[China Aluminum Network] Several key elements influence the performance of aluminum alloys, including vanadium, calcium, lead, tin, antimony, bismuth, sodium, and others. These elements are typically not processed in the manufacturing of finished aluminum coils due to their varying applications. Each of these impurity elements has different melting points, structural properties, and forms distinct compounds with aluminum, which leads to diverse effects on the alloy’s overall performance.
1. Copper: One of the most significant alloying elements, copper contributes to solid-solution strengthening and also enhances the alloy through age-hardening via CuAl2 precipitates. Typical copper content in aluminum plates ranges from 2.5% to 5%, with optimal strengthening achieved between 4% and 6.8%. Most hard aluminum alloys fall within this range.
2. Silicon: In Al-Mg2Si alloys, silicon exhibits a relatively high solubility in aluminum (up to 1.85%), and its solubility decreases gradually as temperature drops. Silicon is commonly added to deformed aluminum alloys, especially for welding materials, and it provides some degree of strengthening.
3. Magnesium: Magnesium significantly enhances the strength of aluminum alloys. For every 1% increase in magnesium, tensile strength can rise by approximately 34 MPa. Adding up to 1% manganese can further improve the strengthening effect while reducing hot cracking tendencies. Manganese also helps refine Mg5Al8 precipitation, improving corrosion resistance and weldability.
4. Manganese: Manganese has a solubility of about 1.82% in solid solution, and as its content increases, so does the alloy’s strength. At 0.8% manganese, elongation reaches a maximum value. Al-Mn alloys are non-heat-treatable, meaning they do not undergo age hardening.
5. Zinc: In Al-Zn systems, zinc shows a high solubility at elevated temperatures (31.6% at 275°C), but this drops sharply to 5.6% at 125°C. While zinc can enhance strength in certain conditions, it also increases the risk of stress corrosion cracking, limiting its use in some applications.
6. Iron and Silicon: Both are common impurities in many aluminum alloys. They form various intermetallic phases depending on their relative proportions. When silicon exceeds iron, β-FeSiAl3 is formed; when iron is higher, α-Fe2SiAl8 appears. Imbalanced ratios may cause cracks during casting, and excessive iron can make the alloy brittle.
7. Titanium and Boron: Titanium is often added as an Al-Ti or Al-Ti-B master alloy. It forms TiAl2 and acts as a nucleation site during solidification, refining the microstructure. The critical titanium content for effective grain refinement is around 0.15%, and boron can lower this threshold even further.
8. Chromium and Niobium: Chromium forms intermetallics like (CrFe)Al7 and (CrMn)Al12, which hinder recrystallization and provide some strengthening. It also improves toughness and reduces stress corrosion cracking. However, chromium can cause discoloration during anodizing. Niobium, when added to high-silicon alloys, refines primary silicon particles, improving ductility and mechanical properties. For example, adding 0.02%–0.07% niobium to hypereutectic Al-Si alloys enhances tensile and yield strengths, along with elongation.
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