Semi-solid injection molding is a common technique in the die-casting industry.
This die-casting technology was developed and put into use in 1977 and commercialized in 1991, making it a well-established process with a history of over 30 years. It primarily processes magnesium metal products.
The semi-solid molding process is suitable for alloy systems with a defined solidification range. The liquidus temperature of the magnesium alloy should be kept as low as possible to avoid excessive heating temperatures that reduce screw life. Currently, the main commercial magnesium alloys used in semi-solid injection molding are AZ91D and AM60B.
Metal Semi-Solid Injection Molding
AZ91D magnesium alloy. It is the most widely used magnesium alloy, with a solidification range of 437-597°C and excellent semi-solid molding capabilities. Japanese researchers have found that the grain size of AZ91D alloy produced using semi-solid injection molding is significantly smaller than that produced using hot-chamber die-casting and slightly smaller than that produced using cold-chamber die-casting. Furthermore, in actual parts, the semi-solid injection molding process exhibits a low casting defect rate and superior mechanical properties. For most common metals and alloys, under normal solidification conditions, due to supercooling of the components, they tend to crystallize in a dendritic rather than granular form. Therefore, it is impossible to obtain a semi-solid metal with a special microstructure under ordinary solidification conditions. Dendrites hinder the flow of molten metal and the relative movement between dendrites, and even in a solid-liquid coexistence state, the aforementioned properties of semi-solid metal are not present. Therefore, how to prepare high-quality billets with non-dendritic structures has become a prerequisite for semi-solid forming. Currently, there are many methods for preparing semi-solid metal billets, including mechanical stirring, electromagnetic stirring, strain-induced melt activation, isothermal heat treatment, solid-liquid mixed casting, the new MIT process, cooling chute method, NRC process, ultrasonic vibration method, etc. Many slurry preparation technologies are still under research or development, such as powder metallurgy, which uses powder metallurgy technology to produce ingots, and then reheats them to melt the low-melting-point components to obtain a semi-solid slurry. This method is particularly suitable for refractory alloys. Other methods include liquidus solidification, ramp cooling, turbulence effects, grain refinement, shear-cooling-rolling, pulse forming, and passive stirring. However, these methods are still in the laboratory research stage and few have been successfully applied in production.
Semi-solid forming technologies can be broadly categorized into two main categories: rheoforming and thixoforming. Rheoforming involves directly die-casting or extruding a semi-solid slurry with a spheroidal structure, prepared using methods such as mechanical stirring or electromagnetic stirring. Thixoforming involves reheating a semi-solid billet with a spheroidal structure, prepared using methods such as mechanical stirring or electromagnetic stirring, to a semi-solid state before die-casting or extrusion.
Thixoforming involves heating a solid material to the desired semi-solid temperature, placing it in a container, and then using a piston to press it into a mold to solidify the resulting part. Under thixoforming conditions, the solid phase volume fraction in the semi-solid alloy slurry is slightly higher than during rheoforming. Under certain conditions, the semi-solid metal exhibits high viscosity due to its static state, maintaining its solid shape. However, when shear force is applied, its viscosity decreases significantly, restoring its fluidity and enabling it to fill the mold cavity.
Thixoforming effectively utilizes this unique thixotropic property of semi-solid metal. The most notable feature of this process is that the deformed billet is solid prior to die-casting, while the shear force generated during die-casting imparts the excellent fluidity required for mold filling. Thixoforming technology solves the problem of connecting semi-solid metal slurry preparation with forming equipment. The storage, transportation, and forming processes of the semi-solid slurry are easily controlled, facilitating automated production.
Thixoforming, therefore, represents a new generation of forming technology for complex, high-density parts, offering enormous potential for application and becoming a next-generation metal forming technology for the 21st century. Products produced using this technology offer excellent mechanical properties, robust corrosion resistance, and long mold life. They also feature low porosity, superior quality, near-net-shape molded parts, and the elimination of melting equipment, making production safer and more environmentally friendly.
Of course, if you have large-volume magnesium die-casting needs, you can partner with Mulan Metal. With 12 years of experience in semi-solid injection molding, Mulan Metal has helped clients in over 30 countries complete magnesium die-casting production and processing, earning widespread recognition. We look forward to hearing from you.
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