Influencing factors of crack formation in low pressure casting of automobile parts

In the casting of automobile parts, the wheel casting is a little different. Because it needs to meet a series of requirements, including impact and fatigue properties, including impact and fatigue properties. More because its failure may directly lead to a devastating disaster, so its security is particularly important. Air tightness is an important requirement for the wheel, because its edge needs to be combined with the tire, which is a component of pressure vessel; high quality surface polishing is also a requirement that the wheel needs to meet, because the wheel needs to play an important role in aesthetic decoration; geometric tolerance and rotation balance, which requires the wheel to have sufficient working stability. However, the casting defects of wheels are a great challenge to the above series of requirements for wheels. At the same time, casting defects are also an inevitable problem in the aluminum alloy casting industry. Casting defects of automobile aluminum alloy continue to challenge metallurgical experts and production engineers, especially for enterprises that emphasize product quality and production cost. So it is very important to study the formation mechanism of casting defects and the improvement measures.

1) The influence of wheel structure on wheel crack is that when there is sharp angle in wheel structure, there will be great stress in this part, which is easy to form crack. If the fillet setting is not reasonable in this part, cracks may occur, which is also the most common cause of hot cracks. In addition to the setting of the inner fillet, if the cross-section of the wheel structure changes suddenly, the cooling speed will be inconsistent during the solidification process, even if there is a better feeding, it will produce a great stress, which will lead to cracking.

2) If the lifting pipe is too long or the holding time is too long, the solidification of liquid metal in the lifting pipe will appear. At the moment when the casting is finished and the wheel casting is ejected, the solidified alloy in the lifting pipe has a large tensile stress, which will lead to cold cracking of the wheel. Therefore, in order to reduce the cracking of the wheel when it is ejected, it is necessary to design reasonable process parameters such as the liquid lifting system and the holding time.

3) The influence of mold temperature on wheel crack mold temperature has a great influence on productivity, casting weight, casting cycle, casting temperature and cooling mode all have an effect on mold temperature. In the process of low-pressure casting, the mold temperature affects the solidification mode of liquid metal, the internal and surface quality of the casting, and it is also one of the main reasons for the defects such as deformation and dimensional deviation of the casting.

Properly increasing the mold temperature is conducive to improving the filling capacity and reducing the stress, because increasing the mold temperature can reduce the heat transfer intensity between the liquid metal and the mold, and increase the flow time. Moreover, the increase of mold temperature will reduce the interfacial tension between the alloy liquid and the mold, improve the filling ability and reduce the filling time. If the mold temperature is too low, the heat exchange strength between the molten metal and the mold is large, which makes the solidification speed faster, and it is easy to cause the different solidification speed of each part of the casting, thus leading to the thermal stress and deformation, and finally leading to the hot crack and residual stress of the finished casting. There is also a bad side to raising the mold temperature. Higher mold temperature will make the crystal structure coarse, and the liquid metal is easy to shrink and inhale, resulting in the defects of porosity and shrinkage porosity. Therefore, in order to improve the mold temperature, we need to consider it comprehensively.