The causes of crack formation in low-pressure casting aluminum alloy wheel hubs are analyzed, and various factors affecting crack generation, such as casting structure, process parameters, mold temperature, etc., are studied. By properly controlling and adjusting these factors, the influence of cracks on the hub castings is eliminated. Thereby improving the economic efficiency of the enterprise.
Aluminum alloy wheels have many characteristics unmatched by steel wheels, so aluminum alloy wheels have been widely used in cars, motorcycles and other vehicles. By 2002, the loading rate of China's car aluminum alloy wheels was close to 45%. Due to the high quality requirements of the automobile wheel hub, the structure itself is suitable for low-pressure casting, and the demand is large, thus greatly promoting the development of low-pressure casting technology. At present, low-pressure casting has become the main process for the production of aluminum alloy wheels. Most of the domestic aluminum alloy wheel manufacturing enterprises use this process.
Low-pressure casting can achieve high mechanization and automation, which not only improves productivity (10~15 type/h), but also reduces many human factors that are not conducive to the production process, improves the yield, and can greatly reduce the labor intensity of workers. However, the quality of low-pressure castings is affected by factors such as process schedule, process parameters, mold structure and manual operation, and the interaction between them. Unreasonable design or improper operation of any link may lead to defects in low-pressure castings. Among them, the crack of aluminum alloy wheel 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 automobile safety. Therefore, the discussion on the causes of cracks in low-pressure casting aluminum alloy wheels is particularly important.
First, the cause of crack formation in low-pressure casting aluminum alloy wheel hub
The crack of the low-pressure casting aluminum alloy wheel hub mainly occurs in the part where the stress is concentrated, or the uneven force due to the wheel jacking out, or the crack caused by the solidification of the liquid at the riser pipe. Cracks are generally classified into cold cracking and hot cracking.
Cold cracking refers to a crack formed when the alloy is below its solidus temperature. In layman's terms, cold cracking occurs when the casting is cooled to a low temperature and the casting stress acting on the casting exceeds the strength or plasticity of the casting itself. Cold cracking occurs on the surface of the casting, and the surface of the crack is slightly oxidized. The thermal cracking is generally considered to occur during the solidification of the alloy. Due to the heat transfer of the shaped wall, the casting always solidifies from the surface. When a large number of branches appear on the surface of the casting and are lapped into a complete skeleton, the casting will undergo solid shrinkage (often expressed by linear shrinkage). However, at this time, there is a liquid metal film (liquid film) which has not been 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 shrunk, so that it does not appear. stress. When the shrinkage of the dendritic layer is hindered, it cannot be freely contracted or subjected to tensile force, and tensile stress occurs. At this time, the liquid film between the dendrites is deformed by the action of stretching. When the tensile stress exceeds the strength limit of the liquid film, the dendrites are pulled apart. However, there are some liquid metals around the cracked part. If the liquid film is pulled slowly and there is enough liquid around it and it will flow in the crack, the crack will be filled and “healed”. There are no hot cracks in the casting. If the crack cannot be "healed" again, hot cracks will occur in the casting. The surface of the thermal fracture is strongly oxidized, giving a dark or black color without metallic luster.
Second, the main factors affecting the generation of cracks
For the same alloy, whether the hub is cracked often depends on the hub structure, process parameters and mold temperature.
1. The influence of improper design of the hub structure on the crack of the hub
(1) Improper fillet size is a common cause of hot cracks in the hub, because the hub generates a large stress at the sharp corners when it cools. In the small fillet area, even if the shrinkage is good and there is no shrinkage, hot cracking will occur.
(2) The sudden change of the cross-section of the hub will result in different cooling speeds. Even if the shrinkage is good, it will cause a large stress, which will cause cracks or cracks after solidification of the hub.
2. The influence of unreasonable process parameters on the crack of the hub
In low-pressure casting, because the holding time is too long, or the liquid riser tube is too long, the liquid in the riser tube solidifies, and the hub casting is subjected to a certain pulling force when it is ejected, thereby causing cold cracking of the hub. Therefore, the design of reasonable dwell time and liquid lift system is of great significance to reduce the crack caused by the wheel when it is ejected.
3. The effect of temperature on the crack of the hub
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 casting. It is one of the main reasons for the dimensional deviation and deformation of the casting, and also has a great impact on productivity. The mold temperature changes depending on the weight of the casting, the die casting cycle, the die casting temperature, and the mold cooling method.
From the perspective of heat transfer, increasing the mold temperature reduces the heat transfer between the metal and the mold and prolongs the flow time. Studies have also shown that increasing the mold temperature also slightly reduces 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, the proper increase of the mold temperature is conducive to the reduction of the stress. If the mold temperature is too low, the casting is cooled too fast in the metal mold, and the solidification speed between the various parts of the casting is different, which causes the cooling of the casting in the mold to be uneven. Thermal stress and deformation result in hot cracking and large residual stress and residual deformation on the finished product, but the higher temperature of the mold is not conducive to obtaining fine crystal structure, liquid metal is easy to inhale and shrink, and the casting is produced. The chances of defects such as stomata, shrinkage and shrinkage are increased. In order to unify this contradiction, the mold temperature can be appropriately increased without casting defects.
Third, improvement measures
(1) Reasonable design of the liquid lift system
Because the holding time is too long, or the liquid riser tube is too long, the liquid in the riser tube solidifies, so that the hub castings are subjected to a certain tensile force when ejecting, thereby causing the hub to be cold-cracked. Therefore, a reasonable design of the liquid lift system reduces cracks. The tendency is of great significance. The liquid lift system refers to the passage of liquid metal into the cavity from the concrete during pouring, including the riser pipe, the insulation sleeve and the casting casting system. The dimensions of these parts directly affect the distance between the liquid level in the crucible and the gate of the casting. The longer the distance, the faster the liquid metal will cool down when passing through the distance, which will easily cause the riser channel to condense early. Therefore, it should be noted that: 1 shorten the distance between the liquid level in the crucible and the gate of the casting. This distance involves several aspects such as equipment, process, and mold. Therefore, it is advisable to reduce the distance by comprehensive consideration. 2 improve the insulation sleeve. Appropriately increase the thickness of the insulation jacket to expand the thickness of the insulation layer; use a material with good insulation properties as a thermal insulation sleeve, such as aluminum silicate fiber felt. The diameter of the 3 liter liquid pipe is appropriately increased. In order to prevent early solidification of the riser tube, the diameter of the riser tube should be appropriately increased.
(2) Well-designed hub structure
When designing the hub structure, sudden changes in the corner structure and section should be avoided. It is advisable to use rounded or uniform thickness structures.
(3) Appropriately increase the mold temperature without casting defects.
(4) Improvement of the sprue bushing