Main factors for the formation of pinholes in aluminum castings

Pinhole is a casting defect that is easy to occur in aluminum castings and has a certain impact on the quality of the casting. Hydrogen is the main cause of pinhole (80-90 of the gas dissolved in the aluminum liquid is hydrogen), and hydrogen is mainly produced by the decomposition of water vapor. Aluminum alloy in the melting process caused by water vapor generation, which is the main factor directly affecting the formation of pinholes. The main factors affecting the formation of pinholes are

In the process of aluminum alloy melting and pouring, the raw materials used, auxiliary materials, crystalline water and aluminum rust AL(OH)2 decomposition in some materials will produce water, there are many kinds of organic and inorganic auxiliary materials in the modeling materials with water, the auxiliary materials in the casting materials, coatings, etc. contain water because of poor preheating, etc. When aluminum alloy melting and pouring, a large amount of gas will be produced due to the decomposition of water vapor. When different melting equipment melts aluminum castings, the amount of air absorption and the degree of forming pores in aluminum alloy are different. New crucible and old crucible with rust and dirt should be blown sand or cleared by other methods before use, and heated to 700℃-800℃ to remove the water and other chemical substances adsorbed in the crucible, which will produce water vapor and form pores when melting and pouring because of containing water.

The tools used for melting, such as pouring ladle, the bell for gas removal, etc., should be cleaned from the residual metal, oxide and other dirt on the surface before use; the tools used for aluminum and magnesium alloys are required to be washed in a melt such as halite before use. If the preheating is poor, the surface absorption of water, will be in the melting and pouring process due to the heating of the formation of water vapor and a large amount of gas, resulting in the formation of aluminum castings pinhole.

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