Theses and Dissertations

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    Design and Construction of a Vacuum Casting Equipment for Aluminium Alloys
    (2015-06-19) Aremo, Bolaji
    This work reports the design and construction of an apparatus for the vacuum casting of aluminium alloys, production of cast components using the apparatus and evaluation and comparison of the quality of cast components produced by vacuum casting. This is with a view to alleviating the problems of poor castability and produce inclusions-free castings. Melting and vacuum requirements of the equipment were determined based on a furnace capacity of 15 kg of aluminium. The heating power requirement was calculated for this mass while refractory requirements necessary for a 15 kg mass and non-slagging application was determined. Low-cost clay-based refractory and insulating bricks were specifically developed from locally-available raw materials and novel production routes were used for their production. The vacuum system was made from locally available materials designed for optimal efficiency required for vacuum casting. Computer Aided Design software was integrated into both sheet metal forming and refractory mould design. The equipment was used to melt and cast small-sized specimens using the lost-wax technique and compared with green sand and ceramic mould cast specimens. Tensile and shrinkage tests and microstructural examination were carried out on all specimens. The results of the tensile strength of the vacuum cast, green sand cast and ceramic mould cast specimens of the aluminium alloy were 123.23, 98.83 and 113.56 MPa respectively. Furthermore, shrinkage defect which was common to both green sand and ceramic mould casting was eliminated in vacuum casting. The continual replenishment of the mould cavity by molten metal from the furnace effectively eliminates shrinkage in the cast component during vacuum casting. In conclusion, the aluminium alloys cast from the designed and constructed vacuum casting apparatus showed improved properties when compared with those from the traditional green sand and ceramic mould.
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    Determination of Optimum Grain Refinement Parameters for Hypoeutectic Aluminium-Silicon Alloys by a Volatile Mould Coating Process
    (2015-06-05) Shittu, Moshood Dehinde
    The study produced hypoeutectic aluminium silicon alloy, established its optimum conditions for grain refinement, pouring conditions and assessed the degree of porosity generated in the cast ingot. This was with a view to evolving a standard for the use of volatile mould coating (VMC) process for the grain refinement of aluminium-silicon alloys. A hypoeutectic aluminium-silicon alloy and an alumina based mould coating containing volatile hexachloroethane (C2CI6) compound were developed. Also, a metallic mould was produced from grey cast iron material. The prepared mould was thereafter coated with the developed alumina-based coating using different concentrations of hexachloroethane. The developed aluminium-silicon alloy was melted at various degrees of melt superheats of 50°C, 75°C, 100°C, 125°C and 150°C. At each superheat, the molten aluminium-silicon alloy was poured into the metal mould at various coating thicknesses ranging from 1 mm to 5mm. The effects of the hexachloroethane concentrations in the mould wash on the grain size, bulk density, apparent porosity and mechanical properties of the developed aluminium — silicon alloy were determined using standard methods. The effects of the degrees of melt superheat and the mould coating thickness on these properties were similarly determined. The results showed that an increase in the hexachloroethane content in the mould coating did not produce any significant change (apparent porosity less than 0.05%) in the amount of open microporosity of the prepared alloy. It was also observed that the Brinell hardness value, the ultimate tensile strength and 0.2% proof stress progressively increased as the concentration of the volatile hexachloroethane content in the mould coating increased up to optimum value of 15wt.%. It was also noted that the addition of up to 15 wt.% hexachloroethane to the alumina-based mould coating gradually reduced the grain sizes of the hypoeutectic aluminium silicon alloy from 31.0μm to about 8.01_m. Increase in amount of the melt superheat up to 150°C resulted in coarsening of the grains of the prepared hypoeutectic aluminium silicon alloy. It was observed that the optimum mechanical properties, grain sizes and surface finish of hypoeutectic aluminium silicon alloy cast in hexachloroethane coated mould were attained when its content in the mould coating was about 15 wt.%. It was concluded that the optimum conditions for grain refinement of hypoeutectic aluminium-silicon alloys using the volatile mould coating (VMC) process were achieved when the hexachloroethane content in alumina-based mould wash was 15 wt.%, with coating thickness and melt superheat of 2mm and 50°C respectively. Furthermore, the volatile compound (hexachloroethane) was effective for use in alumina-based mould coating for grain refinement of hypoeutectic aluminium silicon alloy.
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    Evaluation of Corrosion Co-Inhibition Characteristics of Sodium Tungstate with Sodium Nitrate, Sodium Silicate and Potassium Iodide on Low Carbon Steel in Hydrochloric Acid
    (2015-04-29) Ogundare, Olasupo
    This study has investigated the corrosion co-inhibition characteristics of sodium tungstate with sodium nitrate, sodium silicate and potassium iodide on low carbon steel in 0.085 M hydrochloric acid. This was done with a view to determining which of the oxidizing inhibitors would produce the highest co-inhibition efficiency with sodium tungstate. The inhibition study was carried out using the weight loss immersion technique. The inhibitive potentials of the inhibitors were based on the determination of the corrosion rates, pH of corrosive media and a calculation of the inhibitive efficiencies of the different inhibitors at various levels of concentrations. The adsorption mechanisms of the individual inhibitor and co- inhibitors were determined by various adsorption isotherms for calculating degree of surface coverage with concentration. The uninhibited solution of 0.085 M hydrochloric acid served as the control. The effects of individual inhibitor and co-inhibitors on corrosion rates (mpy) and inhibition efficiencies, pH changes and corrosion potentials at 50 ppm, 100 ppm and 200 ppm concentrations with time of exposure in 0.085 M hydrochloric acid were investigated. The results showed that inhibitive efficiencies of 71% were produced by co-inhibiting sodium tungstate with sodium silicate when compared to 54% produced by co-inhibiting with sodium nitrate after 336 hours of exposure. The co-inhibition of 50 ppm sodium tungstate with 100 ppm sodium silicate was observed to be the optimum concentration. Potassium iodide used alone as inhibitor was observed to follow the Langmuir isotherm mechanism of adsorption. The co-inhibition of sodium tungstate and potassium iodide as well as sodium tungstate and sodium nitrate followed the Freundlich isotherm. The corrosion potential change of the test medium was observed to confer a corresponding change in pH. The trend observed was that as the pH values increased, the corrosion potentials of the medium decreased. The study concluded that the inhibition efficiency of sodium tungstate in hydrochloric acid would be greatly enhanced if sodium silicate were added to the acid-inhibitor medium at the optimum quantity.