Magnesium alloys are being increasing considered for many automotive applications due their low density and high strength to weight ratio. However, joining of these materials by welding and especially to dissimilar materials such as aluminum or steel or mechanically by riveting at room temperature have faced many challenges. Research presented in thesis explores a new hot joining process referred to as Upset Protrusion Joining (or UPJ) as a means of mechanically joining cast magnesium alloy to other similar or dissimilar sheet materials. UPJ is being developed as a rapid and reliable joining method to be implemented in the automotive industry for weight and manufacturing cost reduction. It involves a cylindrical protrusion emanating perpendicular to the flat surface of a cast plate-like magnesium component that is fitted through a hole in another plate or sheet material. The two components are then clamped together, electrically heated and compressed perpendicular to the axis of the protrusion. During this process, the protrusion expands circumferentially to fill the hole as well as the region above the hole thus entrapping the sheet metal between the mushroomed head and the casting. The effect of different UPJ process parameters such as applied current, current duration, compression loading rate and compression distance were studied through experimentation that involved a newly developed computer-controlled experimental UPJ setup. The studies involved two cast magnesium alloys of interest to automotive industry, AM60 and AZ91, with protrusions of 11 mm diameter and 14 mm height on a 2 mm thick plate. Studies of the material properties and UPJ process parameters were performed to find optimal process parameters to achieve satisfactory quality of the joint in terms of post-UPJ joint strength with appearance. Also, microstructural studies, temperature measurements in the protrusion region, and electrical resistivity measurements were performed for the two alloys to fundamentally understand their roles in promoting temperature dependent material flow, strain localization, and fracture in the UPJ process. Lastly, materials specific process window for UPJ process was identified based on the experimental work for creation of robust UPJ joints with acceptable joint strengths in tensile shear mode of failure. This new hot joining method was shown as an industrially viable joining method for cast magnesium component. UPJ is a rapid joining method and provides good joint-strength depending upon joint specifications. This method can be implemented in automotive and other industrial manufacturing environment for joining cast component to a similar or dissimilar wrought sheet component. / Thesis / Master of Applied Science (MASc)

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Thermosonic ball bonding is a key technology in electrical interconnections between an integrated circuit and an external circuitry in microelectronics. Although this bonding process has been extensively utilised in electronics packaging industry, certain fundamental aspects behind all the practice are still not fully understood. This thesis is intended to address the existing knowledge gap in terms of bonding mechanisms and interfacial characteristics that are involved in thermosonic gold and copper ball bonding on aluminium pads. The research specifically targets the fine pitch interconnect applications where a thin metal wire of approximately 20 µm in diameter is commonly used. In thermosonic ball bonding process, a thin gold or copper ball formed at the end of a wire is attached to an aluminum pad through a combination of ultrasonic energy, pressure and heat, in order to initiate a complex solid-state reaction. In this research, the mechanisms of thermosonic ball bonding were elaborated by carefully examining interfacial characteristics as the results of the bonding process by utilising dual-beam focused ion beam and high resolution transmission electron microscopy, including the breakdown of the native alumina layer on Al pads, and formation of initial intermetallic compounds (IMCs). The effect of bonding parameters on these interfacial behaviours and bonding strength is also investigated in order to establish an inter-relationship between them. Interfacial evolution in both Au-Al and Cu-Al bonds during isothermal annealing in the temperature rage from 175ºC to 250ºC was investigated and compared. The results obtained demonstrated that the remnant alumina remains inside IMCs and moves towards the ball during annealing. The IMCs are formed preferentially in the peripheral and the central areas of the bonds during bonding and, moreover, they grow from the initially formed IMC particles. Growth kinetics of Cu-Al IMCs obey a parabolic growth law before the Al pad is completely consumed. The activation energies calculated for the growth of CuAl2, Cu9Al4 and the combination (CuAl2 + Cu9Al4) are 60.66 kJ/mol, 75.61 kJ/mol, and 65.83 kJ/mol, respectively. In Au-Al bonds, Au-Al IMC growth is controlled by diffusion only at the start of the annealing process. A t^0.2-0.3 growth law can be applied to the Au-Al IMC growth after the Al pad is depleted. The sequence of IMC phase transformation in both Au-Al and Cu-Al bonds were investigated. Voids in Au-Al bonds grow dramatically during annealing, however, only a few voids nucleate and grow very slowly in Cu-Al bonds. The mechanisms of void formation, including volumetric shrinkage, oxidation and metal diffusion were proposed and discussed.

Microelectronic interconnections require advanced joining techniques. Direct metal bonding methods, which include thercomsonic and thermocompression bonding, offer remarkable advantages over soldering and adhesives joining. These processes are reviewed in this paper. The progress made in this area is outlined. Some work concerned with the bonding modeling is also presented. This model is based on the joint interface mechanics resulting from compression. Both bump and substrate deformation are taken into account. The improved understanding of the relationship between the deformation and bonding formation may provide more accurate joint evaluation criterion. / Singapore-MIT Alliance (SMA)

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It is widely accepted that the sharing of electrons constitutes a bond. Conversely, molecular interactions that do not involve electron transfer, such as van der Waals forces and electrostatics are defined as "non-bonding" or "non-covalent" interactions. More recently computational and experimental observations have shown situations where the division between "bonding" and "non-bonding" interactions is blurred. One such class of interactions are known as σ-hole interactions. Chapter 1 provides a literature review of investigations into the nature of σ-hole interactions, highlighting the individual contributing factors. Chapter 2 provides a detailed analysis into the nature of chalcogen-bonding interactions. Synthetic molecular balances are employed for experimental measurements of conformational free energies in different solvents, facilitating a detailed examination of the energetics and associated solvent and substituent effects on chalcogen-bonding interactions. The chalcogen-bonding interactions examined were found to have surprisingly little solvent dependence. The independence of the conformational free energies on solvent polarity, polarisability and H-bond characteristics showed that electrostatic, solvophobic or dispersion forces were not dominant factors in accounting for the experimentally observed trends. A molecular orbital analysis provided a quantitative relationship between the experimental free energies and the molecular orbital energies, which was consistent with chalcogen-bonding interactions being dominated by an n→σ* orbital delocalisation. Chapters 3 and 4 both use the molecular orbital modelling approach established in Chapter 2 to investigate the potential partial covalency in H-bonding and carbonyl···carbonyl interactions. H-bonding is generally considered to be an electrostatically dominated interaction. However, computational results have suggested a partial covalent character in H-bonding. The molecular orbital analysis revealed an n→σ* electron delocalisation in all H-bonding systems evaluated. However, no quantitative correlation could be found with experimental free energies. Similarly, the nature of carbonyl···carbonyl interactions has been subject to debate, with electrostatic or an n→π* electron delocalisation having been proposed as the dominant factors. The molecular orbital analysis employed here showed that n→π* delocalisation was exceptionally geometry dependent. Studies of literature systems reveal that n→π* delocalisation contributes to overall stability of a range of systems, with a quantitative link between molecular orbital energy and conformational free energies.

Magnesium alloys are widely used in engineering applications, including aerospace and automobile industries, due to their desirable properties, such as lower density, high damping capacity, relatively high thermal conductivity, good machinability, and recyclability. Researchers have, therefore, been developing new magnesium materials. However, mechanical and corrosion properties are still limiting many commercial applications of magnesium alloys. In this Ph.D. thesis research, I developed Mg-Ti composite materials to offer some solutions to further improve the mechanical behavior of magnesium, such as titanium-magnesium (Ti-Mg) claddings, Mg-Ti multilayers, and Ti particle enforced Mg alloys. Low cost manufacturing processes, such as hot roll-bonding (RB) and accumulative roll-bonding (ARB) techniques, were used to produce Mg-Ti composites and sheets. The microstructural evolution and mechanical properties of composites were investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), nanoindentation, and tensile tests. In the first part of this study, I investigated the bonding strength of the AZ31/Ti to understand the mechanical properties of Mg/Ti composites. Using a single pass RB process, I fabricated AZ31/Ti multilayers with the thickness reduction in a range of 25% to 55%. The hot-rolled AZ31/Ti multilayers were heat-treated at 400 °C for 6, 12, and 24 hours, respectively, in an argon atmosphere. Tensile-shear tests were designed to measure the bonding strength between AZ31/Ti multilayers. Furthermore, the experimental results revealed good bonding of the AZ31/Ti multilayers without forming any intermetallic compounds in the as-rolled and heat-treated AZ31/Ti multilayers. The good bonding between Ti and AZ31 is the result of diffusion bonding whose thickness increases with increasing heat-treatment time and thickness reduction. The shear strength of the Ti/AZ31 multilayer increases with increasing bonding layer thickness. In the second part of this study, I characterized the microstructure and texture of three-layered Ti/AZ31/Ti clad sheets which were produced by single-pass hot rolling with a reduction of thickness 38% (sheet I) and 50% (sheet II). The AZ31 layer in sheets I and II exhibited shear bands and tensile twins {1012}⟨1001⟩ . The shear bands acted as local strain concentration areas which led to failure of the clad sheets with limited elongation. Heat treatment caused changes in the microstructure and mechanical properties of clad sheets due to static recrystallization (SRX) on twins and shear bands in the AZ31 layer. Recrystallized grains usually randomize the texture which causes weaken the strong deformed (0001) basal texture. Twins served as nucleation sites for grain growth during SRX. Tensile tests at room temperature showed significantly improved ductility of the clad sheets after heat treatment at 400°C for 12h. The results showed that the mechanical properties of clad sheets II are better than clad sheet I: The clad sheet II shows elongation 13% and 35% along the rolling direction (RD) for as-rolled and annealed clad sheet, respectively whereas the clad sheet I shows elongation 10% and 22% along RD for as-rolled and annealed clad sheet, respectively. In the final part of this study, I examined the effects of dispersed pure titanium particles (150 mesh) with 0, 2.3, 3.5, 4.9, and 8.6 wt. % on the microstructure and mechanical properties of AZ31-Mg alloy matrix. Mg-Ti composites were processed through three accumulative roll bonding (ARB) steps using thickness reductions of 50% in each pass followed by heat treatment at 400 °C for 12 h in an argon atmosphere. ARB is an efficient process to fabricate Mg-Ti composites. Mechanical properties of Mg- 0Ti and Mg-2.3Ti composite were enhanced by ~ 8% and 13 % in yield strength and ~ 30% and 32 % in ultimate tensile strength, respectively. Meanwhile, the elongation of the composites were decreased by 63% and 70%, respectively. After heat treatment, the results showed a decrease in yield strength and increase in elongation to fracture. The mechanical properties of the Mg-0 and Mg-2.3Ti composite were enhanced: ultimate tensile strength by 9% and 7%, and elongation by 40% and 67%, while the yield strength was decreased by 28% and 36% compared with the initial AZ31. Enhancements of strength and ductility were the results of two mechanisms: a random matrix texture by ARB and ductile titanium particle dispersion.

The aim and objective of this study was to determine the bond strength of a twostep and a single-step dentin bonding agent and to determine the effect of repeated thermal-cycling from 50C and 550C on the bond strength values of these dentin bonding agents.

The aim and objective of this study was to determine the bond strength of a twostep and a single-step dentin bonding agent and to determine the effect of repeated thermal-cycling from 50C and 550C on the bond strength values of these dentin bonding agents.