The gas in liquid aluminum is mainly hydrogen (about 80% ~ 90%) , followed by nitrogen, oxygen, carbon monoxide and so on. Hydrogen is almost insoluble in solid aluminum and highly soluble in liquid. The solubility of hydrogen above and below the solid phase line is 0.65 mL and 0.034 mL per 100 g of liquid aluminum (hydrogen at 0.1 mpa) , which means the solubility difference of hydrogen in solid and liquid phase is 19.1 times, the normal hydrogen content in molten aluminum is 0.1 ~ 0.4 mL per 100g. Because of the different solubility of hydrogen tends to escape from the melt, when the hydrogen pressure is greater than the surface tension and static pressure of the liquid, the formation of bubbles, and then the formation of pinholes in the casting. Therefore, the main problem in aluminum alloy melt purification is the high hydrogen content of aluminum alloy, and the existing methods can not meet the production requirements of high-quality aluminum alloy castings. Generally, the hydrogen content per 100g aluminum is 0.1 ~ 0.2 mL, and the hydrogen content per 100g aluminum is less than 0.06 mL for castings with special requirements (such as aviation castings) .

 
An inclusion is any substance other than a solid or liquid above the liquidus. The common non-metallic impurities in molten aluminum include oxides, nitrides, carbides, boride, etc. . Most of them are in the form of particles. The typical particle size is in the range of 1 ~ 30μm. In addition to from the charge, mainly the melting process of chemical reaction products. The thickness of the oxide film on the surface of aluminum increases from 2 to 10 μm to 200 μm near the melting point, and the oxide film on the liquid surface is not only thicker, but also its structure has changed, however, the outside of the liquid aluminum is loose, with small holes of 5 ~ 10 μm in diameter and filled with hydrogen, air and water vapor. In addition, some undesired primary intermetallic compounds, such as al-zr, al-ti and so on, will appear in the melt of high alloy, and the fe-rich al-fe phase and al-si-fe phase will be formed in the fe-containing al alloy, the al-si-fe phase is a kind of acicular compound, which can seriously destroy the AL matrix and affect the mechanical properties.

The hydrogen content in molten aluminum is greatly influenced by the inclusion. When the inclusion content is 0.002% and 0.02% , the corresponding hydrogen content was 0.2 mL/100g AL and 0.35 mL/100g AL.. On the contrary, when the impurity content in aluminum liquid is very low, the hydrogen content is also very low. Even if the hydrogen gas is artificially introduced, it will precipitate out automatically, it soon returned to its original level. Even if a small amount of inclusion exists, the critical concentration of pore formation can be significantly reduced. On the other hand, the critical hydrogen concentration for pore formation can reach 0.3 ml/100 gal without inclusion (or with very low inclusion content) . Therefore, degassing and impurity removal should be carried out at the same time, equally important. No matter which refining method, the effect of degassing and impurity removal is often both, but each has its own emphasis.