A method to study the history of a double oxide film defect in liquid aluminum alloys
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ALUMINUM alloy castings are commonplace in safety critical applications in the automotive and aerospace industries. It is therefore imperative that internal defects and potential failure mechanisms are fully understood if reliable castings are to be produced consistently. Campbell[1] described the concept of an entrained double oxide film and its deleterious effects on the properties of an aluminum casting. Every time the surface of the metal folds upon itself, e.g., by the action of a breaking wave, the surface oxide film becomes entrained in the bulk liquid. This occurs as a doubled-over oxide film in which the internal surfaces are not bonded, but have a layer of air trapped between the two surfaces (Figure 1) that leads to a crack in the solidified casting. Most reactive liquid metals such as Al and Mg probably contain such defects because of surface disturbance during the transfer and pouring of the melt. The oxide film defects can become convoluted and compact because of internal turbulence in the liquid metal during casting. It has also been proposed that they can open and expand due to a variety of driving forces, such as the precipitation of hydrogen into the atmosphere of the double oxide film defect, and the shrinkage of the liquid metal during solidification.[1] Green and Campbell[2] showed that a bad running system design introduced oxide film defects and reduced the reproducibility of the properties of aluminum alloy castings. Examples of the tangled networks of oxide films found in their castings (demonstrated by energy-dispersive X-ray R. RAISZADEH, formerly Ph.D. Student, with the Department of Metallurgy and Materials Science, School of Engineering, University of Birmingham, Edgbaston, Birmingham, United Kingdom B15 2TT, is Assistant Professor, with the Department of Materials Science and Engineering, Shahid Bahonar University of Kerman, Kerman, Iran. Contact e-mail: [email protected] W.D. GRIFFITHS, Lecturer, is with the Department of Metallurgy and Materials Science, School of Engineering, University of Birmingham. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA. METALLURGICAL AND MATERIALS TRANSACTIONS B
(EDX) analysis) are shown in Figure 2. Nyahumwa et al.[3,4] also showed the effect of entrained oxide film defects on the fatigue lives of Al-7Si-0.3Mg castings. It was suggested that, when a cast Al-7Si-0.3Mg alloy was subjected to a hot-isostatic pressing (hipping) treatment at a temperature close to its eutectic temperature, the applied pressure induced substantial plastic deformation in the casting, causing the double oxide film defects to collapse and their surfaces to be forced into contact. It is known that[3,4] after an incubation period, alumina can transform to magnesium aluminate spinel, MgAl2O4, and the volume change and atomic rearrangement of the crystal structure involved would be expected to encourage diffusion bonding across an oxide-oxide interface characte
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