Aluminum-silicon alloys are widely used in casting production because they offer good fluidity and stable casting behavior.
However, untreated Al-Si alloys often contain coarse eutectic silicon. This silicon may appear as flakes, plates, or needles, which can reduce strength, plasticity, and machining performance.
Proper aluminum alloy melt treatment can refine silicon morphology and improve the final casting quality.
Why Al-Si Alloy Modification Is Important
In many cast aluminum alloys, silicon is the main alloying element. Especially in eutectic and hypoeutectic Al-Si alloys, eutectic silicon has a strong influence on mechanical properties.
Without modification, eutectic silicon becomes coarse and brittle. This can lower elongation and make machining more difficult.
Modification treatment changes coarse silicon into finer particles or fibrous structures. This helps improve strength, ductility, and casting reliability.
Sodium Modification
Sodium modification is a traditional and widely used method for Al-Si alloys. It has good modifying performance and stable results.
A common sodium modifier is based on NaF. During treatment, sodium enters the aluminum melt and modifies eutectic silicon.
However, sodium modification also has several limits.
- Sodium has a short effective time.
- It can burn off or oxidize quickly.
- It may increase melt viscosity.
- It can reduce fluidity.
- It may cause pores, inclusions, and white spots.
For sand casting, the effective time is usually short. The best effect often appears shortly after treatment.
Problems With Sodium Modification
Sodium is highly active in aluminum melt. It can react with moisture and oxygen, causing hydrogen absorption and oxidation.
This may increase pinhole defects and reduce mechanical properties.
Sodium salts may also corrode iron crucibles, melting tools, and furnace equipment. Volatile fluorides and chlorides can pollute the working environment.
For this reason, many foundries now pay more attention to non-sodium modification methods.
Non-Sodium Modification
Non-sodium modifiers include strontium, antimony, tellurium, barium, zirconium, and rare earth elements.
These modifiers are used to overcome the short effective time and process risks of sodium modification.
Strontium modification is one of the most common choices. It can provide a longer effective time and does not corrode tools like sodium salts.
For aluminum-silicon alloy casting, strontium modification can help keep the modification effect more stable during production.
Strontium, Antimony, Rare Earth and Zirconium
Strontium modifiers are often added as master alloys or mixed salts. They are suitable for sand casting and metal mold casting.
Antimony modification can reduce gas absorption and inclusions. It also avoids some corrosion and pollution problems.
Rare earth modifiers can affect both primary aluminum and eutectic silicon. When used properly, they can improve mechanical properties.
Zirconium-based modifiers, such as K2ZrF6, can also improve the structure of aluminum alloy castings. However, operation control is important to avoid slag and pollution.
Incubation Time in Modification Treatment
After a modifier is added, the best modification effect does not always appear immediately.
The waiting period is called the incubation period. It is also known as the gestation period.
This period exists because the modifier must diffuse through the melt. It may also react first with impurities such as phosphorus.
If the incubation time is too long, energy consumption increases. Production efficiency also decreases.
Stirring the melt can help shorten the incubation period. Proper process control also helps achieve a stable modification effect.
High-Silicon Aluminum Alloy Modification
High-silicon aluminum alloys usually contain 11% to 26% silicon. Common grades may contain about 18% to 24% silicon.
These alloys have good wear resistance and low thermal expansion. They are useful when dimensional stability and wear resistance are required.
However, high-silicon alloys contain hard primary silicon crystals. If these crystals are coarse, the alloy may show poor elongation and difficult machining.
That is why primary silicon refinement is critical for high-silicon aluminum alloy production.
Phosphorus Modification for Primary Silicon
Phosphorus is one of the most effective modifiers for refining primary silicon.
It can form AlP particles in the melt. Because AlP has a crystal structure similar to silicon, it can act as a nucleation core.
This promotes the formation of finer primary silicon particles.
Phosphorus treatment is stable and can maintain its effect at high temperature. Pre-treated Al-Si ingots can often be remelted and cast without repeated modification.
This is a major advantage compared with sodium modification.
Grain Refinement Treatment
Grain refinement means adding a small amount of material to the melt to promote nucleation.
This refines the aluminum matrix grains and improves the casting structure.
Fine grains can improve mechanical properties and casting performance. They also help reduce cold shuts, cracks, segregation, and local defects.
For industrial buyers, grain refinement is important when choosing stable aluminum and magnesium alloy materials.
What Buyers Should Check
When purchasing aluminum alloy castings or related materials, buyers should ask suppliers about melt treatment control.
Important questions include:
- What modification method is used?
- Is sodium, strontium, phosphorus, or rare earth treatment applied?
- How is incubation time controlled?
- Is melt temperature recorded?
- Are gas absorption and oxide inclusions controlled?
- Is grain refinement used before casting?
These questions help confirm whether the supplier can provide stable casting quality.
Conclusion
Al-Si alloy modification is a key process for improving casting performance.
Sodium modification offers strong effects but has short life and process risks. Non-sodium modification provides better stability for many production cases.
For high-silicon alloys, phosphorus treatment is especially useful for refining primary silicon.
A reliable supplier should control modification, refining, temperature, and grain structure together. This is the foundation for high-quality aluminum alloy casting and industrial alloy parts.
