In the smelting, processing, and performance control of aluminum-magnesium alloys, anhydrous magnesium chloride (MgCl₂) plays a mainly auxiliary technical role due to its unique chemical and physical properties. Its core function revolves around optimizing melt quality, reducing defects, and ensuring stable production. The following is an analysis of its specific functions and principles:
　 　I. As a core component of the smelting flux, it efficiently removes oxide inclusions
　　During the smelting of aluminum-magnesium alloys, both aluminum (Al) and magnesium (Mg) are active metals that readily react with oxygen in the air to form oxide inclusions such as Al₂O₃ and MgO. If these inclusions remain in the alloy melt, they will severely reduce the alloy's mechanical properties (such as strength and ductility) and processing properties (such as cracking during rolling and welding). Anhydrous magnesium chloride, as a key component of the flux (often used in combination with NaCl, KCl, etc.), removes impurities through the following mechanisms:
　　Chemical reaction removal: At high temperatures (aluminum-magnesium alloy smelting temperature is approximately 650-750℃), MgCl₂ can react with oxides. The resulting complex salts have melting points far lower than the alloy melt and will float to the surface of the melt as a liquid slag, making them easy to remove.
　　Physical adsorption and entrainment: After melting, the anhydrous magnesium chloride flux forms a low-viscosity liquid layer that can encapsulate fine oxide particles, causing them to aggregate, grow, and float, achieving separation from the metal melt.
　　II. Inhibits the oxidation and combustion of magnesium, protecting the melt
　　Magnesium is extremely chemically active and readily reacts violently with oxygen at high temperatures during smelting to form MgO, or even combust, leading to magnesium loss, melt composition deviating from the design value, and the generated MgO inclusions further worsening the melt quality. The role of anhydrous magnesium chloride is:
　　Formation of a protective layer: Molten MgCl₂ has a lower density than the aluminum-magnesium alloy melt (approximately 1.56 g/cm³ vs. alloy approximately 2.6-2.8 g/cm³), forming a continuous liquid film on the surface of the melt, isolating the air (oxygen) from direct contact with the melt, and inhibiting magnesium oxidation and combustion from the source.
　　Reduced combustion risk: Compared to hydrated magnesium chloride (such as MgCl₂・6H₂O), anhydrous magnesium chloride avoids the decomposition of water at high temperatures to produce H₂ and O₂ (which may exacerbate combustion or cause hydrogen embrittlement), further ensuring smelting safety.
　 　III. Improves melt fluidity and optimizes casting performance
　　The fluidity of the aluminum-magnesium alloy melt directly affects the filling capacity of the casting process (such as die casting and sand casting). Insufficient fluidity easily leads to insufficient filling and cold shuts. Anhydrous magnesium chloride can improve fluidity in the following ways:
　　Reduces melt viscosity: As an electrolyte, molten MgCl₂ can interact with metal ions in the alloy melt, weakening the bonds between atoms, reducing melt viscosity, and making the alloy easier to fill the mold during pouring.
　　Refines inclusion distribution: Efficient impurity removal reduces the obstruction of solid inclusions to melt flow, indirectly improving fluidity.
　　 IV. Assists degassing and reduces porosity defects
　　During the smelting of aluminum-magnesium alloys, the melt easily absorbs hydrogen (mainly from moisture in the raw materials and environmental humidity). Hydrogen precipitation during cooling will form pores, leading to a decrease in the mechanical properties of the castings. The degassing mechanism of anhydrous magnesium chloride is:
　　At high temperatures, anhydrous magnesium chloride can react with hydrogen or hydrogen-containing compounds in the melt to produce gases such as HCl. These gases will carry the hydrogen in the melt out during their escape.
　　The molten MgCl₂ layer can also prevent external moisture (a source of hydrogen) from entering the melt, reducing the re-absorption of hydrogen.
　 　V. Adjusts flux properties to suit process requirements
　　Industrial smelting of aluminum-magnesium alloys often uses composite fluxes (such as the MgCl₂-NaCl-KCl system). Anhydrous magnesium chloride, as one of the core components, can be adjusted in proportion to:
　　Lower the melting point of the flux: The melting point of pure MgCl₂ is approximately 714℃. When blended with NaCl (801℃) and KCl (770℃), it can form a low-eutectic (such as MgCl₂-KCl eutectic point of approximately 480℃), making it easier to melt at the alloy smelting temperature and enhancing its activity.
　　Optimize flux density and viscosity: By controlling the MgCl₂ content, the flux density can be slightly lower than the alloy melt (facilitating flotation) and the viscosity can be moderate (beneficial for entraining inclusions), improving the efficiency of impurity removal.
　　Key Considerations: The Necessity of Anhydrous Material
　　Anhydrous magnesium chloride, not magnesium chloride containing water of crystallization (such as MgCl₂・6H₂O), must be used in the smelting of aluminum-magnesium alloys. The reason is:
　　The water of crystallization will decompose into H₂O and HCl at high temperatures. H₂O reacts with the melt to generate hydrogen (exacerbating porosity), and while HCl has a degassing effect, an excess will corrode the furnace equipment.
　　Anhydrous magnesium chloride avoids the risk of melt "hydrogen embrittlement" caused by the introduction of moisture, ensuring alloy purity.
　　Summary
　　Anhydrous magnesium chloride in aluminum-magnesium alloys does not act as an alloying element (it does not participate in composition control), but rather through auxiliary functions such as impurity removal, oxidation prevention, improved fluidity, assisted degassing, and optimized flux performance, ensuring stable smelting, reducing casting defects, and ultimately improving the mechanical properties and processing reliability of aluminum-magnesium alloys. Its core value lies in improving melt quality through physicochemical effects, making it an indispensable process auxiliary material in the industrial production of aluminum-magnesium alloys.