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Causes of cracks in low-pressure casting aluminum alloy wheel hubs and improvement measures

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1. Causes of cracks in low-pressure casting aluminum alloy wheels
The cracks of low-pressure cast aluminum alloy wheels are mainly caused by the stress concentration, or the cracks caused by uneven force when the hub is ejected, or the liquid solidification at the riser pipe. Cracks are generally divided into two types: cold cracking and hot cracking.

Cold cracking refers to the cracks formed when the alloy is below its solidus temperature. In layman's terms, cold cracking is when the casting is cooled to a low temperature, and the casting stress acting on the casting exceeds the degree allowed by the strength or plasticity of the casting itself. Cold cracks mostly appear on the surface of the casting, and the surface of the crack is slightly oxidized; while hot cracks are usually considered to be generated during the solidification of the alloy. Due to the heat transfer of the mold wall, the casting always solidifies from the surface. When a large number of branches appear on the surface of the casting and overlap to form a complete skeleton, the casting will appear solid state shrinkage (often expressed as linear shrinkage). But at this time, there is still a layer of liquid metal film (liquid film) between the dendrites. ), if the shrinkage of the casting is not hindered in any way, the dendrite layer is not affected by force and can shrink freely, and there will be no stress. When the shrinkage of the dendrite layer is hindered, it cannot shrink freely or be subjected to tension , There will be tensile stress, and the liquid film between the dendrites will be stretched and deformed. When the tensile stress exceeds the strength limit of the liquid film, the dendrites will be pulled apart. But the part of the cracked There is still some liquid metal around. If the liquid film is pulled apart very slowly, and there is enough liquid around and it flows into the crack in time, the crack will be filled and "healed", and the casting will not appear hot cracks If the tensile cracks cannot be "healed" again, the castings will appear hot cracks. The surface of the hot cracks will be strongly oxidized, showing a dark or black color with no metallic luster.

2. The main factors affecting cracks
For the same alloy, whether cracks occur in the hub often depends on factors such as the hub structure, process parameters and mold temperature.
1. The impact of improper wheel structure design on wheel cracks
(1) Improper size of the inner fillet is the most common cause of thermal cracks in the hub, because the sharp corners of the hub will generate a lot of stress when it is cooled. The part with a small inner fillet will not shrink even if the feeding is good. , It will also produce thermal cracks.
(2) A sudden change in the section of the hub will result in different cooling speeds.Even if the feeding is good, a large stress will be generated, causing cracks or cracks to appear after the hub is solidified.
2. The impact of unreasonable process parameters on wheel hub cracks
In low-pressure casting, the liquid in the riser tube is solidified due to the too long holding time or the riser tube is too long, and when the hub casting is ejected, it bears a certain pulling force, which causes the hub to produce cold cracks. Therefore, the design of reasonable holding pressure Time and liquid lifting system are very important to reduce the cracking caused by the wheel hub when it is ejected.
3. The influence of temperature on wheel hub cracks
The mold temperature of low pressure casting determines the solidification method of the alloy liquid, and directly affects the internal and surface conditions of the casting. It is one of the main causes of many defects such as dimensional deviation and deformation of the casting, and it also has a great impact on productivity. Mold temperature varies with It changes with changes in casting weight, die-casting cycle, die-casting temperature and mold cooling method.
From the perspective of heat transfer, increasing the mold temperature can reduce the heat transfer strength between the metal and the mold and extend the flow time. Studies have also shown that increasing the mold temperature can also slightly reduce the interfacial tension between the molten metal and the mold. As the mold temperature increases, the filling time is slightly reduced, that is, the filling capacity increases with the increase of the mold temperature. Therefore, an appropriate increase in the mold temperature is conducive to the reduction of stress. If the mold temperature is too low, the casting is in the metal mold The medium cooling is too fast, and the solidification speed between the various parts of the casting is different, which will cause uneven cooling of the casting in the mold, resulting in thermal stress and deformation, resulting in thermal cracking and large residual stress and residual deformation on the finished casting. The higher mold temperature of the tank is not conducive to obtaining the fine crystalline structure, and the liquid metal is easy to inhale and shrink, which increases the chance of casting defects such as porosity, shrinkage and shrinkage. In order to unify this contradiction, you can In the case of casting defects, increase the mold temperature appropriately.

3. Improvement measures
(1) Reasonable design of the lifting system. Because the pressure holding time is too long, or the riser tube is too long, the liquid in the riser tube will solidify, so that the hub casting will bear a certain pulling force when it is ejected, which will cause the hub to produce cold cracks, so the design A reasonable liquid lifting system is of great significance to reduce the tendency of cracks. The liquid lifting system refers to the channel through which the liquid metal enters the cavity from the crucible during pouring, including the riser pipe, the insulation sleeve and the casting casting system. The dimensions of these parts are directly Affect the distance between the liquid level in the crucible and the gate in the casting. The longer this distance is, the faster the liquid metal will cool down when it passes through this distance during pouring, which is likely to cause early condensation of the riser channel. Therefore, you should pay attention to: ①Short the distance between the liquid level in the crucible and the gate of the casting. This distance involves several aspects such as equipment, technology, mold, etc., so it should be considered comprehensively, and it is better to shorten this distance. ②Improve the insulation sleeve Appropriately increase the diameter of the insulation sleeve to increase the thickness of the insulation layer; use materials with good insulation properties as the insulation sleeve, such as aluminum silicate fiber felt. ③The diameter of the riser pipe should be appropriately increased. In order to prevent the riser pipe from solidifying early, it should be Properly increase the diameter of the riser pipe.
(2) When designing a reasonable hub structure, when designing the hub structure, sudden changes in the sharp-angle structure and cross-section should be avoided, and a rounded or uniform thickness structure should be adopted.
(3) In the absence of casting defects, appropriately increase the mold temperature.

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