M.Ing. / This report documents the steps taken to gain insight into the phenomena of high speed machining (HSM) of titanium alloys. This was done by firstly studying titanium alloys and the problems associated with machining titanium alloys. An experimental set-up and procedure was developed for measuring and recording both the machining temperature and component forces. A sufficient set of experimental data was collected through extensive experimentation. The cutting temperatures and component forces in HSM of Ti-6Al-4V were examined simultaneously. The cutting speed was found to be the most influential and limiting parameter on the machining temperature and component forces. A new approach for modeling the temperatures in HSM of titanium alloys was developed. Analytical predictions of the cutting temperatures were performed and used to evaluate the influence of a variation in the process parameters on the cutting temperature. The research provides insight for future work into the phenomena of HSM of titanium alloys . The results of the analytical model were found to be representative and comparable to the experimental data. It is however expected that the deviation between the predicted and measured result may be significantly reduced by changing the experimental approach. It is recommended that a complete set of experiments be done, using a new tool insert for every cut, thus removing the effect of possible tool wear on the experimental data obtained. In addition it is recommended that the iterative solution be improved through more in depth programming, considering the change in both the thermal and mechanical materials properties with a change in temperature. Ultimately the assumptions made in order to simplify the problem addressed in this report needs to be improved upon, in order to analyze data trends and even magnitudes to a greater degree of certainty.

(+)-(2R,3R)-1,1,4,4-Tetraphenylbutane-1,2,3,4-tetraol (TETROL) and its derivatives were reacted with varying molar ratios of titanium isopropoxide (2:1, 1:1 and 1:2 tetraol:titanium isopropoxide) in an attempt to prepare potential titanium-based tetraol catalysts for enantioselective transformations. In each case, infrared and HNMR spectra suggested that the product was formed. We tentatively proposed that the structure of the catalyst was a spiro-type, but we could not determine conclusively what its exact structure was, despite using numerous techniques at our disposal (molecular modelling calculations, H NMR and IR spectroscopy, thermal analyses, powder diffraction, and single crystal X-ray diffraction). The catalyst and derivatives thereof were able to act catalytically for the enantioselective additions of diethylzinc compounds to aldehydes. The effects of temperature and solvent were investigated, and toluene and -78 °C were selected as optimal from the results obtained. (The reaction could, however, not be maintained at this low temperature for extended periods due to the fact that we did not have, at our disposal, the correct equipment. Each 16 h reaction was thus allowed to reach room temperature in each case.) The selectivity for the product 1-phenylpropan-1-ol (when benzaldehyde was the starting aldehyde) varied depending on the nature of the aryl substituents of the titanium-based catalyst. Using 0.2 molar equivalents of the chiral titanates, the highest selectivity was 42 percent (e.e.), but only when excess Ti(O-i-Pr)4 had been added to the reaction mixture. This was achieved with the tetra(ortho-methoxyphenyl)-TETROLate derivative. TETROL and its derivatives were also successful in metal-free catalysis where higher conversions and selectivities were observed, compared to when these were complexed to titanium. The highest selectivity was 70 percent (e.e.), achieved with the tetra(ortho-methylphenyl)TETROL derivative.

Titanium oxide has found different applications in environmental pollution remediation through its use as a photocatalyst for the purification of air and water. However, despite many advantages of TiO2, its use in unmodified form for photocatalytic reactions is hampered by a number of inherent physicochemical properties. The research in this thesis shows that TiO2 containing either copper (Cu) alone or combined copper-activated carbon (Cu-AC) are active in the simultaneous photocatalytic remediation of nitrate and oxalic acid, while pure titania and titania-activated carbon (TiO2/AC) composites found inactive for the reaction. In the case of photocatalytic removal of phenol, titania-thermally reduced graphene oxide (TiO2-TGO) exhibited superior photocatalytic performance than titania-graphene oxide (TiO2-GO) and pure TiO2.

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This project was focused on the generation of porous and nanotubular anodic films on titanium and Ti-6wt.%Al alloy, and investigation of the key factors responsible for a transition between porous and nanotubular morphologies. Advanced analytical techniques were employed for characterisation of the anodic films, in particular scanning and transmission electron microscopies, including analytical transmission electron microscopy, Raman spectroscopy, nuclear reaction analysis, Rutherford backscattering spectroscopy and atomic force microscopy. Preparation of electron transparent sections for analysis by analytical transmission electron microscopy was undertaken using the focused ion beam technique. Initially, the influence of a post-anodizing rinsing treatment, using various media, on the morphology, structure and composition of anodic films generated on titanium in 0.2 M ammonium fluoride in glycerol, containing 0 and 5 vol.% added water, was investigated. Porous anodic films were formed in an electrolyte without added water followed by rinsing with ethanol. It was revealed that the oxide-rich nanotubes are embedded in a fluoride-rich matrix, with fluoride-rich material being more extensive and oxide-rich nanotubes being thinner-walled for the anodic films generated in the electrolyte with no added water followed by rinsing with ethanol compared with those for the films formed with added water to the electrolyte and rinsed similarly. However, post-anodizing rinsing of the former films transforms the porous morphology of the films to a tubular one. The contents of titanium and fluorine were reduced in the case of anodic films with the nanotubular morphology. It was suggested that dissolution of the fluoride-rich matrix occurs during rinsing of the specimens with water, leading to the transition from porous to nanotubular morphologies and subsequent loss of titanium and fluorine. Further work was undertaken to study the effect of ageing in deionised water on the morphology, structure and composition of the anodic films. It was revealed that loss of titanium and fluorine is greater for the films generated in the electrolyte with no added water followed by rinsing with water and ethanol and ageing compared with that for the films formed in the electrolyte with 5 vol.% added water followed by rinsing with water and ethanol and ageing. Finally, the anodic films generated on the Ti-6wt.%Al alloy were investigated. Porous anodic films were formed in the electrolyte without added water followed by rinsing with ethanol whereas the films treated with water disclosed nanotubular morphology. Porous anodic films contained greater amount of fluorine than nanotubular ones. Compositional analysis revealed an increased amount of fluorine for the anodic films generated on the alloy compared with those for the films formed on titanium under all investigated conditions. The difference in film compositions may be related to the difference in composition of the substrates used for anodizing, in particular, to the presence of aluminium as alloying element in the Ti-6wt.%Al alloy.

The current study focuses on phase stability and evolution in the titanium-zinc titanium-copper and titanium-antimony systems. The study utilized the Laser Engineering Net Shaping (LENS™) processing technique to deposit compositionally graded samples of three binary system in order to allow the assessment of phase stability and evolution as a function of composition and temperature the material is subjected to. Through LENS™ processing it was possible to create graded samples from Ti-xSb (up to 13wt%) and Ti-xCu (up to 16wt%). The LENS™ deposited gradient were solutionized, and step quenched to specific aging temperature, and the resulting microstructures and phase were characterized utilizing XRD, EDS, SEM, FIB and TEM. The Ti-Zn system proved incapable of being LENS™ deposited due to the low vaporization temperature of Zn; however, a novel processing approach was developed to drip liquid Zn onto Ti powder at temperatures above β transus temperature of Ti (882 ◦C) and below the vaporization temperature of Zn (907 ◦C). The product of this processing technique was characterized in a similar way as the graded LENS™ depositions. From measurements performed on Ti-Sb it seems that Sb could be a potential α stabilizer in Ti due to the presence of a mostly homogeneous α grains throughout the gradient; however, from XRD it can be understood that a titanium antimonide phase is present. From results obtained from the Ti-Zn samples, it can be surmised that the eutectoid reaction seems to be active, i.e. The eutectoid reaction is kinetically fast, as concluded by the presence of pearlitic structures. Finally, for the Ti-Cu system this work has been attempted to prove or disprove the existence of the Ti3Cu through the use of XRD and TEM SAD patterns. From XRD spectra collected there are peaks belonging to the Ti3Cu orthorhombic phase along with Ti2Cu and α-Ti phase. In addition to the Ti-Cu system displayed structures associated with divorced eutectoid decomposition mechanism, and at low undercooling seems to be prone to forming solid state dendrites.

A combination of experimental and computational methods is used to explore the microstructure and mechanical behavior of cold sprayed 6061 aluminum alloy and Ti-6Al-4V alloy and their substrate materials. A variety of microscopic methods are used for characterization of the microstructure. The indentation size effect and characteristic length of strain gradient plasticity for the substrate materials are determined. An FEA simulation describes the behavior of a worn Berkovich nanoindenter. Stress strain is studied experimentally in the substrate materials for future comparison with bulk cold-sprayed materials. Abaqus FEA models are used to simulate a single particle impact for a particle with an oxide layer using a linear Johnson-Cook plasticity model and a bilinear Johnson-Cook plasticity model. The implications of the results are discussed for cold spray processes.

No description available.

Near-net-shape manufacturing of titanium metal components through powder metallurgy confers various cost-saving benefits, from improved material utilisation to reduced energy consumption. Further savings can be realised by reducing the cost of the titanium metal powder that is used as feed material in powder metallurgy. Titanium metal-powder production costs can be reduced by removing the steps currently needed (i.e. milling, vacuum arc re-melting and atomisation) to convert titanium Kroll sponge into a powder product that is suitable for use in powder metallurgy. This can be potentially achieved through a controlled metallothermic reduction of titanium tetrachloride to produce a titanium metal powder product that can be used directly in powder metallurgy. Consequently, this thesis divides titanium metal-powder into two main categories, namely primary and secondary metal powder products. Titanium metal-powder is classified as a primary product when it can be used directly as feed material in powder metallurgy. In contrast, a secondary metal-powder product requires extra processing steps after chemical reduction before it can be used in powder metallurgy. To illustrate, titanium metal powder produced through plasma spheroidization is an example of a secondary metal powder product. The metallothermic reduction reaction is known to have two dominant reaction mechanisms. These mechanisms have a characteristic product morphology that forms under specific reaction conditions. The first mechanism is responsible for the sponge-like morphology obtained from the Kroll process, while the second mechanism results in an ultrafine precipitate that has a surface area to volume ratio large enough to oxidise in air to the extent that it cannot be regarded as commercially pure. Consequently, a primary titanium metal-powder product has to date not been realised as efforts to achieve particle growth on this ultrafine precipitate have been unsuccessful. This thesis's main objective was to demonstrate that metal particle growth on suspended metal particles is indeed possible through a controlled metallothermic reduction in a molten salt reaction medium. Subsequent efforts resulted in the postulation of a third reaction mechanism that would enable titanium metal particle growth. The postulated growth mechanism is electrochemical and referred to as “autocatalytic electroless deposition on suspended titanium metal particles”. Theory development and modelling efforts indicated that the postulated growth mechanism is possible, but only in a particular and low concentration range where both reagents are present in a meta-stable equilibrium with each other in the molten chloride reaction medium. The concentration range is estimated to be in the range of parts per million for each reagent. It was further shown that more than one product morphology is inherent in the conditions where the postulated mechanism is possible as there is no dominant reaction mechanism at such low reagent concentrations. Therefore, the metallothermic reduction reaction should be regarded as a system of reaction mechanisms at these conditions. Experimental results substantiated the postulated growth mechanism's existence to the extent where β-titanium metal was deposited on the surface of metallised ilmenite particles. The deposited layer was distinguishable from the substrate particle as ilmenite contains α-titanium (i.e. a hexagonally closed packed crystal system). Therefore, controlled titanium metal particle growth is hypothetically possible through a mechanism known as “autocatalytic electroless deposition”. However, further effort is still needed to demonstrate whether a viable primary titanium metal powder product can be produced. / Thesis (PhD (Engineering))--University of Pretoria, 2021. / Department of Science and Innovation / Chemical Engineering / PhD (Engineering) / Unrestricted

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