A Study of the Behaviour of Double Oxide Films in Al Alloy Melts

Abstract:

The mechanical properties of Al castings are reduced by inclusions, particularly double oxide films, or bifilms, which are formed due to surface turbulence of the liquid metal during handling and/or pouring. These defects have been reported not only to decrease the tensile and fatigue properties of Al alloy castings, but also to increase their scatter. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. In order to follow changes in the composition of the internal atmosphere of a double oxide film defect within an Al melt, a series of analogue experiments were carried out to determine the changes in gas composition of an air bubble trapped in a melt of commercial purity Al, subjected to stirring. The bubble contents were analysed using a mass spectrometer to determine their change in composition with time. Also, the solid species inside the bubbles solidified in the melt were analysed. The results suggested that first oxygen and then nitrogen inside the bubble were consumed, with consumption rates of 2.5x10^-6 and 1.3x10^-6 mol m^-2s^-1, respectively. Also, hydrogen diffused into the bubble from the melt at an average rate of 3.4x10^-7 mol m^-2s^-1, although the rate of H diffusion increased significantly after the consumption of most of the oxygen inside the bubble. Based upon these reaction rates the time required for a typical alumina bifilm to lose all its oxygen and nitrogen was determined to be just under 10 minutes.

• Periodical: Materials Science Forum (Volume 765)
• Pages: 260-265
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.765.260
• Online since: July 2013
• Authors: Mahmoud El-Sayed, Hanadi G. Salem, Abdel-Razik Kandeil, William D. Griffiths
• Keywords: Aluminium Castings, Hydrogen Diffusion, Oxide Films

References

[1] J. Campbell, Castings, second ed., Butterworth-Heinemann, 2003.

[2] J. Campbell, Entrainment defects, Mater. Sci. Technol. 22 (2006) 127-145.

[3] N. Green, J. Campbell, Influence of oxide film filling defects on the strength of Al-7Si-Mg alloy castings, AFS Trans. 102 (1994) 341-347.

[4] C. Nyahumwa, N.R. Green, J. Campbell, The concept of the fatigue potential of cast alloys, J. Mech. Behavior Mater. 9 (1998) 227-235.

[5] C. Nyahumwa, N.R. Green, J. Campbell, Effect of mold-filling turbulence on fatigue properties of cast aluminum alloys, AFS Trans. 106 (1998) 215-223.

[6] R. Raiszadeh, W.D. Griffiths, A method to study the history of a double oxide film defect in liquid aluminum alloys, Metall. Mater. Trans. B 37 (2006) 865-871.

DOI: 10.1007/bf02735007

[7] R. Raiszadeh, W.D. Griffiths, A semi-empirical mathematical model to estimate the duration of the atmosphere within a double oxide film defect in pure aluminum alloy, Metall. Mater. Trans. B 39 (2008) 298-303.

DOI: 10.1007/s11663-008-9142-x

[8] M. El-Sayed, H. Salem, A. Kandeil, W.D. Griffiths, Oxide film defects in Al alloy castings, in: 22nd Int. Conf. Computer-Aided Production Engineering, Edinburgh, Scotland. 2011.

[9] M.A. El-Sayed, H.A.G. Salem, A.Y. Kandeil, W.D. Griffiths, Effect of holding time before solidification on double-oxide film defects and mechanical properties of aluminum alloys, Metall. Mater. Trans. B 42 (2011) 1104-1109.

DOI: 10.1007/s11663-011-9571-9

[10] M.W. Chase, JANAF Thermochemical tables, in: Journal of Physical and Chemical Reference Data, Michigan: American Chemical Society/American Institute of Physics for the National Bureau of Standards, Vol. 14, 1985.