1. The Influence of Alloying Elements

Copper Element (Cu)

In aluminum-copper alloys, the maximum solubility of copper in aluminum is 5.65% at 548°C, decreasing to 0.45% at 302°C. Copper is a significant alloying element, contributing to solid solution strengthening. Additionally, the precipitation of CuAl2 during aging exhibits noticeable age hardening effects.

Typically, copper content in aluminum alloys ranges from 2.5% to 5%, with the most effective strengthening observed at copper contents between 4% and 6.8%. Hence, the copper content in most hard aluminum alloys falls within this range.

Al-Silicon (Si) Alloys

In the aluminum-silicon alloy series, the maximum solubility of silicon in the aluminum-rich phase is 1.65% at the eutectic temperature of 577°C. Although solubility decreases with decreasing temperature, these alloys generally cannot be heat-treated for strengthening. Aluminum-silicon alloys exhibit excellent casting properties and corrosion resistance.

Aluminum-magnesium-silicon alloys are made by combining magnesium and silicon with aluminum. The strengthening phase in these alloys is MgSi. The ratio of magnesium to silicon in this phase is 1.73 to 1.

When designing A-Mg-Si alloys, engineers proportion magnesium and silicon contents according to this ratio. Some Al-Mg-5i alloys add a suitable amount of copper to enhance strength, along with chromium to counteract copper’s adverse effects on corrosion resistance.

In Al-Mg2Si alloys, the maximum amount of Mg2Si that can dissolve in aluminum is 1.85% in the aluminum-rich phase. This amount decreases as temperature increases. In deformable aluminum alloys, silicon is added alone to aluminum only for welding materials, where it also contributes to some degree of strengthening.

Magnesium Element (Mg)

In the aluminum-magnesium alloy series, the solubility of magnesium in aluminum decreases with temperature. However, in most industrial deformable aluminum alloys, magnesium content is less than 6%, and silicon content is also low. These alloys cannot be heat-treated for strengthening but exhibit good weldability, corrosion resistance, and moderate strength.

Magnesium significantly strengthens aluminum, with approximately a 34 MPa increase in tensile strength per 1% magnesium addition. Adding less than 1% manganese can provide supplementary strengthening. Thus, the addition of manganese can reduce magnesium content, decrease the tendency for hot cracking, and improve corrosion resistance and weldability. Moreover, manganese can facilitate the uniform precipitation of Mg5Al8, improving corrosion resistance and weldability.

Manganese Element (Mn)

In the Al-Mn alloy series, at 658°C, the most manganese that can dissolve in the solid solution is 1.82%. Alloy strength increases continuously with increasing solubility, reaching maximum elongation at 0.8% manganese content. Al-Mn alloys are non-heat-treatable hardening alloys, meaning they cannot be strengthened through heat treatment.

Manganese can inhibit the recrystallization process of aluminum alloys, raise the recrystallization temperature, and significantly refine recrystallized grains. The refinement of recrystallized grains is mainly achieved through the dispersion of MnAl6 compound particles, which hinder the growth of recrystallized grains. Another function of MnAl6 is to dissolve impurity iron, forming (Fe, Mn)Al6, reducing the harmful effects of iron.

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

Zinc Element (Zn)

In the aluminum-zinc alloy series, at 275°C, the solubility of zinc in aluminum is 31.6%, decreasing to 5.6% at 125°C. When zinc is added alone to aluminum, it provides limited strength improvement under deformation conditions and tends to cause stress corrosion cracking, limiting its application.

Simultaneously adding zinc and magnesium to aluminum to form the strengthening phase Mg/Zn2 significantly strengthens the alloy. Increasing Mg/Zn2 content from 0.5% to 12% notably enhances tensile and yield strengths. In super-hard aluminum alloys where magnesium content exceeds the requirement for forming the Mg/Zn2 phase, the ratio of zinc to magnesium is controlled at around 2.7 to maximize stress corrosion cracking resistance.

Adding copper to Al-Zn-Mg base alloys to form Al-Zn-Mg-Cu series alloys achieves the greatest strengthening effect among all aluminum alloys, making it a vital aluminum alloy material in aerospace, aviation, and power industries.

2. The Influence of Trace Elements

Iron and Silicon Elements (Fe-Si)

Iron is added as an alloying element in Al-Cu-Mg-Ni-Fe series forging aluminum alloys, while silicon is added in Al-Mg-S series forging aluminum and in Al-Si series welding rods and aluminum-silicon casting alloys. Iron and silicon are common impurity elements in other aluminum alloys, significantly affecting alloy properties. They primarily exist as FeCl₃ and free silicon.

When silicon exceeds iron, the B-FeSiA13 (or Fe2S2Al9) phase forms, while when iron exceeds silicon, the α-Fe2SiAl8 (or Fe3i2Al12) phase forms. Improper ratios of iron and silicon can cause cracking in castings, and excessive iron in cast aluminum can induce brittleness.

Titanium and Boron Elements (Ti-B)

Titanium is a commonly used additive in aluminum alloys, added in the form of Al-Ti or Al-Ti-B master alloys. Titanium forms TiAl2 with aluminum, acting as a non-spontaneous nucleation core during crystallization, refining both casting and weld structures. In Al-Ti series alloys, the critical titanium content for the exothermic reaction is approximately 0.15%, which decreases to 0.01% in the presence of boron.

Chromium Element (Cr)

Chromium is a common additive in Al-Mg-Si, Al-Mg-Zn, and Al-Mg series alloys. At 600°C, the solubility of chromium in aluminum is 0.8%, virtually insoluble at room temperature.

Chromium forms metal intermetallic compounds like (CFe)Al7 and (CrMn)Al12, hindering nucleation and growth processes during recrystallization, providing a certain degree of strengthening to the alloy, improving toughness, and reducing stress corrosion cracking susceptibility. However, it may increase quench sensitivity, resulting in a yellow-colored anodized oxide film. Chromium content in aluminum alloys generally does not exceed 0.35% and decreases with increasing transitional element content.

Strontium Element (Sr)

Strontium is a surface-active element that alters the behavior of intermetallic phases in metallurgy. Therefore, using strontium for modification treatment improves alloy plasticity and final product quality.

Due to its long effective modification time, excellent effects, and reproducibility, strontium has replaced sodium in recent years in Al-Si casting alloys. Adding 0.015%-0.03% strontium to extrusion aluminum alloys transforms β-AlFesi phase into α-AlFesi, reducing homogenization time

Understanding the intricate influence of various elements in aluminum alloys is crucial for optimizing material properties and enhancing performance across industries. Whether you’re involved in aerospace, automotive, construction, or any other field utilizing aluminum alloys, harnessing this knowledge can lead to innovations and advancements in product design, manufacturing processes, and end-product quality. Contact us right now: https://gree-ge.com/die-casting/