The flange is the thickest part of the cast aluminum wheel, and it is finally solidified in the cavity. If it is improperly cooled, it will easily cause shrinkage defects and cause waste. Production proves that when the design of the cooling system is reasonable and the quality of the cooling pipeline is excellent, the cause of shrinkage defects is mainly due to improper selection of the cold process parameters or small diameter of the gate channel.
1. Shrinkage defect analysis
The shrinkage produced by the cast aluminum flange can be divided into three types: the first outer surface is invisible, but visible by X-ray inspection; the second outer surface is visible, and the surface around the sprue cup is dark and dark. The area is loosened, or there is a darkening of the flange in the middle of the surface of the flange around the sprue cup; the third is the shrinkage of the flange face, which is visible on the façade of the damper groove; or The appearance is not visible and can be seen by X-ray inspection.
In order to eliminate the above-mentioned shrinkage defects, it is necessary to find the start and stop time and the application pressure of each cooling line. Analyze the thickness and area of ​​the flange, as well as the temperature field formed by the shock absorbing groove and the bolt hole. Master the temperature difference between each section and each part from the gate cup to the spoke filling channel (ie, the feeding channel), and confirm the cooling line. The application of the cold process parameters causes the flange to form an inwardly directed temperature gradient that eventually increases toward the gate cup. The order of solidification of the flange is shown in Figure 1.
Analyze the temperature difference after filling in the flange cavity. The computer numerical simulation method or the equal solid layer analysis method can accurately locate the hot and cold joints. The equal solid layer analysis method is shown in Figure 2.
Figure 2 is a solidified layer presented in the case where the heat absorption capability of each surface of the mold and the aluminum liquid contact is not applied. When the first layer has become solid, the second layer exhibits a solid-like paste. The middle part communicating with the mushy layer is a liquid, that is, a hot joint zone, and the feeding passage of the pouring system to the hot section is already narrow, as shown by the double-dotted circle in the figure, if the heat is not accurately and timely Cold, the cold section (that is, the narrow section of the feeding channel) is insulated, and the feeding channel is blocked, and the shrinkage hole is generated at the hot section, so that the casting is scrapped.
2. Shrinking solutions
(1) Select the appropriate cold-working parameters and insulation measures for the hot and cold sections, so that the equal layer solidification without uncooled and uninsulated becomes non-equal solidification, that is, the solidified layer at the hot joint The solidified layer at the cold junction is thin, which contributes to the incremental temperature gradient from the hot section to the gating system, as shown in Figure 3.
In order to obtain the effect shown in Fig. 3, it is necessary to grasp the start and end time of the cold application of each cold line. As shown in Figure 3, the "cold 1" and "cold 2" are applied for a short time after the filling is completed. When the thickness of each solidified layer in the figure is formed, the "cold 3" is applied. cold.
It should be noted that it is forbidden to apply cold to the lower mold to affect the casting system. The preferred method of lowering the temperature of the pouring system is that the pouring cup, especially the outer surface of the annular pouring cup, is farther from the surface of the mold, that is, the outer surface of the pouring cup and the surface of the hole of the mold pouring cup should be larger. It is easy to thicken the additive and enhance the insulation capacity. When a paste-like additive is used, the shrinkage is preferably as small as possible after heating, otherwise cracks are likely to occur during curing. If the connection between the sprue bush and the sprue cup is not strict, the aluminum liquid has the opportunity to invade the crack of the sprue cup and contact with the mold, causing the mold to absorb the heat of the pouring system through the crack, so that the pouring system is cooled, and when it is serious, the pouring system is caused. Coagulation in advance and blockage of the feeding channel, not only the hot section can not be supplemented, but also the casting system is pulled off during demolding, which cannot be continuously produced, resulting in the mold coming off the machine.
(2) The gate cup connected to the sprue bushing can be made of metal or ceramic gypsum. If the latter is used, it must have a certain degree of toughness and no cracks can be generated during work. Sometimes, due to the low toughness of the gate cup and the lack of tightness of the sealing material, the pouring cup will crack, and the added material will also have cracks, causing the aluminum liquid to invade, causing the mold to absorb heat and the pouring system to cool down, resulting in the pouring system. The temperature is lower than the hot section of the flange, and the crystal is solidified before the hot section, and the heat knot is not replenished to cause shrinkage. The thickness of the seal additive between the gate cup and the mold is shown in Fig. 4.
(3) If the cooling system is designed, manufactured, and installed correctly, it may also be loosened. It may be that the cooling plate or pipeline is blocked, causing water or wind to flow poorly, or even failing at all; or the pressure of the cooling medium may be insufficient. not enough. When it is found that the flange has a shrinkage defect, it is necessary to check whether the data of the flow and pressure display meets the set parameters, and judge whether the cooling pipelines are blocked according to the data displayed by the meter. For the water cooling system, as the use time is extended, the flow path is also gradually tapered, and the flow path outlet is more serious, which is caused by scale formation. For the air cooling system, due to the displacement of the air nozzle (the positioning is not accurate or the positioning is not strong during installation, the vibration is caused by the work), the set cooling function is not achieved and the shrinkage defect is caused.
Because of the water-cooling method, it is easy to scale. Therefore, most enterprises adopt thick pipelines and large wind pressure to ensure that the effect of cold storage is not lower than that of water cooling. Figure 5 is a 16in wheel hub flange cooling system for a foreign hub manufacturer, the flange of which is not loosened. The characteristics of the cooling pipeline are: the cold hole is thick; the air outlet is close to the cold end face; the sealing material is thick.
(4) The shrinkage defect of the flange is related to the diameter of the through hole of the pouring system, in addition to the above reasons. The diameter of the through hole of the sprue sleeve should be appropriately increased to make the temperature of the pouring system higher than the temperature at the hot section of the flange, so as to ensure the solidification of the casting system after the casting system. However, the diameter of the center hole is often small, and the diameter of the through hole of the sprue sleeve cannot be increased, and the diameter of the inlet hole of the sprue sleeve can be increased, and the production practice proves that the effect is good. If the cooling process parameters are appropriate and the flange is still loosened, the diameter of the through hole of the sprue sleeve must be increased. If the enlarged aperture is still ineffective, the mesh may be appropriately thickened, and if the diameter is originally 0.90 mm (20 mesh), it may be changed to 1.60 mm (12 mesh). An enterprise has a large-volume production of hubs. Because the diameter of the through-hole of the sprue sleeve is small, the scrap rate of shrinkage defects is as high as 60% or more. After that, the diameter of the sprue sleeve is changed to coarse, and the flange is basically eliminated. Shrink the defect.
About the author: Su Yiren (1965-), engineer, senior lecturer, School of Mechanical and Electrical Engineering, Wuhan Vocational and Technical College.
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