China Aluminum Industry Net: 1. The influence of impurity elements on aluminum alloys is significant. Vanadium forms refractory compounds such as VAl11 during the casting process, which helps in grain refinement, although it is less effective compared to titanium and zirconium. It also contributes to refining the recrystallized structure and increasing the recrystallization temperature. This makes vanadium a useful element in certain alloy systems, though not the most potent.
Calcium has very low solubility in aluminum but forms CaAl4 compounds, which can enhance superplasticity in specific compositions. Alloys containing around 5% calcium and 5% manganese exhibit this property. Additionally, when combined with silicon, calcium forms CaSi, which is insoluble in aluminum. This reduces the amount of dissolved silicon, slightly improving the electrical conductivity of pure aluminum. Calcium also improves machinability, though CaSi2 does not contribute to strengthening through heat treatment. Small amounts of calcium help remove hydrogen from molten aluminum, which is beneficial for reducing porosity.
Lead, tin, and bismuth are low-melting metals with limited solubility in aluminum. They slightly reduce the mechanical strength of the alloy but improve its machinability. Their presence can also promote feeding during solidification. In high-magnesium alloys, niobium is added to prevent sodium embrittlement, a critical concern in certain applications.
Germanium is primarily used as a modifier in cast aluminum alloys, while its use in deformed alloys is rare. In Al-Mg alloys, replacing niobium can help avoid sodium embrittlement. Niobium is also added to some Al-Zn-Mg-Cu alloys to improve their formability during hot and cold pressing processes. Its role in enhancing the oxide film structure and reducing casting losses is notable.
Barium, although toxic, is not allowed in aluminum alloys that come into contact with food or beverages due to health risks. Germanium levels in solder materials are typically kept below 8 μg/ml. Similarly, niobium content must be controlled in aluminum alloys used for welding to ensure structural integrity and performance.
Sodium is nearly insoluble in aluminum, with a maximum solubility of less than 0.0025%. Its low melting point (97.8°C) makes it prone to segregation during solidification. Sodium tends to adsorb on dendritic surfaces or grain boundaries, forming a liquid layer during hot working. This can lead to brittle cracking and the formation of NaAlSi compounds, but no free sodium remains, so "sodium embrittlement" doesn't occur under normal conditions. However, when magnesium exceeds 2%, it captures silicon, allowing free sodium to precipitate, causing embrittlement. Thus, sodium-containing fluxes are avoided in high-magnesium alloys. To prevent embrittlement, chlorination is used to convert sodium into NaCl, which is removed as slag. Adding cerium or rare earth elements can also help by forming stable compounds like Na2Bi or Na3Sb, thus mitigating the risk.
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