Research On Magnesium And Magnesium Alloys Melting Protection Under The Mixture Gases Containing C₂H₂F₄

Recently years, magnesium alloys are getting more and more widely used in various fields because of its excellent properties. In the processes of magnesium alloys smelting at high temperature in air, however, the surface of magnesium and magnesium alloys will be oxidized rapidly which can result in combustion of molten state. In order to prevent the happen of oxidation and combustion, some ignition-proof methods must be taken in the processes of its smelting. The gas protection method is relatively better one in a variety of ignition-proof methods. The gas protective result containing SF₆ gas is the best of the gas protection methods. SF₆ has a significant impact on the greenhouse effect. So its use is limited gradually. It is important for magnesium alloy melting technology that a new kind protective gas with good protective result and little damaging effect on environment is investigated in future. This paper studies protective results and protective mechanisms of a new kind of protective gas C₂H₂F₄ experimentally.Protective melting experiments of magnesium and magnesium alloys have beendone under protective gases of C₂H₂F₄ mixed gases in different concentration, different temperature and different exposure time. The experimental results showed that the protective effect is distinct. There were obvious protective effects when the mixture contained 0.2%⁰.5% C₂H₂F₄ for pure magnesium, 0.2%⁰.8% C₂H₂F₄ for AZ91 and AM60 magnesium alloys respectively. There was a better protective result with the concentration of C₂H₂F₄ in mixture increasing and the temperature falling. Protective result will heighten when the C₂H₂F₄ mixed gases have 50% CO₂. The costs using C₂H₂F₄ mixed gases is near to the costs using SF₆ mixed gases.The C₂H₂F₄ mixed gases react with the melt and form a surface protective film during the course of protection. Transection and surfaces of the films have been observed with SEM. It is found that the thickness is about 3μm. The thicker the C₂H₂F₄ concentration and the lower the temperature, the more compact the protective film. The components of protective films have been analyzed with energy dispersive spectrometer, the results show that protective films are made up of MgO and MgF₂, including simple substance of carbon. The thicker the C₂H₂F₄ concentration, the lower the temperature and the longer the exposure time, the higher the content of MgF₂ in the surface protective film.Protective result of C₂H₂F₄ mixed gases result from the cover composite filmlayer which is continuous and compact. MgF₂ and single carbon in protective films increase the thickness of protective films. The film prevents the melt magnesium and magnesium alloys contact with oxygen in air of which acts the protective effect.