# Research Progress on the Manufacturing of Screw-Shaped Parts in Screw Compressors
## 4 New Technology of Semi-Solid Closed Extrusion Forming for Screw-Shaped Parts  
Semi-solid metal forming (SSMF) is a novel metal-forming method proposed by Prof. [PERSON] and his team at MIT [61], which was further applied and investigated by researchers [62; 63; 64]. During the SSMF process, the semi-solid materials are first prepared with grains of significantly small sizes and spherical shapes. Then, the complex parts with fine microstructures and excellent performance can be produced by extrusion, forging, rolling or die casting processes [65; 66]. SSMF has the advantages of high flowability in casting combined with the excellent mechanical properties of components processed by solid plastic forming. Therefore, it can realise the near-net shape forming of complex parts [64; 67; 68; 69]. In view of the disadvantages of the common screw manufacturing processes, such as serious material waste in the cutting process; large forming loads in the solid plastic forming process; short die life and casting defects in casting; and low production efficiency and economics in AM, the semi-solid closed extrusion forming (SSCEF) process was first proposed by us for the production of screw-shaped parts, which is shown in Figure 15. The new technology proposed in this paper is based on a thorough review and comparison of the existing technologies, which address some problems of the existing technologies. In this process, the semi-solid spherical grain is firstly prepared by a radial forging strain-induced melt activation (RFSIMA) process, which includes radial forging and an isothermal procedure for long-shaft raw materials. In this process, a large amount of energy is stored in grains in the bar by the radial forging of long-shaft raw materials, which is used to induce the generation of semi-solid spherical grains during the isothermal procedure. After that, the screw rotor is fabricated by the high-efficiency closed extrusion process. Moreover, the dimensional and shape accuracy of the parts formed by the SSCEF process is similar to that of the solid plastic forming process and AM process, and it is necessary to conduct a grinding and polishing process for higher dimensional and shape accuracy. In conclusion, there are five categories of manufacturing processes introduced in this study; among them, only one process in the cutting category (i.e., direct powerful integral grind machining) is fine machining. The rest of the processes all require a grinding or polishing process for a relatively high dimensional and shape accuracy of screw-shaped parts. Therefore, all other processes, not just the SSCEF process, need post-processing equipment. The SSCEF process mainly includes three stages: \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Common** & & & & & & & \\
**Screw** & & & & & & & \\
**Maufacturing** & & & & & & & \\
**Processes** & & & & & & & \\ \hline \hline \multirow{4}{*}{Casting process} & \multirow{4}{*}{RTM method} & (1) & Dimensional & & (1) & The waste gas & & \\  & & & and shape & & generated from the & & \\  & & & accuracy is & & macromolecular & & \\  & & & about ISO IT8 & & materials is seriously & & \\  & & & level; & pollutant & & & \\  & & (2) & Low cost. & (2) & Poor mechanical & & \\  & & & properties of the & & & & \\  & & & & formed products. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._1. Stage I: Radial forging plastic deformation (RFFP). In this stage, the metal bar is radially forged with a suitable area reduction rate to obtain the radial-forged long-shaft metal billet; thus, a large amount of strain energy can be accumulated. 2. Stage II: Semi-solid isothermal procedure (SSIP). In the SSIP process, the long-shaft metal billet after radial forging is heated to a suitable semi-solid temperature to obtain the ideal semi-solid fine and spherical billets. 3. Stage III: Closed extrusion forming (CEF). During the closed extrusion forming process, the upper die is first raised to a certain height.