When a magnesium casting develops coarse grains, shrinkage, inclusions, or inconsistent mechanical properties, the root cause may already be present before the metal enters the mold. Melt cleanliness, dissolved hydrogen, grain-refinement response, treatment temperature, and the order of operations all influence the final casting.
This guide approaches Magnesium Melt Treatment from a defect-control perspective. Instead of listing treatment methods in isolation, it connects common melt-related symptoms with their likely causes and the corrective principles described in the supplied technical material.
Four Melt Conditions Behind Many Casting Problems
- An unstable or excessively coarse grain structure
- Oxide, nitride, or flux inclusions in the melt
- Excess dissolved hydrogen
- An unsuitable treatment sequence or excessive holding time
These conditions can interact. Gas precipitation may restrict liquid feeding during solidification, inclusions can reduce mechanical consistency, and grain refinement may fade if pouring is delayed. Melt preparation must therefore be managed as an integrated control system.
Problem 1: Coarse or Uneven Grains
Magnesium alloys tend to undergo grain coarsening, and some ingots or castings may contain locally abnormal grain sizes. The corrective method depends on the alloy family.
Magnesium-aluminum alloys
Carbon-bearing compounds such as MgCO3, CaCO3, and C2Cl6 decompose at elevated temperature and supply carbon. The carbon reacts with aluminum to form dispersed Al4C3 particles. Their favorable crystallographic relationship with magnesium allows them to act as heterogeneous nucleation sites and refine the matrix.
The effect can weaken if the melt is held too long, stirring or temperature control is inadequate, the charge balance is unsuitable, or zirconium, titanium, or rare-earth contamination interferes. The supplied guidance calls for pouring within approximately 45 minutes after treatment. Strong stirring or a brief rise to about 800°C followed by rapid cooling may improve the response.
Magnesium-zinc and magnesium-rare-earth alloys
These alloys require zirconium-based refinement. Grain size decreases markedly when zirconium in a magnesium-zinc alloy exceeds approximately 0.6%.
| Phase | a | c |
|---|---|---|
| δ-Mg | 3.20 Å | 5.12 Å |
| α-Zr | 3.23 Å | 5.14 Å |
The close crystallographic match allows α-Zr particles to serve as nucleation sites. Because zirconium melts at approximately 1855°C, has a density of about 6.45, and reacts readily with gases and alloying elements, it is normally introduced as a Mg-Zr master alloy. The actual addition described in the source may be three to five times the zirconium required in the final composition.
Problem 2: Nonmetallic Inclusions and Dirty Melt
Magnesium reacts readily with oxygen, nitrogen, and water vapor, producing refractory contaminants such as MgO. Sand and silica contamination must also be prevented because silicon can reduce plasticity and corrosion resistance.
Flux refining cleans the melt by wetting and adsorbing nonmetallic particles. The source recommends approximately 10–15 minutes of settling after treatment. Mixing must create sufficient flux-to-metal contact without generating new oxidation.
C2Cl6 can combine modification, refining, and partial degassing, but it may leave string-like MgCl2– and MgO-rich residue. The described remedies include argon washing or a short treatment with approximately 0.1% RJ-2 flux.
Problem 3: Shrinkage Associated with High Hydrogen
Hydrogen is the principal dissolved gas in molten magnesium. It may enter through moist flux, adsorbed water vapor, corroded return metal, damp tools, or furnace materials. The source states that shrinkage may appear when hydrogen exceeds approximately 14.5 cm³ per 100 g.
| Melt condition | Hydrogen content |
|---|---|
| General production condition | 12 cm³/100 g |
| Corroded condition | 18 cm³/100 g |
| After chlorine degassing | 5 cm³/100 g |
During nonequilibrium solidification, isolated liquid regions may form between growing crystals. Gas precipitation makes these regions more difficult to feed and can intensify dispersed shrinkage. Degassing improves the melt but cannot compensate for an unsuitable feeding system or solidification gradient.
Problem 4: The Treatment Works, but the Casting Is Still Inconsistent
The historical chlorine process in the source was conducted at approximately 725–750°C for 5–15 minutes. Because chlorine degassing can remove the carbon-refinement effect in Mg-Al alloys, degassing must be completed before carbon modification.
Argon or nitrogen purging can provide some degassing, but it does not effectively remove nonmetallic inclusions. Refining and degassing answer different process needs and should not be treated as interchangeable operations.
Defect-to-Process Diagnostic Table
| Observed problem | Likely melt-related cause | Process principle to check |
|---|---|---|
| Coarse grains in Mg-Al alloy | Weak or faded carbon modification | Addition, stirring, temperature, and pouring delay |
| Coarse grains in Mg-Zn or Mg-RE alloy | Insufficient effective zirconium | Mg-Zr recovery and reaction losses |
| Inclusions or string-like residue | MgO, MgCl2, or poor separation | Flux contact, settling, and residue removal |
| Shrinkage porosity | High hydrogen plus poor feeding | Dryness, corrosion, degassing, and solidification gradient |
| Variable mechanical properties | Uneven grains or cleanliness | Alloy-specific refinement and repeatable preparation |
| Treatment loses effectiveness | Wrong order or excessive holding | Degas before modification and pour on time |
Recommended Control Sequence for Mg-Al Alloys
- Clean and dry the charge, tools, flux, and furnace-contact materials.
- Melt under effective surface protection.
- Degas when required.
- Refine to remove inclusions.
- Allow settling and remove residue.
- Apply carbon-bearing grain modification.
- Pour before the refinement effect fades.
Recommended Control Sequence for Mg-Zn and Mg-RE Alloys
- Prepare a clean, dry charge.
- Melt under suitable protection.
- Refine and control hydrogen.
- Add zirconium through a qualified Mg-Zr master alloy.
- Account for reaction and settling losses.
- Allow reaction products to separate before pouring.
Conclusion
Magnesium casting defects cannot be controlled by adding one treatment agent and assuming the melt is ready. Coarse grains require the correct alloy-specific nucleation method. Inclusions require refining and separation. Hydrogen requires prevention and degassing. Shrinkage requires both a sound melt and an effective feeding pattern.
The most reliable magnesium melt treatment program connects the observed defect with its metallurgical cause, then controls charge condition, temperature, treatment order, settling time, and pouring delay as one repeatable process.
Technical and safety note: This article is translated and reorganized from the supplied technical pages. Historical processes involving chlorine, chlorinated compounds, reactive powders, and molten magnesium present severe health, fire, explosion, and environmental hazards. They require approved industrial equipment, current safety data, emissions control, applicable regulatory compliance, and qualified metallurgical supervision.
