Low pressure casting can realize high mechanization and automation, not only improve the productivity (10-15 type / h), but also reduce many human factors that are not conducive to the production process, improve the yield, and greatly reduce the labor intensity of workers. However, the quality of low-pressureis affected by such factors as process plan, process parameters, mold structure and manual operation, as well as the interaction between them. Unreasonable design or improper operation of any link may lead to defects of low-pressure casting parts. Among them, the generation of aluminum alloy wheel crack is an important factor affecting the production cost and production efficiency. Therefore, it is very important to study the cause of the wheel crack in low pressure casting aluminum alloy.
1. Causes of crack formation
1.1 definition of low pressure casting
1.1 definition of low pressure casting
The dry compressed air is introduced into the sealed container filled with metal liquid, which acts on the metal liquid surface to maintain a certain temperature, so that the metal liquid can enter the mold cavity from bottom to top along the riser through the sprue. After the metal liquid fills the mold cavity, the air pressure is increased, and the metal liquid in the mold cavity solidifies and forms under a certain pressure, and then the gas pressure on the liquid surface is relieved, so that there is no gas pressure in the riser The solidified liquid metal falls back to the increasing pot, and then opens the mold to take out the parts.
1.2 low pressure casting aluminum alloy wheel crack
It is mainly produced in the parts with stress concentration, or the cracks caused by uneven stress when the wheel is demoulded, or the liquid solidifies at the liquid lifting pipe. Cracks are generally divided into cold cracks and hot cracks.
Cold crack refers to the crack formed when the temperature of alloy is lower than its solid phase line. Generally speaking, cold crack is produced when the casting is cooled to low temperature and the casting stress acting on the casting exceeds the allowable strength or plasticity of the casting itself. Cold cracks often appear on the surface of castings, and there is slight oxidation on the surface of cracks; hot cracks are generally considered to be produced in the process of alloy solidification, because of the heat transfer effect of mold wall, castings always start to solidify from the surface. When a large number of dendrites appear on the surface of the casting and form a complete framework, the casting will show solid shrinkage (usually expressed as linear shrinkage). However, there is still a layer of liquid metal film (liquid film) between the dendrites. If the shrinkage of the casting is not hindered, then the dendrite layer can contract freely without stress. When the shrinkage of dendrite layer is blocked, it can’t shrink freely or be affected by tensile force, then tensile stress will appear, and the liquid film between dendrites will be deformed by tensile force. When the tensile stress exceeds the strength limit of the liquid film, the dendrite will be pulled apart. However, there are still some liquid metals around the cracked part. If the liquid film is pulled out slowly and there is enough liquid around and flows into the cracked part in time, the cracked part will be filled and “healed” and the casting will not have hot cracks. If the pull crack cannot be “healed” again, hot cracks will appear in the casting. The surface of the hot fracture surface is strongly oxidized, showing a dark or black color without metallic luster.
2. Main factors affecting crack generation
For the same alloy, whether the hub cracks or not often depends on the hub structure, process parameters and mold temperature and other factors.
2.1 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.
2.2 effect of unreasonable process parameters on wheel crack
In the low-pressure casting, the liquid in the riser solidifies due to the long holding time or the long riser, and bears a certain tension when the hub casting is ejected, which results in the cold crack of the hub. 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.
2.3 effect of mold temperature on wheel crack
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 many defects such as dimensional deviation and deformation of the casting, and also has a great impact on the productivity. Mold temperature changes with the change of 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 castings are cooled too fast in the metal mold, and the solidification speed of different parts of the castings is different, the cooling of the 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 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.