Semi-solid die casting is an advanced die-casting process developed based on the properties of semi-solid metal slurries. Its core approach is to utilize the unique rheological properties of metals in a solid-liquid coexistence state (solid fraction 50%-70%) to produce high-performance die-castings through low- or high-pressure molding. Compared to traditional die casting, semi-solid die casting fundamentally changes the filling method of molten metal, effectively solving the problems of air entrainment and oxidation caused by high-speed filling in traditional die casting, and providing a new approach to producing high-strength, high-density die-casting parts.
The preparation of semi-solid slurry is a critical step in the process and directly impacts the quality of die-cast parts. Currently, the mainstream preparation methods include mechanical stirring, electromagnetic stirring, and ultrasonic vibration. Mechanical stirring uses rotating blades to agitate the molten metal, breaking up the grains and forming a uniform semi-solid structure. This method is suitable for small-batch production and can achieve a solid fraction control accuracy of ±5%. Electromagnetic stirring utilizes the Lorentz force generated by an alternating electromagnetic field to stir the molten metal, avoiding the contamination associated with mechanical stirring. It is suitable for large-scale industrial production and can stably prepare aluminum alloy slurries with a solid fraction of 50% to 60%. Regardless of the method used, precise control of temperature (typically 50-100°C below the liquidus temperature) and stirring speed is crucial to ensure the slurry exhibits good fluidity and thixotropy—that is, it remains solid when at rest but exhibits liquid-like fluidity when subjected to shear. This property ensures smoother filling of the semi-solid slurry into the mold cavity and reduces gas entrapment.
The semi-solid die casting process differs significantly from traditional die casting and requires specialized process parameters. During the filling phase, the semi-solid slurry enters the mold cavity at a relatively low speed (1-5 m/s), significantly lower than the 5-50 m/s of traditional die casting. This slow and steady filling method avoids turbulence and air entrainment, allowing sufficient time for gases in the cavity to escape. Due to the higher viscosity of the semi-solid slurry, a higher injection pressure (100-200 MPa) is required compared to traditional die casting to ensure the slurry can fill the complex mold cavity. The holding pressure during the holding phase also needs to be increased, typically to 80%-90% of the injection pressure, and the holding time extended to 5-15 seconds to ensure sufficient compaction of the solid phase particles in the slurry and minimize shrinkage cavities and porosity. Mold temperature control is equally important and should be 20-50°C higher than traditional die casting. For example, the mold temperature for aluminum alloy semi-solid die casting should reach 250-300°C to prevent premature solidification of the slurry during the filling process.
Semi-solid die-casting offers excellent mechanical properties and consistent quality, which are its core advantages. Because the slurry is formed in a semi-solid state, the grain size is significantly refined (from 100-200μm in traditional die-casting to 10-30μm). This can increase the tensile strength of die-casting parts by 15%-30% and their elongation by 50%-100%. For example, 6061 aluminum alloy semi-solid die-castings achieve tensile strength exceeding 300MPa and elongation exceeding 15%, significantly exceeding the 250MPa and 8% of traditional die-casting. Furthermore, semi-solid die-castings achieve a density exceeding 99.5%, with minimal internal porosity and oxide inclusions, allowing for high-temperature heat treatment (such as T6 treatment) and welding, surpassing the performance limitations of traditional die-casting. Furthermore, semi-solid die-casting offers higher dimensional accuracy, with tolerances reaching IT6-IT8 and surface roughness Ra ≤ 0.8μm, reducing subsequent processing costs.
Semi-solid die-casting is increasingly used in the industrial sector, particularly in applications requiring high strength and reliability. The automotive industry is a major application area, with safety components such as engine connecting rods, brake calipers, and suspension brackets manufactured using semi-solid die-cast aluminum alloys. This not only meets high-strength requirements (tensile strength ≥ 280 MPa) but also achieves a weight reduction of over 30%. In the aerospace sector, semi-solid die-cast magnesium alloy parts are used in drone fuselages and satellite structures. With a density of only 1.8 g/cm³ and a specific strength close to that of steel, semi-solid die-casting is used to produce cooling brackets for laptop computers. Complex flow channel structures can be formed in a single step, improving heat dissipation efficiency by 20%. With the maturity of slurry preparation technology and the reduction of production costs, semi-solid die-casting is gradually replacing some traditional die-casting and forging processes, becoming a key option for high-performance metal forming.
The development trend of semi-solid die-casting is characterized by intelligentization and scale. Online monitoring systems control the slurry’s solid fraction and temperature in real time, with accuracies of ±2% and ±3°C, respectively, ensuring process stability. The development of new mixing equipment (such as twin-screw agitators) has increased slurry preparation efficiency by 50%, making it suitable for large-scale production. In the future, semi-solid die-casting will expand to larger and more complex parts, such as battery pack housings (over 1m in size) for new energy vehicles. Integrated die-casting will reduce the number of parts and improve production efficiency. Furthermore, semi-solid die-casting technology for novel alloy materials (such as aluminum-lithium alloys and magnesium-neodymium alloys) will further enhance the specific strength and corrosion resistance of parts, expanding their application range.
