Analysis of Crack Formation Causes in Low-pressure Cast Aluminum Alloy Wheel Hubs

The causes of the formation of cracks in low-pressure cast aluminum alloy wheels are analyzed, and various factors affecting crack generation, such as casting structure, process parameters, mold temperature, etc., are studied, and the impact of cracks on the hub castings is eliminated by rationally controlling and adjusting these factors. In order to improve the economic efficiency of enterprises. Aluminum alloy wheels have many characteristics that cannot be matched by steel wheels. Therefore, aluminum alloy wheels have been widely used in cars and motorcycles. By 2002, the loading rate of China's aluminum alloy wheels for cars was close to 45%. Due to the high requirements on the quality of the automotive hub, the structure itself is suitable for low pressure casting, and the demand is large. Therefore, it greatly promotes the development of low pressure casting technology. At present, low-pressure casting has become the main production method for aluminum alloy wheels. Most aluminum alloy wheel manufacturers in China adopt this technology for production. Low-pressure casting can achieve high degree of mechanization and automation. It can not only increase productivity (10 to 15 type/h), but also reduce many human factors that are unfavorable to the production process, increase the yield, and can greatly reduce the labor intensity of workers. However, the quality of low-pressure casting parts is affected by factors such as process scheme, process parameters, mold structure and manual operation, as well as the mutual influence between them. Irrational design or improper operation of any one link may cause defects in low-pressure casting parts. Among them, the occurrence of cracks in the aluminum alloy hub is an important factor affecting the production cost and production efficiency of the enterprise, and the hub crack is a major hidden danger of the safety of the automobile. Therefore, it is very important to discuss the causes of cracks in low-pressure cast aluminum alloy wheels. I. Causes of crack formation in low-pressure cast aluminum alloy wheels Cracks in low-pressure cast aluminum alloy wheels mainly occur at locations where stress is concentrated, or when the hub is ejected, due to uneven force, or cracking caused by solidification of liquid at the riser. Cracks are generally divided into cold cracking and hot cracking. Cold cracking refers to cracks formed when the alloy is below its solidus temperature. In layman's terms, cold cracking is caused by the fact that castings that act on castings when castings are cooled to low temperatures exceed the strength or plasticity of the castings themselves. Cold cracking occurs on the surface of the casting and the surface of the crack is slightly oxidized. Hot cracking is generally thought to occur during the solidification process of the alloy. Due to the heat transfer of the wall, the casting always solidifies from the surface. When a large number of branches appear on the surface of the casting and overlap into a complete skeleton, the casting will appear solid shrinkage (usually expressed as line shrinkage). However, at this time, there is still a layer of liquid metal film (liquid film) that has not yet solidified between the dendrites. If the shrinkage of the casting is not hindered, the dendritic layer is free from the force and can be freely contracted, and thus it will not appear. stress. When the shrinkage of the dendritic layer is hindered, it cannot be freely contracted or subjected to a tensile force, and tensile stress occurs, and the interdendritic liquid film is 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 are still some liquid metals around the cracked part. If the liquid film is pulled slowly, and there is enough liquid around it and the cracks will flow in time, the cracks will be filled and "healed." Castings do not show hot cracks. If the cracks cannot be re-healed, the casting will have hot cracks. The surface of the thermal fracture is strongly oxidized, showing a dark or black with no metallic luster. Second, the main factors affect the crack generation For the same alloy, whether the hub cracks, often depends on the hub structure, process parameters and mold temperature and other factors. 1. Improper design of the hub structure on hub cracks (1) The improper size of the fillet is the most common cause of thermal cracks in the hub, because the hub will generate a lot of stress at the sharp corners during cooling. In areas where the fillet is small, thermal cracking can occur even if the shrinkage is good without shrinkage. (2) A sudden change in the hub section will result in different cooling rates. Even if the feeding is good, a large stress will occur, causing cracks or cracks in the hub after solidification. 2. Effect of irrational process parameters on cracks in the hub In low-pressure casting, due to the excessive pressure holding time, or the liquid rising in the liquid riser due to the excessive length of the liquid riser, a certain pulling force is applied when the hub casting is pushed out, resulting in a cold crack in the hub. Therefore, designing a reasonable pressure holding time and liquid raising system is of great significance to reducing the cracking caused by the hub during ejection. 3. Influence of temperature on the crack of the hub The mold temperature of the 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 defects such as dimensional deviation and deformation of the casting, and at the same time, productivity. It also has a great influence. 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 point of view of heat transfer, increasing the temperature of the mold can reduce the heat exchange strength between the metal and the mold, and extend the flow time. Some studies have also shown that increasing the mold temperature can slightly reduce the interfacial tension between the molten metal and the mold. As the temperature of the mold increases, the filling time decreases slightly, that is, the filling capacity increases as the temperature of the mold increases. Therefore, the 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 type, the solidification speed between the various parts of the casting is different, will cause uneven cooling of the casting in the mold, resulting in Thermal stress and deformation result in thermal cracking and large residual stress and residual deformation on the finished casting product. The high temperature of the mold is not conducive to obtaining a fine crystalline structure. The liquid metal is easy to inhale and shrink, and the casting is produced. The chances of defects such as porosity, shrinkage and shrinkage increase. To unify this contradiction, the mold temperature can be appropriately increased without the occurrence of casting defects. Third, the improvement measures (1) rational design of liquid raising system due to pressure for a long time, or the liquid riser caused by excessive solidification of the liquid inside the tube, so that the hub casting in the ejection of a certain degree of tension, resulting in the hub to produce cold Cracking, so a rational design of the liquid raising system is of great importance to reduce the tendency of cracks. Liquid system refers to the passage of liquid metal from the crucible into the cavity during pouring, including the liquid riser, insulation jacket and casting system. 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 when it passes through this distance during pouring, and can easily cause the early convergence of the riser channel. Therefore, it should be noted that: (1) Shorten the distance between the inner surface of the crucible and the gate in the casting. This distance involves several aspects such as equipment, craftsmanship, and molds, so comprehensive consideration should be taken to shorten this distance. 2 improve the insulation sleeve. Appropriately increase the insulation cover trade route, in order to expand the thickness of the insulation layer; use good insulation material for insulation cover, such as aluminum silicate fiber felt. 3 Increase the diameter of the liquid riser properly. In order to prevent early solidification of the riser, the diameter of the riser should be increased. (2) A properly designed wheel hub structure When designing a wheel hub structure, sudden changes in the sharp corner structure and cross section should be avoided, and round corners or a uniform thickness structure should be adopted. (3) Appropriately increase mold temperature without casting defects.