[Technology] Smelting Process Characteristics of Magnesium Alloy Melting

The melting of magnesium alloys requires careful control to prevent oxidation or combustion, which can be achieved through either flux-based or fluxless methods. Historically, fluxes were used to remove impurities and create a protective layer on the surface of the melt, but this method had limitations. The protective film formed by fluxes was not always effective in blocking oxygen, leading to significant magnesium loss due to oxidation. Additionally, fluxes could cause inclusions that reduced mechanical properties and corrosion resistance, while also generating harmful gases that damaged equipment and the working environment. To address these issues, the 1970s saw the development of fluxless smelting techniques, where inert gases such as sulfur hexafluoride (SF6) and nitrogen (N2) were used to create a protective atmosphere around the molten metal. These gases prevented direct contact between the melt and air, significantly reducing oxidation. However, the concentration of SF6 had to be carefully controlled—too high, and it could erode the melt; too low, and it would fail to provide adequate protection. In general, both flux and fluxless methods can produce high-quality magnesium alloys when properly executed. Flux protection involves using specific fluxes tailored for each magnesium alloy, ensuring they are applied correctly. During melting, care must be taken to avoid overheating and to manage chlorination processes effectively. Before pouring, the melt should be thoroughly cleaned and deoxidized, particularly removing any chloride compounds that might affect corrosion resistance. After pouring, a thin layer of sulfur powder is often sprinkled on the surface to further reduce oxidation during solidification. In contrast, fluxless smelting avoids the use of fluxes, reducing the risk of inclusions and improving casting quality. This method became especially important in die-casting, where flux usage posed operational challenges. Fluxless techniques have played a crucial role in advancing the use of magnesium alloys by minimizing defects and enhancing performance. Gas protection mechanisms involve using gases like CO2, SO2, and SF6, with SF6 being the most effective. These gases form protective layers on the melt surface, preventing oxidation and improving stability. For example, CO2 reacts with magnesium to form amorphous carbon, which fills gaps in the oxide layer and suppresses magnesium ion diffusion. Similarly, SO2 forms a dense MgS/MgO layer that acts as a barrier against oxidation. SF6, when used in a controlled mixture, creates a stable and effective protective atmosphere, though its environmental impact and cost must be considered. Modern practices include the use of sodium alkyl sulfonate sand as an eco-friendly alternative to fluorine-based materials. This sand improves gas permeability and reduces the risk of oxidation. It is composed of quartz sand, bentonite, boric acid, and sodium alkyl sulfonate, with glycerin sometimes added to control moisture and improve workability. Proper storage and handling of the sand are essential to maintain its effectiveness and prevent defects in the final castings. Despite its benefits, sodium alkyl sulfonate sand can present challenges, such as reduced wet strength and rapid moisture evaporation. Careful mixing and temperature control are necessary to ensure consistent quality. Common issues during production include burning spots on the casting surface, caused by improper flow patterns or excessive recycled sand. Addressing these problems requires optimizing the mold design and controlling the pouring process to minimize turbulence and oxidation. Overall, the evolution of magnesium alloy smelting has focused on improving safety, reducing defects, and enhancing environmental sustainability. By combining advanced gas protection techniques with innovative sand systems, the industry continues to develop more efficient and reliable methods for producing high-quality magnesium castings.

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