Preparation of A357 Alloy Slurry by Pulsed Magnetic Field Processing
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TRODUCTION
SEMI-SOLID metal forming (SSF) was developed by Flemings and co-workers at MIT in the early 1970s.[1] Since then, the semi-solid processing (SSP), including thixoforming and rheocasting, has become a widely studied and accepted process. In recent years, the rheocasting has been paid more attention to during the preparation of semi-solid metals to save cost. Some novel slurry preparation methods, such as electromagnetic stirring, twin screw rheomolding, new rheocasting (NRC), continuous rheoconversion process, liquidus casting, and controlled nucleation method,[2–5] have been introduced. However, many researchers are still seeking new methods to prepare semi-solid slurry to decrease the production cost and simplify the process. The morphology of the primary phase of semi-solid metals plays a very important role in the quality control of SSF. The application of an electromagnetic vibration in the solidification of metals under continuous cooling during casting and other processes is considered an effective method to refine grains and improve the mechanical properties of the alloys, and more attention has been paid to it in recent years.[6–11] Pulsed magnetic field (PMF) processing as a new electromagnetic vibration technology has become one of the most promising new techniques to vibrate the liquid metal. Previous studies have investigated the refinement effect of PMF on the solidification structure. Ban et al. applied a PMF to solidification of 2124 Al alloy and found equiaxed fine grains instead of coarse dendrites when the alloy
was exposed to the magnetic field.[12] Gao et al. studied the structural transformation in pure Al under external PMF. The experiments showed that the morphology of grains was changed from dendrites to globular shapes with PMF.[13] Yang et al. applied PMF during the solidification of pure Mg, AZ31, AZ91D, and superalloy, and grain refinement was achieved with proper thermal control under the action of PMF.[14–17] Li et al. studied the effect of a PMF on the structural transformation of 1Cr18Ni9Ti austenitic stainless steel. The results show that the applications of a pulsed magnetic field can significantly refine columnar crystal spacing, reduce the solidification time, and lead to the increase of the initiation and finishing solidification temperature.[18] Here it should be pointed out that although the effects of PMF on the solidification of metals have been studied extensively, there are few studies on the preparation of semi-solid metal slurry. We therefore use PMF processing technology to prepare semi-solid slurry, because it has the following advantages: (1) the alloy melt would not be polluted because of the noncontact with the melt; (2) the pulse generator equipment is relatively simple; (3) the cost of this method is relatively low; and (4) the efficiency of this method is high. Therefore, semi-solid A357 alloy slurry was prepared by PMF processing. Furthermore, the effect of pouring temperature and vibration power on morphology and grain size of the primary a-Al particles in semi-solid A357 a
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