1. Influence of Alloying Elements in Aluminum Alloys
Copper (Cu)
In aluminum-copper alloys, copper’s maximum solubility in aluminum is 5.65% at 548°C, decreasing to 0.45% at 302°C. Copper plays a crucial role in solid solution strengthening. The precipitation of CuAl₂ during aging contributes significantly to age hardening. Typically, copper content ranges from 2.5% to 5%, with optimal strengthening observed between 4% and 6.8%. Most hard aluminum alloys contain copper within this range.
Aluminum-Silicon (Si) Alloys
In aluminum-silicon alloys, silicon’s maximum solubility in the aluminum-rich phase is 1.65% at the eutectic temperature of 577°C. Although solubility decreases as temperature drops, these alloys generally cannot be heat-treated for strengthening but exhibit excellent casting properties and corrosion resistance.
Aluminum-magnesium-silicon alloys combine magnesium and silicon, forming the strengthening Mg₂Si phase with a Mg-to-Si ratio of approximately 1.73:1. Engineers carefully balance these elements to optimize strength. Some alloys include copper to boost strength and chromium to mitigate copper’s adverse effects on corrosion resistance.
Magnesium (Mg)
Magnesium solubility in aluminum decreases with temperature. Most industrial deformable aluminum alloys contain less than 6% magnesium and low silicon content. While these alloys cannot be heat-treated, they offer good weldability, corrosion resistance, and moderate strength. Magnesium significantly improves tensile strength, increasing it by about 34 MPa per 1% addition. Adding manganese (<1%) further strengthens the alloy and improves corrosion resistance and weldability by facilitating uniform precipitation of Mg₅Al₈.
Manganese (Mn)
In Al-Mn alloys, manganese solubility is about 1.82% at 658°C. Alloy strength increases with manganese content, peaking in elongation at 0.8% Mn. These alloys are non-heat-treatable but benefit from manganese’s ability to inhibit recrystallization, refine grain structure via MnAl₆ particles, and reduce iron impurity effects by forming (Fe, Mn)Al₆ compounds. Manganese is widely used either alone or with other elements in aluminum alloys.
Zinc (Zn)
Zinc’s solubility in aluminum is 31.6% at 275°C but falls to 5.6% at 125°C. Zinc alone offers limited strengthening and tends to cause stress corrosion cracking. However, adding zinc with magnesium forms MgZn₂ phases that greatly enhance tensile and yield strengths. Super-hard aluminum alloys control the Zn:Mg ratio (~2.7) to maximize stress corrosion cracking resistance. The Al-Zn-Mg-Cu alloy series provides the highest strength among aluminum alloys, making it essential in aerospace, aviation, and power industries.
2. Influence of Trace Elements
Iron and Silicon (Fe-Si)
Iron and silicon are common impurities that significantly affect aluminum alloy properties. In Al-Cu-Mg-Ni-Fe forging alloys, iron is added intentionally, while silicon is included in Al-Mg-S forging alloys and Al-Si welding rods and castings. Improper Fe-Si ratios cause phases like B-FeSiAl₁₃ or α-Fe₂SiAl₈ to form, which can lead to casting cracks and brittleness if excessive.
Titanium and Boron (Ti-B)
Titanium, often added as Al-Ti or Al-Ti-B master alloys, forms TiAl₂ particles acting as nucleation cores to refine cast and weld structures. The critical titanium content is about 0.15%, reduced to 0.01% with boron presence.
Chromium (Cr)
Chromium is common in Al-Mg-Si, Al-Mg-Zn, and Al-Mg alloys. At 600°C, its solubility is 0.8%, negligible at room temperature. Chromium forms intermetallics like (CrFe)Al₇ and (CrMn)Al₁₂ that hinder recrystallization, enhance toughness, and reduce stress corrosion cracking but may increase quench sensitivity and alter anodized film color. Chromium content is generally kept below 0.35%.
Strontium (Sr)
Strontium modifies intermetallic phases in aluminum alloys, improving plasticity and final product quality. It has replaced sodium in recent years for Al-Si casting alloys due to its longer modification effect and better reproducibility. Adding 0.015%-0.03% strontium to extrusion alloys converts β-AlFeSi to α-AlFeSi, reducing homogenization time.
Understanding the roles of these alloying and trace elements is essential for optimizing aluminum alloy properties, improving product performance, and advancing manufacturing processes across industries such as aerospace, automotive, and construction.
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