In the process of refining and transportation of molten metal, foreign matter such as oxide scale, intermetallic compounds, and hearth fragments are easily brought into the molten metal to form non-metallic inclusions. These non-metallic inclusions are mainly oxide inclusions. According to the different formation period in the melting and casting process, it can be divided into primary oxide inclusions and secondary oxide inclusions.
Primary oxide inclusions mainly refer to all oxide inclusions formed before molten aluminum is poured. Primary oxide inclusions can be divided into two types according to their shapes: one is the large inclusions with uneven distribution in the macrostructure. This type of inclusions makes the alloy structure discontinuous, reduces the air tightness of the workpiece, and becomes the source of corrosion. Significantly reduce the strength and plasticity of aluminum alloys, and often become the source of cracks in parts. The second type of oxide inclusions refers to small and dispersed inclusions, which cannot be completely removed even after careful purification. They increase the viscosity of the molten metal. Reduce the feeding ability of molten aluminum during solidification, which is easy to cause shrinkage of castings.
Secondary oxide inclusions, also known as endogenous inclusions, are mainly formed during the casting process. In general, the distribution of endogenous inclusions is relatively uniform and the particles are relatively small. The splash and turbulence of molten aluminum during the pouring process are the main sources of secondary oxide inclusions. In the sand mold, the molten aluminum reacts with the moisture in the sand, and the water is decomposed into oxygen and hydrogen. Oxygen reacts with aluminum to form oxide inclusions, and hydrogen dissolves in the molten aluminum.
Molten metal refining flux is mainly used for primary oxide inclusions removal in furnaces. Ceramic foam filter can filter inclusions in molten metal, it mainly used for secondary oxide inclusions removal.