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In the early 1970s, M. Flemigs and D. Spencer from the Massachusetts Institute of Technology (MIT) discovered that alloys in a solid-liquid phase exhibit low apparent viscosity after continuous stirring, with dendrites transforming into granular structures during crystallization. This slurry is highly deformable and can be filled with minimal force into complex cavities, leading to the development of a new metal forming technique known as semi-solid metal forming. This method is categorized into two main types: rheology and thixoforming. Rheology involves applying pressure to a strongly stirred metal slurry, while thixoforming utilizes the thixotropic properties of metals by heating a solidified slurry to a semi-solid state before pressing.
Semi-solid metal forming offers numerous advantages, such as eliminating air and shrinkage holes, improving mechanical properties and mold life, reducing solidification shrinkage, and enhancing dimensional accuracy. The ease of handling and transporting semi-solid metals also supports continuous and efficient automation. Compared to traditional casting methods, it is more energy-efficient and environmentally friendly. Currently, this technology has been implemented in the United States and Western Europe, with the U.S. military including rheocasting in its five-year process development plan. In Japan, from 1988 to 1994, 18 major companies supported the establishment of a specialized research institute to study semi-solid metal properties and technologies, eventually moving toward industrial production. However, in China, research remains at the laboratory stage, with a gap before reaching industrial-scale application.
The rheological properties of semi-solid metals refer to their flow and deformation characteristics under external forces. These properties are crucial for the preparation and forming of semi-solid metals. When the solid content exceeds 0.05 to 0.1, the behavior becomes non-Newtonian. At higher solid fractions (0.5–0.6), the slurry exhibits non-linear viscoplastic behavior with Bingham flow characteristics. While factors like alloy composition, manufacturing conditions, and solid particle shape affect rheology, the solid fraction has the most significant impact. Apparent viscosity is commonly used as an indicator of these properties. Through shear deformation tests under specific cooling conditions, the apparent viscosity of aluminum, copper, and iron semi-solids was measured. The relationship between apparent viscosity and solid fraction was analyzed using regression, resulting in formula (1).
The temperature of the semi-solid metal determines the solid fraction, making temperature control critical in practical applications. Shear strain rate also significantly affects apparent viscosity. For A356 aluminum alloy, the steady-state apparent viscosity was measured using a high-temperature rotational viscometer, yielding formula (2). The results showed that apparent viscosity decreases with increasing shear rate, though the exact relationship depends on material properties. In real molding processes, where filling lasts only seconds, changes in solid particle size, number, and morphology complicate the situation further.
Semi-solid metal slurries exhibit thixotropic behavior, meaning they can transition from a solid-like state to a fluid-like state under shear stress. This property allows for easier handling and shaping. For alloys with dendritic primary crystals, when the solid fraction reaches about 0.3, the material cannot flow, but once the solid fraction exceeds 0.5, mobility is still possible. The grain morphology during solidification significantly influences rheology. Various methods, such as mechanical or electromagnetic stirring, are used to prepare semi-solid metals, with each offering unique advantages in terms of contamination reduction and efficiency.
Semi-solid metal forming has found widespread application in various industries, particularly in automotive parts. Companies in the U.S., Europe, and Japan have adopted this technology to produce components such as engine parts, brackets, and housing. Thixotropic die casting, for instance, is widely used for producing lightweight, high-strength parts. Additionally, semi-solid forging and Injection Molding techniques have been developed, enabling the production of complex shapes with improved mechanical properties and reduced machining requirements.
Overall, semi-solid metal forming represents a promising advancement in metallurgy, offering enhanced performance, efficiency, and sustainability compared to conventional methods. As research continues, the potential for broader industrial adoption grows, paving the way for more innovative and cost-effective manufacturing solutions.