Orientador: Marcelo Augusto Santos Torres / Banca: Antonio Jorge Abdalla / Banca: Carlos Antonio Reis Pereira Baptista / Resumo: Este trabalho tem por finalidade avaliar o efeito do processo de shot peening na taxa de propagação de uma trinca por fadiga já existente. O shot peening é um processo de trabalho a frio, que consiste no jateamento repetitivo com microesferas de aço, cerâmica ou vidro, na superfície de uma peça, produzindo encruamento e induzindo tensões residuais compressivas nas superfícies tratadas. Devido a isso, o shot peening tem-se mostrado um método bastante eficiente para dificultar o início e a propagação de uma trinca por fadiga a partir da superfície do material, melhorando a sua resistência à fadiga. Contudo, estudos de seu efeito sobre uma trinca já existente são pouco conhecidos. Corpos de prova do tipo CT, para a liga aeronáutica de alumínio 2024 - T3, foram inicialmente trincados por fadiga e, em seguida, submetidos ao processo de shot peening em ambos os lados. Após o tratamento, os ensaios foram reiniciados nas mesmas condições. Os ensaios foram realizados com amplitude constante em duas espessuras de corpos de prova e duas razões de carga, R, para analisar possíveis efeitos do estado de tensão na ponta da trinca nos resultados. Duas intensidades de shot peening foram utilizadas para cada valor de R ou espessura analisada. Também foram estudadas duas regiões de aplicação do processo de shot peening (atrás e em volta da ponta da trinca) em dois tamanhos de trincas. Curvas comparativas do comportamento da taxa de propagação de trincas por fadiga, mostrando a influência de cada um dos parâmetros descritos, são apresentadas, e suas diferenças, discutidas. Os resultados encontrados mostram que o processo de shot peening, quando realizado em um tamanho de trinca em pleno estágio II de propagação, apresenta maior influência quando aplicado atrás da ponta da trinca, com menor intensidade, em corpos de prova em estado ... (Resumo Completo, clicar acesso eletrônico abaixo) / Abstract: This work focuses to evaluate the effect of the shot peening process in the propagation rate of a fatigue crack already existing. The shot peening is a cold working process, which consists of repetitive blasting with steel shot, ceramic or glass, the surface of a workpiece, producing hardening and inducing compressive residual stresses on the treated surfaces. Because of this, shot peening has proved a very efficient method for hindering the initiation and propagation of a fatigue crack from the surface of the material, improving its fatigue strength. However, studies of its effect on an already existing crack are little known. Specimens of type CT to the aeronautics aluminum alloy 2024 - T3, were initially cracked by fatigue and then subjected to shot peening process on both sides. After treatment, the tests were resumed under the same conditions. Tests were carried out with constant amplitude in two thicknesses of specimens and two load ratios, R, to analyze possible effects of stress state on the crack tip results. Two shot peening intensities were used for each R value or thicknesses analyzed, and were also studied two application regions of shot peening process (behind and around the crack tip) in two sizes of cracks. Comparative curves of the fatigue crack propagation rate, showing the influence of each of the parameters described are shown, and their differences discussed. The results show that the shot peening process, when performed in a crack size in full stage II propagation, has greater influence when applied behind the crack tip, with lower intensity, in the specimens in plane stress state and lower load ratio. For crack lower (close to the stage I of propagation), the shot peening process applied behind the crack tip induced delay in their propagation rate in all conditions studied. The delay effect on fatigue crack propagation rate ... (Complete abstract click electronic access below) / Mestre

In this study, we have investigated the relationship between the process-microstructure to predict and modify the material's properties. Understanding these relationships allows the identification and correction of processing deficiencies when the desired properties are not achieved, depending on the microstructure. Hence, the co-relation between process-microstructure-properties helped reduce the number of experiments, materials & tool costs and saved much time. In the case of high entropy alloys, friction stir welding (FSW) causes improved strength due to the formation of fine grain structure and phase transformation from f.c.c to h.c.p. The phase transformation is temperature sensitive and is studied with the help of differential scanning calorimetry (DSC) to calculate the enthalpy experimentally to obtain ΔGγ→ε. The second process discussed is heat treatment causing precipitation evolution. Fundamental investigations aided in understanding the influence of strengthening precipitates on mechanical properties due to the aging kinetics – solid solution and variable artificial aging temperature and time. Finally, in the third case, the effect of FSW parameters causes the thermal profile to be generated, which significantly influences the final microstructure and weld properties. Therefore, a computational model using COMSOL Multiphysics and TC-Prisma is developed to generate the thermal profile for different weld parameters to understand its effect on the microstructure, which would eventually affect and predict the final properties of the weld. The model's validation is done via DSC, TEM, and mechanical testing.

The occurrence of hot tearing during solidification is one of the major factors influencing both the quality and productivity of aluminum castings. In order to reduce the formation of hot tears, quantitative information regarding both hot tearing formation and semi-solid deformation is essential. In this study, the mechanisms of hot tearing and semi-solid deformation have been investigated via two novel techniques: x-ray micro-tomography on material deformed in the semi-solid region, and development of a three phase microstructural model based on a geometry derived from a Voronoi diagram with rounded corners and porosity. Numerical techniques were utilized to quantify both the size evolution and orientation of internal damage relative to void growth. In order to conduct the above research, a new semi-solid tensile deformation methodology was devised which uses a two thermocouple control technique to enable accurate measurement of semi-solid tensile strength and ductility. The experimental work was conducted on the aluminum – magnesium alloy AA5182 in the as-cast and hot isostatic pressing (HIP) states. The x-ray micro-tomography technique was used to observe that semi-solid deformation is accommodated by internal damage via growth of as-cast porosity and the nucleation of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and via coalescence between voids. The damage then localizes, causing failure. The finite element semi-solid microstructural model was used to explore the effects of fraction solid, fraction porosity, and grain size on semi-solid constitutive behaviour. The simulations revealed that increased grain size and fraction porosity lead to a reduction in flow stress for a given fraction solid. Furthermore, local strain accumulation was linked to hot tearing, since strain localizes in the liquid very early in the deformation process. Based on the model predictions, a new constitutive relationship was developed over the range 0.75 < fs < 0.95. Together, these two techniques have provided powerful new insight, such as the critical role played by as-cast porosity, on the phenomena of hot tearing and semi-solid deformation in aluminum alloys.

Novelis Inc. recently developed and patented a unique Direct Chill (DC) casting process known as Fusion Novelis Technology. In this process a chill bar is inserted into the DC casting mould which permits for the first time the co-casting of laminate of clad ingots. These ingots can then be rolled down into clad sheet and offer distinct advantages over traditional aluminum clad sheet processing routes (i.e. brazing and roll bonding). The research presented in this Master’s Thesis was done as part of a larger collaborative research and development project with Novelis Inc. The main objective of this research was to investigate the Novelis Fusion Technology and understand it from a scientific viewpoint. The research has been multi faceted and has included: the creation of a thermal fluid model using the commercial software package CFD to model the first the DC and then Fusion casting process, as well as the design and testing of an experimental DC and Fusion caster at the Novelis Global Technology Centre (NGTC) in Kingston, Ontario. This MASc research has been focused on performing both traditional DC (for AA6111, AA3004 and AA4045) and novel Fusion (AA3004/AA4045) casting experiments. First the series of DC casting experiments was performed. During the experiments two arrays of 5 thermocouples were embedded in the ingot during the cast to capture the thermal history of the ingot. Melt poisoning with a zinc rich alloy was also performed as an independent method of determining the sump depth and shape. Other temperature measurements during the experiment (i.e. alloy superheat, mould temperature, cooling water temperature) were done to gather meaningful data for model validation. A series of Fusion casting experiments was performed after the DC casting trials. Three successful Fusion casting trials were performed at NGTC using a lab scale caster with a 152 mm × 381 mm rectangular mould divided in half by a water cooled copper chill bar. For the Fusion casting experiments the AA3003-Core/AA4045-Clad alloy system was chosen since this alloy system has already been commercially produced using this novel technology. In addition to embedded thermocouples in the Fusion cast ingot, and other temperature measurements as for the DC casting experiments, temperature measurements of the chill bar were performed to gather information for model validation. The effect of melt poisoning as the interface of the composite ingot forms was unknown, so only the core of one experimental ingot was poisoned; this gave enough information about the depth and asymmetrical shape of the AA3003-Core sump. The Fusion cast ingots were characterized (both optically and using SEM techniques) at four distinct locations across the width of the ingot, consistent with different thermal histories at the interface and regions where good and poor interfaces were found in the solidified ingot. No clear correlation between thermal history and the quality of the interface could be found indicating that the interface formation during Fusion casting is extremely complicated and other factors such as oxide formation and wetting mechanisms of the AA4045 on the AA3004 need to be understood to gain a more in depth understanding of the conditions necessary to form a defect free interface. Comparisons of the measured thermal histories and sump depth and shape measurements to the model predictions were excellent.

The marked increase in demand for miniaturized consumer products in a broad range of potential applications including medical, telecommunication, avionics, biotechnology and electronics is a result of advancements in miniaturization technologies. Consequently, engineering components are being drastically reduced in size. This coupled with the quest for higher quality components, has imposed more stringent requirements on manufacturing processes and materials used to produce micro components. Hence, the development of ultra precision manufacturing processes to fabricate micro-scale features in engineering products has become a focal point of recent academic and industrial research. However, much attention in the area of micro-manufacturing, especially micro-mechanical machining, has been devoted to building miniature machine tools with nanometer positioning resolution and sub-micron accuracy. There is lack of fundamental understanding of mechanical machining at the micro and nano scale. Specifically, basic understanding of chip formation mechanism, cutting forces, size-effect in specific cutting energy, and machined surface integrity in micro and nano scale machining and knowledge of how these process responses differ from those in macro-scale cutting are lacking. In addition, there is a lack of investigations of micro and nano scale cutting of common engineering materials such as aluminum alloys and ferrous materials. This thesis proposes to advance the understanding of machining at the micro and nano scale for common engineering alloys. This will be achieved through a series of systematic micro and nano cutting experiments. The effects of cutting conditions on the machining forces, chip formation and machined surface morphology in simple orthogonal micro-cutting of a ferrous, P20 mold steel (30 HRC), and a non-ferrous structural alloy, aluminum AL7075 (87 HRB), used in the mold making and rapid prototyping industry will be studied. The data will also be compared with data obtained from conventional macro-scale cutting. In addition, the applicability of conventional metal cutting theory to micro and nano cutting test data will be examined. The analysis will provide a better understanding of machining forces, chip formation, and surface generation in micro and nano scale cutting process and how it differs from macro-scale cutting.

 Molecular Beam Epitaxy is an advanced method for the synthesis of single-crystal thin-film structures. However, the growth behavior varies case by case due to the complicated kinetic process. In this thesis, the epitaxial growth processes of AlxIn1-xN alloy and Bi2Se3 thin-films are studied. Heteroepitaxial growth of AlxIn1-xN alloy on GaN(0001) substrate is carried out in the Nitrogen-rich flux conditions. A series of transient growth stages are identified from the initiation of the deposition. A significant effect of source beam-flux on the incorporation rate of Indium atoms is observed and measured. A correlation between the incorporation rate and the growth conditions (flux ratio and growth temperature) is revealed by the dependence of the growth-rate of the film on beam fluxes. A mathematic model is then suggested to explain the effect, through which the measured results indicating a surface diffusing and trapping process is indicated. Unexpected behavior of the lattice-parameter evolution of the growth front during deposition is also observed, indicating a complex strain-relaxation process of the epilayers. For three-dimensional (3D) topological insulator of Bi2Se3, growths are attempted on various substrate surfaces, including clean Si(111)-(7x7), Hydrogen terminated Si(111), Bismuth induced Si(111) reconstructed surfaces, GaN(0001), and some selenide “psudo-substrates”. The specific formation process of this quintuple-layered material in MBE is investigated, from which the Van der Waals epitaxy growth characteristics inherent to deposition of Bi2Se3 is determined, and the mechanism of the “two-step growth” technique for this material is further clarified. Among the various substrates, those that are inert to chemical reaction with Bi/Se are important for the growth. The epilayers’ lattice-misfit with the substrate is also a crucial factor to the structural quality of the Bi2Se3 epifilms, such as the defects density and the single-crystalline domain size. The effect of a vicinal substrate on suppressing the twin-defects in film is also addressed. Using a suitable substrate and adapting an optimal condition, ultra-thin films of Bi2Se3 with a superior structural quality have been achieved. Multilayered Bi2Se3 structures with ZnSe and In2Se3 spacers are attempted. Finally the high-quality superlattices of Bi2Se3/In2Se3 are successfully synthesized. The hetero-interfaces in the superlattice structure of Bi2Se3/In2Se3 are sharp, and the individual layers are uniform with thicknesses being strictly controlled. The behaviors of strain evolution during the hetero-growth process are finally investigated. An exponential relaxation of misfit strain is observed. And the correlation between the residual strain and the starting surface in the initial growth stage is also identified. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The occurrence of hot tearing during solidification is one of the major factors influencing both the quality and productivity of aluminum castings. In order to reduce the formation of hot tears, quantitative information regarding both hot tearing formation and semi-solid deformation is essential. In this study, the mechanisms of hot tearing and semi-solid deformation have been investigated via two novel techniques: x-ray micro-tomography on material deformed in the semi-solid region, and development of a three phase microstructural model based on a geometry derived from a Voronoi diagram with rounded corners and porosity. Numerical techniques were utilized to quantify both the size evolution and orientation of internal damage relative to void growth. In order to conduct the above research, a new semi-solid tensile deformation methodology was devised which uses a two thermocouple control technique to enable accurate measurement of semi-solid tensile strength and ductility. The experimental work was conducted on the aluminum – magnesium alloy AA5182 in the as-cast and hot isostatic pressing (HIP) states. The x-ray micro-tomography technique was used to observe that semi-solid deformation is accommodated by internal damage via growth of as-cast porosity and the nucleation of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and via coalescence between voids. The damage then localizes, causing failure. The finite element semi-solid microstructural model was used to explore the effects of fraction solid, fraction porosity, and grain size on semi-solid constitutive behaviour. The simulations revealed that increased grain size and fraction porosity lead to a reduction in flow stress for a given fraction solid. Furthermore, local strain accumulation was linked to hot tearing, since strain localizes in the liquid very early in the deformation process. Based on the model predictions, a new constitutive relationship was developed over the range 0.75 < fs < 0.95. Together, these two techniques have provided powerful new insight, such as the critical role played by as-cast porosity, on the phenomena of hot tearing and semi-solid deformation in aluminum alloys.

Aluminum-silicon-copper cast alloys of the 319-type have attained a commercially important status because of their widespread use. Artificial aging treatments are routinely applied to these alloys in order to obtain precipitation hardening and improve their mechanical properties. Standard treatments may not always yield the optimum achievable properties, thus Mg and Sr are commonly added to improve the response of the alloy to aging and to modify the eutectic Si morphology from acicular to fibrous, respectively. The present study was carried out to investigate aging behavior of four 319-type alloys in regard to such mechanical properties as their ultimate tensile strength, yield strength, microhardness, percent elongation and impact toughness. Non-conventional aging cycles were applied so as to evaluate the degree of the improvement in strength obtainable. These treatments, labeled in this study as T6- and T7-type multi-temperature and interrupted aging treatments, involve several heating stages at different temperatures, as opposed to the single stage at constant temperature specifications of the standard T6 or T7 heat treatment regimes. Scanning electron microscopy was used to examine the fracture surfaces of selected tensile-tested samples to compare the fracture behavior. Transmission electron microscopy was used to reveal and identify the tiny precipitates which appear in the microstructure as a result of the precipitation-hardening process due to artificial aging. It was found that the main strengthening phase is theta-Al2Cu in the form of needles; other phases were observed as minor constituents in this alloy, including the binary beta-Mg2Si, the ternary S-CuAlMg 2 and the quaternary Q-Al5Cu2Mg7Si 7. The results show that while Mg and Sr additions improve the properties of the alloy, the standard T6 treatment may not be the best available option to produce optimum properties. In fact, when the peak-aged (T6) condition is desired, the optimum treatment consists of a continuous artificial treatment at 170°C for 8 h; when the overaged (T7) condition is desired, a T7-type multi-temperature treatment consisting of underaging at 170°C for 1 h, then at 190°C for 1 h, and finally overaging at 240°C for 2 h is the best option.

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High cycle fatigue tests were carried out on a medium strength continuous casting AA 5754 Al alloy, and new generation high strength AA 2026 and AA 2099 Al alloys. The effect of texture on fatigue properties and short crack behavior were studied. The strengthening mechanisms were also thoroughly investigated for the two high strength alloys.Texture played an important role in the anisotropy of fatigue strength for the AA 5754 Al alloy. Being a solution strengthened alloy, it had a fatigue strength of 120% σy. High strength Al alloys had a strong tendency for planar slip due to the high density of coherent and shearable precipitates in the alloys. Texture was a key factor controlling the crack initiation and propagation. The crack path and the possible minimum twist angles were measured using EBSD and calculated theoretically by a crystallographic model. Based on the micro-texture measured by EBSD, the crack paths were predicted for the AA 2099 alloy and confirmed by the observed values.The excellent balance of superior fatigue properties and high tensile strength of AA 2026 and AA 2099 was attributed to the reduced population of Fe-containing particles, homogeneously distributed precipitates and dislocations. The addition of Zr coupled with the optimized thermo-mechanical treatment strongly restrained the recrystallization, refined the grain structure and promoted the homogenization of the precipitates. Moreover, the retainment of the deformation texture developed during the hot extrusion provided significant orientation strengthening in the high strength Al alloys.Fatigue cracks tended to initiate at coarse second phase particles on sample surfaces and the crack population varied markedly with the applied stresses in the high strength Al alloys. The relationship between of the crack population and the applied stress level was studied and quantified by a Weibull distribution function. Since the measured cracknumbers were associated with the crack initiate sites (i.e., the weakest links) in an alloy, the fatigue weakest-link density, which is defined as the crack population per unit area when stress close to the ultimate tensile stress, and the weakest-link strength distribution can all be calculated and regarded as a property of the studied materials.