Evolution of Entrainment Defects Formed in Mg-Y Alloy Castings
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NTRODUCTION
DUE to the sustained growth of the global demand for Mg alloys,[1–4] improvements in the quality of Mg-alloy castings have become a significant issue to be addressed, motivating the investigation of entrainment defects (also known as double oxide film defects) formed in Mg alloys, which have been widely accepted to be a major factor affecting the quality and reproducibility of the properties of light alloy castings.[5] Entrainment of the oxidized surface of a liquid light alloy can result in the entrapment of a doubled-over surface film containing a small quantity of the local atmosphere, forming a double oxide film defect in the final castings. The random nature of fluid flow, which influences the random size and orientation of entrainment defects, contributes greatly to the variable nature of casting properties.[5] Most previous investigations into entrainment defects were carried out on aluminum alloys,[5–15] and research into the defects in Mg alloys has been relatively limited.[16,17] Due to the use of a cover gas to avoid the well-known rapid oxidation and burning of molten magnesium alloys in air, oxide films formed on a Mg-alloy melt have a more complicated structure than those formed on an Al alloy.
TIAN LI is with the University of Birmingham, Birmingham, B15 2TT, UK and also with the Xiangnan University, Chenzhou, 423000, Hunan, P.R. China. Contact e-mail: [email protected] J.M.T. DAVIES is with the University of Birmingham. Manuscript submitted March 30, 2020
METALLURGICAL AND MATERIALS TRANSACTIONS A
SF6 is commonly used as a cover gas during the Mg-alloy casting process. Due to the high global warming potential (GWP) of SF6,[18] the use of this cover gas in magnesium casting has been restricted by the European Council and Parliament, and SO2 has consequently become the most widely used cover gas in the European Mg-alloy industry.[19] However, a recent commercial report regarding the SF6 global market in 2014 to 2025 shows that this cover gas is still widely used for the Mg-alloy processing in developing countries, such as China, Brazil, India, etc.[20] The World Resource Institute also reported that China had not announced a mitigation target or a specific national policy that includes the emissions of non-CO2 greenhouse gases (including SF6).[21] It should be noted that Chinese producers dominated the global magnesium metal production (e.g., contributing 82 pct of the global production in 2019).[22,23] Therefore, understanding the entrainment defects formed in a SF6-containing atmosphere is still important. Meanwhile, SF6 is also widely used in academic areas for research concerning Mg alloys. Based on search results from the Web of Science with the key words ‘‘magnesium’’ and ‘‘casting’’, 10 papers published in Apr 2020 used SF6-containing cover gas to cast their Mg-alloy samples,[24–33] while SO2 containing cover gas was only used in one study.[34] Previous studies reported different types of oxide films formed on liquid Mg-alloy surfaces under a SF6containing cover gas. Pettersen et al.[35] foun
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