Many die-casting hardware factories want to solve the reasons for the defects of poor airtightness of aluminum alloy die castings as soon as possible to find out the problems that may cause defects. Therefore, combining several cases in production practice, formulating corresponding measures, and finally finding a solution Scheme.
According to analysis, poor airtightness of die-casting parts means that when a certain pressure is applied to the inside of the die-casting part, leakage occurs from the inside or outside of the casting part, resulting in a pressure drop. If such a die-casting part is used, oil leakage is likely to occur. , Air leakage, water leakage, and other problems. Poor air-tightness is one of the more difficult problems in die casting defects. There are many reasons for it. The following three reasons are shown:
1. Analysis of the causes of air leakage in die castings
Gas in raw materials
Our commonly used die-casting material is aluminum alloy (this article only takes aluminum alloy die-casting as an example). In the production of die-casting parts, since the main component of the gas contained in the aluminum liquid is hydrogen, the level of hydrogen content in the aluminum alloy liquid is related to There is a direct relationship between the number of spores produced in the casting. The pores in the casting not only reduce the mechanical properties and corrosion resistance of the alloy but also reduce the airtightness.
At present, the main method to remove the gas in the aluminum liquid is to pass inert gas such as nitrogen into the aluminum alloy or add a solid degassing agent, etc., so that the hydrogen dissolved in the aluminum liquid can diffuse into the bubble when the bubble floats to the surface of the aluminum liquid, The bubble bursts, hydrogen escapes into the atmosphere, so as to achieve the purpose of removing hydrogen.
The influence of mold gating system
The gating system determines the design quality of a pair of die-casting molds, and it is the main factor that determines the quality of die-casting parts in the later production. As a system, it is composed of many elements, and its purpose is to make the alloy liquid enter the cavity in a suitable flow state to fill the cavity.
At the same time, the gas in the system can be exhausted to the maximum. Therefore, the die-casting mold should have a good pouring system and overflow system.
Since the runner is opened in the direction of the dense side, the molten aluminum is returned to the last dead corner on the upper left side and then returns to produce eddy current and curling phenomenon, which leads to a significant decrease in the quality of the left side of the casting and reduces the airtightness.
The runner design makes each channel of aluminum liquid basically fill at the same time and make up for the phenomenon of insufficient local pouring so that the overall quality of the casting is improved in a balanced manner. Therefore, in the mold making, the design of the runner should use the multi-strand runner method as much as possible, and the aluminum liquid flow should be consistent with the direction of the casting, and collisions should be avoided as much as possible to reduce the occurrence of eddy currents and the occurrence of entrainment due to chaotic filling; at the same time, To fill the cavity with multiple runners, it should be filled at the same time as much as possible, and one or more strands of molten aluminum should not reach the dead corner at the last end and then return to produce eddy currents. In addition, the slag bag and exhaust ducts on the die-casting mold should be reasonably distributed. The proper flow pattern is the guarantee of no collision, entrainment of liquid flow, and stable speed. Otherwise, no matter how good the drainage system is, the gas will still not be discharged.
From the above analysis, it can be seen that various defects inside the casting caused by the poor gating system are the direct cause of the poor airtightness of the casting.
Equipment performance
The pores, shrinkage holes, and cold partitions in die castings are also a major cause of air leakage in die castings, and the performance of the equipment plays a vital role in the production of castings. For products with strict requirements on airtightness, suitable die-casting models must be selected.
At present, the die-casting machine basically uses three-stage injection in the production of aluminum alloy die-casting. In the first-stage injection, the injection punch advances at a slower speed, which is conducive to extruding the gas in the pressure chamber; and In the second-stage injection, the speed of the inner runner is extremely fast, and the aluminum liquid basically fills the cavity. At the same time, if the position of the secondary injection speed comes too early, the casting is prone to defects such as blowholes. If the starting position of the secondary injection speed is too late, the castings are prone to defects such as cold barriers. Generally, it is ideal to choose the starting position of the secondary injection speed where the molten metal in the cup just reaches the entrance of the inner runner. Therefore, this level is the key to the generation of pores, so the higher the speed, the easier it is to generate vortex and form pores.
For example, there are many types of defects that cause air leakage in the die casting of the CG125 right crankcase on a motorcycle engine. In theory, any die casting defect may cause air leakage in the casting. There are many reasons for this kind of problem, and the main reason should be grasped to make adjustments so that the air leakage can be significantly improved. It is an effective method to adjust the die casting process curve.
In order to reduce the shrinkage cavity inside the casting and fill the air leakage channel, the gas in the pressure chamber should be removed as much as possible. In this process, the main idea of controlling the pores of die castings is to achieve by controlling the first and second injection speeds and the first and second switching points. Under the premise of meeting the requirements of casting forming or surface quality, the first-stage injection speed should be as low as possible, and the high speed should be started when the aluminum alloy reaches the inner runner. Through the improvement of the above process, the airtightness of the box body is greatly improved.
Method of operation
During the die-casting process, some coatings have the properties of high volatilization point and large air volume, which have a direct impact on the porosity of the casting. The release agent is mainly sprayed manually, and the dosage is mainly determined by experience. If the spray volume is too much, If the spraying time is too long, it is easy to cause a large amount of gas volatilization. In addition, the low mold temperature and untimely volatilization will result in a large porosity. Therefore, the paint with a low volatilization point and a small amount of gas should be selected in the production process. At the same time, the blowing time can be appropriately extended to ensure that the dynamic and fixed molds are blown dry. For molds with complex shell structures, it is necessary to dry the excess release water or oil in the injection chamber and the mating surface of the punch, the surface of the mold cavity, the core-pulling part, and the runner.
Control of machining allowance
During the forming process of die-casting parts, the cavity is filled at the fastest speed, so that the molten aluminum is rapidly solidified in the mold to form a product, so it is inevitable that there will be pores in the casting caused by the entrainment of the molten aluminum, or due to the combination of solid and liquid. The density is different and shrinks. However, the surface layer of the casting will also form a dense layer of fine grains due to rapid solidification, and the mechanical properties of these fine-grain layers are relatively high. The production process changes, for different castings, the thickness of the dense layer of the structure will be different. In order to ensure the air-tightness of the casting, in the later machining process, a small machining allowance should be used as much as possible.
Reasonably add back charge
The pinhole degree reflects the spatial distribution density of pinholes and is an important factor affecting the airtightness of castings. Due to the combined influence of the small pores and oxidized inclusions in the recycled material, there are more pinholes in the castings. The scrap rate of individual castings in production is relatively high. For the sake of energy saving and consumption reduction, the use of scraps in actual production and the large increase in the amount of recycling of runners will also cause the airtightness of the castings to deteriorate.
Therefore, when producing castings with air tightness requirements, the classification, treatment, and use of the recycled materials should be strictly controlled, so that the ratio of recycled materials and new materials should be used strictly in proportion to meet the quality requirements. Otherwise, excessive use of recycled materials will increase the pinhole degree of castings in subsequent production and fail to meet the requirements of airtightness, which is not conducive to the guarantee of casting quality.
Choose a reasonable pressure chamber fullness
When the punch diameter and die-casting machine are selected, the weight of the molten metal contained in the press chamber is also a certain value, but the weight requirements of the molten metal each poured for different castings are different. If the volume of molten metal poured into the pressure chamber is insufficient (that is, when the pressure chamber is less full), the gas in the pressure chamber cannot be removed as soon as possible during the injection. Under the high-speed push of the injection piston, a turbulent flow is formed. Flow sequentially, which is easy to be involved in gas, resulting in defects such as pores and insufficient pouring. At the same time, due to the existence of excessive oxide scale in the pressure chamber, it is easy to form a partition inside the casting, which causes the local strength of the casting to decrease, and it is easy to leak under the action of a large leak test pressure. Therefore, selecting a reasonable fullness of the pressure chamber can effectively reduce the pore defects in the casting, thereby reducing the air leakage rate of the casting.
2. Examples of improving the airtightness of castings
When strict, 100% leak detection must be done, otherwise it will cause oil leakage during use and affect the normal use of the vehicle. Ensuring airtightness during the production process is the focus of quality inspection.
In the early mold design, since airtightness was not considered a major issue, the air leakage rate during the production of CLQ81 base castings was extremely high, especially after a period of production, severe cracks appeared on the surface of the mold, causing local tension. The injury is also very serious, and the leakage rate of the casting is even worse. Poor airtightness of product parts has become the main bottleneck restricting production (although it can be compensated by later infiltration, the production cost is greatly increased). To solve this problem, we have analyzed the reasons for the leakage of castings.
Due to a large amount of waste parts and runner materials used in the production, the impurity content of the later production castings increases, which is extremely unfavorable to the guarantee of the quality of the castings. At the same time, in the machining process, due to the large machining allowance, more pores and sand holes inside the casting are exposed, which intensifies the air leakage of the casting. In response to the above analysis, we have taken the following measures.
(1) Strictly control the use of recycled materials, blow nitrogen into the molten aluminum, and add powdered refining agent at the same time, so that the molten aluminum in the furnace is fully in contact with the gas and the refining agent, and remove the gas and the refining agent in the molten aluminum as much as possible. Impurities.
(2) In order to ensure the airtightness of aluminum alloy die castings after machining, in the later machining process, we will improve the design of the machining fixture and the positioning accuracy of the machining. Under the premise of ensuring the required size of the drawing, we try our best to reduce the machining allowance.
(3) In order to reduce the surface defects such as cracks and strains in the later stage of the mold, the corresponding parts of the mold are strengthened, and the surface is plated with titanium when making a new core-pulling core. After the above measures have been implemented for a period of time, the air leakage rate of the casting has been greatly reduced, and the impregnation treatment is basically no longer necessary.
3. Conclusion
Through the analysis, it can be seen that solving the problem of poor airtightness detection of castings is relatively difficult in die casting production, and the cause may be caused by the combined effect of various casting defects. Therefore, when dealing with the phenomenon of poor air-tightness, it is necessary to gradually investigate the properties, processes, molds, etc. of the alloy, find out the main reasons, and take targeted measures to effectively improve the air-tightness of aluminum alloy die casting. Sex.