For the same alloy, whether the hub cracks or not often depends on the hub structure, process parameters and mold temperature and other factors.
The influence of improper design of hub structure on hub crack
1) The improper size of the inner fillet is the most common reason for the hot crack in the hub, because the hub will produce a large stress at the sharp corner during cooling. For the part with small inner fillet, even if the feeding is good and there is no shrinkage crack, there will be hot crack.
2) If the cross section of the hub changes suddenly, the cooling speed will never be the same, even if the feeding is good, it will also produce large stress, which will cause cracks or cracks after the solidification of the hub.
Influence of unreasonable process parameters on crack in wheel
In the process of low pressure casting, the liquid in the riser solidifies due to the long holding time or the long riser, and bears a certain tensile force when the runner is ejected, thus causing the cold crack of the runner. Therefore, it is of great significance to design a reasonable pressure holding time and liquid lifting system to reduce the cracking caused by wheel shaft ejection.
The influence of mould temperature on the crack of wheel
The mold temperature of low-pressure casting determines the solidification mode of the alloy liquid and directly affects the internal and surface conditions of the low-pressure casting. It is one of the main reasons for many defects such as dimensional deviation and deformation of the low-pressure casting, and also has a great impact on the productivity. Mold temperature changes with the change of low-pressure casting quality, die-casting cycle, die-casting temperature and mold cooling mode.
From the perspective of heat transfer, increasing the mold temperature can reduce the heat transfer intensity between metal and mold, and extend the flow time. It has also been shown that increasing the mold temperature can slightly reduce the interfacial tension between the molten metal and the mold. With the increase of mold temperature, the filling time decreases slightly, that is, the filling capacity increases with the increase of mold temperature. Therefore, the proper increase of mold temperature is conducive to the reduction of stress. If the mold temperature is too low, the low-pressure castings are cooled too fast in the metal mold, and the solidification speed of different parts of the low-pressure castings is different, the cooling of the low-pressure castings in the mold will be uneven, resulting in thermal stress and deformation, resulting in hot cracks and large residual stress and deformation on the finished low-pressure castings, and the higher mold temperature is unfavorable In order to obtain fine crystal structure, liquid metal is easy to inhale and contract, which increases the chance of producing defects such as porosity, shrinkage porosity and shrinkage porosity. In order to unify this contradiction, the mold temperature can be raised properly without casting defects. Generally, the crack of hub is located on the inner wall of hub or spoke.