Casting is one of the important basic industries in the national economy, but the control, design and process flow of the casting process often rely on empirical judgment, so the quality of castings is difficult to guarantee and the scrap rate is high. Pressure casting is a type of special casting. Its essence is a method in which liquid or semi-liquid metal fills the mold cavity at a high speed under high pressure and solidifies under pressure to obtain castings. In order to obtain high-quality, high-level die-casting parts and ensure that the die-casting parts meet the requirements of smoothness, clear outline, dense structure, and high strength, it is necessary to coordinate and unify the influencing factors in the die-casting process. For aluminum alloy die-casting parts, from production preparation to mass production, there are many links involved and many influencing factors, including materials, molds, equipment, processes, and other aspects.
Porosity defects in die castings and their causes
Workpieces produced by aluminum alloy die-casting are often scrapped due to the presence of pores. There are many reasons for pores. When solving this product quality problem, it is often difficult to start. How to quickly and correctly take measures to reduce the scrap rate caused by pores? This is Issues that all aluminum alloy die-casting manufacturers are concerned about.
In aluminum alloy die-casting production, the causes of pores often fall into the following categories.
Porosity caused by poor refining degassing quality
In aluminum alloy die-casting production, the pouring temperature of molten aluminum liquid is usually between 610 and 660°C. At this temperature, a large amount of gas (mainly hydrogen) is dissolved in the aluminum liquid. The solubility of hydrogen in aluminum alloy is closely related to that of aluminum alloy. Temperature is closely related. In liquid aluminum at about 660°C, it is about 0.69cm3/100g, while in solid aluminum alloy at about 660°C, it is only 0.036cm3/100g. At this time, the hydrogen content in the liquid aluminum is about 0.69cm3/100g. 19 to 20 times. Therefore, when the aluminum alloy solidifies, a large amount of hydrogen is released in the form of bubbles in the aluminum alloy die casting.
Reduce the gas content in the aluminum liquid and prevent a large amount of gas from precipitating and causing pores when the aluminum alloy solidifies. This is the purpose of refining and degassing during the aluminum alloy smelting process. If the gas content in the aluminum liquid is originally reduced, the amount of gas evolved during solidification will be reduced, and the bubbles generated will also be significantly reduced. Therefore, the refining of aluminum alloy is a very important process. If the refining quality is good, there must be fewer pores. If the refining quality is poor, there must be more pores. The measure to ensure the quality of refining is to select a good refining agent. A good refining agent can react to produce bubbles at around 660°C. The bubbles generated are not too violent, but are generated uniformly and continuously. Through physical adsorption, these bubbles are mixed with the refining agent. The aluminum liquid is in full contact, adsorbing the hydrogen in the aluminum liquid and bringing it out of the liquid surface. Therefore, the bubbling time should not be too short, generally 6 to 8 minutes.
When the aluminum alloy is cooled to 300°C, the solubility of hydrogen in the aluminum alloy is only less than 0.001cm3/100g, which is only 1/700 of that in the liquid state. The pores produced by the precipitation of hydrogen after solidification are dispersed and small. The pinholes do not affect air leakage and the machined surface, and are basically invisible to the naked eye.
When the aluminum liquid solidifies, the bubbles produced by the precipitation of hydrogen are relatively large, mostly in the center of the final solidification of the aluminum liquid. Although they are also dispersed, these bubbles often lead to leakage, and in severe cases, the workpiece is often scrapped.
Pores caused by poor exhaust
In aluminum alloy die-casting, due to the poor exhaust channel of the mold and the poor exhaust design structure of the mold, the gas in the cavity cannot be fully and smoothly discharged during die-casting, resulting in the existence of pores in certain fixed parts of the product. The pores formed by the gas in the mold cavity are sometimes large and sometimes small. The inner walls of the pores have an oxidation color caused by the oxidation of aluminum and air. They are different from the pores produced by hydrogen gas precipitation. The inner walls of the hydrogen gas precipitation pores are not as smooth as the air holes and have no oxidation color. It's a bright gray inner wall. For 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 avoided in time.
Porosity caused by air entrainment due to improper die-casting parameters
In die-casting production, the die-casting parameters are improperly selected. The filling speed of aluminum hydraulic casting is too fast, so that the gas in the cavity cannot be fully and smoothly extruded from the cavity in time, and is drawn into the aluminum liquid by the flow of aluminum liquid. Due to the surface of the aluminum alloy, After rapid cooling, it is wrapped in a solidified aluminum alloy shell and cannot be discharged, forming larger pores. This kind of pores are often under the surface of the workpiece. The entrance of the aluminum liquid is less than the final confluence, pear-shaped or oval-shaped, and there are many and large at the final solidification point. For this kind of pore, the filling speed should be adjusted to ensure that the aluminum alloy liquid flow advances smoothly without generating high-speed flow and air entrainment.
Shrinkage holes of aluminum alloy
Aluminum alloys, like other materials, shrink during solidification. The higher the casting temperature of aluminum alloys, the greater the shrinkage. A single pore caused by volume shrinkage exists in the final solidification part of the alloy, which is irregular in shape and serious. Sometimes in a mesh shape. Often in products, it coexists with pores due to hydrogen evolution during solidification. There are shrinkage pores around the hydrogen evolution pores or curling pores, and there are filamentous or mesh-like pores extending to the outside around the bubbles.
For this kind of pores, we should start with the casting temperature to solve it. If the die-casting process conditions permit, the aluminum liquid casting temperature during die-casting should be reduced as much as possible. This can reduce the volume shrinkage of the casting, reduce shrinkage holes and shrinkage porosity. If such pores often appear in the heating part, you can consider adding core pulling or cold iron to change the final solidification part and solve the problem of leakage defects.
Porosity caused by excessive wall thickness difference of product
The product shape often suffers from the problem of excessive wall thickness difference. The center of the wall thickness is where the aluminum liquid finally solidifies, and it is also the most likely place to produce pores. The pores at this wall thickness are a mixture of precipitation pores and shrinkage pores, which are not ordinary. measures that can be prevented.
When designing the shape of the product, consideration should be given to minimizing the problem of uneven or excessive wall thickness, adopting a hollow structure, and adding core pulling or cold iron, or water cooling, or increasing cooling here in the mold should be considered in the mold design. speed. In die-casting production, attention should be paid to the amount of subcooling in areas with large thickness, and the pouring temperature should be appropriately reduced.
From the above classification of pores, it can be seen that there are many reasons why products produce pores in aluminum alloy die-casting production. It is necessary to find out the cause and prescribe the right medicine to solve the problem. The main measures and ways to prevent pores are:
- To ensure the refining and degassing quality of aluminum alloy smelting, use good refining agents and degassing agents to reduce the gas content in the aluminum liquid, and promptly remove oxides such as scum and bubbles on the liquid surface to prevent gas from entering again. in die castings.
- Choose a good release agent. The selected release agent should not produce gas during die casting and have good release performance.
- Ensure that the exhaust of the mold is smooth and not blocked, and the exhaust in the mold is completely discharged. Especially at the final polymerization point of the aluminum liquid, the exhaust channel must be unobstructed.
- Adjust the die-casting parameters, and the mold filling speed should not be too fast to prevent air entrapment. The casting temperature must also be controlled.
- In product design and mold design, attention should be paid to the use of core pulling and cooling to minimize excessive wall thickness differences.
- For pores that often appear in fixed parts, the mold and design should be improved.
New developments in aluminum alloy die-casting technology
In recent years, people have continued to improve vacuum technology in order to solve the problems of pores and shrinkage cavities inside die castings so that they can produce die castings with high strength, high density, weldability, heat treatment, twistability and other properties. In addition to die casting, new technologies such as squeeze casting and semi-solid die casting have been developed, and are generally called "high-density die casting method".
Vacuum die casting technology
The vacuum die-casting method evacuates or partially evacuates the gas in the mold cavity to reduce the air pressure in the mold cavity to facilitate mold filling and eliminate the gas in the alloy melt, so that the alloy melt fills the mold cavity under pressure, and under pressure solidified to obtain dense die castings.
Oxygenated die-casting technology
Most of the gases in die castings are N 2 and H 2 , with almost no O 2 . The main reason is that O 2 reacts with active metals to form solid oxides, which provides a theoretical basis for oxygenated die casting technology. Oxygenated die casting is to fill the mold cavity with oxygen to replace the air before die casting. When entering the mold cavity, part of the oxygen is discharged from the exhaust groove, and the remaining oxygen reacts with the molten metal to generate dispersed oxide particles, forming an instant vacuum in the mold, thereby obtaining a pore-free die casting.
Semi-solid die-casting technology
Semi-solid die casting is a technology that stirs liquid metal when it solidifies, obtains a slurry with a solid component of about 50% or more at a certain cooling rate, and then shapes the slurry through die casting. Currently, there are two processes for semi-solid die casting: flow forming process and thixoforming process. The former is to feed liquid metal into a specially designed injection molding barrel, where shear is applied by a spiral device to cool it into a semi-solid slurry, and then die casting is performed. The latter is to feed solid metal particles or chips into a spiral injection molding machine, and semi-solid metal particles are die-casted under conditions of heating and shearing.
Extrusion die casting technology
Extrusion die casting is also called "liquid metal molding". Its castings have good density, high mechanical properties, and no pouring risers. Some enterprises in our country have applied it in actual production. Squeeze die-casting technology has excellent process advantages. It can not only replace traditional die-casting, squeeze casting, low-pressure casting, and vacuum die-casting processes, but is also compatible with differential pressure casting, continuous casting and continuous forging, and semi-solid rheological casting processes. . Experts believe that extrusion die-casting technology is a cutting-edge new technology that spans multiple process fields, is rich in connotation, highly innovative, and extremely challenging.
Electromagnetic pump low-pressure casting technology
Electromagnetic pump low-pressure casting is a newly emerging low-pressure casting process. Compared with gas-based low-pressure casting technology, it is completely different in terms of pressurization methods. It uses non-contact electromagnetic force to directly act on liquid metal, which greatly reduces oxidation and suction problems caused by impure compressed air and excessive partial pressure, and achieves smooth transportation and filling of liquid aluminum. Prevent secondary pollution caused by turbulence. In addition, the electromagnetic pump system is completely controlled by computer digital elements, and the process execution is very accurate and repeatable. This process has obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization. As research continues to deepen, this technology has become more and more mature.
Conclusion
Due to the uneven wall thickness of aluminum alloy disc and cylindrical die-casting parts, die-casting quality defects such as pores and shrinkage holes are prone to occur. Through continuous pouring experiments, engineers can seek more optimized die-casting process parameters. In this way, reasonable process parameters can be selected, and actual casting cost waste can be avoided, bringing significant economic benefits to the enterprise. It is hoped that this topic can be of some help to actual production. The ultimate goal is to produce qualified die-casting parts to meet the actual production needs of enterprises. Judging from domestic and foreign die-casting research in recent years, with the deepening of theoretical research, especially the development of computer simulation technology, the flow pattern of metal filling the cavity, the solidification process of metal in the cavity, and the There have been great theoretical breakthroughs in the flow pressure of the internal metal liquid, the temperature gradient of the mold, and the deformation of the mold.