Casting of magnesium alloy parts

Magnesium alloy parts can be formed by gravity Casting, low pressure casting and die casting. In recent years, new technologies for thixotropic injection molding have emerged. Among them, the die casting process and equipment are the most mature. At present, most of the magnesium alloy parts at home and abroad are produced by die casting.

1) Die-casting of magnesium alloy parts:

Since the magnesium alloy solution is easily oxidized to cause combustion, the hot cracking tendency during casting is larger than that of the aluminum alloy, so the magnesium alloy is more complicated than the aluminum alloy die casting in terms of melting, casting and temperature control of the die casting liquid. The die casting machine is divided into the hot chamber die casting machine and the cold chamber die casting machine. The hot chamber die casting machine has high production efficiency, which is about twice that of the same capacity cold chamber die casting machine. However, the lock type force is generally below 7840kn, which is usually used for small quality. Thin-wall castings (usually below 2kg), such as the magnesium alloy computer casing of the size of 61 0x61 0nm produced by White Metal Casting of the United States, and the 2.5kg weight bicycle frame produced by Kirt Precision of the United Kingdom is a hot chamber die casting machine. produced.

The cold chamber die casting machine is widely used. In 1990, the United States produced a world-class large-scale magnesium alloy cold chamber die-casting machine with a locking force of 13.72MN. The piece is melted and die-casted. Pick up the robot. The cold chamber die casting machine is suitable for the production of thicker and thicker parts. For example, Audi's 1440x3.5mm and 4.2kg truck dashboard is equipped with an automatic casting mechanism. 24.50MN produced on a cold chamber die casting machine. GM's 1470x300x2mm right-angle bolsters, weighing 1.8Kg, were die-cast from M60B magnesium alloy in a cold chamber die casting machine with a lock force of 21.56 MN. In addition, products such as car seats, frames, and automobile wheels are produced by cold chamber die casting machines. According to reports, in 1992, the number of cold and hot chamber die casting machines used in the production of magnesium alloy castings by the United States and Japan exceeded 160 units. In recent years, the development of magnesium alloy die-casting in Taiwan Province of China has also been rapid. By 2001, there were more than 40 manufacturers, with more than 220 cold and hot chamber die-casting machines and an annual output of 8,600 tons of magnesium alloy products. According to reports, there are currently only 8 manufacturers of magnesium alloy die-Casting Parts in mainland China, and 8 of them are under construction [7], which lag far behind domestic and foreign development needs.

2) Development trend of magnesium alloy die-casting technology

Magnesium alloy die casting is the same as other metal die casting. During the die casting process, the magnesium alloy liquid fills the die casting cavity with high-speed turbulent flow in a dispersed state, so that the gas in the cavity cannot be eliminated, and high-pressure micropores are formed or dissolved in the alloy. These pores may be folded or expanded at high temperatures to cause deformation of the casting or bulging. Therefore, the magnesium alloy die-casting parts produced by the conventional die-casting method, like other alloy die-casting parts, cannot be heat-treated and can not be used at a relatively high temperature. In order to eliminate this defect, improve the quality of die castings, and expand the use of die casting technology, in recent years, some new die casting technologies have been researched and developed, such as vacuum die casting and semi-solid thixotropic die casting technology.

Vacuum die casting is to extract the gas in the cavity during the die casting process to reduce or eliminate the pores and dissolved gases in the die casting, and improve the mechanical properties and surface quality of the die casting. The vacuum die-cast magnesium alloy parts have a minimum wall thickness of 1.5-2.0 mm, a vacuum degree of less than or equal to 80 kPa, a punch speed of up to 10 m/s, a casting strength of 10% or more, and a toughness of 20-50%. A number of major automotive parts such as magnesium alloy automotive wheels and steering wheels have been successfully produced by vacuum die casting.

Semi-solid thixotropic die casting technology

(Thixomolding): The process of thixotropic injection molding is close to injection molding, and its equipment principle is shown in Figure 1. Firstly, the magnesium alloy ingot is processed into fine particles, and the magnesium alloy particles are loaded into the hopper and forcedly transported into the granule. The rotating screw in the granule drives the magnesium alloy particles to move toward the mold, and when it reaches the heating of the granule At the site, the alloy particles are partially melted. Under the shearing action of the spiral, the alloy material with dendritic structure forms a semi-solid alloy with a thixotropic structure. When it accumulates to a certain volume, it is driven at a high speed (5.5m/ s) Injection into a preheated cavity that is evacuated to form. Semi-solid alloys can be flow-formed like thermoplastics under external force, but the temperature, pressure and screw rotation speed of thixotropic injection molding are much higher than those of injection molding equipment. The formed heating system uses a combined process of electrical resistance and induction heating to heat the alloy to 582 ± 2 ° C, a solid phase volume fraction of 60%, and argon gas for protection.

Compared with traditional die casting, thixotropic injection molding does not require liquid metal melting and casting processes, and the production process is relatively clean, safe and energy efficient. No melt is produced, the consumption of raw materials per unit part is greatly reduced, there is no danger of explosion, SF6 gas protection is not needed, and the destruction of ozone layer by SF6 is eliminated. The gas entrapped during the forming process is greatly reduced, and the porosity of the part is less than 0.069%. The formed parts can be heat treated, and the formed parts have high density, good mechanical properties and strong corrosion resistance. Compared with the traditional die-casting, the working temperature is reduced by about 100 °C, which is beneficial to improve the life of the die-casting mold, and has a good consistency in the production process, which reduces the shrinkage rate of the magnesium casting in the mold and reduces the drag-off resistance of the casting. The dimensional accuracy of the casting is improved, the surface quality of the part is good, and the thin and light parts with a wall thickness of 0.7-0.8 mm can be cast. At present, the actuator housing, cover, igniter housing, etc. of the automobile have been produced by this method, and the magnesium alloy used is AZ91D. The highly automated magnesium alloy semi-solid injection molding machine and its production line have developed rapidly in industrialized countries. The magnesium alloy die-casting industry in Taiwan Province of China is also beginning to use a large number of semi-solid injection molding machines. Only Huafu Company was in 2002. It plans to purchase 5-8 new semi-solid casting machines, which will become the mainstream of magnesium alloy castings in the future.