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Method For Removing Shrinkage Cavity And Shrinkage Of Zinc Alloy Die Casting

Posted by: CCHEN JIANG 2020-06-01 Leave a comment

What is the reason for the formation of shrinkage holes in zinc alloy die castings? Is there any way to solve it??
Shrinkage and shrinkage are formed in this way: the liquid alloy poured into the mold, in the subsequent solidification process, if the volume shrinkage caused by its liquid shrinkage and solidification shrinkage cannot be supplemented, solidify on the zinc alloy die casting Some holes are formed in the area. The holes with larger volume are called shrinkage holes, and the small and scattered holes are called shrinkage holes.
1. Shrinkage
It usually appears in the solidified part of the upper part of the casting, and the shape is mostly inverted conical, the inner surface is rough, and usually hidden in the inner layer of the casting.
The casting alloy with narrower crystallization temperature range tends to solidify layer by layer, and the easier it is to form shrinkage cavities. First, the liquid alloy fills the casting mold. Due to the effect of the casting mold, a layer of liquid alloy near the surface of the casting mold quickly solidifies, while the inside is still in a liquid state; as the temperature of the casting continues to decrease, the thickness of the shell continues to thicken The liquid alloy shrinks its volume due to its own liquid shrinkage and the solidification shrinkage of the supplementary shell, which causes the liquid level to drop and causes voids in the zinc alloy die casting. In this way, the casting is solidified layer by layer until it is completely solidified, and a shrinkage cavity is formed in the upper part; the solid shrinkage will slightly reduce the external dimensions of the zinc alloy die casting.
In short, the greater the liquid shrinkage and solidification shrinkage of the zinc alloy, the larger the shrinkage cavity volume.
1. Shrinkage
Shrinkage is the dispersion and fine shrinkage caused by the liquid alloy in the solidified area of ​​the zinc alloy die casting.
According to the distribution pattern, shrinkage is divided into macro shrinkage and micro shrinkage:
(1) Macro shrinkage refers to tiny holes that can be seen with the naked eye or a magnifying glass. It usually appears below the shrink hole.
(2) Miniature shrinkage refers to tiny holes distributed between dendrites, which can only be seen under a microscope. This shrinkage has a larger distribution surface, and it is difficult to completely avoid the entire cross section of the casting. General castings are not treated as non-good products, except for some zinc alloy die castings that require high compactness and mechanical properties.
In short, alloys that tend to solidify layer by layer, such as pure metals, alloys with eutectic composition, or alloys with a narrow crystallization temperature range, have a large tendency to form shrinkage pores and are not easy to form shrinkage; while some alloys that tend to solidify in a paste state Alloys with a wide crystallization temperature range have a small tendency to shrink, but they are prone to shrinkage. Therefore, shrinkage and shrinkage can be converted into each other within a certain range.
Using appropriate technological measures to achieve “sequential solidification” of castings, castings without shrinkage can be obtained.
The so-called sequential solidification refers to the use of some appropriate technological measures to make the part of the casting away from the riser or gate solidify first. In this way, the volume reduction caused by solidification of the first solidified part I of the casting can be supplemented by the liquid alloy of the later solidified part II; the shrinkage of part II is supplemented by the liquid alloy of part III; Supplement, so that the shrinkage of each part of the casting can be supplemented, and the shrinkage hole is transferred to the riser. Remove the riser to get a dense casting.

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