Causes of porosity in aluminum alloy die casting

The workpiece produced in the production of aluminum alloy die casting is often scrapped due to the existence of pores, but there are many reasons for porosity, and it is often impossible to start when solving this product quality problem, how to quickly and correctly take measures to reduce the rejection rate caused by pores, which is the concern of aluminum alloy die casting manufacturers.

1. Classification of pores in aluminum alloy die casting

In the production of aluminum alloy die casting, people often generally call the holes of the product pores, which types of pores are generated, and are not known by people in detail, here we make a simple classification of pores:

1. Refine and remove pores caused by poor gas quality

In aluminum alloy die casting production, the molten aluminum pouring temperature is generally 610oC to 660oC, at this temperature, there is a large amount of gas dissolved in aluminum liquid (mainly hydrogen), the solubility of hydrogen aluminum alloy is closely related to the temperature of aluminum alloy, about 17.25pxj/100g in liquid aluminum liquid at about 660oC. In the solid aluminum alloy of about 660oC, it is only 0.036cm3/100g, and the hydrogen content in the liquid aluminum liquid is about 19-20 times that of the solid. Therefore, when the aluminum alloy solidifies, there is a large amount of hydrogen out in the form of bubbles in the aluminum die casting.

Reduce the gas content of aluminum water, prevent a large number of gases in aluminum alloy solidification and produce pores, which is the purpose of refining and degassing in aluminum alloy melting process. If the content of gas is reduced in the liquid aluminum, then the amount of gas precipitated during solidification will be reduced, and the resulting bubbles will be less and significantly reduced. Therefore, the refining of aluminum alloy is a very important process means, refining quality is good, pores must be less, refining quality is poor, pores must be more. The measure to ensure the quality of refining is to choose a good refining agent, a good refining agent is about 660oC can react to produce bubbles, the generated bubbles are not too intense, but uniform and continuous bubbles, through physical adsorption, these bubbles and liquid aluminum full contact, the longer the better, generally have 6-8 minutes of bubbling time.

When the aluminum alloy is cooled to 300oC, the solubility of hydrogen in the aluminum alloy is only 0.001 cm3/100g below, at this time it is only 1/700 of the liquid state, and the pores generated by the hydrogen precipitation after solidification are dispersed, small pinholes, which does not affect the leakage and processing surface, and can not be seen by the naked eye.

The bubbles produced by hydrogen precipitation during the solidification of liquid aluminum are relatively large, mostly in the final solidification of liquid aluminum, although they are also dispersed, but these bubbles often lead to leakage. Serious results often lead to workpiece scrap.

2, due to poor exhaust gas holes

In aluminum alloy die casting, due to the poor exhaust channel of the mold and the poor design structure of the mold exhaust, the gas in the mold cavity can not be completely discharged smoothly during die casting, resulting in the existence of pores in some fixed parts of the product. This kind of porosity formed by the gas in the mold cavity is large and small, and the inner wall of the porosity is the oxidation color of aluminum and air oxidation, which is different from the porosity produced by hydrogen precipitation. The inner wall of the hydrogen precipitation porosity is not as smooth as the air hole, and there is no oxidation color, but the inner wall of the gray bright.

For the pores 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 can be solved in time.

3, due to improper die casting parameters caused by the gas porosity

Improper selection of die casting parameters in die casting production, aluminum water die casting filling speed is too fast, so that the gas in the cavity can not be completely and smoothly extruded out of the cavity, and the flow of aluminum liquid is involved in the aluminum liquid, because the aluminum alloy surface is cooled rapidly, is wrapped in the solidified aluminum alloy shell, and can not be discharged to form a large porosity. This kind of porosity is often under the surface of the workpiece, the aluminum water inlet is less than the final junction, is pear-shaped or elliptical, and is more and larger at the final solidification.

For this kind of stoma, the filling speed should be adjusted to make the aluminum alloy liquid flow advance smoothly and do not produce high-speed enrolling gas.

4, aluminum alloy shrinkage porosity and shrinkage

Aluminum alloy, like other materials, shrinks during solidification, and the higher the pouring temperature of aluminum alloy, the greater the shrinkage, and the single porosity caused by volume shrinkage is present in the final solidification part of the alloy. It is irregular in shape, and in severe cases it is reticular. In the product, it exists at the same time as the pores precipitated by hydrogen during solidification, there are shrinkage pores around the hydrogen precipitated pores or curling pores, and there are filaments or mesh pores around the bubbles reaching out.

For this kind of porosity, it should be solved from the casting temperature, and the casting temperature of aluminum water during die casting should be reduced as far as possible if the die casting process conditions permit. This can reduce the volume shrinkage of the casting, reduce the shrinkage porosity and shrinkage.

If such pores often appear in the same part, it can be considered to increase the core withdrawal or cold iron, so that it can change the final solidification part to solve the problem of leakage and defects.

5, due to the product wall thickness difference caused by air holes

The shape of the product is often too large wall thickness difference problem, often can not change the shape of the product, in the center of the wall thickness is the final solidification of aluminum water, but also the most easy to produce pores, this kind of wall thickness of the porosity is a mixture of precipitation and contraction of the porosity, not general measures can be prevented.

In the design of the shape of the product, it should be considered to reduce the problem of uneven wall thickness, or too thick, adopt a hollow structure, and consider adding core withdrawal or cold iron, or water cooling, or increasing the cooling speed of the mold here.

In the production of die casting, pay attention to the amount of supercooling in this part, and appropriately reduce the pouring temperature.