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A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Masters of Science in Engineering. University of the Witwatersrand, Johannesburg, 2016 / With the current tough economic times and depressed commodity prices, focus is being placed on achieving higher power densities in gears without increased cost or reduced achievable life and reliability. An investigation into the use of different carburisation methods and processes to minimise post heat treatment distortion, with the aim to reduce and even eliminate the requirement for post heat treatment grinding and grinding stock allowances is presented. The investigation included the processing of test pieces, as per the recommendations of AGMA 2004-B89: Gear Materials, Heat Treatment and Processing Manual. Four alloy steels, namely AISI 3310, AISI 8620, AISI 9310 and 17CrNiMo6, were selected for this research, based on availability, cost and hardenability. The carburising cycles were derived for both gas and vacuum carburising for a 0.10% and 0.20% carbon steel respectively. Two quenching options were applied, oil quenching and intensive quenching. The test pieces were 3-dimensionally measured to determine distortion through the changes in diameter and ovality. The microstructures of the case and core were analysed, as well as effective case depth. This research found that current methods, atmospheric carburising and oil quenching, and steel alloy combinations are inadequate to produce low distortion carburised internal gears, while the use of vacuum carburising and intensive quenching as a process combination can achieve such gears. It was also found that the use of AISI 3310, performed the best for both current and proposed process combinations. It is recommended that future work be conducted, including a full-scaled manufacturing trial with more detailed process adjustments to ensure the quality and repeatability of the final carburised gear. / MT2017

Aluminum-silicon-copper cast alloys of the 319-type have attained a commercially important status because of their widespread use. Artificial aging treatments are routinely applied to these alloys in order to obtain precipitation hardening and improve their mechanical properties. Standard treatments may not always yield the optimum achievable properties, thus Mg and Sr are commonly added to improve the response of the alloy to aging and to modify the eutectic Si morphology from acicular to fibrous, respectively. The present study was carried out to investigate aging behavior of four 319-type alloys in regard to such mechanical properties as their ultimate tensile strength, yield strength, microhardness, percent elongation and impact toughness. Non-conventional aging cycles were applied so as to evaluate the degree of the improvement in strength obtainable. These treatments, labeled in this study as T6- and T7-type multi-temperature and interrupted aging treatments, involve several heating stages at different temperatures, as opposed to the single stage at constant temperature specifications of the standard T6 or T7 heat treatment regimes. Scanning electron microscopy was used to examine the fracture surfaces of selected tensile-tested samples to compare the fracture behavior. Transmission electron microscopy was used to reveal and identify the tiny precipitates which appear in the microstructure as a result of the precipitation-hardening process due to artificial aging. It was found that the main strengthening phase is theta-Al2Cu in the form of needles; other phases were observed as minor constituents in this alloy, including the binary beta-Mg2Si, the ternary S-CuAlMg 2 and the quaternary Q-Al5Cu2Mg7Si 7. The results show that while Mg and Sr additions improve the properties of the alloy, the standard T6 treatment may not be the best available option to produce optimum properties. In fact, when the peak-aged (T6) condition is desired, the optimum treatment consists of a continuous artificial treatment at 170°C for 8 h; when the overaged (T7) condition is desired, a T7-type multi-temperature treatment consisting of underaging at 170°C for 1 h, then at 190°C for 1 h, and finally overaging at 240°C for 2 h is the best option.

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Published Article / The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for traditional dendritic alloys. These heat treatments are not necessarily the optimum heat treatments for SSM processing. The T6 heat treatment of aluminium alloys is a relatively expensive process and a reduction in treatment times would be advantageous. In order to improve the T6 heat treatment cycle for rheocast alloy A356, the effect of various parameters on the quality index were investigated. These included solution treatment time, natural aging time, artificial aging temperature and artificial aging time.

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