Analyze the causes of cracks in low-pressure cast aluminum alloy wheels, study the various factors that affect the cracks, such as casting structure, process parameters, mold temperature, etc., through reasonable control and adjustment of these factors, eliminate the effects of cracks on wheel castings. Thereby improving the economic efficiency of the enterprise.
Aluminum alloy wheels have many unmatched characteristics of steel wheels, so aluminum alloy wheels have been 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, the structure itself is suitable for low-pressure casting, and the demand is large. Therefore, the development of low-pressure casting technology has been greatly promoted. 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 use this process to produce.
Low-pressure casting can achieve a high degree of mechanization and automation, which not only improves productivity (10-15 types/h), but also reduces many artificial factors that are not conducive to the production process, improves the yield rate, and can greatly reduce the labor intensity of workers. However, the quality of low-pressure castings is affected by factors such as process plans, process parameters, mold structures and manual operations, as well as the mutual influence between them. Unreasonable design of any link or improper operation may cause defects in low-pressure castings. Among them, the occurrence of aluminum alloy wheel 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 very important to discuss the causes of cracks in low-pressure cast aluminum alloy wheels.
1. Reasons for the formation of cracks in low-pressure cast aluminum alloy wheels
The cracks of low-pressure cast aluminum alloy wheel is mainly generated in the places where the stress is concentrated, or the cracks caused by the uneven force when the hub is ejected, or the solidification of the liquid at the riser tube. Cracks are generally divided into cold cracks and hot cracks.
Cold cracking refers to cracks formed when the alloy is below its solidus temperature. In layman's terms, cold cracking is caused when the casting cools to a low temperature, and the casting stress acting on the casting exceeds the strength or plasticity of the casting itself. Most of the cold cracks appear on the surface of the casting, and the surface of the crack is slightly oxidized; while the hot crack is generally considered to be generated during the solidification of the alloy. Due to the heat transfer of the mold wall, the casting always starts to solidify from the surface. When a large number of branches appear on the surface of the casting and overlap into a complete skeleton, the solid shrinkage of the casting will occur (often expressed by linear shrinkage). But at this time there is still a layer of liquid metal film (liquid film) that has not been solidified between the dendrites. If the shrinkage of the casting is not hindered by any, then the dendrite layer can be contracted freely without force, and it will not appear. stress. When the shrinkage of the dendrite layer is hindered, it cannot shrink freely or be subjected to tensile force, and tensile stress will occur. At this time, the liquid film between the dendrites will be deformed by the action of 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 away slowly, and there is enough liquid around it to flow into the cracked part in time, then the cracked part will be filled and "healed". Castings will not show hot cracks. If the cracks cannot be "healed" again, the casting will have hot cracks. The surface of the hot fracture is strongly oxidized, showing a dark or black color without metallic luster.
Second, the main factors affecting the generation of cracks
For the same alloy, whether the wheel hub cracks, often depends on factors such as wheel hub structure, process parameters and mold temperature.
1. The influence of improper design of wheel hub structure on wheel hub cracks
(1) Improper fillet size is the most common cause of thermal cracks in the hub, because the hub will generate a lot of stress at the sharp corners when it cools. In the part with a small fillet, even if the packing is good and the shrinkage does not occur, thermal cracking will occur.
(2) Sudden changes in the cross-section of the hub will result in different cooling speeds, and even if the packing is good, it will produce greater stress, causing cracks or cracks after the hub is solidified.
2. The influence of unreasonable technological parameters on the cracks of the hub
In low-pressure casting, the liquid in the riser is solidified due to the long pressure holding time or the riser tube is too long, and it bears a certain pulling force when the hub casting is ejected, resulting in cold cracking of the hub. Therefore, designing a reasonable pressure holding time and liquid lifting system is very important to reduce the cracks caused by the hub during ejection.
3. The influence of the temperature on the crack of the hub
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. The temperature of the mold changes with changes in the weight of the casting, the cycle of die casting, the temperature of the die casting, and the cooling method of the mold.
From the perspective of heat transfer, increasing the temperature of the mold can reduce the heat exchange strength between the metal and the mold and prolong 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 decreases slightly, that is, the filling capacity increases as the mold temperature increases. Therefore, an appropriate increase in mold temperature is conducive to the reduction of stress. If the mold temperature is too low, the casting is cooled too quickly in the metal mold, and the solidification speed of each part of the casting is different, which will cause uneven cooling of the casting in the mold and produce Thermal stress and deformation result in hot cracking and large residual stress and residual deformation in the finished casting. The higher temperature of the mold is not conducive to obtaining a fine crystal structure. Liquid metal is easy to get in and shrink, causing castings to produce. The chance of defects such as stomata, shrinkage and shrinkage increases. To unify this contradiction, the mold temperature can be increased appropriately without casting defects.
3. Improvement measures
(1) Reasonable design of the liquid lifting system due to the long pressure holding time or the long liquid of the liquid riser, which causes the liquid in the liquid riser to solidify, which causes the hub casting to withstand a certain tensile force when it is ejected, which causes the wheel to produce cold cracks. A reasonable lift system is very important to reduce the tendency of cracks. The liquid lift system refers to the channel through which the liquid metal enters the cavity from the crucible during pouring, including the liquid lift pipe, the insulation sleeve and the casting system for casting. The dimensions of these parts 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 cools down when pouring through this distance, and it is easy to cause early condensation of the riser channel. It should therefore be noted:
① Shorten the distance between the liquid level in the crucible and the gate in the casting. This distance involves several aspects such as equipment, process, mold, etc., so comprehensive consideration should be taken 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 is appropriately increased. In order to prevent early solidification of the riser, the diameter of the riser should be increased appropriately.
(2) Reasonably designed hub structure. When designing the hub structure, sudden changes in sharp corner structures and cross sections should be avoided. Rounded corners or structures with uniform thickness should be used.
(3) In the absence of casting defects, increase the mold temperature appropriately.
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