Pressure casting is called die casting for short. Under a certain pressure, the melt can fill the cavity. The structure of the casting is compact, the mechanical property is high, the dimension is precise, and the machining allowance is small. According to the pressure level, it can be roughly divided into two categories: low pressure casting and high pressure casting. The latter has high mold cost, which is especially suitable for mass production of automobile casting products with high efficiency and low total cost. At present, almost all the car transmission case, clutch case, water pump case, carburetor case and steering case are aluminum.
The main parameters of dieare injection force, specific pressure, melt filling speed, filling time, holding time, melt temperature and die temperature. As the melt fills the cavity at high temperature, high speed and high pressure, it will inevitably get involved in gas, so a number of new technologies have been developed, such as vacuum die casting, oxygen filling die casting, low speed filling die casting “precision speed dense” die casting (also known as acurate, rapid, deny), low speed medium pressure filling (NDC) casting, etc., which are all new void free die casting processes. Therefore, it is necessary to control the process parameters scientifically according to the characteristics of the die casting, design and manufacture the mold reasonably (including the pouring system, overflow exhaust system, mold cooling system, mold heating and balance control system), and select the appropriate coating and spraying technology. The temperature of aluminum melt is 40 ℃ ~ 100 ℃ higher than the melting point of alloy in die casting. The temperature and heat balance of die are the most important conditions to achieve high quality casting. For general die casting, the surface temperature of the mold cavity of the casting is about 40% of the melting point of alloy.
The vacuum degree of the gas extraction pipe by vacuum die casting method is 22 kn / m2 ~ 27 kn / m2, and the negative pressure is 4 kn / m2 in 0.15 s. The quality of the vacuum die casting depends on the vacuum degree. Generally, the gas content of the die casting after T6 treatment is less than 0.1ml / (100g al). The air content of the die casting produced by the oxygen filling method is about 0.05 ml / (100 g al), only 1 / 10 of that of the common die casting method, and its alumina content is about 3 times of that of the common die casting. However, there are only a few micropores in the product due to the existence of fine (1 μ m) alumina particles, and the pressure resistance of the product is good.
The pouring speed of low speed filling die casting aluminum melt is about 1 m / s, which is 1 / 20 and 1 / 60 of the conventional die casting speed. The wall thickness of die casting must be more than 6 mm, and the shape should not be complicated. The basic principle of acarad method can be summed up as follows: control of pressure cooling, large section gate, slow filling speed, double punch injection piston. The NDC method adopts the spiral pouring method. The aluminum melt is injected along the spiral sprue of the injection wall, and the air is not easy to mix into the melt. The filling speed is low, and the time is 20 times of that of the traditional die casting method. Because of the solidification under pressure, the structure is dense, and the proper heat treatment is used, the mechanical properties of the casting can be greatly improved. The workpiece that needs the sand core molding can be produced, and the casting has good pressure resistance and air tightness, Japan widely uses this method to produce car air compressor body, motorcycle cylinder head, wheel, exhaust pipe and engine block.