1. Silicon (Si)
Silicon is the main element in most die-cast aluminum alloys. It improves the casting performance of the alloy, generally at a content of 7%-11%. Silicon's functions:
1. Improve the high-temperature fluidity of the alloy
2. Reduce shrinkage
3. Reduce the tendency for thermal cracking
4. Improve wear resistance
However, when the silicon content exceeds 12%, silicon and aluminum form a eutectic, and when impurities such as copper and iron are abundant, hard points of free silicon appear, making machining difficult. High-silicon aluminum alloys severely corrode the crucibles used for casting.
2. Copper (Cu)
The copper content in the alloy is usually 2.5% to 5%. Increasing the copper content can improve the alloy's fluidity, tensile strength, and hardness, but it reduces corrosion resistance and plasticity, and increases the tendency for thermal cracking.
3. Magnesium (Mg)
Adding a small amount (about 0.2-0.3%) of magnesium to high-silicon aluminum alloys can improve strength and yield strength, and improve the machinability of the alloy.
Excessive magnesium content deteriorates casting performance, resulting in low strength and plasticity at high temperatures, large shrinkage during cooling, and thus a tendency for thermal cracking and porosity.
4. Zinc (Zn)
In aluminum-zinc aluminum alloys, zinc improves fluidity and casting performance, and increases tensile strength, but it increases the tendency for thermal cracking and reduces corrosion resistance. It should generally be less than 1.2%.
5. Iron (Fe)
Aluminum alloys adhere strongly to molds. When the iron content is below 0.6%, this adhesion is particularly strong. When it exceeds 0.6%, the adhesion is significantly reduced. Therefore, controlling the iron content within the range of 0.6-1% is beneficial for die casting.
When the iron content is too high, iron exists in the alloy as FeAl3, Fe2Al7, and Al-Si-Fe in flake or needle-like structures, reducing mechanical properties. This structure also reduces the fluidity of the alloy, increases thermal cracking, and reduces corrosion resistance.
6. Manganese (Mn)
Manganese in aluminum alloys reduces the harmful effects of iron and transforms the flake or needle-like structures formed by iron into fine crystal structures. Therefore, aluminum alloys generally allow for less than 0.5% manganese. Excessive manganese content causes segregation, so the manganese content is generally controlled below 0.6%.
7. Nickel (Ni)
Nickel in aluminum alloys increases the strength and hardness of the alloy, but reduces corrosion resistance. Like iron, nickel reduces the corrosion of the alloy on the mold, neutralizes the harmful effects of iron, and improves the weldability of the alloy. When the nickel content is 1-1.5%, the castings can obtain a smooth surface after polishing. Due to the lack of nickel sources, nickel-containing aluminum alloys should be used as little as possible.
From the above, we can see that most alloying elements have both positive and negative effects. One aspect is the quantity, and the other is the combination with other elements. As the ancients said, "Nothing is inherently good or bad; it is the excess that causes harm."
