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.

中文翻译：

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保护气体气氛下Ml19（Mg–Nd–Y–Zn–Zr）镁合金熔化过程中形成的表面层成分的研究

摘要

镁合金无熔剂熔化的最普遍方法是在保护性气体气氛下熔化，该气氛由惰性载气和少量活性气体组成。ML19（Mg–Nd–Y–Zn–Zr）铸造镁合金包含钇和钕。这些金属非常活泼。对类似合金与保护性气体气氛的相互作用的研究很少，并且代表了相当大的实际兴趣。六氟化硫（SF ₆）对全球变暖有强烈影响。因此，这种气体的应用受到限制。有鉴于此，HFC-R134a在一些国家被用作活性气体。保护气体混合物对载气（氩气或氮气）和活性气体（SF _{6的影响）的影响}在这项工作中考虑了在ML19镁合金熔体表面上形成的保护层组成上的HFC-R134a或HFC-R134a）。开发了一种特殊的实验室装置，该装置可在样品加热，熔化和固化期间提供保护性气体混合物与金属的接触，从而排除了周围大气的影响。合金元素的损失被证明是微不足道的，但是当使用氮气作为载气时，合金中的Y和Nd含量却比使用氩气时低。使用SF ₆作为活性气体熔融的合金中的锆含量较低。使用保护性气体SF ₆和HFC-R134a时形成的氧化膜的成分和厚度相似。表面膜主要由氟化镁（MgF₂）具有锆，钇和镁的氧化物，氟化物和氮化物的杂质。使用HFC-R134a时，保护膜的相组成的主要区别在于，无论是化合物形式还是游离态，都存在大量的碳。还确定需要在保护性气氛中彻底添加HFC-R134a，因为混合气体中HFC-R134a的分数增加到1体积％以上会导致熔化期间坩埚内表面的严重腐蚀，这在使用SF时未观察到₆。