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Causes of crack formation in low pressure casting aluminum alloy wheels

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    Analyze the causes of cracks in low-pressure casting aluminum alloy wheels, and study various factors that affect cracks, such as casting structure, process parameters, mold temperature, etc., and eliminate the impact of cracks on wheel castings through reasonable control and adjustment of these factors. Thereby improving the economic efficiency of the enterprise.

   Aluminum alloy wheels have many unmatched characteristics of steel wheels, so aluminum alloy wheels have begun to be widely used in cars, motorcycles and other vehicles. By 2002, the loading rate of aluminum alloy wheels for cars in my country was close to 45%. Due to the high quality requirements of automobile wheels, its structure is suitable for low-pressure casting, and the demand is large, it has greatly promoted the development of low-pressure casting technology. At present, low-pressure casting has become the main process method for the production of aluminum alloy wheels, and most domestic aluminum alloy wheel manufacturers adopt this process.

   Low pressure casting can be highly mechanized and automated, which not only improves productivity (10-15 type/h), but also reduces many human factors that are not conducive to the production process, increases the yield, and greatly reduces the labor intensity of workers. However, the quality of low-pressure castings is affected by factors such as process plan, process parameters, mold structure and manual operation, as well as their mutual influence. Unreasonable design or improper operation of any link may cause defects in low-pressure castings. Among them, the occurrence of aluminum alloy wheel hub cracks is an important factor affecting the production cost and production efficiency of enterprises, and wheel hub cracks are a major hidden danger to automobile safety. Therefore, it is particularly important to discuss the causes of cracks in low-pressure casting aluminum alloy wheels.

   One. Reasons for the formation of cracks in low-pressure casting aluminum alloy wheels

  The cracks of the low-pressure casting aluminum alloy wheel hub are mainly caused by the stress concentration, or the crack caused by uneven force when the hub is ejected, or the liquid solidification at the riser pipe. Cracks are generally divided into cold cracks and hot cracks.

  Cold cracking refers to the cracks formed when the alloy is below its solidus temperature. In layman's terms, cold cracking is caused when the casting stress acting on the casting exceeds the allowable degree of the strength or plasticity of the casting itself when the casting is cooled to a low temperature. Cold cracks mostly appear on the surface of castings, with slight oxidation on the surface of the cracks; 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 castings always solidify 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). However, there is still a layer of liquid metal film (liquid film) between the dendrites at this time. 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 it will not appear. stress. When the shrinkage of the dendrite layer is hindered, it cannot shrink freely or is subjected to tensile force, and tensile stress will appear. At this time, the liquid film between the dendrites will be deformed by stretching. When the tensile stress exceeds the strength limit of the liquid film, the dendrites will be pulled apart. However, there is still some liquid metal around the cracked part. If the liquid film is pulled apart very slowly, and there is enough liquid around and flow into the cracked part in time, the cracked part will be filled and "healed". The casting will not show hot cracks. If the tensile cracks cannot be "healed" again, hot cracks will appear in the casting. The surface of the thermal fracture is strongly oxidized, showing a dark or black color without metallic luster.

   2. The main factors affecting cracks

   For the same alloy, whether the wheel hub has cracks or not depends on factors such as the wheel hub structure, process parameters and mold temperature.

  1. The impact of improper design of wheel hub structure on wheel hub 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 cools. In the part with small fillet, even if the feeding is good and there is no shrinkage, thermal cracking will occur.

  (2) A sudden change in the section of the hub will cause the cooling rate to vary. 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 influence of unreasonable process parameters on wheel hub cracks

  In low-pressure casting, the liquid in the riser tube is solidified due to too long pressure holding time or too long riser tube. When the wheel casting is ejected, it bears a certain pulling force, which causes cold cracking of the wheel hub. Therefore, a reasonable design of the pressure holding time and the lifting system is of great significance to reduce the cracking caused by the wheel hub when it is ejected.  

   3. The influence of mold 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 reasons for the dimensional deviation and deformation of the casting, and it also has a great impact on productivity. The mold temperature changes with the weight of the casting, the die-casting cycle, the die-casting temperature, and the 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 slightly reduce the interfacial tension between the molten metal and the mold. As the mold temperature increases, the filling time decreases slightly, that is, the filling capacity increases as the mold temperature increases. Therefore, a proper increase of the mold temperature is beneficial to the reduction of stress. If the mold temperature is too low, the casting will cool too quickly in the metal mold, and the solidification rate of each part of the casting will be different, which will cause uneven cooling of the casting in the mold. Thermal stress and deformation result in thermal cracking and large residual stress and residual deformation on the finished casting. Higher mold temperature is not conducive to obtaining a fine crystal structure. The liquid metal is easy to inhale and shrink, causing the casting to produce The chance of defects such as porosity, shrinkage and shrinkage is increased. In order to unify this contradiction, the mold temperature can be appropriately increased without casting defects.

   3. Improvement measures

(1) Reasonable design of the lifting system. Because the pressure holding time is too long, or the riser pipe is too long, the liquid in the riser pipe will solidify, so that the hub casting will bear a certain pulling force when it is ejected, resulting in cold cracking of the hub. Therefore, the design A reasonable liquid lifting system is very important 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 liquid lifting pipe, the insulation sleeve and the casting system. The size of these parts directly affects 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 this distance during pouring, which will easily cause the riser channel to condense early.

   therefore should pay attention to:

  ①Short the distance between the liquid level in the crucible and the gate in the casting. This distance involves several aspects such as equipment, craftsmanship, molds, etc. Therefore, comprehensive consideration should be given to shorten this distance.

  ②Improve the insulation cover. Properly 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.

  ③ The diameter of the riser pipe is appropriately enlarged. In order to prevent early solidification of the riser pipe, the diameter of the riser pipe should be appropriately increased.


   (2) Reasonably designed wheel hub structure When designing the wheel hub structure, sudden changes in the structure and cross section of sharp corners should be avoided, and a rounded corner or uniform thickness structure should be adopted.

   (3) In the absence of casting defects, appropriately increase the mold temperature.

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