3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Extrudability of particle-reinforced aluminium metal matrix composites at warm working temperatures (0.3-0.5 Tm)(2018) Gxowa, ZizoThis work evaluates the warm temperature extrudability of aluminium Metal Matrix Composites (MMCs) and Metal Matrix Nano Composites (MMNCs) produced by powder metallurgy. Green and sintered compacts were produced by blending 2124-Al with Al2O3 (5 or 10 vol. %) or SiC (10 or 15 vol. %) powders in a high energy ball mill, cold i.e. ambient temperature compaction and sintering at 490°C for 1 hr. The deformation behaviour of unreinforced 2124-Al, MMC and MMNC green and sintered compacts was studied by performing uniaxial compression tests using a Gleeble 3500®, within the warm working temperature range (170 - 280°C). Strain rates of 0.01 and 5 s-1 were used and the total strain of 0.3 was kept constant. The Abaqus Finite Element modelling (FEM) programme was used to simulate an extrusion process using the results from the uniaxial compression tests as input data. The uniaxial compression test results and the FEM analysis were used to design a warm temperature extrusion process. These results were then validated by performing a laboratory scale extrusion experiment. A more uniform distribution of reinforcing particles in the 2124-Al alloy matrix was achieved in the Al2O3 reinforced MMNCs than SiC reinforced MMCs. Cold compaction of the 2124Al with 10 vol. % Al2O3 powder was unsuccessful as green compacts pressed from this powder fractured. This fracturing was attributed to poor bonding and plastic flow due to the higher density of Al2O3 particles on the surface of the 2124-Al powder. Alternate consolidation techniques, such as spark plasma sintering (SPS), were recommended for the 10 vol. % Al2O3 powder. Deformation behaviour improved significantly when sintered MMC compacts were uniaxially compressed at 280°C, a strain rate of 5 s-1 and a soaking time of 20 minutes. The best deformation, i.e. good ductility which is shown by a large plastic region and high flow stress, was achieved in the 2124-Al with 10 vol. % SiC MMC, as it plastically deformed at the highest stress (~153 MPa) up to the maximum strain of 0.3. Extrudability of the unreinforced 2124-Al was good, while the 5 vol. % Al2O3 reinforced MMNC and SiC reinforced MMCs had poorer warm temperature extrudability, which was attributed to a lack of lubrication during extrusion. The MMCs and MMNC were more difficult to extrude than the unreinforced 2124-Al alloy because they have a higher resistance to deformation as a result of the harder, stiffer reinforcing particles which do not deform easily. The lack of lubrication could have made deformation of MMCs and MMNCs more difficult due to higher friction (increased resistance to deformation) and reduced material flow. The desired good distribution of Al2O3 in 2124-Al achieved in blending was not maintained by cold compaction, uniaxial compression and extrusion. This indicated that an alternate processing route is required for the Al2O3 reinforced MMNCs. Distribution of SiC particles in 2124-Al with 10 vol. % SiC improved slightly due to uniaxial compression. It was observed that in some areas, SiC particles were reasonably dispersed inside slightly deformed 2124-Al grains; illustrating that deformation influenced the distribution of SiC particles in the aluminium alloy matrix. Analysis of small extruded portions of SiC reinforced MMCs showed that extrusion has the potential to improve distribution of SiC particles in 2124-Al grains. However, higher deformation is required to optimise the SiC distribution.Item Evaluating the wear and corrosion resistance of plasma-sprayed tungsten carbide coatings on aluminium-6082 alloy(2017) MacGregor, Oluwadamilola SolomonThe benefits of Al alloys to industry are significant. For the truck loading application in this study, the use of Al can deliver a greater payload than most other metals, due to its low relative density. However, it has poor tribological properties. This study investigated ways to improve the wear resistance of AA6082, an Al alloy widely used in transport. This was done by studying WC-based coatings to improve the wear resistance of the AA6082 surface. To ensure a sufficient WC-substrate bond, Al powder was used as a binder for the coatings. Although these coatings improved the wear resistance of the AA6082, it was imperative to test and establish that their corrosion resistance was not inferior to that of the AA6082 itself. Differences in hardness and tensile properties of the substrate were evaluated for varying ageing durations. The plasma spray technique was used to coat the AA6082 substrate. The heat input from this coating process on an Al alloys with high thermal conductivity could have lowered the hardness and mechanical properties of the AA6082 substrate. Therefore, hardness tests were carried out on the cross-section of each coated substrate. The hardnesses of all coated AA6082 samples were not lowered by the heat input from the plasma spray process. The coatings were varied to contain 20%, 40%, 60%, 80% and 100% volume of WC admixed with Al-102 powder. The wear resistance of AA6082 was significantly improved as WC content increased. From the wear resistance results, both 60% and 80% WC showed the highest wear resistance. The 60% WC coating, which contained a lower quantity of WC than the 80% WC coating, was the preferred option for truck loading applications because of cost. The 60% WC coating had a lower hardness value than the 80% WC coating, giving the 60% WC coating a higher allowance for work-hardening in service as ductile Al-102 is present in higher quantity. Al and its alloys, with no coating, are known to have good corrosion resistance. It was therefore imperative to evaluate and compare the corrosion resistance of the coated samples with AA6082. The lowest corrosion rate of all coated samples exposed to the neutral and acidic media was the 20% WC coating. Generally, all the coated samples had very low corrosion rates in the neutral solution, which is more applicable to typical truck loading conditions. Therefore, the 60% WC coating was most preferred for wear and hardness tests and also had acceptable corrosion behaviour for the proposed truck loading application.Item The design and development of a light-weight aluminium semi-trailer(1991) Elston, Malcolm John Mac GregorWith increasing competition in the commercial vehicle industry, future vehicle designs need to be able to carry greater payloads within the bounds of existing legislation. Increasing the payload efficiency of transport vehicles involves two main areas: a. Optomizing the size and mass distribution of the vehicle for the purpose intended. b. Keeping the tare mass as low as possible through efficient design and the use of lighter and stronger materials. [Abbreviated Abstract. Open document to view full version]