Understanding the role of oxidation in bonding of aluminium alloys

Thesis

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-O₂ system, the oxidation proceeds in the order of amorphous γ-Al₂O₃ - to - crystalline γ-Al₂O₃ - to - α-Al₂O₃; in the case of the Al-Mg-O₂ system, the oxidation proceeds as amorphous γ-Al₂O₃ to MgO to MgAl₂O₄ 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.

Authors: Wu, G

• Publication date: 2014
• DOI: 10.5287/ora-bd6exexyn
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: oxidation
• Subjects: Materials Sciences