answer:
When the metal solidifies and shrinks, the defects caused by the molten metal not effectively feeding the casting are called shrinkage defects, including shrinkage holes, shrinkage porosity, shrinkage, and shrinkage.
1. Features
①Shrinkage cavities: There are irregularly shaped holes on the castings, the hole walls are rough and have dendrites, which are called shrinkage cavities. It usually occurs in the final solidification part of the casting.
② Shrinkage porosity: There are scattered and small shrinkage holes on the section of the casting, sometimes with the help of a magnifying glass, which is called shrinkage porosity. For example, when low pressure casting is used to produce aluminum pistons, sometimes shrinkage occurs at the top of the piston.
③ Looseness: very small holes in the slow solidification zone of the casting. Distributed in and between dendrites, it is a mixed defect of diffuse pores, micro-shrinkage and coarse structure, which reduces the compactness of the casting and is easy to cause leakage.
④ Subsidence: The collapse of the upper plane of the thick end face of the casting or the intersection of the cross-section. There are sometimes shrinkage holes below the shrinkage, and sometimes the shrinkage also appears near the inner shrinkage hole.
⑤Shrinkage: A casting defect that occurs when using water glass limestone sand to produce castings, which is characterized by the expansion of the section size of the castings.
⑥Shrinkage cracks: Cracks caused by improper feeding, blocked shrinkage or uneven shrinkage of the casting. May appear just after solidification or at lower temperatures.
2. Causes
The reasons for the formation of shrinkage cavities and shrinkage porosity: During the solidification process of the molten metal, due to the liquid shrinkage and solidification shrinkage of the alloy, the volume loss caused by the volume shrinkage cannot be compensated, that is, no shrinkage can be obtained, often at the solidified part of the casting. Holes appear. Different from general gravity casting, low pressure casting is filling from bottom to top, and the gate is at the bottom. In order to obtain sufficient feeding of the casting, it is necessary to form a top-down sequential solidification, that is, the solidification is first away from the runner, and the last solidification at the runner, otherwise shrinkage holes and shrinkage defects will occur.
3. Preventive measures (simultaneous solidification or sequential solidification)
Since low-pressure casting and differential pressure casting are all anti-gravity castings, gravity always hinders feeding, so whether for sand casting or metal casting, and for castings that solidify simultaneously or sequentially, the quality of the liquid level pressure control system is good. Damage is the key link in determining the compactness of castings. Especially for metal mold casting of thin-walled parts, the solidification time is not long. When the mold is filled to the top of the mold, the solid phase fraction in the liquid metal already occupies a considerable proportion. At this time, the pressure should be increased rapidly to overcome the negative effect of gravity and perform feeding. At this time, the compactness of the casting is a more critical moment. At present, some liquid level pressurization control systems still pressurize slowly according to the filling speed at critical moments, and some control systems are even worse. They can increase the pressure normally when the pressure is low, but the higher the pressure, the slower the pressure increase speed. The so-called parabolic filling with downward opening.
When the solidification of the liquid metal has basically ended, the control system will raise the pressure of the booster feeding, which is obviously too late, which will not have a good effect on the density of the casting. In production, sometimes the feeding pressure is already high (up to 0.2MPa), but the casting still has shrinkage defects, resulting in a high leakage rate. When the feeding channel is reasonable, this is mainly because the timing of the pressurization of the control system is not well controlled, rather than the so-called "feeding pressure has little effect on the compactness of the casting". For large thin-walled parts, qualified castings have not been cast for more than two years after trial production. When the old-fashioned liquid level pressure control system was replaced with a closed-loop feedback "CLP-3" low-pressure casting liquid level pressure control system, the situation changed greatly, and the original process did not change much, and qualified castings were produced. (Editor/Jing Liwen, Edit/Yanan)
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