Semi-solid forming technology, also known as semi-solid die forging or thixotropic die forging, is a new part processing technology, which is an advanced forming technology between liquid forming (casting) and solid forming (pressure processing). The idea is to use physical or mechanical methods to vigorously stir the solidified metal melt to knock down the solidified dendrite branches and make them evenly suspended in the melt to form a kind of floating spheroidal primary solid. The solid-liquid mixed slurry is rheological slurry, and the volume of the solid phase is usually 50% to 90%.
When casting magnesium alloy castings, it is not only hoped that the melt has good fluidity, can be satisfactorily filled and formed, can be well fed to eliminate defects such as shrinkage and porosity, but also can obtain a fine and uniform grain structure. There is no component segregation or slight segregation, so as to achieve the purpose of high density of castings, excellent structure and performance, and high dimensional accuracy. Semi-solid forming technology is an advanced casting technology, which is in the stage of development and perfection. Although mass production of automobile wheels and electronic parts has been applied in the United States and Europe, the products are not cost-competitive in small batches.
The fluidity of the metal melt during the solidification process is not only closely related to its inherent characteristics, but also related to the morphology, size, and volume fraction of the primary solid phase in the melt. The semi-solid forming technology solves this complex technical problem well. It can cast magnesium alloy parts with excellent performance.
Schematic diagram of magnesium alloy semi-solid injection molding process
Compared with the conventional casting process and pressure processing process, the semi-solid forming method has the following characteristics:
The forming temperature is lower than that of the conventional casting process, energy saving and emission reduction, and the melt has released part of the latent heat during the solidification process, which greatly reduces the thermal shock to the mold and improves the mold life; because the semi-solid slurry is a kind of It is a solid-liquid mixture, the forming pressure is greatly reduced, it is easy to prepare large workpieces, the forming speed is fast, and the parts with complex structures and thin-walled parts can be manufactured; There are coarse dendrites and columnar crystals of traditional castings, and defects such as pores, porosity, and segregation in castings are almost non-existent or greatly reduced. The semi-solid slurry fills the cavity smoothly in the form of laminar flow. Unlike conventional melts, there is no turbulent flow or sputtering. It is beneficial to reduce gas entrapment and slurry oxidation, and significantly improves the internal and surface quality of the casting , It can also reduce the thermal shock and surface erosion on the mold; the mechanical properties of the castings are comprehensively improved, generally more than 20% higher than that of the conventional castings; the scope of application is wide.
Initially, this method was developed for the production of aluminum alloy castings, but the alloy systems with solid-liquid two-phase regions on the binary phase diagram can be semi-solid formed, such as iron-based, aluminum-based, zinc-based, copper-based, magnesium-based, etc. Alloys can be used; it is easy to realize automatic and continuous production, and the process is relatively controllable. Of course, it is not suitable for small batch production. Usually, the annual output should not be less than 500,000 pieces.
In general, the rheological casting process is simple, does not require secondary heating, energy saving and emission reduction, and the cost of castings and forgings is also low. However, the requirements for slurry retention and transportation are strict and difficult, and it is difficult to achieve automatic continuous operation, so the development is slow. Semi-solid forming technology for foreign industrial applications refers to thixotropic casting technology and thixotropic die forging technology. When implementing thixoforming, the first main process is to prepare semi-solid blanks, which increases the production cost. At the same time, secondary heating is required, which consumes electricity and the process is more complicated. However, it is easy to realize automatic continuous production. It has achieved great development, and many commercial production lines have been built abroad, mainly producing aluminum alloy parts.
Scientists transfer the plastic injection molding process to the semi-solid metal forming to form a new rheological-thixotropic injection casting process, which integrates the preparation, transportation, and forming of semi-solid metal slurry, making the semi-solid technology commercialized The production stage becomes a reality. The use of semi-solid forming technology to prepare automotive magnesium alloy parts has broad potential.
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