Pressure casting (abbreviated as die casting) is a special casting method with little cutting and rapid development in modern metal forming processing technology. The essence of the process is to fill the die casting cavity with liquid or semi-liquid metal at a high speed under high pressure, and to form and solidify under pressure to obtain castings.
Characteristics of die casting process: High speed and high pressure are the main characteristics of pressure casting. The commonly used working pressure is tens of MPa, the filling speed is about 16~80m/s, and the time for the metal liquid to fill the mold cavity is extremely short, about 0.01~0.2s. Compared with other casting methods, die casting has the following three advantages:
1. Good product quality
The casting has high dimensional accuracy, generally equivalent to level 6~7, and even up to level 4; good surface finish, generally equivalent to level 5~8; high strength and hardness, the strength is generally 25~30% higher than sand casting, but the elongation is reduced by about 70%; stable size and good interchangeability; can die cast thin-walled and complex castings.
2. High production efficiency
The machine has high productivity. For example, the domestic J1113 horizontal cold air die casting machine can die cast 600 to 700 times in eight hours on average, and the small hot chamber die casting machine can die cast 3,000 to 7,000 times every eight hours on average; the die casting mold has a long life, and a pair of die casting molds, die casting bell alloy, can have a life of hundreds of thousands of times or millions of times; it is easy to realize mechanization and automation.
3. Excellent economic effect
Due to the advantages of die casting, such as smooth surface. Generally, it is not machined anymore but used directly, or the processing volume is very small, so it not only improves the metal utilization rate, but also reduces a lot of processing equipment and working hours; the casting price is cheap; combined die casting can be used with other metals or non-metallic materials. It saves both assembly time and metal.
Die casting is one of the metal forming methods, and it is an effective way to achieve less chipping and no chipping. It is widely used and develops rapidly. At present, die-casting alloys are no longer limited to non-ferrous metals such as zinc, aluminum, magnesium and copper, but are also gradually expanded to die-cast cast iron and steel parts. The size and weight of die-casting parts depend on the power of the die-casting machine. As the power of the die-casting machine continues to increase, the size of the casting can range from a few millimeters to 1-2 meters; the weight can range from a few grams to tens of kilograms. Aluminum castings with a diameter of 2 meters and a weight of 50 kg can be die-cast abroad.
The metal materials used to produce die-casting parts are mostly non-ferrous metals such as aluminum alloys, pure aluminum, zinc alloys, copper alloys, magnesium alloys, lead alloys, tin alloys, etc., and ferrous metals are rarely used.
1. Silicon (Si)
Silicon is the main element of most die casting aluminum alloys. Silicon and aluminum can form a solid solution. At 577°C, the solubility of silicon in aluminum is 1.65%, 0.2% at room temperature, and when the silicon content reaches 11.7%, silicon and aluminum form a eutectic. Improve the high-temperature moldability of the alloy, reduce shrinkage, and have no tendency to hot cracking. When the silicon content in the alloy exceeds the eutectic composition, and there are more impurities such as copper and iron, hard spots of free silicon appear, making cutting difficult. High-silicon aluminum alloy has a serious melting effect on the casting crucible.
2. Copper (Cu)
Copper and aluminum form a solid solution. When the temperature is 548°C, the solubility of copper in aluminum should be 5.65%, which drops to about 0.1% at room temperature. Increasing the copper content can improve the fluidity, tensile strength and hardness of the alloy, but reduce corrosion resistance and plasticity, and increase the tendency to hot cracking.
3. Magnesium (Mg)
Adding a small amount (about 0.2-0.3%) of magnesium to high-silicon aluminum alloy can improve strength and improve the machinability of the alloy. Aluminum alloys containing 8% magnesium have excellent corrosion resistance, but their casting performance is poor, their strength and plasticity are low at high temperatures, and they shrink greatly when cooled, so they are prone to hot cracking and looseness.
4. Zinc (Zn)
Zinc can improve fluidity, increase hot brittleness and reduce corrosion resistance in aluminum alloys, so the zinc content should be controlled within the specified range.
5. Iron (Fe)
All aluminum alloys contain harmful impurities. When the iron content in aluminum alloys is too high, iron exists in the alloy in the form of flaky or needle-like structures of FeAl3, Fe2Al7 and Al-Si-Fe, which reduces the mechanical properties. This structure will also reduce the fluidity of the alloy and increase the hot cracking. However, since the adhesion of aluminum alloys to the mold is very strong, it is particularly strong when the iron content is below 0.6%. When it exceeds 0.6%, the sticking phenomenon is greatly reduced, so the iron content should generally be controlled within the range of 0.6-1%, which is good for die casting, but it cannot exceed 1.5%.
6. Manganese (Mn)
Manganese can reduce the harmful effects of iron in aluminum alloys and can change the lamellar or needle-shaped structures formed by iron in aluminum alloys into fine crystal structures. Therefore, generally, aluminum alloys are allowed to have less than 0.5% manganese. When the manganese content is too high, it will cause segregation.
7. Nickel (Ni)
Nickel can improve the strength and hardness of aluminum alloys and reduce corrosion resistance. Nickel has the same effect as iron, which can reduce the alloy's melting corrosion on the mold, while neutralizing the harmful effects of iron and improving the welding performance of the alloy.
When the nickel content is 1-1.5%, the casting can obtain a smooth surface after polishing. Due to the lack of nickel sources, aluminum alloys containing nickel should be used as little as possible.
8. Titanium (Ti)
Adding a trace amount of titanium to aluminum alloys can significantly refine the grain structure of aluminum alloys, improve the mechanical properties of alloys, and reduce the thermal cracking tendency of alloys.
