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Orientador: Angelo Caporalli Filho / Banca: Valdir Alves Guimarães / Banca: Wyser José Yamakami / Resumo: A Engenharia tem buscado a fabricação de materiais com elevados valores de resistência mecânica e tenacidade. Uma das formas de obter tais propriedades é promover a redução do tamanho de grão do material. A fim de reduzir o tamanho de grãos e obter tais propriedades neste trabalho foi aplicado o processo ECAP (Equal Channel Angular Pressing) em amostras da Liga de Alumínio AA3104, liga de Al-Mn não Tratável Termicamente. As amostras de alumínio foram deformadas plasticamente por cisalhamento simples na região de interseção dos canais. A repetição deste processo resulta no encruamento do material. Os resultados mostram que após o processamento a microestrutura esta com grãos alongados devido ao encruamento do material e por meio de ensaios de dureza Vickers verifica-se um aumento de dureza após 3 passes de ECAP aplicados / Abstract: A constant search in Engineering is the manufacturing of materials with high values of strength and tenacity. One way to obtain such properties is to promote the reduction of grain size of the material. In order to reduce the size of grains and obtain such properties in this work was applied the ECAP (Equal Channel Angular Pressing) process in samples of Aluminum Alloy AA3104, Al-Mn alloy is non-heat treatable. A sample of aluminum was deformed by simple shear in the intersection of the channels that leads to a severe plastic deformation. The repetition of the process results in a material with a cold work hardening. The results show that after the processing ECAP microstructure with elongated grains and by Vickers hardness testing the hardness increase after the three pass of ECAP applied / Mestre

Friction StirWelding (FSW) has been researched for a number of years since its inception in 1991. The work thus far has been based on understanding the material and thermal flow using the standard fixed pin tool. The keyhole resulting during tool extraction in a FSW weld, is a disadvantage and a current limiting factor. Eliminating this effect from a weld using a movable pin tools would make FSW more commercially viable. This dissertation focuses on the design of a novel retractable pin tool, and highlights the problems encountered during the welding of Aluminum plates, Al2024 and Al5083. Previously studied techniques of material and thermal flow were used, to investigate the effect of the tool during extraction in a FSW weld. A prototype retractable tool was designed using parametric and axiomatic design theory, and implementing a pneumatic muscle actuation system. The resulting problems in the calibration of the retractable pin tool and the resulting welds are presented, these results confirming previous studies. The movable pin produced discrepancies the heat generation around the shoulder during a FSW weld. The failure of this tool to produce a reasonable weld showed that previous ideas into the workings of a retractable pin tool requires further investigation, furthermore a fresh approach to the interpretation and understanding of the FSW weld process needs consideration.

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Dissertation submitted in fulfilment of the requirements for the degree Master of Technology: Mechanical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology / Aluminium alloys has seen recent increase usage in the automotive industry. This is due to the global obligation towards carbon emission reduction and fuel efficiency in the transport sector. The good strength-to-weight ratio offered by Al-Si-Cu alloys showed promising results towards the compliance of these environmentally friendly criteria. The enhanced mechanical properties is obtained when the alloy is subjected to the T6 heat treatment process, which cause microstructural changes due to the evolution of intermetallic phases. The process involves solution heat treatment, for dissolving soluble Cu- and Mg-containing phases, the homogenization of alloying elements, and the spheroidisation of eutectic Silicon. It is followed by quenching, for maximum precipitation hardening particle retention in solution, and a further artificial ageing process with the aim to acquire a uniform distribution of small precipitates, for strength improvement. The heat treatment schedule applied in this study was conducted as follows: Solution heat treatment at a temperature of 525°C for 6h Quenching in water of temperature 50°C; Artificial ageing for 8h at a temperature of 175°C, and then after left inside furnace to cool down to room temperature. This is higher than the 520°C, but shorter than the 8-12h, observed in literature. Also, quenching is done at a lower temperature rather than 60°C, and artificial ageing at a higher temperature, rather than the 155°C. This was done to be able to draw a comparison between the MAGMASOFT® simulation, which has this non-adjustable schedule, and the experimental results. The simulated and experimental results were comparable and similar outcomes, but with some discrepancies. Such as the porosity was far more visible and intense in the experimental, than what was predicted by the software. The as-cast and heat treated microstructure revealed the expected evolution of intermetallic particles, such as dissolving of the Al2Cu and the spheroidisation of the eutectic Si phases. Another phase that was identified was the insoluble AlFeSi and other possible Fe-containing phases, which due to the higher solution heat treatment temperature, showed partial fragmentation and dissolution. The study provided practical data about the effect of heat treatment on microstructural evolution and how it affects the properties of the Al-Si-Cu alloy. It also brought to the attention and understanding of how critical pouring temperature is, as it affect the initial nucleation, and cooling rate, and therefore the micro and macro properties.

The stress corrosion behaviour of precipitation hardened Al-9Mg, Al-22Zn and Al-3Mg-6Zn alloys has been studied in aqueous environments and ethanol. The stress corrosion susceptibility defined as the reciprocal of failure time has been investigated as a function of alloy-environment system, isothermal aging treatment, microstructure, applied tensile stress, and temperature using smooth and notched specimens. Constant load tests, load-relaxation tests and tensile tests in different environments have been used to evaluate the stress corrosion characteristics of aluminum alloys. A limited study of Mg-9Al has also been carried out in aqueous environments. The process of stress corrosion generally consisted of three parts: 1) A slow initiation stage 2) a rapid propagation stage 3) mechanical fracture due to tensile overload. With a few exceptions, the initiation time was greater than the propagation time. The crack initiation and propagation rates were stress and thermally activated and could be expressed by a general equation of the form Rate = [formula omitted] where α is the applied tensile stress, Q is the apparent activation energy of the rate controlling process and A(0) and n are constants for a given alloy-environment system. The apparent activation energy of the rate controlling process was different in the two environments. It also changed between initiation and propagation stages. The aluminum alloys when ranked in order of increasing susceptibility were: 1) Al-3Mg-6Zn, 2) Al-9Mg, 3) Al-22Zn. The alloys which were given heat treatments correlating to the presence of coherent or partially coherent phases, were found to be most prone to stress corrosion cracking. The environments placed in an order of increasing aggressiveness were dessicant-dried air, double distilled water, ethanol, ambient air, deionized water and NaCl/K₂CrO₄solution. The ductility of susceptible aluminum alloys was found to be significantly decreased by NaCl/K₂CrO₄and deionized water at low strain rates and enhanced by dessicant-dried air. Fractography showed the cracking to be intergranular in aluminum alloys and transgranular in the Mg-Al alloy. The stress corrosion surface was characterised by a rough or corroded appearance while the mechanically fractured surface exhibited slip steps and dimples caused by void formation. The hydrogen mechanism of cracking was examined in light of hydrogen charging experiments and other evidence and was found to be unsatisfactory. Models involving either dissolution or deformation alone were also inadequate in explaining the present results. Therefore a new model was postulated which involves the generation of a continuous path of chemical heterogeneity by shearing and link up of coherent precipitates followed by their dissolution. The rate controlling step in the deformation process is believed to change during the transition from initiation to propagation. The postulated model is consistent with the present results but its further development must await better knowledge of the kinetics of dissolution of precipitates and that of deformation processes at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The system Zn-0.2 wt. % Al has been investigated to determine under what conditions of strain rate, grain size and temperature the phenomenon of superplasticity may be observed. The analysis and experimental conditions were based on established procedures which have been applied to known superplastic alloys. However the continually decreasing strain rate and grain growth during testing complicate the normal analysis. Based on this study the requirements for superplastic behavior are a fine-grained microstructure, grain boundaries which are relatively free of obstructions and a homolohous temperature of at least 0.42. A model incorporating grain boundary shear and non-continuous grain growth has been proposed to account for the observed superplastic behavior where grain boundary migration is the rate controlling process. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Single crystals and bicrystals have been used to study pitting corrosion and intergranular corrosion of Al and Al-Cu alloys in 0.5M NaCl solution. The critical pitting potential and pit density were examined as a function of a number of factors. These included crystallographic orientation; the bulk solution chemistry including CI- concentration, NO₃- addition and pH; the effect of Cu alloying; and the effects of homogenizing and aging on the alloy crystals. The susceptibility for pitting corrosion was found to depend on crystallographic orientation in pure Al with {111} showing maximum pitting and {011} and {001} exhibiting progressively less pitting. This crystallographic effect was not observed in the Al-3 wt% Cu alloy. The addition of Cu to pure Al was found to raise the Epit and produce a higher pit density on the surface. The increase of CI⁻ concentration was found to enhance pitting corrosion, producing a higher pit density and lowering the Epit. Addition of NO₃- to the solution decreases pitting corrosion, reduces the pit density and substantially shifts the Epit to a more noble potential. A model of pitting corrosion is proposed, based on a local kinetic balance between the repassivation process and the dissolution process at the bare metal surface at the base of a preexisting oxide flaw on the crystal surface. The model successfully accounts for the observed effects of the Cu alloy addition, and the solution composition variations on pitting corrosion. In the alloy bicrystals, it was observed that pitting corrosion in the grain boundary region was dependent on the composition and thermal history of the crystal. In most of the homogenized Al-Cu bicrystals, the presence of the grain boundary did not influence the pitting corrosion. In a 0.1 wt% Cu alloy with a tilt boundary of 28° about the <001> direction preferential pitting along the grain boundary was observed. The preferential pitting is attributed to nonequilibrium depletion of Cu at the high angle tilt boundary. Preferential attack is also observed at grain boundaries in as-grown and in aged bicrystals. This is attributed to Cu segregation in the crystals and the lower value of Epit associated with the Cu depleted regions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This investigation evaluates the abrasive-corrosive wear behaviour of aluminium alloys with the aim of establishing a data base of performance and guide lines for material optimisation. Wear test apparatus and standard tests developed by previous research programmes were utilised (Noel and Allen, 1981; Barker, 1988). Further tests were then devised for a more detailed characterisation of wear behaviour. Tests conducted showed that aluminium alloys have approximately a quarter to half the abrasion resistance of mild steel. Poor microfracture properties of Al-Si cast alloys were observed as a result of coarse and brittle silicon rich phases contained in the aluminium matrix. Non heat-treatable wrought alloys exhibit ductile micro-deformation characteristics whilst heat-treatable alloys, having the best abrasion resistance, possess better combinations of strength, hardness and toughness. Tests with combined corrosion and wear showed that most aluminium alloys are subject to pitting corrosion due to localised differences in electrode potentials at constituent sites. Higher series alloys with a large number of constituent particles exhibit higher pitting densities. Due to the high electrode potentials of silicon phases and copper and zinc solid solutions, the alloys LM6+Sr, 2014 and 7075 have poor corrosion resistance and are subject to localised and pitting attack. As a consequence the alloys 2014, 7075 and LM6+Sr show a decrease in wear performance under abrasive-corrosive conditions. In contrast the good corrosion resistance of the alloys 5083, 6261 and 7017 provide a significant improvement in wear performance under conditions of long corrosion periods with light abrasive intervals. This study concludes that the abrasion resistance of wrought alloys may be optimised by designing an alloy with a good combination of tensile strength, fracture toughness and hardness together with an intermediate microstructural size distribution of second phase particles in the aluminium matrix. Ageing of heat treatable alloys improves abrasion resistance significantly, peak hardness and strength conditions resulting in optimum abrasion properties.

Aluminum alloys are widely used in aircraft industry for their strength and light weight. Those alloys that are hardened by precipitation, especially the Copper-rich of the 2000 series, are prone to corrosion and are protected against it using chromate containing coatings. The primary component of these coating systems is Chromium 6+ (CrVI) that has been found to be very toxic in the environment and carcinogenic, toxic and mutagenic in humans. The Mg-rich primer development is the result of a successful multi-year project funded by the US Air-force with its objective the replacement of coatings based on CrVI with a class of coatings less toxic and with comparable protective performances. The Mg rich primer fulfilled the USAF requirements and it is currently undergoing commercial and military qualifications testing. The use of Mg as one of the active pigments in coatings allows the primer to protect the underlying Al sacrificially, not considered possible for this substrate until now. Mg is anodic to most of the other structural metals and when particulate Mg became available commercially, the concept of the primer was first developed by analogy to Zn-rich coatings for steel. When Mg and Al are in contact and immersed in a corrosive environment, magnesium corrodes preferentially and protects the aluminum.