The condition of a magnesium melt has a direct effect on grain size, mechanical properties, casting performance, porosity, hot cracking, and shrinkage. Oxides, dissolved hydrogen, nonmetallic inclusions, and an unstable grain structure can remain in the liquid metal unless the melt is treated correctly before pouring.
Effective Magnesium Melt Treatment normally combines three functions:
- Grain refinement or modification
- Removal of nonmetallic inclusions
- Degassing and hydrogen control
The appropriate treatment depends on the alloy family. Magnesium-aluminum alloys are commonly refined through carbon-bearing treatment, while magnesium-zinc and magnesium-rare-earth alloys generally require zirconium-based grain refinement.
Why Magnesium Melt Treatment Is Necessary
Magnesium alloys have a tendency toward grain coarsening. Some ingots and castings may also develop locally abnormal grain size. Melt treatment can refine grains, improve mechanical properties and castability, reduce hot cracking and shrinkage, remove inclusions, and lower dissolved hydrogen.
Furnace cleanliness is essential. Sand and silica-bearing contamination must be removed so silicon does not enter the melt. Steel or welded steel-plate crucibles are commonly used because iron has very low solubility in molten magnesium.
Grain Refinement of Magnesium-Aluminum Alloys
Overheating Treatment
An early method was to superheat the melt to approximately 850–900°C, remove the crucible, cool the metal rapidly to pouring temperature, and cast it. The material also describes superheating to approximately 800°C while stirring and strong stirring at approximately 740–780°C followed by holding.
This method requires strict temperature and timing control because excessive high-temperature exposure increases oxidation and process risk.
Carbon Inoculation
Carbon-bearing compounds such as MgCO₃, CaCO₃, or C₂Cl₆ are introduced into the melt. At high temperature, they release carbon, which reacts with aluminum to form dispersed Al₄C₃ particles.
Al₄C₃ and magnesium both have close-packed hexagonal structures, and their lattice constants differ by only approximately 4%. Al₄C₃ can therefore act as a heterogeneous nucleus and refine the magnesium grains.
Factors Influencing Carbon Refinement
Refinement Fading
After treatment, grains become progressively coarser as the melt stands. The supplied process guidance states that pouring should be completed within approximately 45 minutes after modification.
Stirring and Reheating
Stronger stirring or brief reheating to approximately 800°C, followed by rapid cooling to the pouring temperature, can improve the refining effect.
Alloy Chemistry
Manganese can improve the refinement response. Excess fresh metal, excessive additions, unintended zirconium, titanium, or rare-earth elements may reduce the effectiveness of carbon treatment.
Hexachloroethane Treatment
The supplied material describes C₂Cl₆ treatment as providing modification, refining, and degassing:
- Chlorine reacts with magnesium and produces MgCl₂ for refining.
- Carbon promotes Al₄C₃ formation and grain refinement.
- Released gas contributes to hydrogen removal.
The material reports mechanical-property improvements of approximately 10–20% compared with MgCO₃ modification and good properties after two hours of holding.
A disadvantage is string-like residue near the crucible bottom, principally containing MgCl₂ and MgO. The source describes argon blowing or approximately one minute of treatment with about 0.1% RJ-2 flux as removal methods.
Zirconium Refinement of Mg-Zn and Mg-RE Alloys
Carbon treatment is mainly suitable for magnesium-aluminum alloys. Magnesium-zinc and magnesium-rare-earth alloys require zirconium.
When zirconium content exceeds approximately 0.6%, magnesium-zinc grains become significantly finer. At the peritectic temperature, magnesium dissolves only approximately 0.597% zirconium. Additional α-Zr particles remain in the melt and serve as crystallization nuclei.
| Phase | a | c |
|---|---|---|
| δ-Mg | 3.20 Å | 5.12 Å |
| α-Zr | 3.23 Å | 5.14 Å |
Zirconium can also react with hydrogen to form solid ZrH₂, lowering melt hydrogen and helping reduce shrinkage porosity.
Why Zirconium Is Added as a Master Alloy
Zirconium has a melting point of approximately 1855°C and a density of approximately 6.45. It is difficult to dissolve directly in magnesium and reacts readily with gases and alloying elements.
Zirconium is therefore commonly introduced through a Mg-Zr master alloy. The actual addition may be approximately three to five times the zirconium required in the finished alloy.
Refining the Magnesium Melt
Oxygen, nitrogen, and water vapor can form refractory nonmetallic inclusions such as MgO. These inclusions reduce mechanical properties and contribute to shrinkage, porosity, and inconsistent casting quality.
The supplied material describes two refining methods:
- Flux refining
- C₂Cl₆ refining
Flux Refining
Flux refining uses close contact between molten flux and magnesium. A suitable flux should wet and adsorb inclusions, prevent new contamination, carry captured particles into the flux, and separate from the magnesium.
After controlled mixing, the source recommends holding the melt for approximately 10–15 minutes so residue can settle. Temperature must be controlled to maintain flux flow and stability without increasing oxidation.
C₂Cl₆ Refining
C₂Cl₆ can provide modification, refining, and degassing simultaneously. Its decomposition products produce MgCl₂ for refining, carbon for grain refinement, and gas for hydrogen removal.
The source reports an approximate 10% reduction in hydrogen in certain production practice. Its main disadvantage is the formation of MgCl₂- and MgO-containing residue near the crucible bottom.
Why Hydrogen Matters
Hydrogen is the principal dissolved gas in molten magnesium. Its solubility in magnesium is approximately two orders of magnitude greater than in molten aluminum. Rapid cooling can leave solid magnesium supersaturated with hydrogen.
Although gas pores may not always be obvious, hydrogen content is closely related to shrinkage severity.
Hydrogen Content and Shrinkage
The supplied material reports that shrinkage can appear when hydrogen exceeds approximately 14.5 cm³ per 100 g of alloy.
| Alloy condition | Hydrogen content |
|---|---|
| General production condition | 12 cm³/100 g |
| Corroded condition | 18 cm³/100 g |
| After chlorine degassing | 5 cm³/100 g |
Hydrogen may originate from wet flux, adsorbed water vapor, corrosion products, damp charge, wet tools, and atmospheric moisture.
Why Hydrogen Increases Shrinkage
If sequential solidification is not established, several regions may solidify almost simultaneously and create dispersed shrinkage cavities. Gas precipitation further obstructs liquid feeding and makes shrinkage more severe.
A sound process therefore requires both effective degassing and good directional or sequential solidification.
Chlorine Degassing
The supplied process describes chlorine introduced through a graphite tube at approximately 725–750°C for 5–15 minutes.
Above approximately 715°C, liquid MgCl₂ forms and assists removal of oxides and suspended inclusions. Excessive temperature can form too much MgCl₂ and increase flux-inclusion risk.
Chlorine degassing can remove the effect of carbon modification, so the source specifies degassing before carbon treatment.
C₂Cl₆ and Organic Chloride Degassing
C₂Cl₆ and related organic chlorides may be formed into tablets and added without dedicated gas-introduction equipment. They are convenient and provide combined refining effects, although the source states that their degassing performance is weaker than direct chlorine treatment.
Argon and Nitrogen Degassing
Argon or nitrogen can provide some degassing, but cannot effectively remove nonmetallic inclusions. A separate refining operation remains necessary.
Selecting the Treatment Sequence
For Mg-Al Alloys
- Prepare and dry the charge.
- Melt under effective protection.
- Degas when required.
- Refine to remove inclusions.
- Apply carbon-bearing grain refinement.
- Pour before the refining effect fades.
For Mg-Zn and Mg-RE Alloys
- Prepare and dry the charge.
- Melt under effective protection.
- Refine and control hydrogen.
- Add zirconium through a qualified Mg-Zr master alloy.
- Verify effective zirconium content.
- Allow residues to separate before pouring.
Production Control Checklist
- Confirm the alloy family and grain refiner.
- Verify charge and flux dryness.
- Control melt temperature and treatment order.
- Avoid excessive agitation and oxidation.
- Record additions, time, and temperature.
- Allow sufficient settling time.
- Pour before the refinement effect fades.
Conclusion
Magnesium melt quality depends on grain size, inclusion level, dissolved hydrogen, and treatment sequence. Carbon-bearing treatment is principally used for Mg-Al alloys, while zirconium is required for Mg-Zn and Mg-RE systems.
Flux refining removes nonmetallic inclusions, and controlled degassing reduces hydrogen associated with shrinkage. Temperature, timing, moisture control, addition method, settling, and treatment order all influence the final result.
Technical and safety note: The numerical ranges and process descriptions in this article were translated and reorganized from the supplied technical pages. Several described methods involve chlorine, chlorinated compounds, high-temperature molten magnesium, or other hazardous materials. Industrial use requires current regulations, safety data, emission controls, approved equipment, and qualified metallurgical supervision.
