Microstructure and Mechanical Properties of Al-8 pct Si Alloy Prepared by Direct Chill Casting Under Electromagnetic and
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grain refinement is greatly useful for Al-Si alloy castings, because benefits can be obtained in various ways, such as enhancement of mechanical properties, reduction of hot tearing, a better elimination of shrinkage porosity, and a more uniform distribution of second phases.[1] For this reason, grain refinement of Al-Si alloys is always one of the most important subjects in metallurgy. Three different approaches are commonly used to achieve grain refinement: rapid solidification, chemical inoculation, and physical field.[1–3] Fast cooling rate always causes a fine microstructure, but it is hard for industrial application due to its high costs, while addition of grain refiners, such as Al-Ti-B, Al-Ti-C, etc.,[4,5] is a favorable method to get a fine-grained structure. Application of physical fields is also widely used to refine solidification microstructure by inducing fluid flow, for instance, electromagnetic (EM) field,[6–8] mechanical agitation, ultrasonic (US) vibration,[9–12] etc. Among these methods, many investigations have demonstrated that the EM field and US field have a YUBO ZHANG, LI WU, and YING FU, Ph.D. Students, JINCHUAN JIE, Postdoctoral, YIPING LU, Associate Professor, and TINGJU LI, Professor, are with the Laboratory of Special Processing of Raw Materials and School of Material Science and Engineering, Dalian University of Technology, Dalian 116024, Liaoning, P.R. China. Contact e-mail: [email protected] MU LI was formerly a College Student with the Laboratory of Special Processing of Raw Materials and School of Material Science and Engineering, Dalian University of Technology, is now Master Student with the Tokyo Institute of Technology, Tokyo 152-8550, Japan. Manuscript submitted June 6, 2013. Article published online November 16, 2013 2014—VOLUME 45A, APRIL 2014
pronounced influence on the grain refinement of Al-Si alloys, and the individual application of the EM or US field has been realized successfully during semi-continuous casting of Al-based alloys.[13–15] The ultrasonic treatment is beneficial to stimulate nucleation and fracture the dendrites,[16] leading to a fine and equiaxed grain structure. However, the ultrasonic technology is limited by the relatively small treated region. Hence, a wider area of ultrasonic refined effect is required for a better application. The EM field can induce forced convection in the whole liquid, which gives rise to thermal and solute transfer.[6,7] It is considered that such forced convection can enlarge the treated region of the US field by transferring the nuclei and fragments around. For this reason, the application of compound fields including the EM and US fields is supposed to have better grain refinement and microstructural modification for large size castings. Direct chill (DC) casting is a vertical semi-continuous casting process for fabricating cylindrical or rectangular billets, and is widely used in the production of Al-, Cu-, and Mg-based alloys. Due to easy control and efficient production, more than half of aluminum alloys in the world are produced by DC casting. However,
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