The main aim of this work is to study the bonding behaviour of Al alloys as a function of temperature, time and processing conditions from a perspective of oxidation so as to gain a comprehensive knowledge of the oxidation issues during processing and to design better bonding approaches for different alloy systems. Two major parts of work have been carried out during the study: (a) the use of a stacking approach and a double pouring approach to potentially join two Al alloys and their effectiveness are assessed; and (b) a precise investigation of the oxidation mechanisms for Al-Cu and Al-Mg alloys using a combination of theoretical analysis and experimental characterization. The project started with the use of a stacking approach to try to bond two stacked Al alloys. The stacked sample can be viewed as a bi-metal which has an oxide bi-film layer at the bond interface. It was found that the bi-film layer was a physical barrier preventing direct metallic bonding. How this bi-film layer evolves during the bonding process was then investigated. The work then moved on to investigate the oxidation mechanisms of Al-Cu alloys and Al-Mg alloys in greater detail. The thermodynamics, kinetics of oxidation, chemistry and morphology of the oxide scale were particularly studied. Briefly speaking, in the case of the Al-Cu-O<sub>2</sub> system, the oxidation proceeds in the order of amorphous γ-Al<sub>2</sub>O<sub>3</sub> - to - crystalline γ-Al<sub>2</sub>O<sub>3</sub> - to - α-Al<sub>2</sub>O<sub>3</sub>; in the case of the Al-Mg-O<sub>2</sub> system, the oxidation proceeds as amorphous γ-Al<sub>2</sub>O<sub>3</sub> to MgO to MgAl<sub>2</sub>O<sub>4</sub> and the morphology of the oxide scale develops from a protective layer to a porously structured composite layer. A double pouring approach was finally developed to bond Al and Al-5Cu but the method has still not been perfected due to the formation of bi-film defects along the bond interface. Induction melting, squeeze casting, and extrusion bonding were therefore studied as an attempt to reduce the harmful effect of bi-film defects. Although all of the approaches exhibit some limitations, they have potential for future development.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:627827 |
| Date | January 2014 |
| Creators | Wu, Guo |
| Contributors | O‘Reilly, Keyna; Galano, Marina |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:e8d98c0a-f8a8-4ea5-9906-94e7be782f7b |
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