Structural characteristics of water cooling shell and analysis of internal defects of castings

As shown in Figure 1, a low-pressure casting water-cooled housing for electric bus is in the shape of a drum with a diameter of 280mm and a height of 350mm; The wall thickness of the casting is 3-6 mm, and the casting material is A356.2 aluminum alloy. During solidification of aluminum alloy in low pressure casting, shrinkage cavity and porosity are mainly caused by liquid shrinkage and volume shrinkage between liquidus and solidus. Due to the uneven wall thickness of the water-cooled shell structure of electric vehicle, especially the local closed structure of barrel sand core, the feeding channel is often blocked or blocked, the pressure transmission of low-pressure casting is blocked, resulting in the isolated liquid phase area in the local part of the casting; The volume shrinkage of the separated liquid part is characterized by shrinkage porosity and shrinkage cavity. Therefore, it is necessary to carry out high-precision scanning of the tomographic image, focus on the detection of the key position where the barrel shaped sand core section of the shell casting has obvious isolation effect, analyze the solidification and crystallization of the specific area of the aluminum alloy casting, and take targeted process improvement measures.

As shown in Figure 2, through the solidification process analysis of AnyCasting software, it can be seen that there are several wide positions in the cross-section structure of barrel shaped sand core, which cause structural separation on the cylinder wall around the shell, reduce the wall thickness connection part between the shell body and the peripheral shape, and produce lagging independent liquid phase at four positions a, B, C and D of aluminum alloy casting during solidification and crystallization, These isolated liquid regions block the normal crystallization pressure transfer of low pressure casting, and slow down the heat emission of this part of the region. The volume shrinkage of the separated liquid phase during final solidification is characterized by shrinkage porosity and porosity. In view of this, in practice, circulating water cooling beryllium copper alloy inserts are added to the outer wall of a, B, C and D to accelerate the local solidification of castings; In order to achieve the normal solidification sequence of low-pressure casting, the process parameters such as the pressurization speed of low-pressure casting are adjusted to improve the effect of pressure transfer and the solidification and crystallization quality of casting, so that the closed heat is transmitted to the heat dissipation device. These optimization measures need to be verified by the enlarged image of the scanning electron microscope.

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