This work establishes the relationships between several key Friction Stir Welding process parameters and the resulting microstructural and flexural properties of the welded joint. A series of four single parameter experiments were run. The parameters investigated were pin diameter, feedrate, shoe temperature, and pressure time. Butt welds were made in 6 mm thick stress-relieved extruded polypropylene sheet. Three-point bend tests were used to determine the ultimate flexural strength and coincident strain. The maximum bend angle before failure was used to label the welds as "good or bad." An optical microscope capable of cross polarization was used to examine and photograph the weld microstructure. Welds were evaluated according to 1) DVS bend angle, 2) flexural properties, and 3) weld microstructure. All welds made surpassed the DVS requirements for classification as a "good weld" established for hot-gas, extrusion, and laser welding processes. Most welds met the bend angle requirement for hot-plate welds. Welds created for this work maintained 80-92% of base material flexural strength. In the majority of the welds, the strength was between 85 and 90% of base material. The FSW joints showed a flexural strength of 10500 psi, compared to a base material strength of 12400 psi. Four microstructural zones were found to exist in the FSW joints. These were: 1)advancing interface, 2) retreating interface, 3) bottom disturbance, and 4) central zone. Several common microstructure types and defects were found to exist in the welds. These were: 1) spherulites, 2) voids, 3) root defects, 4) flow lines, and 5) onion skin. A distinct correlation was observed between weld microstructure and flexural properties. Those welds whose microstructure most nearly resembled the base material demonstrated the best flexural properties. This can be accomplished by operating with a low feedrate, a high shoe temperature, and a large pin.

Texture evolution is a vital component of many computational tools that link structure, properties and processes of polycrystalline materials. By definition, this evolution process involves the manipulation, via rotation, of points in orientation space. The computational requirements of the current methods being used to rotate crystalline orientations are a significant limiting factor in the drive to merge the texture information of materials into the engineering design process. The goal of this research is to find and implement a practical rotation algorithm that can significantly decrease the computation time required to rotate macroscopic and microscopic crystallographic textures. Three possible algorithms are considered in an effort to improve the computational efficiency and speed of the rotation process. The first method, which will be referred to as the Gel'fand method, is based on a paper, [1], that suggests a practical application of some of Gel'fand's theories for rotations [2]. The second method, which will be known as the streamline method, is a variation on the Gel'fand method. The third method will be known as the principal orientation method. In this method, orientations in Fourier space are written as linear combinations of points on the convex surface of the microstructure hull to reduce the number of points that must be rotated during each step in the texture evolution process. This thesis will discuss each of these methods, their strengths and weaknesses, and the accuracy of the computational results obtained from their implementation.

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No description available.

No description available.

L'une des façons d' augmenter les performances mécaniques des pièces moulées est de réduire la taille moyenne des grains. L'utilisation des effets de la vibration du moule lors de la solidification permet d'agir en ce sens, soit sur l'affinage des grains. La présente étude porte donc sur l'affinage des grains par vibro-solidification d' alliages binaires Al-Mg. Les vibrations sont transmises au moule à l'aide d'une poutre d'acier vibrant à sa propre fréquence de résonance. Cinq alliages avec des teneurs en magnésium comprises entre 0% et 12,90/0 poids, ont été moulés dans un moule préchauffé à des températures variant entre 200°C et 400°C. Pour chacune des conditions de moulage avec et sans vibration, la microstructure des pièces moulées et le degré de remplissage du moule ont été caractérisés. Les résultats obtenus montrent que la vibro-solidification ne contribue pas de façon significative à l'affinage de la structure granulaire des alliages étudiés, mais contribue à un degré supérieur de remplissage des moules. Le taux de refroidissement en surface et au coeur des pièces moulées n'est pas influencé significativement par les vibrations. Par contre, la présence de vibrations lors de la coulée apporte une plus grande uniformité dans les résultats obtenus. Finalement, une augmentation de la teneur en Mg de 0 à 12,9% dans l'alliage contribue à réduire la taille moyenne des grains d'environ 26 % dans les pièces vibrées comparativement à environ 23 % dans les pièces non vibrées. De plus, les pièces solidifiées avec vibrations montrent un fini de surface plus rugueux et des bavures plus apparentes sur le pourtour des pièces moulées à cause d'un accroissement de la fluidité de l'alliage.

Mechanical properties of structural materials are highly correlated to their microstructure. The relationship between microstructure and mechanical properties can be established experimentally. The growing need for structural materials in industry promotes the study of microstructural evolution of materials by design using computational approaches. This thesis presents the microstructural evolution of two different structural materials. The first uses a genetic algorithm approach to study the microstructural evolution of a high-temperature nickel-based oxide-dispersion-strengthened (ODS) alloy. The chosen Ni-20Cr ODS system has nano Y2O3 particles for dispersion strengthening and submicron Al2O3 for composite strengthening. Synergistic effects through the interaction of small dispersoids and large reinforcements improved high-temperature strength. Optimization considered different weight factors on low temperature strength, ductility, and high temperature strength. Simulation revealed optimal size and volume fraction of dispersoids and reinforced particles. Ni-20Cr-based alloys were developed via mechanical alloying for computational optimization and validation. The Ni-20Cr-1.2Y2O3-5Al2O3 alloy exhibited significant reduction in the minimum creep rate (on the order of 10-9 s-1) at 800oC and 100 MPa. The second considers the microstructural evolution of AA 7050 alloy during friction stir welding (FSW). Modeling the FSW process includes thermal, material flow, microstructural and strength modeling. Three-dimensional material flow and heat transfer model was developed for friction stir welding process of AA 7050 alloy to predict thermal histories and extent of deformation. Peak temperature decreases with the decrease in traverse speed at constant advance per revolution, while the increase in tool rotation rate enhances peak temperature. Shear strain is higher than the longitudinal and transverse strain for lower traverse speed and tool rotation rate; whereas for higher traverse speed and tool rotation rate, shear and normal strain acquire similar values. Precipitation distribution simulation using TC-PRISMA predicts the presence of η' and η in the as-received AA 7050-T7451 alloy and mostly η in the friction stir welded AA7050 alloy, which results in the lower predicted strength of friction stir welded alloy. Further, development of modeling assists in process optimization and innovation, and enhances the progression rate. Accelerating the development process requires coupling experimental methods with predictive modeling. The overall purpose of this work was to develop an integrated computational model with predictive capabilities. In the present work, an application tool to predict thermal histories during FSW of AA7050 was developed using COMSOL software.

Innovative materials with novel properties are in great demand for use in the criticalcomponents of emerging technologies, which promise to be more cost-effective and energyefficient.A controversial issue with regard to manufacturing complex industrial products isto develop advanced materials with optimised manufacturability in addition to the requiredmechanical and physical properties. The objective of this research study was to develop andoffer new solutions in material-processing-related issues in the field of mechanical andelectrical engineering. This was achieved by investigating the new opportunities affordedby a recently developed rheocasting method, RheoMetalTM process, with the goal of comingto an understanding of the critical factors for effective manufacturing process. A study of the evolution of microstructure at different stages of the rheocasting process,demonstrated the influence of multistage solidification on the microstructural characteristicsof the rheocast components. The microstructural investigation onquench slurry showed itconsists of the solute-lean coarse globular α-Al particles with uniform distribution ofalloying elements, suspended in the solute-rich liquid matrix. Such inhomogeneous slurryin the sleeve seems to play a critical role in the inhomogeneity of final microstructure. Inthe rheocast component, the separation of the liquid and solid parts of slurry during fillinginfluenced on the microstructural inhomogeneity. The relationship between the microstructural characteristics and properties of the rheocastcomponents was investigated. The study on the fracture surfaces of the tensile-testedspecimens showed that the mechanical properties strongly affected by microstructuralinhomogeneity, in particular macrosegregation in the form of near surface liquid segregationbands and subsurface porosity. The thermal conductivity measurement showed variation ofthis property throughout the rheocast component due to variations in the ratio of solute-leanglobular α-Al particles and fine solute α-Al particles. The result showed silicon in solidsolution have a strong influence (negative) on thermal conductivity and precipitation ofsilicon by heat treatment process increase the thermal conductivity. / RheoCom

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

This paper investigates the market microstructure of the Taiwan Stock Exchange Capitalization weighted Stock Index (TX) futures contracts traded on the Taiwan Futures Exchange which quite recently switched from an electronic periodic call auction market to an electronic continuous auction market. No doubt it is a rare opportunity for us to deeply look into market quality under different trading mechanisms. Using time-stamped transaction data of trades and quotes covering the period from January 2001 to September 2002, overall behavior for all TX Futures contracts are explored first—including intraday and daily patterns in the bid-ask spreads, volume, trade size, volatility, liquidity ratio and other characteristics. Next, in order to observe whether long-term contracts and short-term contracts have different patterns, the sample is divided into two groups—quarterly expiration contract months (March, June, September, and December) and non-quarterly expiration contract months, and the intraday/daily patterns are displayed. Moreover, since TAIFEX transferred trading mechanism on July 29th 2002 from an electronic periodic call auction market to an electronic continuous auction market, intraday/daily patterns are separately illustrated and compared before and after July 29th 2002, and ANOVA F-Statistic and Kruskal-Wallis tests are also taken to provide more insights into time-varying behavior under two different kinds of market trading mechanisms. The empirical results indicate that the most active periods correspond to the TAIFEX’Ss opening five-minute interval (8:45-8:50), TSEC’Ss opening five-minute interval (9:00-9:05), and TAIFEX’S closing five-minute interval (13:40-13:45) with wide spreads and large trade sizes. In 54 five-minute intervals for the regular trading session of both TAIFEX and TSEC from 9:00 a.m. to 1:30 p.m., the behavior of spreads, volume and trade sizes mainly reveal U-shaped patterns. The average trading volume within each time interval plunges, except within the final 5 minutes closing procedure interval, after TAIFEX transferred trading mechanism from an electronic periodic call auction market to an electronic continuous auction market with wider spreads and narrower volatility, in general. Moreover, intraday patterns of the average volume under new microstructure exhibit a right angular U-shape while intraday patterns of volume under old microstructure reveal a smooth U-shape. The evidence suggests a conjecture that the transfer of market trading mechanism might result in informed traders’ altering their intraday behavior and might lessening their trading desire. Further evidence in confirmation of this statement is left to future work. / This paper investigates the market microstructure of the Taiwan Stock Exchange Capitalization weighted Stock Index (TX) futures contracts traded on the Taiwan Futures Exchange which quite recently switched from an electronic periodic call auction market to an electronic continuous auction market. No doubt it is a rare opportunity for us to deeply look into market quality under different trading mechanisms. Using time-stamped transaction data of trades and quotes covering the period from January 2001 to September 2002, overall behavior for all TX Futures contracts are explored first—including intraday and daily patterns in the bid-ask spreads, volume, trade size, volatility, liquidity ratio and other characteristics. Next, in order to observe whether long-term contracts and short-term contracts have different patterns, the sample is divided into two groups—quarterly expiration contract months (March, June, September, and December) and non-quarterly expiration contract months, and the intraday/daily patterns are displayed. Moreover, since TAIFEX transferred trading mechanism on July 29th 2002 from an electronic periodic call auction market to an electronic continuous auction market, intraday/daily patterns are separately illustrated and compared before and after July 29th 2002, and ANOVA F-Statistic and Kruskal-Wallis tests are also taken to provide more insights into time-varying behavior under two different kinds of market trading mechanisms. The empirical results indicate that the most active periods correspond to the TAIFEX’Ss opening five-minute interval (8:45-8:50), TSEC’Ss opening five-minute interval (9:00-9:05), and TAIFEX’S closing five-minute interval (13:40-13:45) with wide spreads and large trade sizes. In 54 five-minute intervals for the regular trading session of both TAIFEX and TSEC from 9:00 a.m. to 1:30 p.m., the behavior of spreads, volume and trade sizes mainly reveal U-shaped patterns. The average trading volume within each time interval plunges, except within the final 5 minutes closing procedure interval, after TAIFEX transferred trading mechanism from an electronic periodic call auction market to an electronic continuous auction market with wider spreads and narrower volatility, in general. Moreover, intraday patterns of the average volume under new microstructure exhibit a right angular U-shape while intraday patterns of volume under old microstructure reveal a smooth U-shape. The evidence suggests a conjecture that the transfer of market trading mechanism might result in informed traders’ altering their intraday behavior and might lessening their trading desire. Further evidence in confirmation of this statement is left to future work.