Semi solid forming is an advanced metal forming process proposed by Professor m.c.flemings of MIT in 1970’s. Semi solid metal forming can be divided into rheological forming and thixoforming. Rheoforming is a process in which the ideal semi-solid slurry with fine and round primary particles uniformly suspended in the liquid phase is obtained by applying severe stirring in the solidification process of liquid metal, and then the forming process is carried out. In thixoforming, the liquid metal is refined to make non dendritic ingot, and then the ingot is cut into small billets according to the need, and then the billets are reheated to semi-solid temperature for forming. Compared with traditional liquid forming process, semi-solid forming process has the advantages of low forming temperature, long die life, improving production conditions and environment, refining grain, reducing porosity and shrinkage cavity, improving microstructure compactness, and improving casting quality.
In addition, the process combines the advantages of solidification and plastic forming, that is, the forming temperature is lower than the liquid state, the deformation resistance is lower than the solid state, and the large-size, complex thin-walled parts with high precision and quality requirements can be formed with a large amount of deformation. Therefore, semi-solid forming technology is known as one of the most promising material forming technologies in the 21st century.
At present, semi-solid casting technology is widely used in the production of aluminum alloy parts. The United States, Italy, Japan, Germany and other countries are at the leading level of research and industrial application. However, the semi-solid thixocasting technology developed by Dow Chemical Company is the only one used in the actual production of magnesium alloy parts.
The technology has the advantages of simple process and high degree of automation. It has been used in the production of magnesium alloy automobile parts, computer memory parts, laptop shell and frame, mobile phone shell, aerospace and other industrial instruments. However, the raw material of this process is magnesium chips, which is troublesome to prepare, high cost and high porosity (1.0% ~ 1.7%). In addition, it also has the disadvantages of high equipment investment and maintenance costs, long production cycle and so on. Therefore, while researching and developing thixotropy casting, Cornell University of the United States put forward rheo casting technology. In this technology, liquid magnesium alloy slurry was prepared by shear stirring of outer cylinder and screw rod, and then shot casting was carried out, so as to shorten the process flow, reduce the cost of casting, reduce the porosity of casting, and facilitate the recovery of waste products and surplus materials. However, the process has not yet reached the level of practical application.