Aluminum alloy in the smelting process, pouring out of the furnace, the deterioration of the process, the high air flow rate during purging and casting process, the aluminum alloy liquid will be strongly disturbed. The turbulence of the liquid metal surface will pull the oxide film on its surface to expand, fold, and break. Oxide film exposed at the exposed alloy surface cleanliness, will be oxidized to produce a new oxide film. Folding oxide film will make it toward the dry side of the atmosphere on the surface of the bonded to each other, and wrapped in a small amount of air between the two drying surfaces, as 'oxide film sandwich'. Oxide interlayer easy to get involved in liquid metal, but also in the turbulence of the liquid metal was squeezed into small groups. Since Al2O3 has a melting point higher than that of aluminum alloy liquid by more than a thousand degrees Celsius, and has high chemical stability, small clusters do not fuse and do not dissolve in aluminum alloy. Although the density of Al 2 O 3 is slightly higher than that of aluminum alloy, the density of oxide film interlayed in air is relatively close to that of aluminum alloy. Therefore, in addition to the long holding time in a large holding furnace oxide film interlayer may sink, the general casting production conditions, will be more stable suspension in the aluminum alloy liquid. An aluminum alloy liquid that has been suspended with an oxide film interlayer is again disturbed, and more oxide film interlayer is generated. Casting process, the alloy smelting, pouring out from the furnace, metamorphic treatment, purification, pouring and other operations will have a strong disturbance of aluminum alloy liquid, aluminum alloy liquid in addition to retain the original oxide film interlayer, but also because of Again disturbed and continue to add new oxide film sandwich. Therefore, the liquid metal into the cavity contains a large number of tiny oxide film sandwich. After the molten metal is filled in the cavity, it is in a quiescent state, and the oxide film sandwich squeezed into clusters gradually expands into small pieces. After the liquid metal is cooled to below the liquidus, the nucleation and growth of the dendrites are also the factors that promote the interlayer expansion of the oxide film extruded. Casting solidified, a large number of small pieces of oxide film sandwich itself is a small crack, starting from the role of cutting the metal matrix, of course, will reduce the mechanical properties of the alloy, but more harmful to the induced stomatal and shrinkage of the production. As the temperature of the liquid metal gradually decreases, the solubility of hydrogen in the molten metal decreases. However, it is very difficult for hydrogen to precipitate out of the liquid metal in the form of pores. When a new phase (gas phase) is generated in a homogeneous liquid phase, it always consists of aggregating several atoms or molecules first, and its volume is small. This tiny new phase, its specific surface area (ie, the unit volume of the surface area) is extremely large, to produce a new interface, you need to work on it, which is the new phase interface energy, that is, its surface area and surface tension The product of It is practically impossible to get such a large amount of energy during liquid cooling of an aluminum alloy. Even if the core of a new phase is created, it takes a great deal of energy to grow up and only grows when the new phase size exceeds a certain threshold. The size of the core less than the critical phase of the new phase can not grow up, only to go away.
