Protecting molten Magnesium and its alloys


Magnesium is having a high affinity to oxygen in the molten state and unlike other metals it is not forming a stable protecting film on the surface of melts. To avoid burning it is therefore necessary to protect molten Mg and its alloys. To achieve this, nowadays mostly SF6 is used in combination with carrier gases like dry air, N2, CO2, or even Ar. However, SF6 has been identified as a highly potent greenhouse gas with a global warming potential > 23,000 more than CO2. Moreover, SF6 is cracked at temperatures higher than 750 °C and toxic fluorine is set free. Fortunately fluorine immediately reacts with Mg vapour and forms stable MgF2. Due to the different threats caused by SF6 the European Union (EU) will ban the use of SF6 for high pressure die casting of magnesium alloys as of January 1st, 2018. Alternatives are already available. But most of them are fluorinated hydrocarbons or other fluorinated chemicals. They are also under discussion in the EU due to the risk that comes with fluorine. SO2 is already recommended as a possible SF6 replacement but is also having its own restrictions with maximum working space concentrations. It might be necessary to reuse again fluxes, but they also have limitations. If magnesium and its alloys shall be further used and processed in the EU, alternative ways of magnesium melt protection need to be established within the near future. This contribution will discuss available methods to protect molten Mg and their consequences.
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