Microstructure Evolution of a Semisolid Magnesium Alloy Slurry Obtained via an Internal Rapid Cooling Stirring Process (
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MAGNESIUM alloys are some of the lightest structural materials and have been widely used in vehicle manufacturing, electronic communications, aerospace, and other fields. However, their poor ductility and insufficient room-temperature strength limit their applications.[1] Lightweight alloys are often formed as semi-solid metals to reduce casting defects and because their non-dendritic structure improves the quality of wrought alloys.[2] Semi-solid forming can be categorized as either thixoforming or rheoforming. Rheoforming transforms liquid metal into a semi-solid slurry with a low solid fraction, which is then formed in molds under pressure. Due to its rapid process, low forming pressures, and the formation of complex parts, rheoforming is a promising casting process.[3] The key to
XIANGJIE YANG, ANSHAN YU, and BIN ZHENG are with the School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, P.R. China and also with the Key Laboratory of Near Net Forming in Jiangxi Province, Nanchang 330031, P.R. China. HONGMIN GUO is with the Key Laboratory of Near Net Forming in Jiangxi Province and also with the School of Material Science and Engineering, Nanchang University, Nanchang 330031, P.R. China Contact e-mail: [email protected] Manuscript submitted December 22, 2019.
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successful rheoforming is the preparation of a semi-solid slurry with a fine-grain microstructure in the liquid phase.[4] Recently, many techniques have been developed to prepare semi-solid slurries with low solid fractions. One method is to inhibit dendrite growth using an induced field, such as through mechanical stirring,[5] electromagnetic stirring,[6] or ultrasonic vibration.[7] Another method is to control nucleation using techniques such as low superheat casting[8] and the addition of grain refiners.[9] However, the production volume of semisolid slurries cannot be used to form large components, and discontinuities and the high cost of preparing semi-solid slurries limit its application. The SSR[10] method relies on low superheating and precise high-temperature control, and the RSF[11] method requires replacing the EEM block and discontinuities. Internal rapid cooling stirring process (IRCSP) is based on the heat balance principle and provides a simple method to obtain non-dendritic slurries and quickly homogenize alloy melts without needing to control the temperature. A slurry with a low solid-phase fraction with many free crystals can be obtained through internal rapid chilling and rapid stirring. Factors such as the solid fraction, solid-phase morphology, and solid-phase distribution play vital roles during semi-solid processing because they determine the rheological properties and quality of obtained semi-solid alloys.[12–14] Semi-solid slurry particles often agglomerate and uniformly disperse under high shear flow[15] or ultrasonication,[16] and well-dispersed spherical particles
show good rheological properties and excellent product quality. Therefore, to understand
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