At present, the application of magnesium profile and deformation magnesium alloy in automobile is less (the average magnesium consumption is less than 3 kg), the main application is casting (the single car reaches 30-40 kg) [6], the magnesium alloy produced by pressure casting accounts for about 80% of the total magnesium alloy for automobile.
1. Pressure casting
According to the research and application at home and abroad, there are many casting methods for magnesium alloy. At present, most of automobile parts are produced by pressure casting process. The castings produced by pressure casting process have the characteristics of clear profile, high surface quality, accurate and stable size, small machining allowance, fine structure, high strength and hardness. In addition, the thin-walled castings with complex shapes can be produced by the pressure casting process with high production efficiency.
In the pressure casting of magnesium alloy, the specific heat capacity of magnesium alloy is small, the filling speed of the mold cavity is fast, the metal flow in the dispersion state is highly mixed with the air in the mold cavity, the gas is too late to be discharged, and finally the defects such as porosity, insufficient pouring and cold separation are formed in the pressure casting parts. In order to eliminate these defects and improve the quality of pressure casting parts, new processes such as vacuum pressure casting, oxygen filled pressure casting, squeeze casting, semi-solid thixo injection molding and casting forging double control forming have been developed in recent years in the United States and Japan.
2. Vacuum pressure casting
The vacuum pressure casting of magnesium alloy is to use the vacuum technology to empty the air in the model cavity, and at the same time to help reduce the air pressure in the model cavity, so that the filling molding and the gas in the alloy melt are discharged, so that the alloy melt fills the model cavity under the pressure, and solidifies under the pressure to obtain the dense parts. Vacuum pressure casting can not only eliminate the surface porosity and porosity, improve the surface quality, but also improve the casting density, solve the coating peeling, electroplating blistering, blind hole material shortage and other technical problems. In recent years, with the improvement of vacuum pressure casting process of magnesium alloy, the minimum wall thickness of the casting can reach 1.5 mm, the vacuum degree ≤ 80 K Pa, the filling time is greatly reduced, and the strength and toughness of the casting are correspondingly improved.
3. Oxygen filled pressure casting
The oxygen filled pressure casting of magnesium alloy is to make full use of the characteristics that the molten magnesium alloy is easy to react with oxygen or other active gases. Before the liquid metal filling the pressure casting mold cavity, the active gas is filled to replace the air in the mold cavity. When the molten magnesium enters the mold cavity by hydraulic pressure, the active gas reacts with it to generate metal oxide particles and disperse in the casting, so as to eliminate the gas in the casting. Oxygen filled die casting can reduce or even eliminate the porosity of the die casting, refine the structure of the casting, so as to improve the density and mechanical properties of the casting, and make it possible to carry out heat treatment and strengthening. AM60 magnesium alloy wheel hub was mass produced by oxygen filled pressure casting in Japan Light Metal Co., Ltd., which is 15% lighter than aluminum wheel hub.
4. Squeeze casting
Using high pressure to squeeze molten magnesium into the mold cavity at low speed, the method to get the casting after solidification and small plastic deformation is squeeze casting. The castings obtained by this process have high density, excellent mechanical properties and high dimensional accuracy, which can improve the porosity and porosity of castings. In addition, the obtained castings can be heat treated directly to obtain finer grains. Jiang Deping and long Siyuan of Chongqing University have done a series of research on the magnesium alloy wheel hub. The magnesium alloy wheel hub made by squeeze casting has good mechanical properties, shorter technological process and higher production efficiency. Because the pressure of squeeze casting can change the solid solubility and melting point of magnesium alloy, researchers have developed some magnesium alloy systems suitable for squeeze casting, such as HTMi magnesium alloy in England, which has excellent strength and creep resistance at 180 ℃.
5. Semi solid Thixomolding
Thixoforming of magnesium alloy is a semi-solid thixoforming process which makes full use of the principle of injection molding. Mold and forming materials are similar to die casting process, and the process is similar to injection molding process. This process not only makes the casting structure compact, solidification shrinkage small, forming temperature low and size accuracy high, but also has a series of advantages such as simple operation, safety, high efficiency and environmental protection [17,19], which is suitable for the production of parts with complex shape, high density, thin wall and high performance. Thixotropic injection molding technology is the only semi-solid forming technology used in the industrial production of magnesium alloy, which has great application potential.
6. Casting forging double control forming
The casting forging dual control forming of magnesium alloy combines the advantages of liquid pressure casting and forging [16], in the forming process, the precise control of casting shape, size and service performance can be realized at the same time. In essence, on the basis of pressure casting, when the liquid metal in the mold cavity is not fully solidified, a deformation force is applied immediately to break the solidified dendrite structure and strengthen the fine grain, so that the mechanical properties of the parts can be significantly improved, which not only solves the defects of shrinkage and porosity of the rough casting parts, but also solves the defect that forging can not produce complex and high-precision parts, It is mainly used to produce automobile parts with complex shape and high strength requirements.