percent weld gap of the plate thickness. Final welds were made using three rotational speeds and feed rates with sufficient plunging to prevent root defects. Analysis of the results were detailed which include vickers microhardness test, tensile test and metallographic observation to access the suitability of the weld structure. From the set of tool-pins designed, the flare tool-pin gave a well-defined weld nugget with improved stirring at the weld root. Also, with a concave shoulder, right hand threaded tool-pin and counterclockwise flutes undergoing a clockwise spindle rotation, plasticized material flow was upward which was beneficial in reducing the amount of plate thinning. The right hand thread counter clockwise flute with a flute machined in the foot exhibited superior tensile strength for welds containing 30 percent weld gap.

The thesis includes a summary of important theory in the fields of impact engineering and plasticity theory, and a literature study is carried out on aluminium designations, alloying and heat treatment. The true stress-strain curves of aluminium alloy AA6070 in O-, T4-, T6-, and T7-configurations are identified by tensile tests where the behavior is continuously measured to fracture. The aluminium was delivered as 20 mm rolled plates; microstructural images and strain ratios are reported. Material constants are found by direct calibration and inverse modeling. Ballistic tests are done in a laboratory using 7.62 mm APM2 bullets, 20 mm blunt projectiles and 20 mm ogival projectiles (CRH = 3). From these tests the ballistic limit curves and the ballistic limit velocities are found for all temper/projectile combinations. In the material tests it was shown that the O-temper is the most ductile temper and consequently almost no fragmentation takes place in the ballistic tests for this temper. The T6-temper proved to be brittle, and fragmentation was commonly seen in the ballistic tests. The degree of fragmentation is found to be of vital importance for the ballistic performance. The Cockcroft-Latham fracture criterion is implemented by using the plastic work to fracture. Numerical analyses are performed with the IMPETUS Afea Solver and LS-DYNA, with 3D-models and 2D axisymmetric models, respectively. Ballistic limit curves and the ballistic limit velocities are calculated on the basis of the numerical results and then compared to the experimental values. Limited sensitivity studies are conducted on mesh size, heat-expansion dependency, strain-rate dependency, etc. Overall the results from 2D axisymmetric models are found to be consistent with previous studies, and the 3D-analyses carried out with the IMPETUS Afea Solver gave some good results. The IMPETUS Afea Solver proved to be a user-friendly finite element program with some powerful features. In addition to the numerical studies, a thorough derivation of the Cylindrical Cavity Expansion Theory (CCET) is given. Results from CCET are good for 7.62 mm APM2 bullets. A case-study where the ballistic performance is determined without conducting any experiments is also conducted. The results are promising, but less conservative than the original simulations, due to the inability to calibrate a fracture criterion. Some suggestions for further work within the field of impact engineering and the application of the finite element method are provided at the end of the thesis, followed by an appendix that includes graphical representations and photographs of the multiple material tests and program-codes written for use in the thesis.

El presente trabajo estudia de forma comparativa la tendencia a la fisuración en caliente de la aleación de aluminio AA6063-T6 de 2.8 milímetros de espesor mediante el proceso GTAW sin metal de aporte empleando tres condiciones de enfriamiento. Se elaboraron diferentes procedimientos de soldadura para analizar la soldabilidad del material mediante la obtención de cupones con descarga de líneas de calor sometidos a bajo aporte térmico (menor a 325 KJ/m), mediano aporte térmico (350 KJ/m), y a alto aporte térmico (mayor a 400 KJ/m); modificando en cada caso la velocidad de disipación del calor mediante el uso de un respaldo de acero, un respaldo de cobre y un respaldo de cobre refrigerado con agua, con la finalidad de analizar su relación con la tendencia a la fisuración en caliente. La inspección visual y los ensayos microestructurales realizados sobre los cupones con descarga de líneas de calor mostraron que bajo condiciones normales de soldeo no se logra evitar la aparición de fisuración en caliente. No obstante, si aseguramos que la soldadura presente una velocidad de enfriamiento superior al que normalmente se produce en procesos de soldadura, se podrá eliminar la aparición de fisuración en caliente. Ello se puede conseguir utilizando un respaldo de cobre, el cual presentó buenos resultados a bajo y mediano aporte térmico. Si bien es cierto, el respaldo de cobre refrigerado con agua logra eliminar la fisuración en caliente incluso para alto aporte térmico, no se recomienda su uso en soldadura debido a que este es un método poco práctico y difícil de utilizar en construcciones soldadas. Por otro lado, se realizaron cupones soldados a tope sin el empleo de metal de aporte y con enfriamiento mediante respaldo de cobre. Estos cupones fueron sometidos a ensayos de líquidos penetrantes, doblado y tracción, comprobándose su integridad y obteniendo una resistencia promedio de 145 MPa. Estos resultados fueron comparados y cumplieron con las exigencias del código de soldeo estructural de aluminio AWS D1.2, concluyéndose que cuando se desee soldar la aleación de aluminio AA6063-T6 de 2.8 milímetros de espesor mediante el proceso GTAW y sin el uso de metal de aporte, se deberá garantizar una velocidad de enfriamiento superior al que se obtiene normalmente en procesos de soldadura, lo que se consigue asegurando un buen contacto entre el metal base y el respaldo de cobre a fin de favorecer una buena disipación de calor.
Tesis

This thesis deals with the concept of ECAP applied to a commercial Al Mg-Si alloy (AA6082.50). A detailed analysis of the strains introduced by ECAP in a single passage through the die has been made, based on direct measurements on partially pressed samples.

Further, the mechanical properties of ECAP’ed samples have been investigated. The effect of initial material temper and post-ECAP artificial aging was investigated in addition to the effect of strain accumulation and processing route.

Finally, a detailed study of the textural and microstructural development was made. The continuous evolution of texture and microstructure was followed through the ECAP deformation zone up to an accumulated strain of 2 (2 passes) by route A, and linked to strain measurements from the same zone.

Strain measurements on sectioned samples have validated the plane strain assumption for ECAP. The shear angle has been measured and some typical features of ECAP have been corroborated, i.e. friction and material temper affect the strain distribution, the strain homogeneity and the workpiece corner angle, friction being the most significant here. Also, new conclusions have been drawn. The analysis of material element deformation histories along path lines reveals that ECAP can be interpreted as the combination of shearing and stretching (i.e. tension and/or compression). Furthermore, the final shear strain angle obtained in ECAP appears to be friction and material temper independent in the zone of homogeneous deformation.

The 6082 alloy has been successfully processed by ECAP at room temperature to strains ε =6 to ε =8. The alloy has been pressed in the solutionized, T4, homogenized and soft annealed states. The highest tensile strength was obtained from the solutionized material, followed by T4, homogenized and soft annealed. This behaviour is linked to the solid solution content prior to ECAP and the potential for dynamic precipitation during ECAP processing.

The tensile elongation to failure drops to a constant level between 4% and 9% after ECAP and is highest for the soft annealed and lowest for the solutionized material. The ductility in the solutionized material can, however, recover to ~18% elongation to failure (i.e. an increase by a factor 2-3) by low temperature heat treatment with only a small drop in tensile strength.

Soft annealed and ECAP’ed material has been compared to cold rolling to similar strains. The tensile strength response to accumulated strain is similar, but the ductility and post uniform deformation is different. However, the ECAP’ed material can be processed to higher strains and, thus, achieving higher strength.

The tensile yield strength behaviour of the homogenized and ECAP’ed material can be described by a simple relation to the grain size and the fraction high and low angle boundaries.

The typical texture components related to ECAP of aluminium, pressed by route A, have been identified. The typical ECAP texture starts to develop already at ~25% strain and increases in intensity during the first pass. In the second pass, two of the stable texture components continue to increase in intensity, while the other texture components decrease.

The microstructural development during the first pass is dominated by deformation banding leading to grain-subdivision. The average linear intercept distance (grain size) is reduced from ~80μm to ~4μm for high angle boundaries and from ~10μm to ~0.7μm for low angle boundaries.

During the second pass, the linear intercept distance is further reduced to ~1.8μm for high angle and ~0.3μm for low angle boundaries. Deformation twins are observed during the second pass and are believed to play an important role in severe plastic deformation when the grains reach the sub-micron or nano-metre scale.

The deformation banding have been explained in terms of the low energy dislocation structure (LEDS) theory, and has been shown to be an important mechanism in the early stages of grain subdivision, and is further believed to be the main source of high angle grain boundary formation by grain subdivision down to a grain size of approximately ~0 6μm, when other deformation mechanisms may be energetically more favourable.

Advancements in friction stir welding (FSW) have enabled the development and testing of a lightweight automotive bumper-beam/crash-box assembly. Previously, a test fixture to dynamically (crash) test the functionality of advanced bumper assemblies fabricated by FSW was developed. This FSW development work included microstructural examination and static mechanical testing. Results from coupon-level development were compared against results from component-level testing of prototype articles using micrographs and an advanced electronic (signal/frequency analysis) non-destructive evaluation (e-NDE) technique in order to detect weld anomalies primarily in the form of voids. Due to the geometry of the welded part joint, conventional mechanical testing methods (tensile and peel test) were not applicable. Therefore, a wedge test was devised to test the relative toughness of the FSW joint. From recorded data, toughness plots were calculated to select the best joint from three weld tools, each having the same basic threaded probe and WiperTM shoulder designs, and differing only in probe features. In addition to the basic tool configuration, one tool had a set of partial CounterFlowTM grooves, and the other had a set of partial straight flats. Each also had a special geometrical feature added to the tip of the tool probe, referred to as a concentrating tip, to improve metal flow at the end of the probe in order to inhibit void formation. Traditional sled testing for low-speed bumper requirements was performed at the General Motors (GM) Research and Development (R&D) facility in Detroit, Michigan, and drop tower tests were performed using an FSW test fixture at the National Institute for Aviation Research (NIAR) at Wichita State University (WSU). These dynamic tests were performed on bumpers using both FSW and gas metal arc welding (GMAW). Finite element analysis (FEA) was used to compare the predicted damage to the actual damage sustained by the bumpers fabricated by GMAW and FSW, respectively.
Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering

El presente trabajo de tesis estudia las diferencias en las propiedades mecánicas que se presentan entre las uniones soldadas, de la aleación de aluminio AA6063 de 2,8 mm de espesor, en diferentes estados de tratamiento térmico, mediante el proceso GMAW empleando como material de aporte ER5356 (Al-5Mg). Los estados de tratamiento térmico en los que se soldó la aleación de aluminio AA6063 fueron tres: solubilizado y templado (W); solubilizado, templado y envejecido natural (T4); y solubilizado, templado y envejecido artificial (T6). Los cupones que fueron soldados en estado W, recibieron un tratamiento de envejecimiento luego del soldeo, equivalente a los estados T4 y T6; pasando a ser denominados W-T4 y W-T6. Los parámetros de soldeo empleados no variaron al momento de soldar, manteniendo constantes el amperaje a 124 A, el voltaje de 17,9 V, la velocidad de alimentación del material de aporte y velocidad de soldeo. La inspección visual realizada a los cupones, posteriores al proceso de soldeo, comprobó que el proceso fue realizado satisfactoriamente, al no mostrar discontinuidades, ni fisuras externas ni porosidades. Los ensayos mecánicos de tracción mostraron que la máxima resistencia a la tracción de las uniones soldadas en los diferentes estados varía entre los 192 MPa y 202 MPa, coincidiendo como zona de rotura en la ZAC; y los ensayos de dureza mostraron que la mínima dureza de las uniones en los diferentes estados, varía entre los 43 HV y 67 HV, coincidiendo de la misma manera en la ZAC como zona de mínima dureza. Se corroboró que realizando un tratamiento térmico posterior al soldeo, mejoran las propiedades mecánicas perdidas, obteniéndose los estados W-T4 y W-T6. Sin embargo, esta mejora en las propiedades mecánicas de la aleación, no es significativa, en comparación a soldarlo en los estados T4 y T6. De estos resultados obtenidos en el presente trabajo de tesis, se concluyó que para la aleación AA6063, se deberá soldar directamente en los estados T4 y T6.
Tesis

A model has been developed to predict the yield stress of the aluminum alloy AA6111 after multi-step heat treatments which involve combinations of ambient temperature ageing and high temperature artificial ageing. The model framework follows the internal state variable framework where the two principal state variables are i) the volume fraction of clusters which form at ambient temperature and ii) the volume fraction of metastable phases which form during high temperature ageing. The evolution of the these state variables has modeled using a set of coupled differential equations. The mechanical response (the yield stress) is then formulated in terms of the state variables through an appropriate flow stress addition law. To test the model predictions a series of experiments were conducted which examined two scenarios for multi-step heat treatments. In general, good agreement was observed between the model predictions and the experimental results. However, for the case where a short thermal excursion at 250oC was applied immediately after the solution treatment, the results were not satisfactory. This can be understood in terms of the importance of the temperature dependence for the nucleation density of metastable precipitates.

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A Soldagem Ponto Fricção e Mistura Mecânica (Friction Stir Spot Welding - FSSW) é uma técnica de soldagem para união pontual de juntas sobrepostas no estado sólido. Por não envolver fusão das chapas que estão sendo soldadas, proporciona melhores propriedades mecânicas em comparação com a soldagem convencional. Outras vantagens são pouca distorção térmica, não necessidade de preparação da superfície e não geração de fumos (tecnologia limpa), o que vem tornando o processo atrativo para a industria automotiva. Neste trabalho foi feito um estudo do desempenho mecânico de juntas sobrepostas soldadas pelo processo FSSW da liga AA6060-T5. Propriedades metalúrgicas e mecânicas de juntas FSSW em chapas de 3,2 mm de espessura foram investigadas, em termos de dureza, resistência ao cisalhamento, macroestrutura e área soldada. Planejamento de experimentos (DOE) foi implementado, pelo Método Taguchi, para determinar as combinações de parâmetros de soldagem a serem testadas e para a interpretação dos resultados. Os parâmetros de processo estudados foram velocidade de rotação, tempo de permanência e velocidade de penetração, em três níveis cada, formando uma matriz ortogonal L9. Com os resultados do Método Taguchi foram escolhidos os dois parâmetros mais significativos e aplicou-se um planejamento Fatorial completo 2³. Para cada condição de soldagem, foram produzidas quatro amostras, uma para avaliação macroestrutural e microdureza e três para testes mecânicos de cisalhamento. Os resultados mostraram que soldas de qualidade são possíveis para a liga e equipamentos usados nesse estudo. Resistências ao cisalhamento superiores a 2,4kN foram observadas, associadas a longos tempo de soldagem. O tempo de permanência e a velocidade de rotação são os parâmetros de maior influência na resistência ao cisalhamento. Os tempos de soldagem muito longos causam diminuição da resistência ao cisalhamento devido a ascentuação do gancho. O aumento da velocidade de rotação leva ao fenômeno de transição chamado de stick/slip, o que limita a sua magnitude. Equações empíricas para prever a força de cisalhamento em função dos parâmetros de soldagem são satisfatórias quando obtidas por interpolação quadrática.
The Friction Stir Spot Welding (FSSW) is a welding technology to produce overlap spot joints in the solid state. The absence of bulk melting leads to enhanced mechanical properties in comparison to conventional welding. Other advatages are little thermal distortion, no need of surface preparation and no fumes (clean technology), all contributing to the attractiveness of this process in the industry, specialy the automotive. This work aimed the study of the mechanical performance of FSSW joints produced with the AA6060-T5. Mechanical and metallurgical properties of FSSW produced in sheets 3,2mm thick were investigated, in terms of hardness, shear tensile strength, macrostructure and welded area. Design of Experiments (DOE) was applied by means of the Taguchi Method to determine the welding parameters to be tested and to understand the results. The welding parameters tested were rotational speed, dwell time and plunge rate, three levels each which led to a L9 orthogonal matrix (9 welding configurations). With results of the Taguchi Method, the two welding parameters more significant (dwell time and rotational speed) were selected for a complete factorial 2³ assessment. For each welding configuration five samples were produced, one for macrostructural and hardness assessment and four for mechanical tests. The results showed that sound welds are possible with the selected alloy and welding equipment. Shear strength higher than 2,4kN, was observed, associated to long welding time. The dwell time and rotational speed are the parameters most effective on the joint shear strength. Too long welding times lead to poor joint strength due to the hook. There is a limit to the rotational speed associated to the transition phenomenon known as slick/slip. Empirical equations for predicting joint strength in terms of welding parameters were sucsessfuly obtained by quadratic interpolation.

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Financiadora de Estudos e Projetos
In 1991, it was developed a solid state welding process called Friction Stir Welding - FSW, intended to increase the number of applications for the aluminum and its alloys. As every welding process the presence of defects should be avoided to not compromise the quality of the welded joint. Some typical defects originating from FSW process are found in some joints depending of the welding parameters. Thus, this study has the purpose to evaluate the quality of welded joints in 3 mm thick 6013-T6 friction stir welded joints in function of the obtained defects, using different welding parameters. The characterization was based on metallography by OM and SEM of the welded joints in order to analyze the obtained defects. The mechanical properties were evaluated by tensile, microhardness and fatigue tests.Welds joints were intentionally produced to contain inner defects in the weld nugget like tunnel shaped along the joint and lack of root penetration of the welding tool. All welded conditions, the microhardness profiles show a big drop in hardness when crossing from the HAZ zone into the center of each weld, while the tensile and fatigue tests showed a strong defects influence in the mechanical properties. The lack of penetration weld defect had the worst results of mechanical properties compared to tunneling defects, and the sound weld joint exhibited the best results of mechanical properties for all conditions investigated.
Visando ampliar o número de aplicações para o alumínio e suas ligas, foi desenvolvido em 1991, um processo de soldagem no estado sólido denominado Friction Stir Welding. Como em todo processo de soldagem a presença de defeitos deve ser evitada, pois compromete a qualidade da junta soldada. Alguns defeitos típicos oriundos do processo FSW são encontrados em algumas juntas dependendo dos parâmetros de soldagem escolhidos. Assim sendo, este trabalho teve como principal objetivo estudar a influência de defeitos nas propriedades mecânicas de juntas soldadas pelo processo Friction Stir Welding da liga de alumínio 6013-T6, correlacionando as variações microestruturais obtidas com os diferentes parâmetros de soldagem adotados. Para tanto foram utilizados diferentes parâmetros de soldagem para a obtenção de juntas soldadas que apresentem defeitos distintos. A caracterização metalográfica se baseou em macrografias e micrografias via M.O. e MEV das juntas soldadas, de forma a analisar os defeitos obtidos. As propriedades mecânicas foram avaliadas por ensaios de tração, microdureza e fadiga. Foram obtidas soldas com defeitos internos a lente de soldagem em forma de vazios ao longo do cordão e soldas com falta de penetração na região da raiz. Em todas as condições foi observada uma redução brusca nos valores de microdureza na região da junta soldada, enquanto os ensaios de tração e fadiga revelaram forte influência da presença de defeitos na junta. A solda que apresentava como defeito a falta de penetração na raiz apresentou resultados de propriedades mecânicas muito inferiores às condições que apresentaram vazios internos a lente de soldagem. A solda que não continha defeitos apresentou os melhores resultados de propriedades mecânicas em relação a todas as condições investigadas.

O objetivo deste trabalho é apresentar a seqüência de desenvolvimento de um processo de forjamento a quente com uma liga de alumínio AA6351. Métodos experimentais, numéricos e computacionais são utilizados em conjunto buscando a redução do tempo de desenvolvimento do processo de conformação, reduzindo as chances de confecção incorreta de ferramental, conformação insatisfatória, capacidade dos equipamentos inadequada, entre outros. Os programas comerciais de simulação numérica MSC.SuperForge 2005® e MSC.SuperForm 2005®. Este é alimentado com parâmetros obtidos através de métodos experimentais, como curvas de escoamento e fator de atrito, e métodos teóricos como coeficientes de transferência de calor, com o objetivo de melhorar a qualidade dos resultados obtidos através da simulação numérica. As peças produzidas são comparadas com os resultados obtidos via simulação computacional com o intuito de validar os resultados obtidos e apresentar a viabilidade de utilização de métodos numéricos para desenvolvimento de processos de conformação mecânica. O desenvolvimento de geratrizes pré-formadas é realizado através de simulação numérica resultando em uma redução de aproximadamente 59% no volume de material perdido em forma de rebarba.
The purpose of this work is to present the sequence of a hot forging process development from AA6351 aluminum forged part. Experimental, numeric and computational methods are used together so as to reduce the product’s development time, chances of incorrect tools construction, incorrect formation and inadequate equipment capacity, among other things. The MSC.SuperForge 2005® commercial numeric simulation software is used. This software is fed with parameters acquired through experimental methods, like flow-stress curves and friction curves, and numeric methods, like heat transfer coefficients, with the purpose of increase the results quality got through numeric simulation. Produced parts are compared with computational simulation results with the purpose to validate the results got and present the feasibility of the use of numeric methods to development of mechanical formation processes. Pre-formed billets development is realized through numeric simulation resulting in a material volume reduction of approximately 59% in material volume loss like flash.

Uma nova tecnologia de soldagem, denominada Friction Spot Welding (FSpW) utiliza uma ferramenta rotativa especial que transfere energia térmica e mecânica ao material sendo soldado. O processo se destina à soldagem de juntas sobrepostas, primariamente para ligas leves, mas, em teoria, é aplicável a qualquer material que apresente boa plasticidade. A crescente competitividade tem motivado as indústrias à otimização de processos, o que tem levado ao uso cada vez mais expressivo de ligas leves nas indústrias automobilística e aeronáutica. Soma-se a isto a necessidade de redução do consumo de combustível em veículos, dada a crescente preocupação com as questões do meio ambiente. Estes fatores tornam o processo FSpW um grande atrativo seja na substituição ou no uso complementar de tecnologias consagradas como soldagem a ponto por resistência elétrica e a LASER (indústria automobilística) e rebitagem (indústria aeronáutica). Neste trabalho foi feito um estudo da aplicabilidade do processo FSpW para a liga de alumínio 6181-T4 que tem forte apelo na indústria automotiva. O estudo teve como objetivo avaliar os efeitos dos parâmetros de processo, na microestrutura, na geometria da junta e no desempenho mecânico, além de buscar a otimização do processo para a liga em estudo. Tal otimização foi feita por meio do estudo dos mecanismos de fratura das soldas sob solicitação monotônica de cisalhamento e de tração. Juntas sobrepostas foram produzidas utilizando condições de soldagem variadas, por meio de diferentes combinações de velocidade de rotação e tempo de processamento. Os resultados obtidos mostraram que soldas de alta resistência e de reprodutibilidade são possíveis com este novo processo. O botão de solda FSpW da liga 6181-T4 é constituído de três elementos (União Principal, União Secundária e Cunha). As características destes elementos, somada às transformações metalúrgicas decorrentes do processo, afetam fortemente o comportamento em fratura das juntas e, conseqüentemente, o seu desempenho mecânico.
A new welding technique referred to as Friction Spot Welding (FSpW) suitable to produce overlap joints uses a spindle to transfer mechanical and thermal energy to the material being welded. Although initially developed for lightweight alloys this welding technique is suitable to virtually any material presenting some degree of plasticity. The process has shown potential applicability to complementary use together with other well established techniques such as resistance and LASER spot welding and riveting. In this work a study of the application of the FSpW process for the automotive aluminium alloy 6181-T4 was performed. Overlap joints were produced with different welding conditions by using different combinations of tool rotation speed and welding time. The objectives were the assessment of the welding parameters on the strength, microstructure and geometry of the joints, as well as the optimization of the process. The welded joints were assessed mechanically and metallurgically aiming the investigation of the mechanisms of joint formation and the optimization of the welding parameters. The methodology used for the optimization of the welding parameters was the fracture mechanisms of the joints under shear and tension loads. The results revealed that sound welds with high strength and toughness are possible to be produced with the 6181-T4 alloy using this process. Moreover, the metallurgical investigation revealed that the spot joint is comprised by three elements: Primary Union, Secondary Union and Rim. It was found that the characteristics of these three elements in addition to the metallurgical transformations imposed to the material play a fundamental role on the fracture behaviour of the joints and thus, on its mechanical behaviour.

Il presente progetto di Tesi Magistrale è il risultato del lavoro svolto all’interno del gruppo di ricerca coordinato dal Prof. Lorenzo Donati, Dipartimento di Ingegneria Industriale “DIN” dell’Università di Bologna. L’obiettivo è quello di sviluppare un modello per la stima della dimensione dei grani della lega di alluminio AA6063 ricristallizzatisi staticamente durante il processo di estrusione diretta di un profilo industriale prodotto dall’azienda svedese “Hydro Extrusions Finspång”. A lato della campagna sperimentale condotta nei laboratori del Dipartimento, è stato infatti redatto in linguaggio LUA un algoritmo, ottimizzato tramite il modello di Levenberg – Marquadt, allo scopo di ricavare le costanti sperimentali necessarie per la modellazione del processo in esame. Tale algoritmo è stato successivamente applicato in ambiente QForm – Extrusion allo scopo di ricavare, per via computazionale, l’evoluzione della microstruttura ottenuta durante lo svolgimento del processo di estrusione. I dati numerici così ottenuti sono stati infine confrontati con i valori sperimentali ricavati durante la campagna d’indagine allo scopo di poter validare il modello così proposto e fare le dovute considerazioni sulla sua applicabilità in termini di strumento previsionale da utilizzarsi durante le fasi di progettazione e/o di ottimizzazione del processo di estrusione.

Extruded profiles of AlMgSi-alloys are widely used in various industries such as the automotive industry, aerospace and in the oil and gas industry. To tailor the alloys for specific needs and applications, more information about the behaviour of alloying elements is needed. In this study, extruded profiles of AA6082 have been examined. Seven different alloys with increasing content of Mn have been subjected to three homogenisation procedures. The purpose of the different homogenisation procedures was to obtain different densities of dispersoids, while still maintaining the same level of elements in solid solution. The study have examined the effect of Mn and homogenisation procedure on grain structure, strength and corrosion behaviour.Results from this study showed that the density of dispersoids greatly affect the final grain structure after extrusion. Homogenisation variants that contained a high density of dispersoids showed low degree of recrystallisation. Increasing amount of Mn also contributed to a lower degree of recrystallisation. In as extruded condition variants with a high density of dipersoids were harder than variants with lower density, despite equal chemistry and elements in solid solution. At peak strength, increased content of Mn led to a reduction in strength.

Dentre as melhorias nos processos produtivos para a manufatura de aeronaves nas empresas, a soldagem a laser tem sido destacada como uma das tecnologias mais promissoras. A mudança de rebitagem para soldagem do revestimento da fuselagem e dos reforçadores tem contribuído para a redução de peso e dos custos das aeronaves, bem como diminuído o seu tempo de fabricação. Este trabalho pretende contribuir ao entendimento da metalurgia de soldagem autógena (sem uso de metal de adição e em um único passe) de uma liga de alumínio de alta resistência (AA6013) com um laser a fibra (2 kW). A geometria escolhida ée solda topo, sem chanfro, para facilitar a análise dos resultados. Foram estudados os efeitos dos parâmetros de processo, velocidade e potência do laser, além de dois valores diferentes de espessura das chapas, realizados com um desenho de experimentos sobre a microestrutura e qualidade da solda. O estudo mostrou que ée possível obter soldas com boa qualidade macroscópica, em relação à regularidade do cordão e furos, e com isenção de trincas ou porosidades em velocidades em torno de 100 mm/s e com potência do laser de cerca de 1 kW. Também foi verificado que a zona afetada pelo calor ée bastante concentrada, com região de liquação em torno de 50 mm.

There is a current trend in increased use of aluminum extrusion alloys in automotive applications. This trend is driven by the need to reduce the vehicle weight, which in turn, is to decrease energy use and/or emissions of the vehicles. In this work, Al-Mg-Si alloys, and especially variants of AA6082 with different Mn and/or Cr additions have been studied. The objectives of the study are to i) experimentally characterize the evolution of the constituent particles (phase, volume fraction and size) and dispersoids (chemistry, crystal structure, size and volume fraction), ii) rationalize the mechanisms of dispersoid evolution and iii) develop a physically based constitutive law for the high temperature flow stress. The characterization of microstructure evolution during homogenization was done using a combination of i) transmission electron microscopy, ii) field emission gun scanning electron microscopy, iii) electron microprobe microanalysis and iv) electrical resistivity measurements. The high temperature flow stress was characterized by uniaxial compression tests. The main results on microstructure evolution during the process of homogenization show that i) there is a transformation of the constituent particles from the β to α phase during homogenization and a concurrent speriodization of the particles, ii) dispersoids with the size range of 20-200 nm and a volume fraction of 0.25 – 1.3 % are initially formed during homogenization but they eventually dissolve as Mn is transported to the constituent particles and iii) a steady-state flow stress of between 20 and 45 MPa was measured for the test temperature between 550 °C and 580 °C with strain rates of 0.1 – 10 s-¹. The evolution of dispersoids during homogenization was rationalized by considering their nucleation, growth and coarsening. It is proposed that dispersoid coarsening initially involves long-range diffusion of Fe from the constituent particles to dispersoids and later Mn and Fe diffuse to the constituent particles. The Kocks-Chen constitutive model was extended to include the role of dispersoids on high temperature flow stress using an Orowan type model for precipitation hardening. This was found to predict the flow stress ± 5 % in ≈ 95 % of the cases that were studied.
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