Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--Ohio State University, 2006. / Full text release at OhioLINK's ETD Center delayed at author's request

This dissertation presents the results of an investigation into the corrosion resistance of friction stir welding (FSW) for aerospace structures. Two of the most common aerospace aluminum alloys, 2024 and 7075, were investigated. In the as-welded condition, both alloys were found to be highly susceptible to exfoliation corrosion, and 7075 was found to be susceptible to stress corrosion cracking as well. The goal of this research was to identify proper initial temper selection and postweld aging treatments for enhancing the corrosion resistance of both 2024 and 7075 alloys, and their dissimilar joints. A large number of heat treatments were investigated for 7075 in the T6 and T73 tempers, including retrogression re-aging (RRA). Heat treatments were also investigated for 2024-T3 and 2024-T81. Samples were evaluated for resistance to exfoliation corrosion using optical microscopy. Microhardness, electrical conductivity, tension, and fatigue crack propagation tests were also performed on the samples. Beneficial heat treatments were found for both alloys as well as for their dissimilar joints. / "December 2005." / Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering

The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

In this work, three aluminum sheet alloys, AA5754, AA5182 and AA6111, which are prime candidates for replacing mild steel in automobile structures, are tested in tension at quasi-static and high strain rates. <br /><br /> In order to characterize the constitutive response of AA5754, AA5182 and AA6111 at high strain rates, tensile experiments were carried out at strain rates between 600 s^-1 and 1500 s^-1, and at temperatures between ambient and 300°C, using a tensile split Hopkinson bar (TSHB) apparatus. As part of this research, the apparatus was modified in order to provide an improved means of gripping the sheet specimens. Quasi-static experiments also were conducted using an Instron machine. <br /><br /> The experimental data was fit to the Johnson-Cook and Zerilli-Armstrong constitutive models for all three alloys. The resulting fits were evaluated by numerically simulating the tensile experiments conducted using a finite element approach.

The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

Aluminum and its alloys are used in many aspects of modern life, from soda cans and household foil to the automobiles and aircraft in which we travel. Aluminum alloy systems are characterized by good workability that enables these alloys to be economically rolled, extruded, or forged into useful shapes. Mechanical properties such as strength are altered significantly with cold working, annealing, precipitation-hardening, and/or heat-treatments. Heat-treatable aluminum alloys contain one or more soluble constituents such as copper, lithium, magnesium, silicon and zinc that individually, or with other elements, can form phases that strengthen the alloy. Microstructure development is highly dependent on all of the processing steps the alloy experiences. Ultimately, the macroscopic properties of the alloy depend strongly on the microstructure. Therefore, a quantitative understanding of the microstructural changes that occur during thermal and mechanical processing is fundamental to predicting alloy properties. In particular, the microstructure becomes more homogeneous and secondary phases are dissolved during thermal treatments. Robust physical models for the kinetics of particle dissolution are necessary to predict the most efficient thermal treatment. A general dissolution model for multi-component alloys has been developed using the front-tracking method to study the dissolution of precipitates in an aluminum alloy matrix. This technique is applicable to any alloy system, provided thermodynamic and diffusion data are available. Treatment of the precipitate interface is explored using two techniques: the immersed-boundary method and a new technique, termed here the sharp-interface method. The sharp-interface technique is based on a variation of the ghost fluid method and eliminates the need for corrective source terms in the characteristic equations. In addition, the sharp-interface method is shown to predict the dissolution behavior of precipitates in aluminum alloys when compared with published experimental results. The influence of inter-particle spacing is examined and shown to have a significant effect on dissolution kinetics. Finally, the impact of multiple particles of various sizes interacting in an aluminum matrix is investigated. It is shown that smaller particles dissolve faster, as expected, but influence the dissolution of larger particles through soft-impingement, even after the smaller particles have disappeared.

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Thesis--Auburn University, 2009. / Abstract. Includes bibliographic references (p.83-85).

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 198-205).