Talk about generation of pores in die cast aluminum alloys

Time:2020-05-19 08:19:56 / Popularity: / Source:

die cast aluminum alloys 
Pressure casting is a type of special casting. It is a method to obtain a casting by filling a cavity at a certain speed under action of liquid or semi-liquid metal and pressure. For die cast aluminum alloys, many factors are involved in production process, including materials, molds, equipment, and process parameters. In order to obtain high-quality die casting parts, to ensure that surface of die casting parts is smooth, clear in outline, dense in organization, and excellent in performance, it is necessary to comprehensively control all influencing factors in the process of die-casting.

1. Classification of die cast aluminum alloys

Die cast aluminum alloys have good useability and process performance, so they have developed rapidly and are widely used in various industrial fields. Their dosage is much higher than other non-ferrous alloys, and they occupy an extremely important position in production of die-casting. Die cast aluminum alloys can be divided into Al-Si, Al-Cu, Al-Mg, Al-Zn and other alloys according to basic elements contained. Most of China's die cast aluminum alloys are mainly Al-Si alloys.
aluminum alloy die casting 

2. Classification and causes of pores in die cast aluminum alloys

Die cast aluminum alloys workpieces are often discarded due to existence of pores. There are many reasons for pores. How to quickly and correctly take effective measures to reduce rate of rejects caused by pores is a problem that manufacturers are concerned about. In the production of aluminum alloy die casting, according to causes of pores, they are generally divided into following categories:

2.1 Pores produced by poor refining

In production of aluminum alloy die casting, pouring temperature of aluminum liquid is generally 610 ~ 660℃. At this temperature, a large amount of gas (mainly H2) is dissolved in aluminum liquid, solubility of hydrogen is closely related to temperature of aluminum liquid. Solubility in aluminum liquid at about 600℃ is about 0.69cm3 / 100g, and it is only 0.036cm3 / 100g in solid aluminum alloy at about 660 ℃. At this time, hydrogen content in liquid aluminum liquid is about 19 times that of solid. Therefore, when aluminum alloy is solidified, a large amount of hydrogen precipitation exists in the form of bubbles in aluminum alloy die-casting.
Refining and degassing process in aluminum alloy smelting process can reduce gas content in aluminum liquid and prevent gas from precipitating during solidification of aluminum alloy to generate pores. If gas content in molten aluminum is low, amount of gas precipitated during solidification will be reduced, and bubbles generated will also be significantly reduced. Therefore, refining of aluminum alloy is a very important process means. Good refining quality will inevitably have fewer pores. On the other hand, poor refining quality will inevitably have more pores. Measure to ensure quality of refining is to choose a good refining agent. A good refining agent can react to generate bubbles at about 660℃. Bubbles generated are not too intense, but bubbles are generated uniformly and continuously. Through physical adsorption, these bubbles are in full contact with aluminum liquid, hydrogen contained in molten aluminum is taken out of liquid surface. Therefore, bubbling time should not be too short, generally 6-8min.
When aluminum alloy is cooled to 300℃, solubility of hydrogen in aluminum alloy is only less than 0.001 cm3 / 100g, and at this time it is only 1/700 of that in liquid state. Pores generated by precipitation of hydrogen after solidification are dispersed, and pinholes are small, which do not affect air leaks and processing surface, it is almost invisible to naked eye.

2.2 Porosity caused by improper die casting parameters

During aluminum hydraulic casting process, if filling speed is too fast, gas in cavity cannot be completely and smoothly discharged out of cavity in time, and it is wrapped in a solidified aluminum alloy shell to form a large air hole. Such pores are often pear-shaped or oval below surface of workpiece. For such pores, filling speed should be adjusted so that aluminum alloy liquid flow can be smoothly advanced without generating high-speed flow and entrainment.

2.3 Pores due to poor exhaust

In aluminum alloy die casting, due to poor exhaust passage of mold, mold exhaust design structure is poor, gas in cavity cannot be completely discharged during die casting, resulting in air holes in some fixed parts of product. Pore formed by gas in mold cavity are large or small. Inner wall of pores is oxidized by aluminum and air. Unlike pores generated by hydrogen evolution, inner walls of hydrogen pores is not as smooth as air pores and has no oxidation color, but a gray inner wall. For air holes caused by poor exhaust, exhaust passage of mold should be improved, and it can be avoided after cleaning residual aluminum skin on exhaust channel of mold in time.

2.4 Porosity caused by excessive product wall thickness difference

Shape of product often has a large difference in wall thickness. At the center of wall thickness is the place where aluminum liquid finally solidifies. It is also the place where pore are most prone. It is difficult to prevent. When designing shape of product, it should be considered to minimize problem of uneven wall thickness or excessive thickness, and a hollow structure is adopted. In the design of mold, consideration should be given to adding a core or cold iron, or water cooling, or increasing cooling rate of mold here. In diecast productions, pay attention to amount of supercooling in thick part and reduce pouring temperature appropriately.

2.5 Air hole

Aluminum alloy shrinks when it solidifies. The higher aluminum alloy's casting temperature, the more obvious this shrinkage is. Single pores due to volume shrinkage are present in final solidified part of alloy, which is irregular in shape and reticulate in severe cases. It generally exists at the same time as pores precipitated by hydrogen gas during solidification, there are shrinkage holes around hydrogen precipitation pores or volume pores, and there are filamentous or reticulated pores that extend to outside around bubbles.
For such pores, we should start from casting temperature. If conditions of die casting process allow, we should try to reduce temperature of molten aluminum casting. This can reduce volume shrinkage of casting, reduce shrinkage holes and shrinkage. If such pores often appear in heating part, you can consider adding core pulling or cold iron to change final solidified part to solve problem of leakage defects.

3. Measures to prevent pores

From above classification of pores, it can be known that there are many reasons for pores in production of die cast aluminum alloys. Cause must be found before solve problem. Measures and ways to prevent pores are mainly:
1) Ensure quality of refining and degassing of aluminum alloy smelting. Use good refining agents and degassing agents to reduce gas content in aluminum liquid, timely remove oxides such as liquid scum and bubbles to prevent gas from being brought into die casting again;
2) Select a good release agent, which should not generate gas in die casting and have good release properties;
3) Ensure that exhaust of mold is unobstructed and that gas in mold is completely exhausted, especially exhaust channel must be clear at final polymerization point of aluminum liquid;
4) Adjust die casting parameters, filling speed should not be too fast, and casting temperature should be controlled;
5) In product design and mold design, attention should be paid to use of core pulling and cold iron to minimize wall thickness differences;
6) Improve mold design for air holes that often appear in fixed parts.

4. Conclusion

Due to its own physical properties, production process parameters, mold design, uneven wall thickness and other reasons, die cast aluminum alloy castings are prone to defects such as blow holes and shrinkage. Through continuous parameter optimization and equipment improvement, first pass rate and quality of products can be better improved, production costs can be reduced, and significant economic benefits can be brought.
With in-depth study of aluminum alloy die casting theory, especially development of computer simulation and control technology, it make great theoretical breakthrough in flow shape of metal in filled cavity, solidification process of metal in cavity, flow pressure of metal liquid in cavity, temperature gradient of mold, deformation of mold, etc. It is believed that die casting aluminum alloys will have a better market prospect.

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