# 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
The semi-solid metal billet is provided into the die cavity of the bottom die. The upper die is lowered to complete the closing of the upper and bottom dies. At last, both the left and right extrusion rods move towards each other at the same time to axially extrude the semi-solid billet, producing the screw rotors. It can be found from Figure 15 that the process parameters of SSCFF mainly include the radial forging temperature, the area reduction ratio, isothermal temperature, holding time, die temperature and the axial extrusion speed. High-precision and high-quality aluminium alloy screw rotors with high quality can be produced by this SSCFF process, with the optimal process parameters selected based on a deep thermo-fluid analysis of the forming process. The process parameters selected for the screw need to consider the semi-solid temperature and the filling behaviour during the process, which is dependent on the metal material and its chemical components. Based on the working principle, the similarities and differences between our novelty SSCEF process and the traditional screw forming processes are shown in Table 2. Therefore, this proposed process has the following advantages:  
1. A modified semi-solid billet preparation process, RFSIMA, was used to provide the ideal semi-solid materials. It included the RFPD and SSIT processes. This new process can efficiently prepare \"long-axis\" semi-solid spherulite materials of different sizes and specifications. 2. Compared with mechanical stirring, electromagnetic stirring, ultrasonic stirring and other liquid-phase semi-solid material preparation processes, no oxidation or impurity phase is introduced into the semi-solid materials because the RFSIMA process is a solid-phase method for preparing the semi-solid material. 3. The prepared long-shaft semi-solid billets can be directly placed into the mould cavity for the formation of a screw rotor, which can shorten the process flow passage and improve the production efficiency compared with the AM process. 4. The near-net formation of the screw rotor can be accomplished by the SSCEF process, which can noticeably improve the material utilization rate compared with cutting process. The produced parts via this process also have the advantages of high density and excellent mechanical properties equivalent to forgings, and their mechanical properties are higher than those of castings. Moreover, the proposed SSCEF process has a  
Figure 15: Scheme for the SSCEF process of screw complex profile parts: (**a**) RFPF; (**b**) SSP; (**c**) CEF. longer die life than casting due to the low forming temperature, and the performance of parts can be further improved if it is processed by heat treatment. * Compared to the common manufacturing process, materials including aluminium alloys, magnesium alloys, 45 steel and 304 stainless steel can also be used in the SSCEF process for the forming of screw-shaped parts of screw machineries, such as screw pumps, screw compressors, screw vacuum pumps and screw expanders.