Advancements in Magnesium Alloy Die Casting Technologies
Magnesium alloy components can be produced through various casting methods, including gravity casting, low-pressure casting, and die casting. In recent years, innovative techniques such as thixotropic injection molding have emerged, with die casting remaining the most established process. Currently, die casting is the predominant method for fabricating magnesium alloy parts both domestically and internationally.
1) Die Casting of Magnesium Alloy Components
The die casting process for magnesium alloys presents unique challenges due to the material's susceptibility to oxidation and combustion, which increases the likelihood of hot cracking compared to aluminum alloys. Consequently, the melting, casting, and temperature control processes for magnesium alloys are more intricate. Die casting machines are categorized into hot chamber and cold chamber types.
Hot chamber die casting machines are known for their high production efficiency, operating at approximately double the output of cold chamber machines of similar capacity. However, they typically feature a locking force below 7840 kN, making them suitable for smaller, lightweight castings (generally under 2 kg). Examples include magnesium alloy casings for computers and lightweight bicycle frames produced using hot chamber machines.
Cold chamber die casting machines are more widely utilized, particularly for larger components. A notable example is a large-scale magnesium alloy cold chamber die casting machine developed in the United States in 1990, which has a locking force of 13.72 MN. These machines are ideal for creating thicker parts, such as truck dashboards and automotive components. Reports indicate that by 1992, the combined total of cold and hot chamber die casting machines in the U.S. and Japan exceeded 160 units. In Taiwan, the magnesium alloy die casting industry has also seen rapid growth, with over 40 manufacturers and more than 220 machines producing around 8,600 tons of magnesium alloy products annually. Conversely, mainland China currently has only eight manufacturers of magnesium alloy die castings, highlighting a significant gap in production capacity compared to global standards.
2) Trends in Magnesium Alloy Die Casting Technology
Magnesium alloy die casting shares similarities with other metal die casting processes. During casting, the molten magnesium alloy fills the mold cavity at high speeds, which can trap gas and lead to the formation of high-pressure micropores within the alloy. These defects can compromise the mechanical integrity of the castings, limiting their ability to undergo heat treatment and perform under elevated temperatures. To address these issues and enhance the quality of die castings, new technologies such as vacuum die casting and semi-solid thixotropic die casting have been developed.
Vacuum die casting involves removing gas from the mold cavity during the casting process to minimize the presence of pores and dissolved gases, thereby improving the mechanical properties and surface finish of the castings. Parts produced via vacuum die casting can achieve wall thicknesses as thin as 1.5-2.0 mm, with notable improvements in casting strength and toughness. Major automotive components, including wheels and steering systems, have been successfully manufactured using this technique.
Semi-solid thixotropic die casting, also known as Thixomolding, closely resembles injection molding. This process begins with the processing of magnesium alloy ingots into fine particles, which are then heated and partially melted to form a semi-solid alloy. This material is subsequently injected into a preheated mold cavity at high speed. The semi-solid alloys can flow under pressure, similar to thermoplastics, allowing for the creation of complex shapes with high precision.
Thixotropic injection molding offers several advantages over traditional die casting processes. It eliminates the need for liquid metal melting, resulting in a cleaner, safer, and more energy-efficient production environment. The process significantly reduces raw material consumption and minimizes risks associated with explosions. Furthermore, the porosity of the final parts is substantially lower, allowing for heat treatment and resulting in superior mechanical properties and corrosion resistance. The operating temperature during thixotropic molding is approximately 100 °C lower than conventional die casting, which enhances the lifespan of the molds and improves overall consistency in production. This method has enabled the production of lightweight components with wall thicknesses as low as 0.7-0.8 mm, including various automotive parts.
As the magnesium alloy die casting industry continues to evolve, there is a growing trend toward the adoption of semi-solid injection molding technologies. This shift is evident in regions such as Taiwan, where manufacturers are increasingly investing in advanced semi-solid casting machinery to meet the rising demand for high-quality magnesium alloy components.
