Filtration and Purification Methods used to Remove Inclusions

Inclusions and gases in the metal have a significant impact on the strength, fatigue resistance, corrosion resistance, and stress corrosion cracking performance of the material. Effectively controlling the oxidized inclusions in the melt to improve the quality of cast rods and rolled plates has become a common goal pursued by the metallurgy, casting and material industries of all countries. Currently, filtration and purification methods are widely used to remove inclusions in aluminum alloy melts.

The inclusions in aluminum alloy come directly from the charge, and most of them are formed during the melting and pouring process, mainly oxide inclusions. All inclusions before casting are called primary oxidation inclusions, which can be divided into two categories according to their size: One is the large inclusions with uneven distribution in the macrostructure, which makes the alloy structure discontinuous and reduces the compactness of the casting. It becomes the source of corrosion and the source of cracks, thereby significantly reducing the strength and plasticity of the alloy. The other is the fine dispersed inclusions, which cannot be completely removed after refining, which increases the viscosity of the molten metal and reduces the feeding capacity of the molten aluminum during solidification. The secondary oxidation inclusions are mainly formed during the pouring process. During pouring, the molten aluminum contacts the air, and the oxygen reacts with the aluminum to form oxidized inclusions. During the smelting process, the aluminum alloy contacts various components in the furnace gas to form compounds such as Al2O3. Al2O3 in molten aluminum will increase the hydrogen content of aluminum alloy melt. Therefore, the content of Al2O3 in molten aluminum has a great influence on the formation of pores in aluminum castings.

Filtration and Purification

In recent years, filtration and purification methods of molten aluminum have been studied at home and abroad, such as vacuum dynamic processing, ultrasonic continuous degassing purification and corundum ceramic filters, which have received good results. However, these process methods are more complicated and costly, and it is difficult to popularize them in industrial production. As for the metal filter fiber cloth filter, it can only remove the large inclusions in the aluminum alloy melt, but the inclusions below the micron level cannot be removed, and the metal filter will also pollute the aluminum alloy. The use of foam ceramic filter plates can filter out fine inclusions and significantly improve the mechanical properties and appearance quality of the castings.

The ceramic foam filter plate has a multi-layer network and multi-dimensional through holes, and the holes communicate with each other. When filtering, the molten aluminum carries inclusions and flows along the tortuous channels and pores, and is directly intercepted, adsorbed, deposited and so on when it comes into contact with the foamed framework of the filter plate. When the melt flows in the hole, the channel of the filter plate is curved, the melt flowing through the channel changes the flow direction, and the inclusions collide with the hole wall and adhere firmly to the hole.

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