The eight main elements that affect the performance of aluminum alloys are vanadium, calcium, lead, tin, bismuth, antimony, beryllium, and sodium. Depending on the intended use of the finished aluminum coil, different elements are added during processing. These impurity elements, because of their varying melting points and different structures, form different compounds with aluminum, and thus have different effects on the performance of the aluminum alloy.
1. Metal Element: The Influence of Copper
Copper is an important alloying element and provides some solid solution strengthening. In addition, the CuAl2 that precipitates during aging has a significant aging strengthening effect. The copper content in aluminum plates is usually between 2.5% and 5%. When the copper content is between 4% and 6.8%, the strengthening effect is optimal, which is why most hard aluminum alloys have copper content within this range.
2. Metal Element: The Influence of Silicon
For Al-Mg2Si alloy systems, according to the alloy phase diagram, the maximum solubility of Mg2Si in aluminum is 1.85%, and it decreases slowly with temperature reduction. In deformed aluminum alloys, silicon by itself is mainly added to aluminum plates for welding materials, but adding silicon to aluminum also provides a certain strengthening effect.
3. Metal Elements: The Effect of Magnesium
Magnesium has a significant strengthening effect on aluminum. For every 1% increase in magnesium, the tensile strength rises by about 34 MPa. If less than 1% manganese is added, it can boost the strengthening effect. Therefore, after adding manganese, the magnesium content can be reduced, which also lowers the risk of hot cracking. Additionally, manganese can help Mg5Al8 compounds precipitate more evenly, improving corrosion resistance and weldability.
4. Metal Elements: The Effect of Manganese
Manganese has a maximum solubility of 1.82% in solid solution. The strength of the alloy keeps increasing with more manganese dissolved, and the elongation reaches its peak at 0.8% manganese. Al-Mn alloys are naturally age-hardened without heat treatment, meaning they cannot be strengthened through heat treatment.
5. Metal Elements: The Effect of Zinc
In the Al-Zn alloy system, the solubility of zinc in aluminum is 31.6% at 275°C in the aluminum-rich region of the equilibrium phase diagram, but drops to 5.6% at 125°C. When zinc is added to aluminum alone, its contribution to strengthening the alloy under deformation is very limited, and it also tends to cause stress corrosion cracking, which limits its use.
6. Metal Elements: The Effects of Iron and Silicon
Iron in Al-Cu-Mg-Ni-Fe forged aluminum alloys and silicon in Al-Mg-Si forged aluminum and Al-Si welding rods and aluminum-silicon forging alloys are added as alloying elements. In other aluminum alloys, silicon and iron are common impurities that significantly affect the alloy's properties. They mainly exist as FeCl3 and free silicon. When silicon content is higher than iron, the β-FeSiAl3 (or Fe2Si2Al9) phase forms, whereas if iron content is higher than silicon, the α-Fe2SiAl8 (or Fe3Si2Al12) phase forms. Improper iron-to-silicon ratios can cause cracking in castings, and too much iron in cast aluminum can make the castings brittle.
7. Metal Elements: The Effects of Titanium and Boron
Titanium is a commonly added element in aluminum alloys, usually added in the form of Al-Ti or Al-Ti-B master alloys. Titanium forms the TiAl2 phase with aluminum, serving as a non-spontaneous nucleation center during crystallization and helping refine the forged and welded structure. In Al-Ti alloys, the critical titanium content for forming a superconducting reaction is about 0.15%, but if boron is present, this reduces to as low as 0.01%.
8. Metal Elements: The Effects of Chromium and Strontium
Chromium forms intermetallic compounds like (CrFe)Al7 and (CrMn)Al12 in aluminum sheets, which hinder the nucleation and growth processes of recrystallization. This provides some strengthening to the alloy, improves alloy toughness, and reduces sensitivity to stress corrosion cracking. However, it also increases quenching sensitivity and makes the anodized film appear yellow. The addition of chromium in aluminum alloys is generally no more than 0.35%, and it decreases as the amount of transition elements in the alloy increases. Adding 0.015%–0.03% strontium to aluminum alloys for extrusion transforms the β-AlFeSi phase in cast ingots into a Chinese-character-shaped α-AlFeSi phase, reducing the homogenization time of cast ingots by 60%–70%, and improving mechanical properties and plastic workability of the material, as well as surface roughness of the products. For high-silicon (10%–13%) deformed aluminum alloys, adding 0.02%–0.07% strontium can minimize the primary crystals and significantly enhance mechanical properties: ultimate tensile strength σb rises from 233 MPa to 236 MPa, yield strength σ0.2 increases from 204 MPa to 210 MPa, and elongation δ5 goes from 9% to 12%. In hypereutectic Al-Si alloys, strontium addition can reduce the size of primary silicon particles, improve plastic workability, and allow smooth hot and cold rolling.
