Magnesium has a very strong chemical activity. Oxygen, nitrogen, and water vapor in the air react with magnesium to form non-metallic inclusions such as refractory magnesium oxide. The existence of non-metallic inclusions not only seriously deteriorate the mechanical properties of the alloy, but also associated with shrinkage, porosity and other defects. The purpose of magnesium alloy refining is to eliminate the inclusions and purify the melt. The current production mainly uses refining fluxes for refining.
The flux refining method is to wash the magnesium melt with molten iron I1, and the inclusions are wetted by the full contact between the flux and the melt, and are polymerized in the flux and precipitated with the flux at the bottom of the crucible. To achieve this goal, the flux should have good wetting, adsorption and inclusion ability.
The refining process should also be designed correctly to prevent new inclusions. The refining temperature is generally controlled at 710-7300C. After refining, it is allowed to stand for 10-60 minutes to fully settle the slag.
The use of hexachloroethane (C2CQ as refining 4'1 for refining) is also used in the production. The use of C2CL6 to treat the magnesium melt has both the effect of modification and refining. Especially in the case of fluxless refining of the Mg-Al alloy in a protective atmosphere like a protective atmosphere, It also shows its superiority.The mechanism is that C2C16 rapidly decomposes chlorine, carbon, etc. in the magnesium melt, chlorine and magnesium can produce manganese chloride, gCl), and plays a refining role, while carbon plays a role of grain refinement. effect. In addition, the gas generated by the decomposition of C2C16 also has the effect of removing nitrogen. The main problem of C2C16 refining is the production of flocculent suspended slag in the lower part of the crucible. The major components of the C2C16 refining process are MgC12 and MgO after X-ray diffraction and electron probe analysis. The solution is to purify the nitrogen gas by purging again, so that the slag floats, sinks, or is refined with a flux for 1 min. Magnesium and magnesium alloys are easily affected by the surrounding media during the smelting process, thereby affecting the quality of the alloy melt, resulting in castings Porosity, inclusions, slag inclusions, and shrinkage defects have occurred. Therefore, the magnesium alloy melt needs to be purified. It can usually be controlled from the following three aspects: the correct use of flux, the protection of the melt surface, and the thorough purification of the melt.
1 outgassing. The gas dissolved in the molten magnesium is mainly hydrogen. The nitrogen in the magnesium alloy is mainly derived from the moisture in the flux, the moisture adsorbed on the metal surface, and the moisture that the metal corrodes. The solubility of hydrogen in magnesium melt is 2 orders of magnitude larger than that in aluminum melt, and the tendency of precipitation when solidified is not as serious as that of aluminum. The solubility of nitrogen in melt is 1.5 in solid state. Nitrogen can be made by rapid cooling. Supersaturated solid dissolves in magnesium, so the problem of outgassing is often less of a concern. However, the gas content in magnesium alloys is closely related to the degree of shrinkage in the castings. This is due to the large spacing of the magnesium alloys, especially in the unbalanced state, where the crystallization spacing is greater, and therefore, if solidification is not established during the solidification process, With a temperature gradient, the melt solidifies almost at the same time, forming finely dispersed pores that are not easily supplemented by external metals, causing partial vacuum. Under the action of vacuum suction, the gas easily precipitates out there, and the precipitated gas further hinders the melting. The liquid fills the hole and eventually shrinks even more. Tests have shown that under the production conditions, shrinkage occurs in the magnesium alloy when the nitrogen content of 100g magnesium exceeds 14.5cm3.
The traditional degassing process method is similar to the chlorine-based gas method used in aluminum smelting. Chlorine is introduced into the magnesium melt through a graphite tube and the treatment temperature is 725-7500C. When the temperature is 5-15min o and the temperature is higher than 750°C, liquid MgC12 is generated, which is favorable for the removal of chlorides and other suspended inclusions. If the temperature is too high, too much MgC12 is formed and the possibility of flux inclusion increases. Chlorine degassing will eliminate the metamorphic effect of Mg-Al alloy plus “carbonâ€. Therefore, degassing with chlorine should be arranged. Carbon is carried out before the deterioration process.
Hexachloroethane is commonly used in production (C2CQ and hexachlorobenzene and other organic chlorides are used to degas the magnesium melt. These chlorides are pressed into the melt in sheet form, which is more convenient than chlorine gas degassing, and does not require special Ventilation device and other advantages, but the degasification effect of C2C16 is not as good as chlorine.
Nowadays, in the production, refining is performed by adding nitrogen or nitrogen to the side and adding the refining agent, which can effectively remove the non-metallic inclusions in the melt and degas. Not only does the refinement work well, but it also shortens work time.
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