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Melting Characteristics of Aluminum Alloys

The melting of aluminum alloys is characterized by high heat consumption, easy oxidation, high hydrogen absorption tendency, and susceptibility to metallic impurity pickup.

1) High heat consumption: Although aluminum has a low melting point, it features a large latent heat of fusion, high specific heat, low blackness and strong heat reflectivity. Compared with other commonly used metals such as iron and copper, it consumes more heat during melting.

2) Easy oxidation: Aluminum has a strong affinity for oxygen and oxidizes rapidly to form aluminum oxide (Al₂O₃). The aluminum oxide film formed on the melt surface has a protective effect, yet once damaged, the oxide film will enter the melt and be extremely difficult to remove. This is because Al₂O₃ is not easily reducible and its density is close to that of the aluminum melt. Suspended in the melt, it will enter the ingot along with the melt, adversely affecting the quality of processed materials. More seriously, aluminum oxide is an excellent carrier for various gases, and its presence will cause the aluminum melt to absorb a large amount of hydrogen. Therefore, minimizing oxidation is a crucial issue in the melting process of aluminum alloys.

3) High hydrogen absorption tendency: Aluminum and its alloys have a strong ability to absorb gases (mainly hydrogen), especially in furnace atmospheres containing water vapor or reducing gases. Although the absolute amount of hydrogen absorbed by aluminum and its alloys is small, the solubility of hydrogen in the solid and liquid phases differs greatly at the melting point, leading to a high tendency of porosity and shrinkage cavity formation during ingot solidification. Therefore, minimizing hydrogen absorption is another key issue in the melting of aluminum and its alloys.

4) Susceptibility to metallic impurity pickup: Some alloying elements in aluminum and its alloys have high chemical activity. They can not only absorb iron dissolved directly from iron crucibles and tools, but also displace metallic impurities such as iron, silicon and zinc from many oxides in the furnace lining and many chlorides in the flux. Once these metallic impurities enter the aluminum melt, they cannot be removed. Moreover, the more melting times, the higher the impurity content, and the greater the impact on alloy performance. In severe cases, the grade of pure aluminum is reduced, and the alloy composition exceeds the standard leading to scrapping. Therefore, preventing contamination by metallic impurities is the third important issue in the melting of aluminum and its alloys.