Spheroidization of Eutectic Silicon in Direct-Electrolytic Al-Si Alloy
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INTRODUCTION
AS is well known, the mechanical properties of Al-Si alloys are greatly controlled by the morphologies of Si aggregates. Fine nodular Si particles offer excellent ductility, higher strength, and impact.[1] Therefore, in past decades, many studies were carried out to improve Si morphologies. Three technologies, high cooling rate, impurity modification, and heat treatment, are often used. However, the high cooling rate is inapplicable in sand casting and for large castings. Adding modifiers such as Na, Sr, or others into molten alloy separately or in combination does not spheroidize eutectic Si particles under cast condition, as done in nodule graphite cast iron.[2] In general, the heat treatment produces rounded ends or spheroidal shapes in Si phase either in hypoeutectic[1,3–6] or near-eutectic Al-Si alloys.[7–13] The heat treatment consists of solutionizing at temperatures close to the eutectic temperature for extended period, quenching, and artificial aging. The solution temperature and duration are two important factors influencing the change in Si morphology. For Al-7Si-Mg alloy, the as-cast microstructure in modified manner is not altered significantly in casting heated at 793 K (520 C), even after 800 minutes.[5,6] The solutionizing temperature of 813 K to 823 K (540 C to 550 C) is necessary in foundries to spheroidize eutectic Si particles. Much research has demonstrated that as the Si level increases to 12 pct, a heating
RUYAO WANG, Professor, and WEIHUA LU, Associate Professor, are with the Department of Materials Science and Engineering, Donghua University, Shanghai 200051, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted July 17, 2012. Article published online January 24, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature much higher than 818 K (545 C) is required to achieve the Si sphere.[7,8,10,11,13] Note that when solution temperature is increased to 823 K (550 C), it becomes imperative that the temperature in heat-treated parts be preciously controlled to avoid grain boundary melting and large Si grains often are observed, strongly degrading the mechanical properties of castings. The high temperature required to gain complete spheroidization of Si phase limited the usage of near-eutectic Al-Si alloys with nodular Si in industry. It remains desirable to find a method that offers the advantage of Si spheroidization at a lower temperature of heating with a short holding time combined with excellent mechanical properties. At the beginning of this century, the authors reported that Si aggregates in eutectic Al-Si alloy, which is modified with complex additives of Ti, P, B, and mischmetal followed by heat treatment at 783 K to 793 K (510 C to 520 C) for 8 to 10 hours, are broken down into sphere, greatly improving the ductility and tensile strength.[14,15] However, rare earth elements are needed to modify alloy, restricting its application in industry. In our previous study,[16] it was found that 10-kg ingot of direct-electrolytic Al-Si alloy (D
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