Abstract
Liquid Al and Mg-base alloys are so reactive that it is reasonable to assume that the surface layer is always oxidized. If liquid aluminium entered a mould cavity with a velocity greater than a critical value, the surface skin of the liquid metal would fold over onto itself and be submerged into the bulk liquid with a volume of air entrapped within it, creating what is called a bifilm defect. This defect not only acts as a crack but also it is recognized to initiate hydrogen porosity in the solidified casting, which has been found to have detrimental effects on the tensile and fatigue properties of the castings produced. Previous research suggested that during solidification, the hydrogen, in excess of the solubility limit, comes out of the solution and diffuses into the bifilm gap, expanding it into a pore. Also, placing liquid metal in a vacuum may cause its entrained bifilms to expand, enhancing their buoyancy and therefore their floatation to the surface of the melt. In this work, a casting from an A356 Al alloy was allowed to solidify under vacuum. The solidified casting was sectioned into two halves, and the internal surfaces of the pores were investigated using an SEM to determine their relationship with double oxide film defects.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Campbell J (2003) Castings, 2nd edn. Butterworth-Heinemann, London
Campbell J (2006) Entrainment defects. Mater Sci Technol Ser 22:127–145
Dispinar D, Campbell J (2004) Critical assessment of reduced pressure test. Part 1: porosity phenomena. Int J Cast Metal Res 17:280–286
Griffiths WD, Raiszadeh R (2009) Hydrogen, porosity and oxide film defects in liquid Al. J Mater Sci 44(13):3402–3407
El-Sayed MA, Salem H, Kandeil A, Griffiths WD (2013) A study of the behaviour of double oxide films in Al alloy melts. Mater Sci Forum 765:260–265
El-Sayed MA, Salem H, Kandeil A, Griffiths WD (2014) Determination of the lifetime of a double-oxide film in Al castings. Metall Mater Trans B 45(4):1398–1406
El-Sayed MA, Salem H, Kandeil A, Griffiths WD (2011) Effect of holding time before solidification on double-oxide film defects and mechanical properties of aluminum alloys. Metall Mater Trans B 42(6):1104–1109
El-Sayed MA, Griffiths WD (2014) Hydrogen, bifilms and mechanical properties of Al castings. Int J Cast Metal Res 27(5):282–287
Fox S, Campbel J (2000) Visualisation of oxide film defects during solidification of aluminium alloys. Scr Mater 43:881–886
Raiszadeh R, Griffiths WD (2010) The behaviour of double oxide film defects in liquid Al alloys under atmospheric and reduced pressures. J Alloy Compd 491(1-2):575–580
El-Sayed MA, Hassanin H, Essa K (2015) Effect of casting practice on the reliability of Al cast alloys. Int J Cast Metal Res
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
About this article
Cite this article
El-Sayed, M.A., Hassanin, H. & Essa, K. Bifilm defects and porosity in Al cast alloys. Int J Adv Manuf Technol 86, 1173–1179 (2016). https://doi.org/10.1007/s00170-015-8240-6
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00170-015-8240-6