Precise Analysis of Microstructural Effects on Mechanical Properties of Cast ADC12 Aluminum Alloy
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TRODUCTION
FOR environmental reasons, reductions in automotive exhaust gases, such as carbon dioxide and nitrogen oxides, are required. With this aim, automotive parts made of heavy steel have been replaced with lighter metals such as aluminum alloys. The Al-Si-Cu alloy family (ADC12) is widely employed in the automotive industry because of its good castability, high strength, and low density. In recent years, there has been increasing use of automotive parts made of cast Al alloy, a number of which are produced by high-pressure and high-speed casting, i.e., die casting. High-pressure die-casting technology has the advantage of providing good mechanical properties because of the high cooling rate used in the process.[1] However, die-cast ADC12 components possess complicated microstructures and exhibit various defects, making them unsuitable for the automotive industry, and the use of cast ADC12 alloys to replace steel in the manufacture of safety–critical
MITSUHIRO OKAYASU, Associate Professor, SHUHEI TAKEUCHI, Ph.D. Candidate, and MASAKI YAMAMOTO, Undergraduate Student, are with the Department of Materials Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime, 790-8577, Japan. Contact e-mail: [email protected] HIROAKI OHFUJI, Associate Professor, is with the Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho. TOSHIHIRO OCHI, Director, is with the Ochi Foundry Inc., 1016 Tanouwagata Tanbara-cho, Saijo, Ehime, 791-0522, Japan. Manuscript submitted June 2, 2014. Article published online 19 January 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
parts in automobiles has been considerably restricted because of their lower strength and lower ductility. From our previous work, it appears that failure in cast ADC12 alloys is strongly influenced by a-Al grains (through their effect on secondary dendrite arm spacing, SDAS), eutectic phases, crystal orientation, and casting defects. Mechanical properties of high-pressure die-cast ADC12 alloy have been investigated, and it has been found that failure (or crack initiation) occurred from casting defects, for example, cold flakes and porosity.[2,3] There are various casting defects in die-cast ADC12 alloys, such as porosities, shrinkage cavities, oxide layers, and cold flakes.[2,3] Zhao et al.[4] have investigated the tensile properties of ADC12 die-cast plate samples, which are directly influenced by porosity and serrated faces. It has been reported by Jana et al.[5] that fatigue cracks in cast Al alloy emanate from the edges of porosities as a result of the high stress concentration there. Mechanical properties are also affected by microstructural characteristics, such as a-Al grains and eutectic phases. Many investigators have found that SDAS is a significant factor determining the mechanical properties of cast Al alloys, with smaller SDAS leading to better mechanical properties, i.e., grain-boundary strengthening. It is also considered that high mechanical properties could be obtained in the Al alloys with smaller SDAS due to the decremen
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