What is the cause of cavitation corrosion in aluminum alloy die casting
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Cavitation is also called cavitation. It is more common when the interaction between a moving fluid and a gas creates a negative pressure (vacuum hole) bubble in the fluid. When the negative pressure bubble encounters the surface of the solid in the process of movement or flow, the annihilation of the bubble releases a huge blasting energy, which makes the surface of the solid form a concave hole. This phenomenon is commonly seen in blades used in seawater. The surface of the blades forms negative pressure bubbles due to the interaction of water and swirling air, which annihilate on the blade surface to form cavitation pits, which are commonly referred to as "pockets".
Cavitation also occurs in the production process of aluminum alloy high pressure die casting. For die-casting aluminum alloy appearance parts products, such as communication products, sometimes the surface of the mold forms concave pits (also called "pockets") after producing hundreds or thousands of products, which is also caused by cavitation. Correspondingly, there will be protruding pits on the surface of the aluminum alloy parts, and the macroscopic shape is shown in Figure 1.
It should be pointed out that the "pockmarks" formed on the surface of aluminum alloy die castings, in addition to "cavitation", there are other reasons, such as: the surface of the mold is not completely polished after EDM, or the surface of the mold is too bright, This makes the release agent unable to form a good protective film on the surface of the mold, resulting in point-like erosion and mold sticking on the surface of the mold. However, the pits produced by "cavitation erosion" on the surface of the mold are different in shape from such defects, and it is necessary to have enough experience to make a correct judgment.
Aluminum alloy die casting is to inject liquid aluminum alloy into the entire cavity of the mold at a high speed by a punch to obtain the desired product. During the flow of molten aluminum, it needs to pass through the runner to reach the inner gate and then spray into the mold cavity. The runner of the mold is the only path for the molten aluminum to flow. The design principle is that the cross-sectional area of the runner should be in a shrinking state, so that the air in the runner can be discharged to the front end of the molten aluminum during the flow of the molten aluminum.
The main reason for cavitation is that the mold design violates the above design principles. On the runner path through which the aluminum alloy liquid flows, the cross-sectional area of the runner expands. When the molten aluminum flows through the expanding inner runner, there will be volume expansion and sudden pressure drop, resulting in the formation of negative pressure (or hollowed out) cavities in the molten aluminum. When the injection is pressurized, the cavity bursts in the runner and inside the mold cavity. When it bursts, it forms an internal explosion, which can generate huge energy. When the product is produced, the continuous multiple implosions cause fatigue deformation on the surface of the metal mold or runner, forming pits that are visible to the naked eye. Correspondingly, protruding pits are formed on the surface of the casting product.
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