Removal of Bifilms from Al Melts by Stirring in Unbaffled and Baffled Crucibles
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s may form in molten aluminum alloys by any liquid displacement that produces surface turbulence.[1] This defect, which consists of two oxide layers and a volume of air trapped between them, and is submerged in the liquid metal, is one of the most important factors, which reduces the mechanical properties and increases the variabilities in the properties of castings. The oxygen and nitrogen of the trapped atmosphere within the defect were found[2] to be consumed by the surrounding liquid Al in a couple of minutes or less[3] through the cracks that might form on the oxide layers during the movement of the defect in the liquid stream. Subsequently, the two dry sides of the defect come into contact with each other and become bound together,[4–7] and hence, the most deleterious effect of the defect (i.e., acting as a crack) is eliminated to some extent. Using a reduced pressure test (RPT) technique, Ahmadpour et al.[8] found that the mechanical stirring of the melt accelerated the removal of bifilms from an A356 liquid metal. Bartar-Esfahani et al.[9] also found
HAMED BAGHERPOUR-TORGHABEH, RAMIN RAISZADEH, and HAMID DOOSTMOHAMMADI are with the Department of Metallurgy and Materials Science, School of Engineering, Shahid Bahonar University of Kerman, JomhooriEslami Blvd., Kerman, Iran. Contact e-mails: [email protected], [email protected] Manuscript submitted February 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
the same effect when an Al-0.05 wt pct Sr melt was subjected to mechanical vibration. Bagherpour-Torghabeh et al.[10] realized that the mechanical stirring of Al melts containing 0.3, 0.7, and 4.5 wt pct Mg at 85 RPM had a significantly positive effect on accelerating the healing rate of bifilms in the liquid. The acceleration of healing by mechanical stirring was attributed to (1) an increase in the rate of formation of cracks on the oxide layers of the bifilms, which accelerated the consumptions of O and N of the trapped atmosphere and the diffusion of H into it, and (2) provision of enough Mg for the transformation of the alumina layers of the bifilms to spinel. The faster the consumption of gases within the atmosphere of the defect and the availability of sufficient amounts of Mg for the transformation, the more the opportunity for the two layers of the defect to become bound with each other.[4] The increased rate of diffusion of hydrogen into the trapped atmosphere also accelerated the expansion and removal of bifilms through the liquid surface.[10] The above-mentioned researchers found that the mechanical stirring of the melt with a relatively high speed of 220 RPM caused the oxide layer on the surface of the melt to be entrained into the bulk liquid metal as bifilms, and hence increased the porosity of the samples. One of the most simple and effective methods to suppress the formation of vortex and entrainment of air into the stirred tanks is the use of four flat vertical metal plates set with symmetry on the tank wall. These plates are called ‘‘baffles.’’[11,12]
Determination of the critical stirring speed
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