[Technology] Smelting Process Characteristics of Magnesium Alloy Melting

The melting of magnesium alloys requires careful control to prevent oxidation or combustion, which can significantly affect the quality and performance of the final product. Two primary methods are used: flux-based and fluxless smelting. Before the 1970s, flux smelting was widely adopted, where a protective layer of flux was applied to the surface of the molten metal to shield it from oxygen. However, this method had several drawbacks. Flux could lead to inclusions, reducing mechanical properties and corrosion resistance, while also generating harmful gases that damaged equipment and worsened working conditions. To address these issues, the fluxless smelting process emerged in the early 1970s. This technique uses a protective gas atmosphere, typically a mixture of sulfur hexafluoride (SF₆) and nitrogen (N₂), to isolate the melt from air. The concentration of SF₆ must be carefully controlled—too much can erode the metal, while too little fails to provide adequate protection. When properly implemented, both flux and fluxless methods can produce high-quality magnesium alloy castings. In flux smelting, the use of appropriate flux is essential, as each magnesium alloy may require a specific type. During the melting process, care must be taken to avoid overheating and to manage the addition of materials like chlorine, which needs to be collected effectively. After pouring, sulfur powder is often sprinkled on the melt to reduce oxidation during solidification. Fluxless smelting, particularly with SF₆, has proven highly effective in minimizing oxidation losses. However, SF₆ has environmental concerns due to its greenhouse effect and potential toxicity. For example, at high temperatures, SF₆ can decompose into toxic gases like SO₂, HF, and S₂F₁₀. Therefore, strict safety measures and monitoring are required to ensure safe operation. Another alternative is the use of sodium alkyl sulfonate sand in casting, which acts as a non-toxic protective medium. This sand provides good air permeability and helps prevent oxidation by forming a protective layer around the molten metal. It also reduces the need for fluorine-based additives, making it more environmentally friendly. The preparation of sodium alkyl sulfonate sand involves mixing quartz sand, bentonite, boric acid, and the sulfonate compound. Proper moisture control is crucial, as excessive water can reduce strength and increase the risk of defects. Glycerin is sometimes added to extend the working time of the sand and improve its flame-retardant properties. During production, common issues include burning spots on the casting surface, caused by improper flow of molten metal or residual carbon layers from recycled sand. To mitigate this, proper design of the gating system and careful handling of the sand are necessary. Additionally, maintaining the correct temperature and gas concentration is vital to ensure consistent protection throughout the process. Overall, advancements in smelting and casting techniques have significantly improved the reliability and quality of magnesium alloy products. Whether using flux, fluxless methods, or alternative sands, the key lies in precise control of the environment and materials to achieve optimal results.

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