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A necessidade de redução de custos de produção torna o caster mais vantajoso que o processo de placas, porém a aplicação dos produtos obtidos via vazamento contínuo é limitada pelas condições microestruturais. A alta taxa de resfriamento no processo de vazamento, que pode ultrapassar 300 °C/s, resulta em uma condição de supersaturação dos elementos em solução sólida. Quando as chapas são recozidas, ocorre precipitação na forma de dispersóides que atrasam a recristalização. Quando o material obtido via caster segue o processo convencional de placas laminadas a quente, com recozimento intermediário e têmpera obtida por reduções pré-definidas, o resultado é uma chapa com aspecto superficial e desempenho na estampagem prejudicados pela granulação grosseira. Para otimizar o processo é necessário aprimorar o conhecimento dos fenômenos de encruamento, recuperação, recristalização e crescimento de grãos além de relacioná-los aos processos industriais. A prática usual de processo para o material de caster consiste em um tratamento térmico de homogeneização para precipitar os elementos em solução sólida e melhorar a distribuição dos precipitados para que existam núcleos viáveis para a recristalização. O processo de homogeneização agrega um elevado custo na produção de chapas e pode ser substituído em determinadas aplicações por um tratamento de recristalização parcial que evita o crescimento de grãos. Para o presente trabalho, as amostras foram retiradas de rolos caster brutos de fusão e de placas laminadas a quente, nas ligas AA1200 e AA3003. As bobinas foram laminadas a frio até 0,88 mm e as amostras sofreram tratamentos térmicos em temperaturas entre 150 e 500 °C, com intervalos de 50 °C. O intervalo entre 200 e 450 °C foi melhor detalhado e os tratamentos térmicos foram feitos a cada 10 °C. Todos os tratamentos tiveram duração de 1 hora. Após tratamentos, foram feitas medidas de condutividade elétrica e dureza, ensaios de tração, Erichsen, análises químicas e caracterização microestrutural. O decréscimo das propriedades mecânicas em função do aumento da temperatura do tratamento térmico mostra o efeito da recuperação e recristalização nas amostras de chapas. A evolução das propriedades em função da temperatura é diferente quando se compara caster e placa, devido principalmente à taxa de resfriamento durante o vazamento do caster que mantém os elementos em solução sólida. A diferença é muito mais acentuada na liga AA3003, devido à baixa solubilidade sólida do manganês no alumínio, que precipita durante o tratamento térmico e atrasa a recristalização. Com a evolução das propriedades mecânicas em função da temperatura foi possível determinar a faixa ideal de tratamento térmico para a obtenção das têmperas objetivadas. / The necessity of reducing productions costs makes the continuous casting more advantageous than plates process, however, the application of the products obtained through continuous casting is limited by microstructural conditions. The high rate of cooling in the continuous casting, which can exceed 300 °C/s, results in a condition of supersaturation of the elements in solid solution. When the plates are annealed, precipitation occurs in the form of dispersoids that delays the recrystallization. When the continuous casting material follows the conventional process of hot-rolled plates, with intermediate annealing and hardness achieved by pre-defined reductions, the result is a plate with superficial appearance and performance in drawing hampered by the coarse graining. To optimize the process is necessary to improve the knowledge of the phenomena of cold working, recovery, recrystallization and grain growth in addition to relate them to industrial processes. The practice of process for continuous casting material consists in homogenization heat treatment to precipitate elements in solid solution and improve the distribution of precipitates in order to provide viable nuclei for the recrystallization. The process of homogenization adds a high cost in the production of plates and can be replaced in certain applications for treatment of partial recrystallization that prevents the grain growth. For this paper, the samples were taken from continuous casting rolls as cast and hot-rolled plates, in the alloys AA1200 and AA3003. The coils were cold-rolled up to 0.88 mm and the samples have undergone thermal treatment at temperatures between 150 and 500 °C, with intervals of 50 °C. The interval between 200 and 450 °C was better detailed and heat treatments were performed every 10 °C. All treatments had duration of 1 hour. After treatment, measures of electrical conductivity and hardness were made, besides traction tests, Erichsen, chemical analysis and microstructural characterization. The decrease of mechanical properties as a function of increasing heat treatment temperature shows the effect of recovery and recrystallization in samples of hot-rolled plates. The development of properties depending on the temperature is different when continuous casting and hot-rolled plates are compared, mainly due to the rate of cooling during the casting that keeps the elements in solid solution. The difference is more pronounced in the alloy AA3003, due to the low solid solubility of manganese in aluminum, which precipitates during the heat treatment and delays the recrystallization. With the evolution of mechanical properties according to the temperature, it was possible to determine the optimal range of heat treatment to obtain the desired hardness.

Understanding high temperature interfacial phenomena with Al-alloys is essential for improving corrosion performance of refractories in melting/holding furnaces. Both physical and chemical properties are known to influence wetting and corrosion behaviour. However, uncertainties exist regarding the influence of SiO2 in refractory compositions on interfacial reactions/mechanisms, particularly when present along with non-wetting chemical additives like BaSO4, CaF2 and AlF3. An experimental study was conducted to clarify the interfacial phenomena of Al-alloy7075 with high-alumina refractories at extreme furnace temperatures of 1250??C and 815??C, using classical sessile drop approach and industrial cup tests respectively. At 1250??C, Al-alloy reacted more intensely with SiO2 compared to Al2O3. The interfacial behaviour of SiO2-Al2O3 system with Al-alloy was strongly dependent on SiO2 percentage, such that when upto 25% silica was present, wetting was reduced due to the presence of both original and newly formed corundum. Formation of mullite and originally present silica, along with decreasing corundum contents increased wetting in systems where silica varied from 25-45wt% and more than 45wt% respectively. Moreover, the nature of silica did not influence wetting when present in concentrations less than 20wt%. Different additives produced varying interfacial reactions in the Al-alloy/high-alumina refractory system. AlF3 did not improve the wetting resistance, except when present in high concentrations (>10wt%) in the refractory; this improvement attributed to corundum-rich matrix formation resulting from silica loss as gaseous fluorides. Low CaF2 amounts (<3wt%) improved the wetting resistance due to corundum presence and anorthite formation in the refractory. As CaF2 content exceeded 5wt%, proportion of glassy phases increased, hence enhancing interfacial reactions. However unlike CaF2, low BaSO4 levels (<5wt%) decreased the wetting resistance due to barium silicate formation, while high BaSO4 concentrations (≥10wt%) increased the wetting resistance due to celsian formation. Also, CaF2 dominated interfacial mechanisms when present along with BaSO4 in the refractory. The effect of additives on modifying wetting resistance was found to strongly vary with SiO2 levels of the refractory. The study demonstrated that additive effect is also influenced by treatment temperatures such that generally higher additive amounts are required at lower temperatures for improving the wetting resistance of high-alumina refractories.

xiv, 223, xv leaves : ill. ; 26 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agricultural Biochemistry and Soil Science, 1972

"February 2003" / Bibliography: leaves 160-174. / xi, 174 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This study assessed separately the effects of Aluminium and of low pH on the growth of gigaspora margarita and its symbiotic function in the growth of cowpea (vigna unguiculata L. Walp,). / Thesis (Ph.D.)--University of Adelaide, Dept. of Soil and Water, 2003

<p>The conductive area of a stationary electrical contact consists of many small contact spots. The contact spots can be described as narrow metal bridges across the interface between two conductors. Deterioration of a contact is closely related to processes occurring in these microscopic areas. The objective of this work is to clarify the basic mechanisms affecting the reliability and degradation of an electrical contact by closely studying the contact spots.</p><p>Particular emphasis has been put on examining the contact spots occurring in aluminium-aluminium interfaces when passing AC. Due to the small thermal capacity of a contact spot the temperature in the contact spot cycles with twice the frequency of the applied AC. The minimum temperature attained during one cycle being the bulk temperature of the conductor, the maximum temperature can in heavily stressed contacts be close to the melting point of the conductor material. In the idealized contact used in the present investigation, this thermal cycling may initiate thermal fatigue processes in the contact spot region.</p><p>Microscopic cracks appear in the contact spot region as the result of thermal fatigue processes. The cracks not only lower the mechanical strength of the conductor in close vicinity of the contact spot, they also cause a higher electrical resistivity in these areas.</p><p>Alloying a tracer metal in one of the electrodes of the contact allows for closer investigation of the crack propagation as it occurred during the experiments. The tracer migrates into the heated contact spot region of the opposite electrode where the cracks constitute a barrier impairing further diffusion into the electrode. Further information on the thermal fatigue processes in the contact spot region is obtained when examining metallographic images of cross sectioned contact spots. Here it can be seen that the repetitive thermal stresses give rise to clearly localized recrystallisation in the contact spot region.</p><p>Thermal fatigue is a well known failure mechanism in mechanical structures. It has to the authors knowledge not previously been associated with the deterioration of contact spots. However, the mechanical damage in the contact spot region caused by the temperature cycling also has a detrimental effect on the electrical behaviour of the contact. It is therefore suggested that thermal fatigue is of considerable importance to the reliability and degradation of stationary electrical contacts.</p>

<p>Following a short introduction, the state-of-the-art fatigue life assessment methods based on the nominal stress, the structural hot spot stress and the effective notch stress, as well as the crack propagation analysis are explained and discussed.</p><p>Thereafter, the effect of mean stress on crack propagation was investigated by exemplifying crack propagation data from different aluminium alloys. In this context, Walker's equation received wide attention. In addition, a new, simple equation to express the mean stress dependency of crack propagation data has been proposed.</p><p>In order to generate crack propagation data for the 6082-T6 aluminium alloy, investigate the behaviour of short cracks and find the mean stress dependency of crack growth in this alloy, crack propagation tests of sub-millimetre cracks subjected to different stress ratios have been carried out. The crack depth was successfully monitored on-line employing a special direct current potential drop technique. The crack propagation data generated verified the long crack propagation data given in Eurocode 9. In addition, useful experience was gained to improve future tests. Some evidence was found of accelerated crack growth near the threshold. Based on the data obtained and literature data a Walker exponent γ= 0.78 has been calculated for the aluminium alloy 6082-T6.</p><p>The fatigue life of welded aluminium T-joints has been estimated using the approaches introduced in Chapter 2. The results have been compared with experimental data from T-joints subjected to 4-point bending. In addition, a series of T-joints subjected to transverse bending has been fatigue tested and the results have been compared with predictions using the same fatigue life assessment methods.</p><p>The nominal stress gave a conservative prediction in case of the 4-point bending loading. Since no applicable detail category for the transverse bending load case exists, a suitable detail category has been suggested.</p><p>The structural hot spot stress approach resulted in conservative predictions (factor 4-10 in life) for both load cases. </p><p>The predictions from the effective notch stress approach are in excellent agreement with the experimental results, where the method using a 1 mm weld toe radius gave the best results. It has been shown that the method can be successfully applied to joints with wall thicknesses down to 3 mm.</p><p>The fatigue life predicted with the crack propagation approaches is in excellent agreement with the experimental results from the 4-point bending load when the<i> M</i><i>k</i> factor was used. Applying the local stress distribution, the predictions are conservative for both load cases.</p><p>The slopes of all predicted S-N curves, especially from the stress based methods, are steeper than the experimental curves.</p>

<p>The purpose of this work has been to investigate the ultimate capacities of aluminium plates. The aluminium plates are supposed to be part of a marine structure. Their behaviour has been explored by taking advantage of non-linear finite element simulations. The loading conditions included axial compression, transverse compression, pure shear loading, axial compression in combination with transverse compression and axial compression in combination with shear loading.</p><p>Different patterns of heat affected zones have been studied with particular attention on conventionally welded plates and extruded, welded plates. The plates have been given realistic boundary conditions in addition to boundary conditions believed to represent the extremities in plate collapse behaviour. The initial deflection patterns chosen, were generally conservative, and the variations in collapse capacities for different initial deflection patterns have been investigated. The initial deflection amplitudes were systematically varied to cover all values of the initial deflection amplitudes likely to occur.</p><p>In addition to studying different patterns of the heat affected zones, systematic variations of the breadths of the heat affected zones have been performed. Systematic variations in the magnitudes of the residual stresses and systematic variations of the reductions of the 0.2 percent tensile proof stresses in the heat affected zones have also been carried out. Three different aluminium alloys were studied. They were believed to represent a high, medium and low utilised aluminium alloy. The aspect ratio of the plates was varied to cover both plates that were completely square to plates with a length equal to five times the breadth of the plates.</p><p>It was found that axially loaded plates with heat affected zones along loaded edges had lower ultimate capacities than plates with heat affected zones along unloaded edges, and if the plates had heat affected zones along loaded edges, having additional heat affected zones along unloaded edges did not alter the buckling capacities. The reduction in ultimate capacity could be modelled to vary linearly with the breadth of the heat affected zones, and the reduction in ultimate capacity could be modelled to vary linearly with the relationship between the value of the 0.2 percent tensile proof stress in the heat affected zones and the 0.2 percent tensile proof stress of the base material.</p><p>The ultimate capacities of transversally loaded plates could be found as the sum of a square plate and a plate strip, even if the plates contained heat affected zones. Square plates and plate strips with relevant patterns of heat affected zones had to be employed. Biaxial interaction curves showed a considerable slenderness dependence for all values of the aspect ratio investigated. The biaxial interaction curves were only slightly affected by introduction of different patterns of heat affected zones. The stress corresponding to equal elastic and plastic strain was better suited as the normalising stress than the more commonly used 0.2 percent tensile proof stress. One design curve could be used to represent all the different aluminium alloys investigated. Changing the aluminium alloy did not change the shape of the biaxial interaction curves.</p><p>Based on the detailed study of the collapse behaviour of aluminium plates carried out, and established practice, new ULS design formulas for axial compression, transverse compression, pure shear loading, axial compression in combination with transverse compression and axial compression in combination with shear loading were proposed. The new design formulas accounted for reductions in ultimate capacities due to heat affected zones, including heat affected zones along loaded edges. They took full advantage of non-linear residual strength for slender plates; and the slenderness dependence and aspect ratio dependence for biaxial interaction curves were systematically incorporated.</p>