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A continuous process for the production of ceramic materials has been studied. This method reacted metal and oxidizer in a diffusionally controlled process, demonstrated by reacting a magnesium particle stream and hot water vapor. Many small rich hydrogen/air diffusion flames provide an atmosphere of hot water vapor, hydrogen, and nitrogen for oxidizing the magnesium, which reacts with the water vapor in the form of a diffusion flame. The burner that provides the hot atmosphere has been characterized thermally using thermocouple measurements and a model that gives the true temperature from the measured values. A model was developed that gives the flame profile for the parallel flow geometry of particle stream combustion used in this study as defined in the cartesian coordinate system. / Graduation date: 1995

Magnesium alloy tubes have good formability at elevated temperatures. In this paper, firstly, uniaxial tensile tests are conducted to evaluate the flow stress of AZ61 magnesium alloy at different temperatures and strain rates. Secondly, a hydraulic warm forming machine with axial feeding, counter punch and internal pressure is designed and manufactured. Using this testing machine with the FEM results, experiments of hydraulic forming of AZ61 magnesium alloy tubes at different temperatures are carried out. The effect of loading paths on the product shape and formability at different temperature are discussed.

Membrane proteins are key players in many biological processes. Since most membrane proteins are assembled into oligomeric complexes it is important to understand how they interact with each other. Unfortunately however, the assembly process (i.e. their social life) remains poorly understood. In the work presented in this thesis I have investigated when and how membrane proteins assemble with each other and their cofactors to form functional units. We have shown that that cofactor insertion in the hetero-tetrameric cytochrome bo3 occurs at an early state in the assembly process. We also found that the pentameric CorA magnesium ion channel is stabilised by different interactions depending on the magnesium ion concentration in the cell. These studies indicate that the assembly of a functional unit is a dynamic process, which is a result of many different forces. By studying the assembly of membrane proteins we have obtained a deeper insight into their function, which cannot be explained by static crystal structures. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p>

Microstructural evolution during heat treatment and the precipitation hardening response of Mg-Ca-Zn alloys were investigated. The binary Mg-2.5Ca alloy was chosen as the base alloy and the effects of adding one and two wt% zinc on the microstructural characteristics and precipitation hardening of the alloy system were studied. The as-cast microstructure of all three alloys showed dendritic solidification of α-Mg and the formation of the eutectic mixtures and/or multiple phases within the interdendritic regions. Homogenization heat treatment of the binary alloy removed the dendritic structure of α-Mg and spherodized the lamellar eutectic of α-Mg+Mg2Ca. Homogenization heat treatment in the ternary Mg-Ca-Zn alloys resulted in the formation of α-Mg grains with Mg2Ca and zinc-containing particles mainly dispersed along the grain boundaries. The EDS analysis suggested that zinc is incorporated in Mg2Ca particles. To study the precipitation hardening response of the alloys, homogenized alloys were aged at 175°C, 200°C and 220°C. At all three isothermal aging temperatures, the binary alloy showed only a slight increase in hardness, i.e. from 50 VHN in the homogenized state to approximately 53 VHN when peak aged. On the other hand, adding zinc was found to promote the age hardening response of the ternary alloys and caused the hardness to increase up to 70 VHN at the peak-aged condition. To further study the precipitation hardening behavior of the alloys, DSC and IC studies were conducted on the homogenized, as–quenched, alloys. The DSC result of the binary alloy showed only one exothermic heat effect, while the ternary alloys showed multiple exothermic peaks. Analysis of the DSC and IC traces, along with the evaluation of the slight increase in microhardness, suggested that non-coherent equilibrium precipitates formed in the aged binary alloy. In addition, it was suggested that the formation of coherent precipitates during aging can be the cause of the pronounced increase in hardness in the aged ternary alloys. The IC traces of the alloys were used to evaluate the kinetics of precipitation in the ternary alloys. It was concluded that increasing aging temperature from 175°C to 220°C greatly increased the precipitation rate. Finally the JMAK model was fit to the experimentally analyzed data for the evolution of the relative volume fraction of precipitates. It was found that the experimentally analyzed data was reasonably well described by the JMAK model. The corresponding JMAK kinetic parameters k and n were relatively close for the two ternary alloys at any aging temperature. The temperature dependence of k was modeled using the Arrhenius-type rate relationship. This analysis resulted in a smaller value for the apparent activation energy in the ternary alloy containing the higher zinc level, i.e. in Mg-2Ca-2Zn alloy.

Due to finite fossil fuel resources and the impact on our environment of burning fossil fuels, the automotive industry has been investigating ways to reduce the overall weight of automotive vehicles. This has led to increased interest in ways that light weight alloys such as magnesium can be used in fabrication of automotive parts and manufacturing processes such as welding that would enable increased use of magnesium. The objectives of this project were to characterize and determine the weldability of 2 mm thick AZ31B-H24 magnesium alloy by three different laser beam welding processes: a 4 kW Nuvonyx ISL-4000L high power diode laser, a 5 kW Trump TLC-1005 CO2 laser, and a 10 kW YLR-10000-WC fibre laser. The diode laser operated with a 0.9 by 12 mm spot size and with a maximum power density of 37 MW/m2. Due to its low power density, the diode laser was restricted to conduction-mode welding which produced wide fusion zones. The AZ31B magnesium laser welds exhibited a number of defects including hydrogen porosity, solidification cracking, liquation cracking, high vaporization rates, molten expulsions, and poor weld bead quality due to low surface tension. It was found that the majority of these defects could be controlled through the proper use of clamping and shielding of the weld pool and joint preparation and surface cleaning prior to welding. The as-received base material was delivered with a dark grey hydrated oxide layer. This surface condition was found to increase the overall diode laser beam absorption but was detrimental to the welding process when disrupted. When incorporated into the weld pool, the oxide created weak facets where solidification cracks would initiate or acted to localize strain during tensile testing. Proper joint preparation was required to produce a high quality diode laser weld: machining of the joint interface to remove interfacial gaps, chemical cleaning with acetone and ethanol to remove residual oils or grease, and stainless steel wire brushing to remove the oxide. Diode laser welds made using 3 kW power and 0.75 m/min welding speed achieved approximately 60% of the base metal’s ultimate strength and less than 15% of the base metal ductility. The reduced strength and ductility were attributed primarily to the weld defects which acted as strain localizers during plastic deformation and the lack of strain hardening in the weld metal. Both the CO2 and fibre lasers beams had focal spot sizes of 300 μm diameters and maximum power densities of 70 and 140 GW/m2, respectively. At these power densities, the CO2 and fibre lasers operated in keyhole-mode and produced welds which had narrower columnated fusion zones. The CO2 laser keyhole-mode welds exhibited keyhole instability and bulk material loss through vaporization that resulted in macro-porosity, under-fill, and generally poor weld bead quality. Welds produced using 5 kW power and 8 m/min welding speeds achieved approximately 70% of the base metal’s ultimate strength. The highest quality fibre laser welds were produced at 2 kW power and 100 mm/s welding speeds. These defect-free welds achieved transverse tensile strengths that were 86% of the base metal’s ultimate strength. The 14% loss of strength was attributed to the difference in temper of the base metal and the weld metal. The base material was received in a half-hard H24 temper and the as solidified weld metal is naturally in the softer F temper. This also resulted in a corresponding 15% reduction in hardness. Failure always occurred in the softened fusion zones of the welded samples where the measured hardness was reduced to an average 60 VHN25 from the base metal’s 75 HVN25. The fibre laser weld samples also experienced the greatest extension of any of the tested welds with a cross head displacement of 30% of the base metal. The extreme reduction in overall cross head displacement was attributed to the lower strength of the fusion zone. This led to strain localization in the transverse tensile specimens and premature failure that occurred prior to plastic deformation of the surrounding base material. Proper joint preparation was found to be critical when laser welding AZ31B magnesium sheet. Machined interfaces were required to minimize the gap and degreasing and stainless steel wire brushing were required for removal of the pre-existing hydrated oxide in order to produce sound laser welds. Helium shielding gas was found to improve the weld bead surface quality compared to argon. The keyhole-mode welds produced with the CO2 and fibre lasers were superior compared to the conduction-mode welds produced with the diode laser. This was due to the narrower fusion zone and reduced bulk material loss. Of the three laser welding processes examined in this study, the fibre laser produced the highest quality, strongest, and most ductile welds when analyzed in transverse tensile testing. However, direct comparisons between the CO2 and fibre laser welds could not be made because they were made using different joint preparations and welding conditions.

Microstructural evolution during heat treatment and the precipitation hardening response of Mg-Ca-Zn alloys were investigated. The binary Mg-2.5Ca alloy was chosen as the base alloy and the effects of adding one and two wt% zinc on the microstructural characteristics and precipitation hardening of the alloy system were studied. The as-cast microstructure of all three alloys showed dendritic solidification of α-Mg and the formation of the eutectic mixtures and/or multiple phases within the interdendritic regions. Homogenization heat treatment of the binary alloy removed the dendritic structure of α-Mg and spherodized the lamellar eutectic of α-Mg+Mg2Ca. Homogenization heat treatment in the ternary Mg-Ca-Zn alloys resulted in the formation of α-Mg grains with Mg2Ca and zinc-containing particles mainly dispersed along the grain boundaries. The EDS analysis suggested that zinc is incorporated in Mg2Ca particles. To study the precipitation hardening response of the alloys, homogenized alloys were aged at 175°C, 200°C and 220°C. At all three isothermal aging temperatures, the binary alloy showed only a slight increase in hardness, i.e. from 50 VHN in the homogenized state to approximately 53 VHN when peak aged. On the other hand, adding zinc was found to promote the age hardening response of the ternary alloys and caused the hardness to increase up to 70 VHN at the peak-aged condition. To further study the precipitation hardening behavior of the alloys, DSC and IC studies were conducted on the homogenized, as–quenched, alloys. The DSC result of the binary alloy showed only one exothermic heat effect, while the ternary alloys showed multiple exothermic peaks. Analysis of the DSC and IC traces, along with the evaluation of the slight increase in microhardness, suggested that non-coherent equilibrium precipitates formed in the aged binary alloy. In addition, it was suggested that the formation of coherent precipitates during aging can be the cause of the pronounced increase in hardness in the aged ternary alloys. The IC traces of the alloys were used to evaluate the kinetics of precipitation in the ternary alloys. It was concluded that increasing aging temperature from 175°C to 220°C greatly increased the precipitation rate. Finally the JMAK model was fit to the experimentally analyzed data for the evolution of the relative volume fraction of precipitates. It was found that the experimentally analyzed data was reasonably well described by the JMAK model. The corresponding JMAK kinetic parameters k and n were relatively close for the two ternary alloys at any aging temperature. The temperature dependence of k was modeled using the Arrhenius-type rate relationship. This analysis resulted in a smaller value for the apparent activation energy in the ternary alloy containing the higher zinc level, i.e. in Mg-2Ca-2Zn alloy.

Due to finite fossil fuel resources and the impact on our environment of burning fossil fuels, the automotive industry has been investigating ways to reduce the overall weight of automotive vehicles. This has led to increased interest in ways that light weight alloys such as magnesium can be used in fabrication of automotive parts and manufacturing processes such as welding that would enable increased use of magnesium. The objectives of this project were to characterize and determine the weldability of 2 mm thick AZ31B-H24 magnesium alloy by three different laser beam welding processes: a 4 kW Nuvonyx ISL-4000L high power diode laser, a 5 kW Trump TLC-1005 CO2 laser, and a 10 kW YLR-10000-WC fibre laser. The diode laser operated with a 0.9 by 12 mm spot size and with a maximum power density of 37 MW/m2. Due to its low power density, the diode laser was restricted to conduction-mode welding which produced wide fusion zones. The AZ31B magnesium laser welds exhibited a number of defects including hydrogen porosity, solidification cracking, liquation cracking, high vaporization rates, molten expulsions, and poor weld bead quality due to low surface tension. It was found that the majority of these defects could be controlled through the proper use of clamping and shielding of the weld pool and joint preparation and surface cleaning prior to welding. The as-received base material was delivered with a dark grey hydrated oxide layer. This surface condition was found to increase the overall diode laser beam absorption but was detrimental to the welding process when disrupted. When incorporated into the weld pool, the oxide created weak facets where solidification cracks would initiate or acted to localize strain during tensile testing. Proper joint preparation was required to produce a high quality diode laser weld: machining of the joint interface to remove interfacial gaps, chemical cleaning with acetone and ethanol to remove residual oils or grease, and stainless steel wire brushing to remove the oxide. Diode laser welds made using 3 kW power and 0.75 m/min welding speed achieved approximately 60% of the base metal’s ultimate strength and less than 15% of the base metal ductility. The reduced strength and ductility were attributed primarily to the weld defects which acted as strain localizers during plastic deformation and the lack of strain hardening in the weld metal. Both the CO2 and fibre lasers beams had focal spot sizes of 300 μm diameters and maximum power densities of 70 and 140 GW/m2, respectively. At these power densities, the CO2 and fibre lasers operated in keyhole-mode and produced welds which had narrower columnated fusion zones. The CO2 laser keyhole-mode welds exhibited keyhole instability and bulk material loss through vaporization that resulted in macro-porosity, under-fill, and generally poor weld bead quality. Welds produced using 5 kW power and 8 m/min welding speeds achieved approximately 70% of the base metal’s ultimate strength. The highest quality fibre laser welds were produced at 2 kW power and 100 mm/s welding speeds. These defect-free welds achieved transverse tensile strengths that were 86% of the base metal’s ultimate strength. The 14% loss of strength was attributed to the difference in temper of the base metal and the weld metal. The base material was received in a half-hard H24 temper and the as solidified weld metal is naturally in the softer F temper. This also resulted in a corresponding 15% reduction in hardness. Failure always occurred in the softened fusion zones of the welded samples where the measured hardness was reduced to an average 60 VHN25 from the base metal’s 75 HVN25. The fibre laser weld samples also experienced the greatest extension of any of the tested welds with a cross head displacement of 30% of the base metal. The extreme reduction in overall cross head displacement was attributed to the lower strength of the fusion zone. This led to strain localization in the transverse tensile specimens and premature failure that occurred prior to plastic deformation of the surrounding base material. Proper joint preparation was found to be critical when laser welding AZ31B magnesium sheet. Machined interfaces were required to minimize the gap and degreasing and stainless steel wire brushing were required for removal of the pre-existing hydrated oxide in order to produce sound laser welds. Helium shielding gas was found to improve the weld bead surface quality compared to argon. The keyhole-mode welds produced with the CO2 and fibre lasers were superior compared to the conduction-mode welds produced with the diode laser. This was due to the narrower fusion zone and reduced bulk material loss. Of the three laser welding processes examined in this study, the fibre laser produced the highest quality, strongest, and most ductile welds when analyzed in transverse tensile testing. However, direct comparisons between the CO2 and fibre laser welds could not be made because they were made using different joint preparations and welding conditions.

In an effort to improve the fuel efficiency of automobiles, car designers are investigating new materials to reduce the overall vehicle weight. Magnesium alloys are good candidates to achieve that weight reduction due in part to their low density and high specific strength. To support their introduction into vehicle body structures, the dynamic behavior of magnesium alloys must be determined to assess their performance during a crash event. In this work, the tensile high strain rate behavior of AZ31B magnesium alloy sheets was characterized. Two different temper conditions were considered: AZ31B-O (fully annealed) and AZ31B-H24 (partially hardened). Three different sheet thicknesses were considered for the O temper condition, 1.0, 1.6 and 2.5 mm, while the H24 temper was 1.6 mm in thickness. The sheet condition of the magnesium alloys implies an in-plane anisotropy induced by the rolling process. Therefore, both the rolling and transverse directions were investigated in the current research. In order to characterize the constitutive behaviour of AZ31B-O and AZ31B-H24 magnesium alloy sheets, tensile tests were performed over a large range of strain rates. Quasi-static experiments were performed at nominal strain rates of 0.003s-1, 0.1s-1 and 1s-1 using a servohydraulic tensile machine. Intermediate strain rate experiments were performed at 30s-1 and 100s-1 using an instrumented falling weight impact (IFWI) apparatus, and high strain rate experimental data at 500s-1, 1000s-1 and 1500s-1 was collected using a tensile split Hopkinson bar (TSHB) apparatus. Elevated temperature experiments (up to 300°C) were also performed at high strain rates using a radiative furnace mounted on the TSHB apparatus. The tensile experiments show a significant strain rate sensitivity of the constitutive behavior of both the O and H24 temper conditions. The two tempers exhibit an average increase of stress level of 60-65 MPa over the range of strain rates considered. As the strain rate increases, the strain rate sensitivity of both tempers also increases. The strain rate has a different effect on the ductility of the two material conditions. The ductility of AZ31B-O is significantly improved under high strain rate deformations, whereas the AZ31B-H24 exhibits similar ductility at low and high strain rates. Both material conditions presented a strong in-plane anisotropy, with an average stress level in the transverse direction higher than in the rolling direction by 15 MPa and 35 MPa for the O and H24 tempers, respectively. The thermal sensitivity for both tempers at high strain rates was obtained. The two material conditions exhibit a clear thermal softening. From room temperature to 250°C, the loss in strength at 5% plastic strain was found to be 55 MPa and 125 MPa for the AZ31B-O and AZ31B-H24 materials, respectively. The thickness of the AZ31B-O sheets has a mild effect on the measured constitutive behavior. The flow stress increases with increasing thickness. An average difference of 10-15 MPa was seen between the flow stress of the 1.0mm and 2.5mm sheets. However, similar strain rate sensitivity was seen for the three thicknesses. The experimental data was fit to three constitutive models: the Johnson-Cook model, its modified version with a Cowper-Symonds strain rate sensitivity formulation, and the Zerilli-Armstrong model. The three models were evaluated by numerical simulation of the TSHB experiment under various testing conditions. It was found that the Zerilli-Armstrong model was the most accurate in predicting the flow stress of the different material conditions. However, finite element models incorporating the three constitutive fits failed to predict necking in the specimen.

The demand for lightweight materials in automobiles has been motivated by two factors: fuel economy and air pollution reduction. One of the first steps taken in automotive vehicle weight reduction was the use of aluminum alloys for both structural and non-structural parts. Although magnesium alloys, that have one fourth the density of steel and one third that of aluminum, have also been used in automobiles, however, their applications were limited to non-structural parts. Recently, interest has been focused on using magnesium alloys as structural materials for automotive load-bearing components. Load-bearing components in automobiles are usually subjected to multiaxial cyclic loading. Fatigue is considered to be a significant cause of ground vehicle component failure. Therefore, for magnesium alloys to be used for these components, an understanding of their fatigue behaviour is necessary. In this study, series of monotonic and cyclic tests were conducted on smooth specimens machined from AZ31B magnesium extrusion section. Two loading modes were considered in this investigation, axial and torsional. Monotonic tensile and compressive tests were performed at three different orientations, longitudinal (LD), i.e., parallel to the extrusion direction, 45° and transverse (TD) directions. Monotonic torsion tests were performed on specimens that were machined along the LD. Similarly, cyclic axial and torsional as well as multiaxial axial-torsional tests were performed on specimens that that were machined along the LD. Three different phase angles were considered for multiaxial tests: in-phase, and 45° and 90° out-of-phase. It was found that monotonic axial stress-strain behaviour is direction dependent due to the different deformation mechanisms involved. Significant yield anisotropy and sigmoidal-type hardening were observed. Twinning-detwinning deformation was considered as the major cause of these behaviours. On the other hand, monotonic torsional stress-strain curve had a linear hardening behaviour. Cyclic axial behaviour was found to be affected by twinning-detwinning deformation. Its most significant characteristics are: yield asymmetry, power-like hardening in compressive reversal and sigmoidal-type hardening in tensile reversal. This unusual behaviour was attributed to the contribution of three different deformation mechanisms: slip, twinning and detwinning. Due to yield asymmetry, significant positive mean stress was observed especially at LCF. Cyclic hardening was also observed and it was found to be associated with a substantial decrease in plastic strain energy density. Cyclic shear behaviour was symmetric and did not exhibit any of the aforementioned behaviours in cyclic axial loading. Two major observations were made from multiaxial tests. First, additional hardening due to nonproportionality was observed. Second, phase angle has no effect on fatigue life. Three fatigue life models were considered for multiaxial fatigue life prediction: Smith-Watson-Topper, Fatemi-Socie and Jahed-Varvani. The first two models are based on strain and are evaluated on specific critical planes. The third model is based on energy densities calculated from hysteresis loops. Strain- and energy-life curves had knees and pronounced plateaus. Therefore, it was not possible to model the entire fatigue life using Coffin-Manson-Type equations. Low cycle fatigue lives were predicted within ۬x scatter bounds using the Fatemi-Socie and the Jahed-Varvani models for all loading conditions which was not the case with Smith-Watson-Topper model. Total energy, the sum of plastic and positive elastic strain energy densities, was found to correlate fatigue lives for several wrought Mg-alloys under different loading conditions.