Melting magnesium alloys requires strict control of oxidation, moisture, furnace materials, protective atmospheres, and flux condition. Molten magnesium has a strong chemical affinity for oxygen, and the oxide film formed on its surface is not sufficiently dense to stop continued oxidation.
For this reason, safe and stable Magnesium Alloy Melting depends on isolating the melt from air, preventing contact with water, and maintaining an effective protective layer throughout melting, holding, and pouring.
Why Molten Magnesium Oxidizes Easily
Magnesium has a stronger chemical affinity for oxygen than aluminum. When one gram-atom of oxygen combines with magnesium, approximately 143 kcal of heat is released, compared with approximately 127 kcal for aluminum.
Magnesium forms a loose, porous oxide film with a compactness coefficient of approximately 0.79. Aluminum oxide has a compactness coefficient of approximately 1.28. Because the magnesium oxide film is not dense, it cannot effectively block oxygen or reaction products, so oxidation continues.
How Temperature Changes Oxidation
At relatively low temperature, magnesium oxidation is limited. Above approximately 500°C, oxidation accelerates. Once the temperature exceeds magnesium’s melting point of about 650°C, exposure to oxygen can cause rapid oxidation and burning.
Magnesium oxide is a good thermal insulator, so heat may accumulate at the reaction interface. The rising local temperature accelerates oxidation further. Surface protection must therefore remain continuous during charging, melting, holding, alloying, refining, transfer, and pouring.
Reaction Between Magnesium and Water
Both solid and liquid magnesium can react with water. The reaction becomes faster as temperature increases:
Mg + H₂O → MgO + H₂↑ + heat
Mg + 2H₂O → Mg(OH)₂ + H₂↑ + heatWhen molten magnesium contacts water, magnesium oxide and hydrogen are generated. Heat from the reaction can vaporize and expand additional water, causing severe burning, splashing, or explosion. Furnace charge, tools, fluxes, crucibles, and ladles must therefore be dry.
The magnesium-water reaction is also an important source of hydrogen in the melt and is closely related to shrinkage porosity in castings.
Reaction with Nitrogen and Inert Gases
3Mg + N₂ → Mg₃N₂Magnesium nitride is powdery and does not form a protective surface film. Nitrogen alone cannot reliably prevent oxidation or burning.
Argon, helium, neon, and similar gases do not react chemically with magnesium. They can isolate the melt from oxygen, but do not form a protective film or completely prevent magnesium evaporation.
Carbon Dioxide Protection
Mg + ½CO₂ → MgO + ½CIn dry, pure carbon dioxide, magnesium oxidizes slowly. Carbon produced by the reaction can fill pores in the oxide layer and increase its compactness.
Below approximately 700°C, temperature has relatively little effect on oxidation in dry carbon dioxide. Around 700°C, a thin film may form. At higher temperature, the film thickens, hardens, loses compactness, cracks, and eventually loses protection. Air or water vapor mixed into the gas also reduces its effectiveness.
Sulfur Dioxide Protection
3Mg + SO₂ → 2MgO + MgS
2Mg + SO₂ → 2MgO + SSulfur dioxide forms a relatively compact MgS-MgO surface film that can suppress oxidation. However, the film may crack if sulfur dioxide disappears from the atmosphere, and protection fails above approximately 750°C. Sulfide inclusions and reported explosion risks have limited its use.
Sulfur Hexafluoride Protection
At elevated temperature, SF₆ can react with magnesium and form a dense surface layer containing magnesium oxide and magnesium fluoride. The magnesium fluoride component improves film compactness.
- Below approximately 0.01% by volume, protection is insufficient.
- Between approximately 0.01% and 1%, a protective effect is described.
- Above approximately 1%, protection does not improve significantly.
- Excess concentration may increase equipment corrosion.
At around 500°C and above, SF₆ may decompose into toxic lower fluorides. Gas selection and process control must comply with current safety and environmental requirements.
Comparison of Protective Atmospheres
| Atmosphere | Principal effect | Main limitation |
|---|---|---|
| Argon, helium, neon | Isolate the melt from oxygen | Do not form a film or prevent evaporation |
| Carbon dioxide | Forms a MgO-carbon-containing film | Requires dry, pure gas; weakens at high temperature |
| Sulfur dioxide | Forms a MgS-MgO film | Toxicity, inclusions, film breakdown, and explosion risk |
| Sulfur hexafluoride | Forms a dense MgO-MgF₂-containing film | Decomposition products, corrosion, and environmental concerns |
Why Flux Is Used
Covering
Molten flux spreads across the magnesium surface and forms a continuous covering layer. It separates the melt from air, suppresses magnesium-oxygen and magnesium-water reactions, and helps extinguish local burning.
Refining
Molten flux wets and adsorbs nonmetallic inclusions. The density difference between flux and magnesium helps transfer inclusions from the metal into the flux.
A useful flux requires suitable melting point, density, viscosity, surface behavior, chemical stability, and inclusion-wetting ability.
Main Components of Magnesium Alloy Flux
- Magnesium chloride, MgCl₂
- Potassium chloride, KCl
- Calcium fluoride, CaF₂
- Barium chloride, BaCl₂
- Sodium chloride, NaCl
- Calcium chloride, CaCl₂
The Role of Magnesium Chloride
Magnesium chloride is the principal component in many magnesium alloy fluxes. It provides covering action and refining ability. Its melting point is approximately 708°C, but mixed salts form lower-melting compositions.
A mixture described in the material contains approximately 44–52% MgCl₂ and 32–46% KCl, with a melting point of roughly 400–480°C.
Molten MgCl₂ spreads across the magnesium surface, wets oxide films, wraps oxide particles, transfers them into the flux, and helps suppress oxidation and burning.
2MgCl₂ + O₂ → 2MgO + 2Cl₂
MgCl₂ + H₂O → MgO + 2HCl
2HCl + Mg → MgCl₂ + H₂
Mg + Cl₂ → MgCl₂
MgCl₂ + 5MgO → MgCl₂·5MgOThe Roles of KCl, BaCl₂, and CaF₂
Adding KCl to MgCl₂ lowers melting point, surface tension, and viscosity. It also improves stability by reducing MgCl₂ evaporation and suppressing hydrolysis during dehydration.
Barium chloride may be used as a weighting agent to increase the density difference between flux and magnesium. It also increases flux viscosity.
Calcium fluoride can increase flux density, modify viscosity, and improve stability and refining capacity:
CaF₂ + MgCl₂ → MgF₂ + CaCl₂MgF₂ can combine with magnesium oxide, while small fluoride additions modify interfacial tension between the flux and the molten metal.
Moisture Control for Fluxes
Magnesium chloride is strongly hygroscopic. If a flux absorbs excessive moisture, MgCl₂ reacts with water and forms MgO. Oxide can collect at the flux-metal interface, covering and refining deteriorate, and sparking or violent reaction may occur.
Fluxes, tools, furnace charge, and all melt-contact materials must remain dry. Formulation, dehydration, storage, and addition procedures must be controlled as part of the melting process.
Practical Magnesium Melting Checklist
Before Melting
- Confirm the alloy and charge composition.
- Check iron and silicon limits where required.
- Dry the charge, tools, ladles, and flux.
- Confirm protective-gas supply and dryness.
- Inspect the furnace and crucible for contamination.
During Melting
- Maintain a continuous protective cover.
- Avoid unnecessary disturbance of the surface.
- Control temperature within the selected range.
- Prevent water contact.
- Monitor flux condition and coverage.
- Remove inclusions without exposing large melt areas.
During Holding and Pouring
- Maintain protection during transfer.
- Keep receiving tools and molds dry.
- Minimize turbulence and oxide entrainment.
- Prevent interruption of the protective atmosphere.
- Handle dross and residues under approved procedures.
Conclusion
The naturally formed magnesium oxide film is porous and cannot provide complete protection. Stable melting requires dry materials, a controlled atmosphere, suitable flux, correct temperature, and disciplined handling.
Flux provides both a covering layer and a refining mechanism. Magnesium chloride is the main functional component, while potassium chloride, barium chloride, calcium fluoride, and other salts adjust melting point, density, viscosity, stability, and refining performance.
Technical and safety note: The information and numerical ranges in this article were translated and reorganized from the supplied technical pages. Some practices involve toxic, corrosive, environmentally restricted, or highly reactive materials. Process design must follow current regulations, supplier safety data, equipment requirements, and qualified metallurgical supervision.
