The oxide film on the surface of the magnesium alloy liquid is loose, and magnesium is an active element, which has a strong binding force with oxygen. During smelting, it reacts with oxygen, water vapor and nitrogen in the atmosphere to form MgO, Mg3N2 and H2. MgO and Mg3N2 become impurities, and the inclusions are often accompanied by shrinkage porosity and pores. In addition, due to the use of flux, the tendency to generate flux inclusions is large, and it will become a source of corrosion for magnesium alloy castings. Therefore, preventing these two types of inclusions is an outstanding issue. In order to prevent the reaction between the molten magnesium and the atmosphere, there must always be a covering agent to protect it during the smelting process. In order to remove oxide impurities in the molten magnesium, a sufficient amount of refining agent should be sprinkled for refining. A smooth circulation flow is generated to ensure that the refining agent can fully absorb the inclusions and then settle at the bottom of the crucible. In order to improve performance, grain refinement is adopted. Different types of magnesium alloys have different refining treatments. It is best to refine twice before and after grain refinement.
Magnesium alloy fluxes usually use salts with low melting point, low density, moderate viscosity, and stable chemical properties. Currently, the commonly used flux is anhydrous carnallite (MgCl2—KCl) with some fluoride salts and chloride salts added. During smelting, the flux melts into a liquid state and spreads evenly on the surface of the magnesium alloy liquid to form a continuous and complete covering layer, preventing the magnesium alloy from reacting with oxygen and water in the air. During the use of flux, a large amount of irritating gas (such as HCl, Cl2, etc.) will be generated, and it is easy to cause flux inclusions, which will reduce the mechanical properties and corrosion resistance of the alloy. At present, the application of flux protection method in foreign countries is decreasing day by day.
The gas shielded smelting technology began in the mid-1970s. This technology has greatly improved the purity of magnesium alloys and is now commonly used in countries all over the world. Commonly used protective gases are SF6, CO2, SO2, N2, etc. or their mixed gases. SF6 is a harmful gas, whose greenhouse effect is 24,000 times that of CO2, and will be banned in the future. Studies have shown that 0.2%~0.3% volume fraction of SO2 mixed with dry air can play a good role in melting protection, and there is no obvious odor. Germany’s OTTUJUN company and Volkswagen have cooperated in the study of replacing SF6 and CO2 with argon as the protective gas for magnesium alloy melting, and have achieved good results, and are currently in the production test stage. France Rrochot has successfully developed a protective gas composed of CO2 oxidizing gas (oxidizing gas) + argon + xenon. In addition, adding 0.001%~0.003% Be can form a dense protective oxide film on the surface of the alloy liquid, effectively preventing further oxidation of the alloy liquid. Due to the combustion reaction of molten magnesium with oxygen and water vapor during the smelting process, special attention should be paid to safety. Once a magnesium leakage accident occurs, do not splash water. If the leakage is not serious, the ingot should be lifted out of the crucible immediately, or put into a container filled with dry MgO powder; if the magnesium leakage is serious, a large amount of dry flux can be directly sprinkled to cover the burning liquid surface.