Electronic Theses and Dissertations (Masters)

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Characterisation and surface finish evaluation of Direct Energy Deposited AlCoCrCuFeNi High Entropy Alloys
(University of the Witwatersrand, Johannesburg, 2024-01) Modikwe, Thembisile Patience; Mathe-Maleboho, Ntombi; Maledi, Nthabiseng
This study focused on the use of direct energy deposited (DED) techniques for the fabrication of AlCrCoCuFeNi high entropy alloy (HEAs) samples. HEAs have become a ground-breaking research field that provides solutions to complex problems in the aerospace industry. The industry requires improvement in the application of structural materials that are well-functioning at a low cost for example turbine blades. The fabrication of HEAs via DED commonly produces poor surface finish Ra in the range of 5 μm - 20 μm due to the layer-by-layer deposition method, as a result, it fails the industrial application requirements where the usual range of roughness tolerance required in the industry ranges from Ra is 0.8 μm < Ra < 1.6 μm thus, the need to deploy post-processing methods. This study focused on electropolishing (EP) and centrifugal barrel finishing (CBF) of AlCrCoCuFeNi-HEA samples. The polishing was performed using 80% methanol and 20% per-chloric acid solution used as the electrolyte. The samples were polished for 30 and 60 seconds in a Struers LectroPol-5 electrolytic polishing and etching device. The surface removal at 1200W for 30 sec on sample a was 50.29%, 58.65% for sample b, and 75.48% for sample c. The surface removal at 1400W for 60 sec on sample d is 63.25%, 45% for e, and 49.19% for f. The samples were polished for 7 and 14 hours in a CB320-CBF. During the period of 14 hours, a surface removal where the proportion of material removed for sample a was 55.37%, sample b was 43.13%, and sample c was 32.2% at a laser power of 1200W. After 7 hours of polishing, sample d achieved a surface removal of 86.02%, sample e achieved a surface removal of 43.18%, and sample f achieved a surface removal of 90% at a power of 1400W. Oxidation tests were conducted in static air at 1000˚C for 200h. The presence of FCC, BCC, and Fe2O3 oxide scales resulted in a noticeable increase in mass, with Fe2O3 scales being the most prevalent.
Experimental heat transfer coefficients for the cooling of oil in horizontal internal forced convective transitional flow
(University of the Witwatersrand, Johannesburg, 1981) Rogers, Douglas Gordon; Van der Merwe, D.F.
Influence of copper on the corrosion and mechanical properties of Grade 4 titanium for biomedical applications
(University of the Witwatersrand, Johannesburg, 2022-12) Hadebe, Nomsombuluko Dayanda Elizabeth; Cornish, Lesley; Chown, Lesley H.; Smit, Melanie; Mwamba, Alain
This study assessed the effect of Ti2Cu and its proportions on the corrosion resistance, and compared the results to Grade 4 commercially pure titanium. The Thermo-Calc program with the TTTI3 (Ti-alloy) database was used to predict the phases. Materials Studio software was used to model the crystal structures and XRD patterns of the phases of Ti-Cu alloys. Ti-Cu samples with 0, 5, 15, 25, 33, 40, 47 and 50 wt % Cu were produced. Composition, microstructures, phases, hardness and corrosion resistance were studied in the as-cast and annealed conditions (750° and 900°C water quenched). The CP Ti samples comprised basket-weave acicular microstructures. The Ti-5Cu samples comprised lamellar (αTi) and Ti2Cu phases. The Ti-15Cu, Ti-25Cu and Ti-33Cu alloys comprised (αTi) dendrites and sparse eutectic of Ti2Cu and (αTi). The ((βTi) dendrites decomposed to (αTi) and Ti2Cu, and could not be retained due to insufficient fast quenching. The Ti-40Cu and Ti-47Cu samples had minor titanium oxide dendrites which solidified first and then Ti2Cu nucleated on them and grew as dendrites, surrounded by the Ti2Cu + TiCu eutectic. In the Ti-50Cu sample, TiCu was the true primary phase and grew as needles, and was subsequently surrounded by a coarse TiCu + Ti2Cu eutectic. No Ti3Cu phase was observed. The microstructures of the as-cast alloys agreed with the Cu-Ti phase diagram of Ansara et al. (2021) and Dyal Ukabhai et al. (2022) with the congruent formation of Ti2Cu, as well as no Ti3Cu. The addition of copper to titanium increased the hardness, while annealing decreased the hardness of the Ti-Cu alloys. Addition of copper above 5 wt % Cu and annealing decreased the corrosion resistance of the samples, but since copper ions in liquid solutions promote the antimicrobial activity, some corrosion is necessary to allow the copper ions to be available. The corrosion tests showed that the corrosion rates obtained were very low, below 0.13 mm/y, which is an acceptable corrosion rate for biomaterial applications. Ti-5Cu showed the best corrosion resistance.
Investigating the effect of size, density and shape of Iron ore particles on batch jig performance
(University of the Witwatersrand, Johannesburg, 2024-02) Dzaringa Kisembo, Daniel Elvis; Woollacott, Lorenzo
The gravity separation method is one of the oldest methods of mineral beneficiation that takes advantage of the difference in the specific gravity of particles that are being separated. The separation occurs in a fluid medium, usually water, and involves floating off lighter material to leave behind denser ones. There are several types of gravity separation techniques, and they vary according to the equipment that is used for the separation or the property of the medium that is being used; the main gravity separation methods that are widely used for the beneficiation of Iron ore are Jigging and Dense Medium Separation (DMS). In this research, the jigging method is selected to investigate the concentration of an Iron ore by using a batch laboratory Jig; the jigging method was preferred for its simplicity and availability, generally Jigging has several advantages, some of which include cost effectiveness and simplicity of operation and its minimum impact on the environment. During the beneficiation of minerals using the jigging method of ore concentration, several feed material characteristics affect the efficiency, such as the particle density, size and shape. The aim of this research was to investigate the effect of these feed properties on jig performance. Tests were conducted on a Hematite ore sample using a batch jig to gain a deeper understanding of how the density, the size and shape of particles affect segregation. The iron ore samples were screened and any extremely small particles were removed, maintaining a particle size range between 2.8 and 10 mm. The results showed that particles were stratified on the basis of their specific gravity, denser particles reported toward the bottom layer of the bed and separated more efficiently. Less denser particles reported more toward the upper layer of the bed and were less efficiently separated. Coarser particles tend to report to the bottom layer of the bed and finer particles to the top product layer. Particles that were flatter and more elongated tended to end up in the bottom layer of the bed more often, while more rounded particles were not as likely to be found in the bottom layer.
Selection of a technique to separate carbon dioxide from methane for recovery of natural gas at Lake Kivu
(University of the Witwatersrand, Johannesburg, 2024-02) Ntini, Hermann Ekini; Nkazi, Diakanua; Mukaya, Elie
Lake Kivu is situated between the Democratic Republic of Congo (DRC) and Rwanda. It is known to contain large amount of dissolved carbon dioxide and methane. It is termed a killer lake due to the toxic nature of these gases, which could emerge on the surface during a catastrophic eruption and cause massive devastation in this region. Extracting these toxic gases proves to be crucial to avoid natural disasters and to afford economic benefits in the form of electricity generation or energy export.
Thermo-mechanical processing and testing of titanium alloys for potential dental applications
(University of the Witwatersrand, Johannesburg, 2022-12) Nape, Kgetjepe Tlhologelo; Chown, Lesley H.; Cornish, Lesley
New titanium alloy compositions were identified for potential dental implants on the basis of having two-phase microstructures for good mechanical properties and by avoiding problematic elements to increase biocompatibility. The Thermo-Calc program with the TTTI3 (TT Ti-alloy) database was used to calculate new Ti compositions, without toxic Al and V as alloying elements. The aim was to mimic the α+β phase proportions in Ti-6Al-4V and Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) (an analogue for Ti-6Al-4V). Copper (Cu = 1, 3, 5 and 10 wt%) was varied to give the Ti2Cu phase, which gives good hardness and antibacterial properties. A cost analysis was done and the less expensive Ti-6Nb-4Zr-xCu and Ti-8Nb-4Zr-xCu (x = 0 and 5 wt%) compositions were selected for experimental work. The samples were made by arc-melting and prepared for microstructural studies to understand the influence of alloying elements, and to compare with the commercial Ti-6Al-4V and reported Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) alloys. Hot deformation of the as-received Ti-6Al-4V and TTNZ alloys was investigated, using a Gleeble 3500® Thermo-mechanical Simulation Facility, at 850°C and 950°C and strain rates of 0.1 s-1 and 10 s-1. The as-cast Ti-6Nb-4Zr-xCu and Ti-8Nb-4Zr-xCu (x = 0 and 5 wt%) alloys comprised αTi and βTi, with Ti2Cu once Cu was added, although EDX indicated some inhomogeneity. The XRD analyses identified αTi and small amounts of βTi with solid solution (shifted peaks), with some Ti2Cu. The Ti-8Nb-4Zr alloy (285 ± 7 HV) had similar hardness to Ti-6Nb-4Zr (280 ± 13 HV), and was considered the better alloy. Adding 5 wt% Cu increased the hardness due to Ti2Cu. With the Gleeble, deformation at 950°C and 10 s-1 led to a finer Ti-6Al-4V microstructure, whereas finer Ti-10.1Ta-1.7Nb-1.6Zr (TTNZ) microstructures occurred at 850°C and 10 s-1. The XRD of all deformed Ti-6Al-4V and Ti-10.1Ta-1.7Nb-1.6Zr samples indicated αTi and βTi, with shifted βTi peaks. The Ti-6Al-4V (324 ± 9 HV) deformed at 850°C and 0.1 s-1 had higher hardness than both deformed TTNZ samples. Higher flow stress were obtained at higher strain rate (10 s-1) and lower temperature (850°C). The Ti-6Al-4V alloy had higher flow stress than the TTNZ alloy. Therefore, the TTNZ alloy was considered better, due to its lower flow stress, which indicated better formability. The new alloys had similar hardnesses to Ti-6Al-4V, and were higher than for TTNZ, suggesting that they might have similar properties to Ti-6Al-4V.
Upgrading Semi-Soft Coking Coal by Hydrothermal Treatment: Caking and structural properties
(University of the Witwatersrand, Johannesburg, 2024-02) Ndumo, Jabulile; Bada, Samson
Based on the current challenges faced by the metallurgical industry in South Africa in importing quality reductants, there is an urgent need to investigate a new approach to enhance the semi-soft coal available in the country. Importing prime coking coal has increased the steel price, resulting in many downstream operations involving steel closing down in the country. With a surplus of semi-soft coking coal in South Africa, this research sought to look into this kind of abundant coal to enhance its property as a reductant for blast furnace applications. For this reason, a study was conducted on two Southern African coals, Grootegeluk (semi-soft coking coal) and Moatize (higher quality coal). Both coals were individually hydrothermally treated and then blended at different ratios to further upgrade their metallurgical properties. The as-received Moatize coal showed properties that were more of prime coking coal with high total carbon content (76.50%), a crucible swelling index of nine, a maximum dilatation of 59% and volatile matter of 20.39%. It was a highly vitrinite coal with a vitrinite reflectance of 1.28%, a higher micropore volume than mesopore volume and a very low maximum fluidity of 24 dial divisions per minute (ddpm). According to the initial test, the Grootegeluk coal sample had a crucible swelling index of 5.5, a high volatile matter of 35.02% and a low vitrinite reflectance of 0.72%. In addition, the sample had a maximum dilatation of -10%, a maximum fluidity of 3ddpm and a higher mesopore volume than the Moatize coal. Hydrothermal treatment was conducted on the coal samples at numerous temperatures (100ºC to 200ºC), at various residence times (30 to 90 minutes) and at different coal masses (300 to 600grams (g)). According to the results, the optimal hydrothermal conditions were 200ºC, 90 minutes and 600g. Another hydrothermal treatment was performed at a higher temperature and residence time of 280ºC and 180 minutes. The same sample mass of 600g was used and the result showed no further improvement. The coal samples were then blended at various Grootegeluk/Moatize ratios (15% to 50% Grootegeluk), and further hydrothermal treatment tests were carried out based on the optimum conditions achieved. Both the hydrothermal test and the blending of the coal led to a coal with volatile matter ranging from 21.46% to 23.79%, which is a required specification for metallurgical application. The total carbon of the enhanced coal blend also ranged from 68.8% to 82.10%, with total sulphur below 1%. The mesopore-micropore ratio of the treated blend was higher than the individual coal samples, which is what is expected of a metallurgical coal. Based on these findings, coke was produced and analysed to identify a coke capable of withstanding blast furnace conditions. Using the particle reactivity index (PRI), proximate analysis and the pore size distribution, 90-(50% Grootegeluk+50% Moatize)-C product was identified as the coke with the least PRI and high fixed carbon. Further investigation showed that the blending and hydrothermal treatment affected the coal’s physiochemical, rheological and micro-molecular properties. The study has established that metallurgical properties of the locally mined semi-soft coking can be enhanced solely and when mixed with a hard coal. Even though the 90-(50%GG+50%M)-C did not meet the overall specifications required for use in the blast furnace, it was identified as a suitable reductant for other metallurgical applications.
Wear Reduction and Media Density Optimization for the Single Stage Pipe Densifier at Sishen Iron Ore’s Beneficiation Plants
(University of the Witwatersrand, Johannesburg, 0202-02) Botha, Simone; Kabezya, Kitungwa
The depleting high-grade iron ore mining supply at Sishen Mine in the Northern Cape, South Africa, has given rise to its beneficiation plants operating at higher media densities to upgrade lower-grade ore. In this study, densification was numerically modelled using an MPPIC model and experimentally tested using a 200-mm diameter centrifugal densifier from two local suppliers – Multotec and HMA. Shear stress, wear rate, separation efficiency and media losses were measured at increasing operating densities and differing vortex finder sizes. Optimum operating conditions were established. It was found that a feed density of 3.60 t/m3 and a shear stress of 9.70 e-3 N/m3 at the inlet using a vortex finder diameter size of 30 mm exhibited favourable performance in terms of media densification and downstream recovery. The practical significance of this is proven in terms of wear rate and its predictability to provide a consistent overflow of below 1.20 t/m3 media to the recovery circuit. Furthermore, information about ideal operating conditions in terms of inlet pressure and controls to identify premature failures were established.

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Browsing Electronic Theses and Dissertations (Masters) by School "School of Chemical and Metallurgical Engineering"

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