Effect of Holding Time Before Solidification on Double-Oxide Film Defects and Mechanical Properties of Aluminum Alloys
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f the most important casting defects affecting the reproducibility of mechanical properties of aluminum castings is the double oxide film defect, created because of surface turbulence of the liquid metal, common during metal transfer and pouring in the shape-casting process. When the liquid metal surface is exposed to air, a surface oxide film forms. As a result of surface disturbance, the liquid metal surface can be folded over onto itself, which causes the oxidized surfaces of the folded-over metal to come together but not to fuse. A layer of the local atmosphere is also trapped creating a double oxide film defect or ‘‘bifilm’’ which can be entrained into the bulk metal, as shown in Figure 1.[1,2] Such entrained double-oxide film defects represent one of the easiest possible initiating features for cracks because their unbonded inner surfaces can be separated with little force. Also, gas dissolved in the liquid metal can precipitate inside the bifilm, initiating porosity.[3] In addition, double-oxide films are favorable sites for the
MAHMOUD AHMED EL-SAYED, Assistant Lecturer, is with the Department of Industrial and Management Engineering, Arab Academy for Science and Technology and Maritime Transport, Abu Qir, Alexandria 21599, Egypt. HANADI A.G. SALEM, Professor, is with the Department of Mechanical Engineering, American University in Cairo, New Cairo 11833, Egypt. ABDELRAZEK YOUSSEF KANDEIL, Dean, is with the Arab Academy for Science and Technology. W.D. GRIFFITHS, Senior Lecturer, is with the School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact e-mail: W.D.Griffi[email protected] Manuscript submitted May 31, 2011. Article published online September 20, 2011. 1104—VOLUME 42B, DECEMBER 2011
nucleation and growth of intermetallic compounds. These effects not only reduce the elongation, tensile strength, and fatigue properties of aluminum alloy castings but also increase their variability. Nyahumwa et al.[4] suggested that because of the transformation of the oxide layer from c-Al2O3 to a-Al2O3 (a process thought to take several hours), cracks can be introduced into the oxide, which allows the liquid aluminum to come into contact with, and react with, the gas inside the oxide film defect (presumably mainly oxygen and nitrogen). This mechanism could result in the consumption of the atmosphere inside the bifilm and possibly contribute to its deactivation. The rate of consumption of the internal atmosphere was examined by Raiszadeh and Griffiths,[5] who trapped an air bubble inside an Al melt and monitored its change in volume with time using real-time X-ray radiography. Their results showed that the oxygen in the trapped air should be consumed first to form Al2O3, and then the nitrogen would react to form AlN. These reactions started immediately (with no need for an initiating phase transformation). Also, if the initial hydrogen content of the melt was higher than the equilibrium associated with the ambient atmosphere, the t
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