Investigation into the Surface Layer Composition Formed on the Ml19 (Mg–Nd–Y–Zn–Zr) Magnesium Alloy during Melting under Protective Gas Atmospheres

Abstract

The most currently widespread method of flux-free melting of magnesium alloys is melting under a protective gas atmosphere consisting of inert carrier gas with a small additive of active gas. The ML19 (Mg–Nd–Y–Zn–Zr) casting magnesium alloy contains yttrium and neodymium; these metals are rather active. The interaction of similar alloys with protective gas atmospheres is studied poorly and represents considerable practical interest. Sulfur hexafluoride (SF6) strongly affects global warming; therefore, the application of this gas is limited. In connection with this fact, HFC-R134a is used as active gas in several countries. The influence of protective gas mixtures consisting carrier gas (argon or nitrogen) and active gas (SF6 or HFC-R134a) on the composition of protective layer formed on the surface of the ML19 magnesium alloy melt is considered in this work. A special laboratory setup providing the contact of the protective gas mixture with metal during heating, melting, and solidification of the samples, which excluded the influence of surrounding atmosphere, was developed. The loss of alloying elements turned out to be insignificant, but the Y and Nd content in the alloy when applying nitrogen as carrier gas turned out to be lower than when using argon. The zirconium content was lower in alloys melted using SF6 as active gas. The composition and thickness of oxide films forming when using protective atmospheres SF6 and HFC-R134a are similar. The surface film consists mainly of magnesium fluoride (MgF2) with impurities of oxides, fluorides, and nitrides of zirconium, yttrium, and magnesium. The main distinction of the phase composition of the protective film when applying the HFC‑R134a is the presence of a considerable amount of carbon both in the form of compounds and in the free state. It is also established that a thorough dosage of HFC-R134a in protective atmospheres is required because an increase in its fraction in the gas mixture above 1 vol % leads to severe corrosion of the inner crucible surface during melting, which was not observed when using SF6.