Die-cast magnesium matrix composites
Die-cast magnesium-based composites (Mg-based composites) utilize a magnesium alloy matrix and reinforced phases to form high-performance structural materials. Magnesium’s low density ( 1.74 g/cm³ , only two-thirds that of aluminum ) and the high strength of its reinforcements make them an ideal choice for lightweighting applications, holding an irreplaceable position in aerospace, high-end automotive, and other fields. The magnesium alloy matrix itself possesses high specific strength and excellent vibration damping properties, while the addition of reinforcements such as ceramic particles and carbon fiber significantly enhances the material’s high-temperature stability and wear resistance. For example, AZ91 Mg-based composites reinforced with silicon carbide particles achieve a specific strength of 200 MPa・cm³/g , far exceeding the 150 MPa・cm³/g of conventional aluminum alloys .
Controlling the material interface is key to optimizing the performance of die-cast magnesium-based composites. Magnesium is a highly chemically active metal that readily undergoes interfacial reactions with reinforcement phases at high temperatures, forming brittle phases (such as MgO and Mg3N2), which degrades material performance. To address this issue, researchers have employed surface modification techniques for the reinforcement phase. For example, a sol-gel method is used to coat silicon carbide particles with a SiO2 coating, which effectively inhibits interfacial reactions and increases interfacial bonding strength by over 30%. Matrix alloying is also an important approach. Adding rare earth elements (such as Nd and Gd) to magnesium alloys can refine grain size and improve interfacial compatibility, increasing the impact toughness of the composite to over 25 J/cm².
The die-casting process is quite challenging for forming magnesium-based composites, requiring precise control of process parameters. The melting point of magnesium alloys is around 650°C, and the die-casting temperature is typically between 680 and 720°C. However, at these temperatures, the reinforcement phases tend to agglomerate, and the magnesium liquid is susceptible to oxidation and combustion, necessitating the use of an inert gas (such as a mixture of SF6 and N2) for protection. Injection pressure is a key factor influencing the density of composite materials. Research has shown that when the injection pressure is controlled at 60-80 MPa, the porosity of the casting can be reduced to less than 1%. Conventional magnesium alloy die-casting only requires a pressure of 40-50 MPa. Furthermore, the mold temperature must be maintained between 200 and 250°C to prevent poor filling due to rapid solidification of the magnesium liquid.
The application value of die-cast magnesium-based composites is becoming increasingly prominent in high-end manufacturing. In aerospace, satellite brackets made of composite materials can reduce weight by over 40%, significantly lowering launch costs. In new energy vehicles, the moment of inertia of composite motor rotors is 25% lower than that of aluminum alloy components, improving motor response speed and increasing range. According to industry reports, the Airbus A350 uses approximately 80kg of magnesium-based composite components, reducing fuselage weight by 5%. The use of this material in the battery housing of the Tesla Model S has reduced weight by 15kg and increased range by 8km. These examples fully demonstrate the irreplaceable role of magnesium-based composites in high-end manufacturing.
Despite its promising prospects, the industrialization of die-cast magnesium-based composites still faces many challenges. The cost of the magnesium alloy matrix is 1.5-2 times that of aluminum alloy, and the investment in reinforcement phase dispersion equipment is huge, resulting in high prices for composite materials. The room temperature plasticity of the material is poor, and the elongation is usually less than 5%, which limits its application in components subjected to impact loads. In the future, with the advancement of magnesium resource smelting technology and the improvement of the recycling system, the cost of raw materials is expected to decrease. The application of nano-reinforcement phases and gradient composite material design will further enhance the plasticity of the material. It is estimated that by 2030, the global market size of die-cast magnesium-based composites will exceed US$5 billion, becoming an important growth point in the field of lightweight materials.
