Formation of shrinkage cavity and porosity in valve casting

Formation of casting shrinkage cavity and shrinkage cavity during the solidification process of liquid metal in the mold, due to the shrinkage of liquid, solidification shrinkage and volume reduction, if the shrinkage can not be supplemented, a hole will be formed in the last solidified part of the casting, which is called shrinkage cavity.

1.Shrinkage cavity the shrinkage cavity is usually hidden in the upper part of the casting or the last solidified part, sometimes exposed by machining. In some cases, the shrinkage cavity is produced on the upper surface of the casting, showing obvious pit, which is also called “open shrinkage cavity”. The shape of the shrinkage cavity is characterized by irregular shape, but it is mostly close to the inverted cone, and its inner surface is not smooth. The formation process of shrinkage cavity is shown in Figure 3-9. After the liquid metal fills the mold, the metal close to the surface of the mold cavity will soon drop to the solidification temperature and solidify into a shell due to the endothermic of the mold. As the temperature continues to drop, the solidified layer thickens, and the remaining liquid inside, due to the shrinkage of the liquid and the solidification shrinkage of the supplementary solidified layer, the volume is reduced, and the liquid level drops, there is a gap in the casting. The temperature continues to drop, the shell continues to thicken, and the liquid level continues to drop. When the inner part is completely solidified, a shrinkage cavity is formed in the upper part of the casting. The casting with shrinkage cavity is cooled to room temperature at the end of solidification temperature, and the external dimension is slightly reduced due to solid shrinkage.

2.In essence, shrinkage porosity is to disperse concentrated shrinkage pores into a large number of small shrinkage pores. For the same shrinkage volume, the distribution area of shrinkage porosity is much larger than that of shrinkage cavity. Although the formation of shrinkage porosity is also due to the lack of liquid shrinkage and solidification shrinkage of the alloy, the specific factors have their own particularity compared with the concentrated shrinkage cavity. The formation of shrinkage porosity can be illustrated by a cylindrical casting. The casting begins to solidify from the outer layer at first, but the front edge of solidification is uneven. Because the heat dissipation conditions of the casting are similar in the circumferential direction, the front edge of solidification in the later stage of solidification almost reaches the center at the same time, forming a simultaneous solidification area. In this region, the remaining liquid is divided into many small liquid regions by the uneven solidification front. Finally, when these small liquid areas with large amount solidify and contract, shrinkage porosity will be formed due to lack of feeding.

When the crystallization interval of the alloy is large, in addition to the increase of shrinkage porosity due to the above reasons, micro shrinkage porosity will also occur on a larger area. At this time, crystallization is carried out at the same time in a wide solidification area, including growing solid and liquid metal (see Figure 3-11). The primary crystal often grows in a dendrite shape, so that the liquid is divided into many small liquid areas. If the feeding conditions are not good, the casting will also produce shrinkage porosity, which is even smaller. It should be observed under the microscope. It is difficult to avoid the micro shrinkage in the casting completely. Generally, the general casting is not regarded as a defect. However, if the casting is required to have high air tightness, prevent leakage under pressure, or consider physical and chemical properties, it is necessary to try to prevent or reduce the micro shrinkage.

From the formation of shrinkage cavity and porosity, we can see that:

① The larger the liquid shrinkage and solidification shrinkage of the alloy (such as cast steel, white cast iron, aluminum bronze, etc.), the larger the shrinkage volume, the easier the casting to form shrinkage cavity.

② The higher the pouring temperature is, the larger the liquid shrinkage is (generally, the volume shrinkage increases by about 1.6% for every 100 ℃), and the easier it is to produce shrinkage cavity.

③ The alloy with large crystallization interval is easy to form shrinkage porosity; the alloy with pure metal or eutectic composition has little tendency of shrinkage porosity, and most of them are easy to form concentrated shrinkage porosity.

Shrinkage porosity, especially Microshrinkage, is widely distributed, which is difficult to supplement and find. Centralized shrinkage is easier to check and repair, and it is also easier to take process measures to prevent. Therefore, alloy with near eutectic composition or small crystallization interval is often used to produce castings. Any form of shrinkage cavity makes the mechanical properties of the casting significantly reduced. Shrinkage porosity can also affect the air tightness, physical and chemical properties of the casting. Therefore, shrinkage cavity and shrinkage porosity are the major defects of castings, which must be prevented by adopting appropriate technological measures according to the technical requirements of castings.

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