Low pressure casting aluminum wheel hub crack formation causes and improvement measures

Low-pressure casting can achieve a high degree of mechanization and automation, improve productivity (10 ~ 15 type / h), it can also reduce many human factors that are detrimental to the production process, improve the rate of finished products, and greatly reduce the labor intensity of workers. However, the quality of low pressure casting by the process plan, process parameters, mold structure and manual operation and other factors, any design unreasonable or improper operation of the link may lead to low pressure casting defects. The generation of aluminum wheel cracks is an important factor that affects the production cost and productivity of enterprises. Therefore, it is particularly important to discuss the causes of cracks in low pressure casting aluminum wheels. Next, I will discuss with you the formation and elimination measures of low pressure casting aluminum alloy automotive wheel cracks in China.

The main factors affecting crack generation

For the same alloy, whether the wheel cracks or not often depends on the wheel construction, process parameters and mold temperature.

1. The effect of improper wheel structure design on wheel cracks

(1) Improperly sized inside fillets are the most common cause of thermal cracking in hubs, because the sharp corners of the hub generate high stresses when the hub cools. A small inner fillet can cause thermal cracking even if the shrinkage is good.

(2) Sudden changes in hub cross-section can lead to different cooling rates, which can generate large stresses even if the shrinkage is good, resulting in cracks or crazing of the hub after solidification.

2. The effect of unreasonable process parameters on wheel cracks

In low-pressure casting, due to too long holding time or too long liquid-raising pipe, the liquid in the liquid-raising pipe solidifies, and the wheel casting is subjected to a certain tensile force when ejecting, which leads to cold cracking of the wheel hub. Therefore, the design of reasonable pressure holding time and ascending liquid system is of great significance to reduce the cracking of wheel hub when ejecting.

3. Influence of mold temperature on wheel cracks

Low-pressure casting mold temperature determines the solidification mode of the alloy liquid, directly affects the casting's internal surface condition, is one of the main causes of dimensional deviation and deformation and other defects, also has a great impact on productivity. Mold temperature with casting quality, die casting cycle, die casting temperature and mold cooling method and change.

From the heat transfer point of view, increasing the mold temperature can reduce the intensity of heat transfer between the metal and the mold and extend the flow time. It has also been shown that increasing the mold temperature can also slightly reduce the interfacial tension between the liquid metal and the casting. As the mold temperature increases, the filling time decreases slightly, i.e., the filling capacity increases with the mold temperature. Therefore, an appropriate increase in mold temperature is conducive to stress reduction. If the mold temperature is too low, the casting is cooled too quickly in the metal type, the casting solidification speed of each part of the casting is different, the casting cooling is not uniform, resulting in thermal stress and deformation, leading to the finished casting thermal cracks, residual stress and residual deformation, high mold temperature is not conducive to the organization of the small crystalline, the liquid metal is easy to absorb and contraction, increase the casting to produce porosity, shrinkage and shrinkage defects. In order to unify this contradiction, the mold temperature can be increased appropriately in the absence of casting defects. In order to unify this contradiction, the mold temperature can be appropriately increased in the absence of casting defects. The general wheel crack site is in the spoke or the inner wall of the wheel.

Improvement measures

(1) Reasonable design of the liquid-raising system. Due to too long holding time or too long lift tube, the liquid in the lift tube solidifies, and the wheel casting is subjected to a certain tensile force when it is ejected, which leads to cold cracking of the wheel hub. Therefore, a reasonably designed liquid-raising system is of great significance in reducing the cracking tendency. Liquid-raising system refers to the liquid metal channel that enters the cavity from the crucible, including liquid-raising pipe, insulation sleeve and casting pouring system. The dimensions of these components directly affect the distance between the liquid level in the crucible and the gate in the casting. The longer the distance is, the faster the liquid metal cools during pouring, which can easily lead to early solidification of the ascending tube channel. Therefore, attention should be paid to: ① Shorten the distance between the liquid level in the crucible and the casting gate. The distance involves equipment, process, mold and other aspects, so it should be considered comprehensively and the distance should be shortened; ② Improve the heat preservation sleeve. Appropriately increase the inner diameter of the insulation sleeve, expand the thickness of the insulation layer, the use of aluminum silicate fiber felt and other materials with good thermal insulation properties as a thermal insulation sleeve; ③ Appropriately increase the diameter of the ascending liquid tube. In order to prevent early solidification of the ascending liquid pipe, the diameter of the ascending liquid pipe should be increased appropriately.

(2) Design reasonable hub structure. When designing the hub structure, the sharp angle structure and the sudden change of cross section should be avoided, and the structure with rounded corners or uniform thickness should be used.

(3) The temperature of the mold should be increased appropriately without casting defects.

The quality of low-pressure casting wheels can be improved by analyzing the causes of defects in low-pressure casting wheels