Causes and solutions of pores in aluminum alloy castings

In the production process of aluminum alloy castings, stomata defects often occur due to improper operation or unskilled processes. Once the blowhole appears, it will often cause the casting to be scrapped due to quality problems and increase production costs. However, there are many reasons for the blowhole defects in aluminum alloy castings. Many companies always feel overwhelmed when they encounter problems and have no way to start. This article will start with the reasons for the occurrence of porosity in aluminum alloy castings, and analyze the solutions to the porosity defects.
Generally speaking, the blowhole defects of aluminum alloy castings are caused by the following types:

1. Pores produced by poor refining degassing quality
In the production of aluminum alloy casting, the pouring temperature of molten aluminum is usually 610 to 660 ℃, at this temperature, a large amount of gas (mainly hydrogen) is dissolved in the aluminum liquid. The solubility of hydrogen in aluminum alloys is similar to that of aluminum alloys. The temperature is closely related. It is about 0.69cm3 / 100g in the liquid aluminum liquid at about 660 ° C, but only 0.036cm3 / 100g in the solid aluminum alloy at about 660 ° C. At this time, the hydrogen content in the liquid aluminum liquid is about solid 19 to 20 times. Therefore, when the aluminum alloy solidifies, a large amount of hydrogen precipitates in the form of bubbles in the aluminum alloy casting.
To reduce the gas content in the aluminum liquid and prevent a large amount of gas from being precipitated when the aluminum alloy solidifies to generate pores. This is the purpose of refining and degassing in the aluminum alloy melting process. If the content of gas is originally reduced in the aluminum liquid, the amount of gas evolved during solidification will be reduced, and the bubbles generated will be significantly reduced. Therefore, the refining of aluminum alloys is a very important technological means. The refining quality is good, the porosity is bound to be less, the refining quality is poor, and the porosity is bound to be more. The measure to ensure the quality of refining is to choose a good refining agent. A good refining agent can react to generate bubbles at about 660 ° C. The generated bubbles are not too violent, but bubbles are generated evenly and continuously. Through physical adsorption, these bubbles and The aluminum liquid is in full contact, and the hydrogen in the aluminum liquid is adsorbed to bring it out of the liquid surface. Therefore, the bubbling time should not be too short, generally 6 ~ 8min bubbling time.
When the aluminum alloy is cooled to 300 ℃, the solubility of hydrogen in the aluminum alloy is only 0.001cm3 / 100g or less, which is only 1/700 of that in the liquid state. The pores generated by this hydrogen precipitation after solidification are dispersed and small The pinhole, which does not affect the air leakage and processing surface, is basically invisible to the naked eye.
When the aluminum liquid solidifies, the bubbles generated due to the hydrogen evolution are relatively large, mostly in the heart where the aluminum liquid finally solidifies. Although they are also dispersed, these bubbles often cause leakage, and often cause the workpiece to be scrapped.

2. Pores caused by poor exhaust
In aluminum alloy casting, due to the poor exhaust channel of the mold and the poor structure of the exhaust design of the mold, the gas in the cavity cannot be completely and smoothly discharged during die casting, resulting in the existence of air holes in certain fixed parts of the product. The pores formed by the gas in the mold cavity are large and small. The inner walls of the pores are oxidized by the oxidation of aluminum and air. Unlike the pores generated by hydrogen precipitation, the inner walls of the hydrogen pores are not as smooth as the air pores, and there is no oxidation color. It is a gray inner wall. For the air holes caused by poor exhaust, the exhaust channel of the mold should be improved, and the residual aluminum skin on the exhaust channel of the mold should be cleaned in time to avoid.

3. Pores due to improper die casting parameters
In the die-casting production, the die-casting parameters are not properly selected, and the aluminum hydraulic casting mold filling speed is too fast, so that the gas in the cavity cannot be completely and smoothly squeezed out of the cavity, and the liquid flow of the aluminum liquid is involved in the aluminum liquid. Rapid cooling, encased in a solidified aluminum alloy shell, unable to expel large pores. This type of air hole is often below the surface of the workpiece, and the aluminum liquid inlet is less than the final confluence, which is pear-shaped or elliptical, and there are many and large at the final solidification. For this type of pores, the filling speed should be adjusted so that the aluminum alloy liquid flow advances smoothly, without high-speed flow and gas.

4. Aluminum alloy shrink holes
Aluminum alloy, like other materials, shrinks during solidification. The higher the casting temperature of aluminum alloy, the greater the shrinkage. The single pore caused by volume shrinkage is present in the final solidified part of the alloy, which is irregularly shaped and serious. When it is mesh-like. Often in the product, it coexists with pores due to hydrogen evolution during solidification, there are shrinkage pores around the hydrogen evolution pores or coil pores, and there are filamentary or mesh pores extending outwards around the bubbles.
For this type of porosity, it should be solved from the casting temperature. When the conditions of the die-casting process allow, try to reduce the molten aluminum casting temperature during die-casting. This can reduce the volume shrinkage of castings, reduce shrinkage holes and shrinkage. If such pores often appear in the heating part, you can consider adding core pulling or cold iron to change the final solidified part to solve the problem of leakage defects.

5. Pores caused by excessive wall thickness difference
The shape of the product often has a problem of excessive wall thickness difference. In the center of the wall thickness is the place where the aluminum liquid finally solidifies, and it is also the most prone to generate pores. The pores at this wall thickness are a mixture of precipitated pores and contracted pores, not ordinary What the measures can prevent.
When designing the shape of the product, the problem of uneven wall thickness or excessive thickness should be considered as far as possible. A hollow structure is adopted. In the mold design, the addition of core pulling or cold iron, or water cooling, or the cooling of the mold should be considered. speed. In the production of die-casting, it is necessary to pay attention to the amount of supercooling in thick parts and to reduce the pouring temperature appropriately.
From the classification of the above-mentioned pores, we can see that there are many reasons for the production of pores in the production of aluminum alloy castings. It is necessary to find out the causes and treat the symptoms to solve the problem. The measures and ways to prevent stomata defects are:
(1) To ensure the quality of refining and degassing of aluminum alloy smelting, choose good refining and degassing agents to reduce the gas content in the aluminum liquid, and remove oxides such as liquid scum and bubbles in time to prevent the gas from being brought in again In die casting.
(2) Choose a good mold release agent. The selected mold release agent should not produce gas during die casting and have good mold release performance.
(3) Ensure that the exhaust of the mold is unobstructed, and the exhaust is smooth, and that the gas in the mold is completely discharged, especially the exhaust channel must be unobstructed at the final polymerization place of the aluminum liquid.
(4) Adjust the parameters of the die-casting parts, the filling speed should not be too fast, to prevent gas. The casting temperature should also be well controlled.
(5) In product design and mold design, attention should be paid to the use of core pulling and cooling to minimize the excessive wall thickness difference.
(6) The air holes that often appear in fixed parts should be improved from the mold and design.