The behaviour of double oxide film defects in the processing of liquid Mg alloys


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Li, Tian (2017). The behaviour of double oxide film defects in the processing of liquid Mg alloys. University of Birmingham. Ph.D.

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The global demand for Mg alloys continually grew in the last 20 years, motivating a wide interest in the improvement of the mechanical properties of Mg-alloy castings. In addition, the existence of double oxide film defects, which were widely recognised as a major factor in the quality and reproducibility of the properties of light-alloy castings, has been demonstrated in Mg-alloy castings. Thus it became important to understand behaviour of double oxide film defects formed in Mg-alloys.

In the work reported here, three different Mg alloys (commercial pure Mg, AZ91 alloy, and Mg-Y alloys) and two cover gases (SF6/air and SF6/CO2), were used, in order to involve different doubled oxide films which may have different behaviours. Direct and cross-sectional observations of the double oxide film defects formed the Mg-alloy castings protected by different cover gases were obtained via a Scanning Electron Microscopy (SEM), and the focus ion beam milling (FIB) technique. In addition, oxide films growing on the corresponding Mg-alloy melt surfaces were also investigated. Based on the observed film structures in conjunction with a thermochemical calculation, evolution processes of the different double oxide film defects were suggested.

The quality of Mg-alloy castings was evaluated by the Weibull modulus, which is popularly used to discriminate “good” and “bad” castings. A shortcoming of the traditional Weibull estimation method (i.e. linear least square method) was demonstrated, and a new estimation method was therefore come up with. The Weibull modulus result revealed that air can confer an improvement in the quality of AZ91 castings, compared with CO2.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Metallurgy and Materials
Funders: None/not applicable
Subjects: T Technology > TN Mining engineering. Metallurgy


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