Numerical calculation of the electromagnetic expulsive force upon nonmetallic inclusions in an aluminum melt: Part I. Sp
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I. INTRODUCTION
THE presence of nonmetallic inclusions in aluminum has an adverse effect on the reliability and performance of the end product and may lead to increased porosity, a drastic reduction in mechanical properties, poor machinability, loss of fluidity, and lack of pressure tightness.[1–3] With the increasing trends toward high quality, down-gaging, and recycling of aluminum products, the demand for improved melt cleanliness levels continues to grow. As a result, the elimination of inclusions is central to both the primary and secondary aluminum industries. It is believed that inclusions down to 5 mm in size[4] and of several parts per million in volume[5] are harmful to premium products. Although a number of melt-refining methods for the removal of inclusions prior to casting exist, efficient and consistent removal of small inclusions is very difficult to attain.[6] Based on the basic separation mechanisms, gravity sedimentation may be effective for inclusions larger than 90 to 100 mm. The flotation technique may be effective for inclusions of up to about 30 to 40 mm.[7] Removal of inclusions of less than 30 mm may be accomplished by filtration using ceramic foam filters,[5] but with limited and controversial filtration efficiency.[1,8] Electromagnetic separation can, by comparison, remove even micrometer-sized inclusions, in principle, by using high-intensity force fields at an almost constant rate.[9,10] The concept of this innovative method is based on the following phenomenon: when a nonconductive particle is immersed in an enclosed conductive fluid traversed by a homogeneous electric current and a homogeneous magnetic field perpendicular to that current, it will experience a resultant expulsive DA SHU and TIAN-XIAO LI, Doctoral Candidates, BAO-DE SUN and YAO-HE ZHOU, Professors, and JUN WANG and ZHEN-MING XU, Associate Professors, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, People’s Republic of China. Manuscript submitted February 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
force in the opposite direction of the electromagnetic force exerted on the fluid itself, partly due to the quasi-buoyant surface force and partly to a hydrodynamic force arising from the liquid streaming in its surrounding area, owing to the distortion of the electromagnetic field.[11,12] Since electromagnetic separation makes use of the difference in electrical conductivity between the inclusion particle and the melt, rather than the density difference, it is particularly appropriate for the case of molten aluminum, in which inclusions suspended are very similar to the melt in density.[13,14] While substantial research have been done on the different types of electromagnetic separators[9,15–19] and the effects of various operating parameters on the separation efficiency,[20] which clearly demonstrates the potential and effectiveness of electromagnetic separation, fundamental work concerning the origin of the electromagnetic expulsive force remains scarce. In
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