Fundamental Characterization of the Quality and Performance of Important Primary Aluminum Casting Alloys

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Primary Al-7%Si-Mg foundry alloys such as A356 and 357 have been used extensively in structural applications where higher and more consistent mechanical properties are a key requirement to uphold the safety standards of the cast components. Addition of Mg increases strength via precipitation hardening to provide specific combinations of strength and ductility. Iron, usually present as an impurity, has a negative influence on the alloy properties due to the formation of iron intermetallics, notably the brittle beta-iron A15FeSi phase in the form of platelets which cause feeding-related problems, resulting in porosity formation and early fracture. The formation of the pi-iron A19FeMg3Si5 phase is also undesirable, as it ties up Mg and lowers the alloy strength/ductility. The target, therefore, is to keep the Fe level as low as possible. It has been reported recently that Al-9%Si alloys are more tolerant to iron than 356 and 319 alloys. The ductility of Al-9%Si-Cu-Mg alloys showed vast improvements compared to those containing low Si levels, even in the presence of high Cu, Mg, and Fe contents. While the reason for this is yet to be understood, this is a significant observation, with immense potential for alloy development. Another area of interest is that related to the production of Be-free 357 (Al-7%Si-Mg) alloys. These alloys, containing high Mg levels, are used in castings which require high yield strengths. However, the high Mg also leads to the formation of the pi-phase. While Be is used to eliminate this phase, its toxic nature has made it necessary to seek other alternatives. The proposed research program will address these two main concerns through (i) a fundamental characterization of Al-9%Si alloys (359 and 354 type alloys) using the quality index concept to optimize the tensile properties, and (ii) a comprehensive study of pi-A19FeMg3Si5 phase formation in Al-Si-Mg alloys through a fundamental characterization of the alloy microstructure and related mechanical properties of Be-free 357 alloys, with emphasis on the effect of iron impurities. The application of these findings by the aluminum industry and castings producers would be of distinct benefit to the Canadian economy.