Porosity casting defects are holes that appear in the interior or surface of the casting, and the cross section is circular, elliptical, waist circular, pear shaped or needle shaped, which exist in isolation or in groups. Large pores usually exist in isolation, and small pores usually exist in groups. The casting defects with air holes exposed on the surface of castings are called surface air holes; The waist round dispersed pores located under the skin of the casting are called subcutaneous pores; Pores scattered inside the casting and shaped like needles are called surface pinholes. The pore wall is generally smooth, and the pore casting defects often coexist with the slag inclusion shrinkage casting defects.

Classification and formation principle of blowhole casting defects: according to the causes of formation, blowhole defects are divided into involved blowholes, invaded blowholes, reaction blowholes and plate outlet blowholes. Entrapment pores: pores formed by entrapment of gas in the filling process of molten metal, which are mostly isolated circular or oval. Invading air hole: the air hole casting defect formed by the gas generated by sand mold, sand core, coating and chill invading the surface of the casting, which is mostly round or oval in size.

2Al+3H2O==Al2O3+6H

Mg+H2O==MgO+2H

The solubility of hydrogen in Al Si alloy is shown in Figure 1:

Reactive blowhole defect: a blowhole casting defect caused by a certain chemical reaction between some components in the molten metal or between the molten metal and the sand mold or coating, which is located on the surface of the casting in groups.

Precipitated blowhole defect: a blowhole casting defect formed by the precipitation of gas dissolved in liquid metal during the casting molding process. Usually, it is distributed at the hot spot in small circular, oval or needle shaped groups, and it is distributed in the local area of the whole end face of the rotating part.