Microstructure and Thermal Stability of A357 Alloy With and Without the Addition of Zr
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JMEPEG DOI: 10.1007/s11665-017-2921-2
Microstructure and Thermal Stability of A357 Alloy With and Without the Addition of Zr Yu-Chih Tzeng, Vun-Shing Chengn, Jo-Kuang Nieh, Hui-Yun Bor, and Sheng-Long Lee (Submitted January 2, 2017; in revised form July 27, 2017) The principal purpose of this research was to evaluate the effects of Zr on the microstructure and thermal stability of an A357 alloy that has been subjected to an aging treatment (T6) and thermal exposure (250 °C). The results show that the addition of Zr had a significant influence on the refinement of the grain size, which enhanced the hardness and tensile strength of the A357 alloy under the T6 condition. During thermal exposure at 250 °C, the rodlike metastable bÕ-Mg2Si precipitates transformed into coarse equilibrium phase b-Mg2Si precipitates, resulting in a significant drop in the hardness and tensile strength of the T6 heat-treated A357 alloy. However, after thermal exposure, coherent, finely dispersed Al3Zr precipitates were found to be formed in the T6 heat-treated A357 alloy. The addition of 0.1% Zr played a critical role in improving the high-temperature strength. Consequently, the A357 alloy with the addition of Zr demonstrated better mechanical properties at room temperature and high temperature than the alloy without Zr, in terms of both microstructure and thermal stability. Keywords
aluminum, heat treatment, microscopy, optical metallography
1. Introduction The aluminum casting alloy A357 is a hypoeutectic Al-Si alloy that has been widely used for the fabrication of weightsensitive components in industry because of its attractive comprehensive properties, such as castability, weldability, excellent specific strength and corrosion resistance (Ref 1). Iron is a common impurity in Al-Si alloys, where it readily forms intermetallic compounds with Al, eutectic Si and other elements (Ref 2). Under normal casting conditions, the formation of the acicular b-Al5FeSi phase has deleterious effects on the ductility and toughness of the alloy. It is well accepted that the addition of Mn, Cr or Be can neutralize the embrittling effect of b-Al5FeSi, by modifying the acicular shape into a less harmful, more compact form (Ref 3, 4). However, the strength of the A357 alloys is only effectively preserved at temperatures below 250 °C (Ref 5) because, above that temperature, the coarsening of the major strengthening bÕÕMg2Si and bÕ-Mg2Si precipitates becomes unstable. They coarsen rapidly and then dissolve, causing a decrease in the performance of the alloy. This results in a reduction in the mechanical properties and limits high-temperature applications
Yu-Chih Tzeng, Department of Power Vehicle and Systems Engineering, CCIT, National Defense University, Taoyuan, Taiwan; Vun-Shing Chengn, Optometry, College of Health Sciences, Central Taiwan University of Science and Technology, Taichung, Taiwan; Jo-Kuang Nieh and Hui-Yun Bor, Materials and Electro-Optics Research Division, National Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan; and Sheng-Long
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