The role and impact of various elements in aluminum alloys on the properties of aluminum materials

Copper Element

In aluminum-copper alloys rich in aluminum at 548°C, the maximum solubility of copper in aluminum is 5.65%. When the temperature drops to 302°C, the solubility of copper is 0.45%. Copper is an important alloying element and provides a certain solid solution strengthening effect. Additionally, the CuAl2 precipitated during aging has an obvious aging strengthening effect. The copper content in aluminum alloys is usually between 2.5% and 5%. The strengthening effect is optimal when the copper content is between 4% and 6.8%, so the copper content in most hard aluminum alloys falls within this range.

Silicon Element

In the Al–Si alloy system, the silicon-rich part of the Al–Si alloy has a maximum solubility of 1.65% in the solid solution at the eutectic temperature of 577°C. Although the solubility decreases as the temperature drops, these types of alloys generally cannot be strengthened by heat treatment. Aluminum-silicon alloys have excellent casting properties and corrosion resistance.

If magnesium and silicon are simultaneously added to aluminum to form an Al-Mg-Si alloy, the strengthening phase is MgSi. The mass ratio of magnesium to silicon is 1.73:1. When designing the composition of an Al-Mg-Si alloy, the content of magnesium and silicon in the matrix is configured according to this ratio. In some Al-Mg-Si alloys, to improve strength, an appropriate amount of copper is added, while an appropriate amount of chromium is simultaneously added to counteract the adverse effects of copper on corrosion resistance.

In the Al-rich part of the Al-Mg2Si alloy system equilibrium phase diagram, the maximum solubility of Mg2Si in aluminum is 1.85%, and it decreases slowly with decreasing temperature.

In deformable aluminum alloys, silicon is added to aluminum alone only for welding materials, but adding silicon to aluminum also has a certain strengthening effect.

Magnesium element

Although the solubility curve in the aluminum-rich section of the Al-Mg alloy equilibrium phase diagram indicates that the solubility of magnesium in aluminum decreases significantly with decreasing temperature, in most industrially used wrought aluminum alloys, the magnesium content is less than 6%, and the silicon content is also low. Such alloys cannot be strengthened by heat treatment, but they have good weldability, good corrosion resistance, and moderate strength.

The strengthening effect of magnesium on aluminum is significant; for every 1% increase in magnesium, the tensile strength increases by approximately 34 MPa. If less than 1% manganese is added, it may provide additional strengthening. Therefore, adding manganese can allow for a reduction in magnesium content while also reducing the tendency for hot cracking. In addition, manganese can promote the uniform precipitation of the Mg5Al8 compound, improving corrosion resistance and welding performance.

Manganese Element

In the Al-Mn alloy system, the partial equilibrium phase diagram shows that at the eutectic temperature of 658°C, the maximum solubility of manganese in the solid solution is 1.82%. The strength of the alloy continuously increases with the increase in solubility, and when the manganese content is 0.8%, the elongation reaches its maximum. Al-Mn alloys are non-age-hardening alloys, meaning they cannot be strengthened by heat treatment.

Manganese can prevent the recrystallization process of aluminum alloys, increase the recrystallization temperature, and significantly refine the recrystallized grains. The refinement of recrystallized grains is mainly achieved by the MnAl6 compound dispersoids, which hinder the growth of recrystallized grains. Another function of MnAl6 is its ability to dissolve impurity iron, forming (Fe, Mn)Al6, thereby reducing the harmful effects of iron.

Manganese is an important element in aluminum alloys. It can be added alone to form Al-Mn binary alloys, but more often it is added together with other alloying elements. Therefore, most aluminum alloys contain manganese.

Zinc element

In the Al-Zn alloy system, according to the Al-rich side of the equilibrium phase diagram, the solubility of zinc in aluminum is 31.6% at 275°C, while at 125°C, its solubility decreases to 5.6%.

When zinc is added to aluminum alone, the improvement in the strength of the aluminum alloy under deformation conditions is very limited. At the same time, there is a tendency for stress corrosion cracking, which restricts its application.

Adding both zinc and magnesium to aluminum forms the strengthening phase Mg/Zn2, which significantly strengthens the alloy. Increasing the Mg/Zn2 content from 0.5% to 12% can markedly improve tensile strength and yield strength. When the magnesium content exceeds that required to form the Mg/Zn2 phase in ultra-hard aluminum alloys, the stress corrosion cracking resistance is maximized when the zinc-to-magnesium ratio is controlled at around 2.7.

If copper is added to the Al-Zn-Mg base to form Al-Zn-Mg-Cu alloys, the strengthening effect is the greatest among all aluminum alloys, making it an important aluminum alloy material in the aerospace, aviation, and power industries.