Study of electromagnetic separation of nometallic inclusions from aluminum melt
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I. INTRODUCTION THE use of aluminum alloys has been expanding recently due to two large markets: automobile manufacturing and the aerospace industry. High-quality castings for stress-critical components and scrap recycling for the sake of sustainable development are receiving more and more attention all over the world. Inclusions, the most known defects, are detrimental to both the properties and the reliability of aluminum castings.[1,2,3] Therefore, their elimination is thought to be critical to premium castings as well as the reuse of scrap for quality products. A number of methods exist for the removal of inclusions from melts prior to casting, the basic mechanisms of which are based on one or more of the following principles: sedimentation, flotation, interception, or electromagnetic forces.[4] An effective unit for inclusion removal must separate inclusions down to the smallest sizes from the melt (5 mm or even less), as smaller inclusion particles are preferable to the larger ones in regard to pore nucleation.[5,6] Gravity sedimentation may be effective for inclusions larger than 90 to 100 mm. The flotation technique may be effective 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,[8] but with limited and controversial filtration efficiency.[1,9] Electromagnetic separation can, by comparison, remove even micrometer-sized inclusions in principle by using high-intensity force fields at almost constant rate.[10,11] Therefore, electromagnetic methods are thought to be a promising way to meet the current demand for higher metal cleanliness level. Although the concept of electromagnetic separation was proposed long ago,[12,13,14] and several methods by exerting different types of electromagnetic field, including DC electric field with steady magnetic field,[15] AC magnetic field,[16] D. SHU and T.X. LI, Graduate Students, B.D. SUN and J. WANG, Associate Professors, and Y.H. ZHOU, Professor, are with the School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, People’s Republic of China. Manuscript submitted September 24, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
and AC electric field,[17] were also presented, the effectiveness of each process has not been theoretically clarified yet. The present article is mainly concerned with the calculation of effects of various operating parameters on the removal efficiency of inclusion particles for each method. The migration rate of inclusion particles is also computed as well as compared with the gravity settling velocity. Based on the calculation results, a brief comparison is made in conclusion. II. SETTLING UNDER GRAVITY FIELD It is well known that holding in a furnace or tundish for a long time is beneficial to metal cleanliness. This phenomenon is attributed to the settling of inclusion particles due to their larger density than that of the melt. In this case, the ultimate settling velocity of inclusion particles can be determined by Stokes law,
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