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Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Gall, Kenneth; Committee Member: McDowell, David; Committee Member: Thadhani, Naresh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Dissertation submitted in fulfilment of the requirements for the degree Master of Technology: Mechanical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology / Aluminium alloys has seen recent increase usage in the automotive industry. This is due to the global obligation towards carbon emission reduction and fuel efficiency in the transport sector. The good strength-to-weight ratio offered by Al-Si-Cu alloys showed promising results towards the compliance of these environmentally friendly criteria. The enhanced mechanical properties is obtained when the alloy is subjected to the T6 heat treatment process, which cause microstructural changes due to the evolution of intermetallic phases. The process involves solution heat treatment, for dissolving soluble Cu- and Mg-containing phases, the homogenization of alloying elements, and the spheroidisation of eutectic Silicon. It is followed by quenching, for maximum precipitation hardening particle retention in solution, and a further artificial ageing process with the aim to acquire a uniform distribution of small precipitates, for strength improvement. The heat treatment schedule applied in this study was conducted as follows: Solution heat treatment at a temperature of 525°C for 6h Quenching in water of temperature 50°C; Artificial ageing for 8h at a temperature of 175°C, and then after left inside furnace to cool down to room temperature. This is higher than the 520°C, but shorter than the 8-12h, observed in literature. Also, quenching is done at a lower temperature rather than 60°C, and artificial ageing at a higher temperature, rather than the 155°C. This was done to be able to draw a comparison between the MAGMASOFT® simulation, which has this non-adjustable schedule, and the experimental results. The simulated and experimental results were comparable and similar outcomes, but with some discrepancies. Such as the porosity was far more visible and intense in the experimental, than what was predicted by the software. The as-cast and heat treated microstructure revealed the expected evolution of intermetallic particles, such as dissolving of the Al2Cu and the spheroidisation of the eutectic Si phases. Another phase that was identified was the insoluble AlFeSi and other possible Fe-containing phases, which due to the higher solution heat treatment temperature, showed partial fragmentation and dissolution. The study provided practical data about the effect of heat treatment on microstructural evolution and how it affects the properties of the Al-Si-Cu alloy. It also brought to the attention and understanding of how critical pouring temperature is, as it affect the initial nucleation, and cooling rate, and therefore the micro and macro properties.

The heat-transfer phenomena in the quenching process have been studied using stainless steel and mild steel specimens in brine (3% by wt. NaCl), water, oil and air, under controlled conditions. The experimental data were analysed using a simple mathematical model of the quenching process to study the relationship between the surface temperature of the specimen and the surface heat-transfer coefficient. The influence on this relationship, of important variables such as initial specimen temperature, quenchant temperature, surface oxidation, etc. has been studied. The results from the experimental data are in good agreement with the results of previous workers. The results obtained in this work, together with Jominy-test data and the mathematical model, were used to determine the necessary quenching conditions required to obtain a desired thermal history or mechanical property at a given position in a 4 inch diameter steel grinding-ball. A steel ball was then quenched under the above determined conditions and an examination of the ball section showed that the desired property was indeed present at the given location. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate