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It is an optical figure showing silicon nodule well-distributed through Al-matrix. It isn’t either composite or in-situ composite. It is a new cast light metal—primary-free hypereutectic Al-Si alloy with nodular eutectic silicon crystal. How can get it?
Nodular silicon crystal in Al-14Si alloy Optical. 500×
Over several decades the difficulty to nodulize the primary silicon crystal in Al-Si alloy puzzled metallurgist and foundrymen. Adding element or compound in molten alloy can’t spheroidize the faceted silicon crystal as done in nodular cast iron. High undercooling is capable of granulating the primary silicon. However, it is inapplicable. Heating at high temperature only makes silicon polyhedron round in some degree. The existing technology can’t solve the mystery. We are forced to go in other direction, i.e. depressing the precipitation of primary silicon in solidification.
The hypereutectic alloy is able to display different structure i.e. hypoeutectic, eutectic and hypereutectic structure depending upon alloying elements and undercooling in freezing. If hypereutectic alloy would exhibit eutectic structure, the difficulty to nodulize the coarse silicon would be over. In fact it is impossible to achieve completely eutectic structure in sand or large castings. This idea seems to be a fantasy. At the end of last century direct-electrolytic Al-Si alloy(DEASA) was successfully developed. New Al-Si alloy displays the eutectic structure in modified manner with high undercooling in ingot. Occasionally it was found that the hypereutectic alloy made from DEASA also remains high undercooling in freezing, thus providing an ability to depress the precipitation of primary silicon, forming primary-free quasi-eutectic structure in modified manner. In this case the quasi-eutectic silicon crystal displays fibrous shape, not nodular. How the modified eutectic structure can transforms into nodule is a new problem. Heating is an effective and simple way often used in industry. Experimental demonstrated that a high temperature near eutectic point is required. Fortunately the quasi-eutectic structure is an undercooled structure. At lower temperature of 510℃ the unstable fibrous phase successfully transforms into nodular shape. A new hypereutectic Al-Si alloy with completely nodular eutectic silicon was born. It would represent a superior potential to challenge the increasing demand for lightweight all in automobile components in near future and a new way of producing new alloy offering high strength, toughness and wear-resistance.
Wang Ruyao, Professor, Institute of Materials Science and Engineering, Donghua University, Shanghai, China
Lu Weihua, Associate Professor, Institute of Materials Science and Engineering, Donghua University, Shanghai, China
A paper about the study appeared in International Journal of Materials Science and Application. Title of article is “Hypereutectic Al-Si alloy with completely nodular eutectic silicon: microstructure and process.”