Influences of Super-Gravity Field on Aluminum Grain Refining
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n that grain refining during the casting process is one of the effective methods of improving the material mechanical properties, such as hardness, toughness, fatigue resistance, etc. Much research has focused on grain refining, and the methods used can be classified into three categories: the addition of grain refiner, the increase of cooling rate, and the application of external force.[1] Some literature also defined the latter two methods as the change of solidification conditions. In general, external forces include agitation, mechanical or ultrasonic vibration, mold rotation, rotary magnetic field, and so on. Mold rotation, or rather centrifugal casting, actually is a solidification process under the super-gravity field. There are many studies regarding grain-refining mechanisms of vibration and agitation, but only a few focus on the super-gravity field. It is proposed that mechanical or electromagnetic agitation can cause a turbulent flow in molten metal, which results in the rupture of the dendrite at the growth interface. The fragments from the dendrite become nucleation particles and enhance crystal multiplication. The mechanical or ultrasonic vibration only refines the grains by generating cavitations and LIXIN ZHAO, Doctoral Student, is with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, and the China Graduate University of Chinese Academy of Sciences, Beijing, P.R. China, 100049. ZHANCHENG GUO, Professor, is with the Key Laboratory of Ecologic and Recycle Metallurgy, University of Science and Technology Beijing, Beijing, P.R. China, 100083. Contact e-mail: [email protected] ZHI WANG, Vice Professor, and MINGYONG WANG, Doctoral Student, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, P.R. China, 100190. Manuscript submitted July 9, 2009. Article published online January 13, 2010 670—VOLUME 41A, MARCH 2010
negative pressure, undercooling local liquid, and developing nucleation.[2] Grain refining under the rotary magnetic field is attributed partially to the super-gravity generated by the field.[3] However, for mold rotation, or for centrifugal casting, its grain-refining mechanism has not been discussed in detail, though it has been widely used in foundry industries, especially for the regular cylindrical as-cast, such as steel tube, wheel hub, and automobile cylinder, due to the advantages of refined equiaxed grains and uniform and dense structure. Among the studies about centrifugal casting, most of them have been focused on the influences of cooling rate, grain refiner, pouring temperature, and other factors, but few on the influence of different rotating speeds (or different super-gravity fields). Yeh and Jong[4] have investigated the effect of different rotating speeds on the grain refining of the aluminum alloy. Their results indicated that the grain size in the equiaxed zone decreased with the incre
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