In die casting, the formation of shrinkage cavities and porosit

  • When the die casting structure is reasonable or not, the quality of the casting as well as the feasibility and simplicity of the production process are greatly influenced. Because the structure of die casting is equivalent to the production characteristics of investment casting, it serves as an example of a reasonable structure for some investment casting. As part of the die casting process's quality assurance measures, it is frequently necessary to adjust the process rib and hole on the investment casting to meet specific requirements. The reference size of the process rib design serves as an example of the process hole's application. Die casting parts have the ability to cast extremely complex parts. After a minor structural improvement, several parts assembly parts and weldments originally produced by others can be directly cast into a single investment casting in order to improve the production efficiency and accuracy in the process of use. The pouring temperature of metal mold casting should be properly controlled during the casting process. If the pouring temperature is too high, the cooling will be slow and gradual, the crystallization of the product will be coarse, and the mechanical die cast parts of the product will be low, making it easy to form pinholes, blowholes, and other defects. If the pouring temperature of metal mold casting is too low, it will directly result in defects such as insufficient pouring and cold shut. The casting temperature is thus controlled in accordance with the casting structure and casting process characteristics, but the maximum casting temperature is kept to a bare minimum. Die casting is a process in which the quality of the casting is directly affected by the casting process. Therefore, when designing the casting process and mold, we should consider the shrinkage rate in advance and constantly adjust the size of the metal mold.

    The following are the steps involved in the formation of shrinkage cavities and porosity in die casting.

    The post-solidification part of the casting will develop holes if the volume reduction caused by liquid shrinkage and solidification shrinkage cannot be compensated for during the subsequent cooling and solidification process of the liquid alloy poured into the mold. The larger volume is referred to as the shrinkage cavity, and the smaller and more dispersed pores are referred to as the shrinkage porosity.

     


    1. Cavity shrinkage
    Most of the time, it will appear in the upper part of the casting or the post-solidification part, and its shape will be primarily an inverted cone with a rough inner surface, which will be hidden within the inner layer of the casting in most instances.

    The greater the narrower the range of crystallization temperatures, the greater the tendency of the casting alloy to solidify layer by layer, and the greater the ease with which a shrinkage cavity can form. The liquid alloy is poured into the mold first. Because of the cooling effect of the mold, a layer of liquid alloy near the surface of the mold solidifies quickly, but the inner part of the mold remains in the liquid state; with the continuous decrease in casting temperature, the thickness of the shell is continuously thickened, and the volume of the internal liquid alloy is continuously reduced due to the liquid shrinkage of the alloy and the solidification shrinkage of the supple metal; with the continuous die casting aluminum in casting temperature, the thickness of the shell is continuously thickened, and the volume of the internalAs a result, the casting solidifies layer by layer until it is completely solidified, forming a shrinkage cavity in the upper part. As the casting cools to room temperature, solid shrinkage will cause the overall dimension of the casting to be slightly reduced.

    2. Porosity decreases as a result of shrinkage.
    It is the dispersed and fine shrinkage cavity caused by the failure of liquid alloy in the casting's solidification area that is referred to as shrinkage porosity.

    The distribution of porosity can be divided into two categories: macro porosity and micro porosity.

     


    (1) The term "macro shrinkage" refers to the small holes that can be seen with the naked eye or with a magnifying lens. It is most often found below the shrinkage cavity.
    In this case, the term "micro porosity" refers to the small pores that are distributed among dendrites and can only be seen under a microscope. Even throughout the entire section of casting, the distribution of shrinkage porosity is widespread and difficult to avoid completely. They are not considered defects in general castings, with the exception of some castings that must meet strict requirements for compactness and mechanical properties.

    In a nutshell, alloys that tend to solidify layer by layer, such as pure metals, eutectic alloys, or alloys with a narrow crystallization temperature range, tend to form shrinkage cavities and are difficult to form shrinkage porosity. However, some alloys that are prone to mushy solidification, such as alloys with a wide range of crystallization temperatures, have a low tendency to produce shrinkage cavities, but are more prone to producing shrinkage porosity. Consequently, shrinkage and porosity can be transformed into one another within a certain range of values.