Deep filtration of molten aluminum using ceramic foam filters and ceramic particles with active coatings
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The inclusions in molten aluminum were removed using the deep filtration of ceramic foam filters and ceramic particles with active coatings. The results of tensile tests showed that the elongation of the filtered tensile specimen S6 increases by 17.93 pct, but the tensile strength does not. The scanning electron microscope (SEM) examination showed that the secondary cracks and dimples in the filtered tensile specimen S6 were finer and more homogeneous than those in the unfiltered tensile specimen S0. In addition, metallographic examination showed that there were only a few inclusions of approximately 6 mm in diameter in the filtered specimen S6, but more inclusions of approximately 40 mm in diameter were found in the unfiltered specimen S0. The filtration efficiency of the coated ceramic particles was higher than that of the coated ceramic foam filters. The active coating could effectively capture the inclusions and dissolve Al2O3 in them during filtration. I. INTRODUCTION
MOLTEN aluminum is so active that it can easily chemically react with O2 and H2O to form Al2O3 inclusions and H during melting practice. However, some other inclusions, such as TiB2, Al4C3, MgAl2O4, etc., form by the electrolytic process. Under turbulent flow conditions, the inclusions are distributed unevenly in the molten aluminum. They may associate with cracks or be the locations of crack initiation in solidified casting. Aluminum products containing these inclusions will probably exhibit poor ductility or toughness. Therefore, the purification of the molten aluminum is one of the most important processes for improving the quality of aluminum products.[1,2] The inclusions of 30 mm in diameter are hardly removed from the molten aluminum using glass fibrous webs or foam ceramic filters. Fu et al.[3–7] purified industrial, commercial pure aluminum using a mixture of KCl-NaCl and small quantities of fluoride, carbonate, sulfate, and a fusing agent. During casting, this mixture forms a layer of molten flux. When the melted aluminum droplets pass through this molten flux layer, the inclusions on the surface of the large aluminum droplets can be removed, but the inclusions inside the large aluminum droplets may not be in contact with the molten fluxes. Besides, a thin film of liquid aluminum on the surface of the inclusions hinders chemical reaction between the inclusions and the molten fluxes, as is confirmed by the chemical-reaction kinetics, which predicts a low efficiency of inclusion removal in this case. For the deep filtration, the inclusions are hardly removed from the molten aluminum by using uncoated ceramic particles of large sizes, and uncoated fine ceramic particles are easily washed away by molten aluminum and may form new inclusions. However, the cake mode is the mechanism of capturing the inclusions by micropores of the ceramic M. ZHOU, D. SHU, K. LI, W.Y. ZHANG, H.J. NI, B.D. SUN, and J. WANG are with the School of Materials Science and Engineering, Shanghai University, Shanghai 200030, People’s Republic of China. Contact email: jd
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