Research Progress on the Manufacturing of Screw-Shaped Parts in Screw Compressors

Abstract

Screw compressors are highly researched and developed prospects in industry because of their long service life, high transmission efficiency, low footprint and low vibration. As the key core part of the screw compressor, the screw is a typical, long-shaft, complex profile part. Its processing method, manufacturing accuracy and quality have an extremely important impact on the performance of the whole screw compressor. In this work, the research progress on the manufacturing of screw-shaped parts in screw compressors is summarised from the aspects of the cutting process, solid plastic forming, casting and additive manufacturing. The merits and demerits of these manufacturing processes are provided and discussed, which is conducive to the development of the high-efficiency, precise and high-performance forming process of screw-shaped parts. Additionally, a novel forming process is proposed to solve the problems of serious material waste and low production efficiency for the screw-shaped parts. In the proposed process, the semi-solid spherical grain is firstly prepared by radial forging and the isothermal treatment of long-shaft raw materials. The large strain energy can be stored in the bar by the radial forging of long-shaft raw materials, which is used to induce the generation of semi-solid spherical grains with the assistance of the isothermal procedure. After that, the screw is fabricated by the high-efficiency semi-solid closed extrusion process.

1. Introduction

In 2013, a strategic policy of “Industry 4.0” was published in Germany to achieve the new industrial revolution [1,2,3]. “A National Strategic Plan for Advanced Manufacturing” was proposed in the US, targeted towards enhancing the merits of its modern manufacturing industry with intensified technology [4,5]. The “New Robot Strategy” was published in Japan to enhance the international competitiveness of the manufacturing industry [2,6]. “Made in China 2025” was published in China to implement the strategy of manufacturing power. Its fundamental objective is to change the situation of “large but not strong” in the manufacturing industry in China [7,8]. Therefore, the manufacturing industry has attracted more and more attention in many countries.

Mechanical basic parts are indispensable in the manufacturing industry for various pieces of equipment. The key characteristics of the equipment and related products, such as the machine performance, durability and reliability, are determined by the quality of the basic parts [9]. With the development of modern industrial technology, screw types of machinery, such as screw pumps, screw vacuum pumps, screw extruders and especially screw compressors, have become the key foundation and core parts in many high-end equipment manufacturing industries, playing a crucial role in the safe and reliable operation of high-end equipment.

As a positive displacement compressor, the screw compressors shown in Figure 1a,b have mainly been applied as air compressors or freezing air conditioning compressors [10,11,12]. They are also widely used in aerospace, rail transit, ocean engineering, the petrochemical industry, new energy automobiles and other fields due to their advantages, including a long service life, high transmission efficiency, small floor area and low vibration [13,14]. The screw compressor mainly includes female and male screw rotors and the compressor housing. The screw rotors rotate, meshing with each other. The compressor housing accommodates the screw rotors. An example of a screw rotor for the screw compressor with dimensions can be seen in Figure 1c. Therefore, the screw rotor, which belongs to typical long-shaft complex profile parts, is the key core component of the screw compressor. The processing method, manufacturing accuracy and quality have an extremely important impact on the performance of the whole screw compressor.

The manufacture of screw rotors is often achieved by milling, roughing and subsequent machine grinding processes [15]. However, the shape of the screw rotors is complex, resulting in high difficulty in the manufacturing process. Consequently, it is difficult to guarantee machining accuracy, which is an extremely important factor to reduce the service life of screw machinery. Therefore, the manufacture of screw rotors is a challenging task because it has a high requirement for particular machines or instruments. It is essential to develop novel manufacturing technology for screw-shaped parts.

This paper summarises the research progress on the manufacturing of screw-shaped parts from the aspects of the cutting process, solid plastic forming, casting and additive manufacturing. The pros and cons of different manufacturing processes are provided and discussed, which can contribute to the development of the high-efficiency, precise and high-performance forming process of complex screw-shaped parts. Moreover, a new process is proposed in this review to solve the problems of serious material waste and low production efficiency in forming complex screw-shaped parts.

2. Common Manufacturing Process for Screw-Shaped Parts

2.1. Traditional Cutting Process

At present, the production mode of complex screw-shaped parts is generally determined based on the complex configuration of the screw surface profile through special manufacturing methods. Besides that, it may use high-price screw machine tools, screw grinders, five-axis machining centres and other special equipment, where the bar material is subject to multiple milling or grinding processes. As shown in Figure 2, the common milling methods for screw-shaped parts mainly include finger-type cutter milling, disc-type cutter milling and whirlwind milling [16,17,18,19]. Among them, whirlwind milling is one of the most widely used cutting processes. The ordinary three-axis machine tool has been replaced by cyclone milling machines in many medium and small enterprises. Four-axis or five-axis machine tools are generally used in large enterprises to complete the milling of complex screw profile parts [19].

As the screw rotor is crucial to the assembly of screw compressors, precision in the machining of the screw rotor directly and significantly influences the compressor performance. Therefore, solving the machining accuracy problem of rotor teeth is the key to improving the compressor performance. Bergström [16] used a finger milling cutter to mill a screw rotor on a flexible multifunctional machine tool, improving the machining efficiency and manufacturing accuracy of the screw product. Achmad et al. [20] suggested an insert-cutting trajectory (ICT) process, which was able to generate a designed profile and the cutting-point surfaces of the rotor by the milling procedure. They reported that generating surface cutting marks on the rotor profile was successfully accomplished by the multiple-inserts method for edge cutting in the milling process. The reliability and practicability of the new ICT method was verified by experiments.

Bizzarri et al. [21] introduced the methodology of a computer numerically controlled (CNC) machine method with a five-axis flank, which is named double-flank milling. In this process, the cutting tool tangentially contacts the workpiece at two end surfaces, which is more flexible compared to traditional milling. The results indicated that a high accuracy was obtained in the machining process in cases with different rotor benchmarks with the specially designed tool. Tran [22] proposed a processing methodology for producing a screw rotor with a complex profile that had changes in the tooth pitch by CNC turning. Investigation results from simulations revealed that the proposed process method is effective in flexibly realising the manufacturing of the screw rotor by varying the production of the tooth profile. Luu et al. [23] proposed a new skiving method with internal cylindrical cutters for the precise production of screw rotors with a CNC skiving instrument. It was reported that the error of the screw rotor skived by the proposed methodology was significantly low, even close to zero. Arifin et al. [15] used the blade-cutting trajectory method to quickly calculate the profile and cutting path of screw rotors, based on which a new prediction model was established. The reliability and application potential of the cutting trajectory method were verified by the milling results of screw rotors under different cutting conditions.

With the progress of screw processing technology, grinding has gradually become a popular method of screw finishing or direct powerful integral machining. The grinding methods of screw-shaped parts can be mainly divided into wheel grinding and abrasive belt grinding. Figure 3 shows the equipment and schematic diagram of the RX120 grinding machine tool produced by KAPP company (İzmir, Turkey) [24]. As shown in Figure 3b, a grinding wheel with cubic boron nitride (CBN) is used in this equipment. The grinding capacity of this equipment is significantly high, as it can not only complete the rough and fine machining of the screw but also completes the real-time detection and correction of machining errors. Shen [25] used the wavelet smoothing theory to process challenge points in the curvature and smooth line of the profile with a forming tool in order to realise the finish of a smooth grinding profile of the screw rotor. The interpolation of the high distance between points was accomplished through the use of a cubic spline for profile forming, obtaining a smooth grinding profile of the screw rotor. Zhao [10] proposed a grinding method with high precision for the manufacture of screw rotors through the use of a CNC grinding wheel dressing. The experimental results showed that the deviation of the screw rotor profile produced from the designed one was below ±10 um.

In summary, the milling of screw-shaped parts has the advantages of overall manufacturing and a short process flow, but more than 60% of the materials will be wasted during milling, resulting in a low material utilization rate [26]. Additionally, the design and selection of cutters and the accurate control of their motion tracks are still difficulties faced by various enterprises. Although grinding has gradually become the main method for the fine machining or integral machining of complex screw profile parts both domestically and abroad, the powerful integral grinding process of screws has the disadvantages of low machining efficiency and high requirements for equipment performance.

2.2. Solid Plastic Forming Process

Compared with the traditional cutting process, the solid plastic forming process has almost no material loss, and can also significantly improve production efficiency. Therefore, many scholars have conducted a lot of research on the solid plastic forming process for screw-shaped parts. In general, the screw-shaped part is first rough produced through the solid plastic forming process, and then finished through machining with the grinding and polishing process.

The cross-wedge rolling (CWR) process is a new forging method for metal materials with a near-net-shape for the production of shaft parts, such as screw rotors, steeped axes and shafts [27,28,29,30,31]. Figure 4 shows in detail how the CWR process is achieved [32]. As shown in Figure 4, the upper and bottom dies move oppositely, driving the billet rotation, where the screw rotors can be formed under the hole of the dies during the CWR process. Peng et al. [32] carried out a finite element analysis considering the plastic deformation phenomenon for the CWR process to produce the spiral shaft parts using the DEFORM-3D software with the version of 10.2. Simulation results indicated that the forming quality of spiral shaft parts was good and the CWR process of spiral shaft parts was feasible. Yang et al. [26] proposed the fixed axis cross-rolling (FACR) process, as shown in Figure 5, to form a screw compressor rotor. The metal materials of the billet produced continuous plastic deformation under the pressure of rotating rolling tools arranged parallel to the axis to obtain the required section shape. The screw compressor rotor with asymmetric spiral teeth and steps can be directly formed by the FACR process, which obviously improve production efficiency and the material utilization rate.

The multi-roller rotary feed forming (MRRFF) method [33,34] is a modified method for forming screw-shaped parts, and its schematic diagram is shown in Figure 6. During the MRRFF procedure, rolling wheels are radially located at the side of the workpiece. The workpiece rotates at a constant speed around itself and simultaneously moves in the direction of its axis. Shi et al. [34] examined the effect of process parameters on the product quality of the MRRFF process for uniform-wall-thickness spiral tubes. The experimental result revealed that a relatively high forming precision and relatively low damage value of 304 stainless was obtained under the rolling depth of 3.3 mm and formation speeds below 15 mm/s.

Bulzak et al. [35,36] analysed the forward extrusion forming (FEF) process of twist drills, as shown in Figure 7. During the extrusion process, the starting materials were heated to a suitable temperature for hot forming. When the material reached the required temperature, it was pushed by the punch into the special hole formed via the fixed and movable dies. Then, the formation of the drill in the cross-section was achieved by twisting. Based on the results of the theoretical–experimental tests, they revealed that the production of twist drills is accomplished by this extrusion procedure. It was also found that a small extrusion force was required with a small helix angle.

Figure 8 shows the working principle of the rotary cold extrusion forming (RCEF) procedure for processing screw rods. In the RCEF process, the push rod shifts in the direction of the rod axis, as represented by the green arrow, while the die circumferentially rotates at a specific speed, as indicated by the red arrow. Li et al. [37] investigated the RCEF process of screw rods by simulations via the Deform-3D software. The results demonstrated the feasibility of the production of either hollow or solid screw rods with a high uniformity in wall thickness by the RCFF process. They found that when the belt height where the die works was large, the required force and torque for the extrusion process were also high.

The helical tube incremental die forging forming (IDFF) process, as shown in Figure 9a, was proposed by Jin et al. [38] for the production of helical tubes. The billet was fed based on a designed value of length and a set angle ratio simultaneously when the dies opened. After the billet was fed, the closing of dies started, through which the billet was deformed by the process. The feasibility of this process was validated by data from simulations and experiments in their work. The experimental results, as shown in Figure 9b, revealed that a deviation of 0.31% was observed in the length value of pitches when manufacturing helical tubes at a pitch length of 400 mm.

2.3. Casting

Casting [39], including gravity casting [40], low-pressure casting [41], high-pressure casting [42], continuous casting [43], etc., is a versatile process for roughly fabricating complex parts, and then the final part with high dimensional accuracy can be obtained by finishing machining with the grinding and polishing process. Moreover, the rough-forming efficiency of casting is great [44]. Figure 10 shows a mould coated with sand and the produced screw rotors with high dimensional and shape accuracy [45]. However, products formed by casting would have some internal defects, such as shrinkage porosity and gas porosity, which may reduce the reliability of screw rotors [46].

To reduce the weight of the screw rotor, the resin transfer moulding (RTM) [47,48] method was used by Jung et al. [49]. It can produce screw rotors applied to air compressors with composite materials. Figure 11 shows a schematic diagram of the equipment for the RTM process, which mainly comprises a resin pool, a set of moulds and some resin flow pipes. Firstly, epoxy resin is evenly stirred by the agitator. Secondly, a certain pressure supplied by a compressor is applied to the resin pool, through which the epoxy resin is injected into the mould cavity along the pipes and solidifies to form a female and male screw rotor, meeting the design requirements. The screw castings formed with this process have the merits of low cost, high precision and a smooth surface. However, the waste gas generated from the gasification of macromolecular materials is seriously pollutant, which is contrary to the green manufacturing concept.

2.4. Additive Manufacturing Process

Additive manufacturing (AM) is a versatile technology for rough production applied in a number of application areas [50,51,52,53]. Moreover, the parts produced by the AM process usually require subsequent manufacturing steps, including grinding or polishing, to obtain higher dimensional and shape accuracy. According to the works of Tomasz et al. [54], additively manufactured parts, such as heat pipes, thrusters, magnetic shields, etc., can be used in different applications in space. Additionally, AM can be used for prototyping rapidly. Zhao et al. [55] used selective laser melting (SLM) to additively manufacture (3D-print) female and male screw rotors made from a TC4 titanium alloy powder. The dimensional and shape accuracy of the produced screw rotors shown in Figure 12 reached a high quality, the same as rotors after coarse grinding. To decrease the size and capital investment, Lee et al. [56] applied fused deposition modelling (additive manufacturing) to produce the blade of a controllable-pitch Archimedean screw (CPAS) for generating hydropower, the process of which is illustrated in Figure 13. The designed CPAS was used for a hydropower generation system. The performance evaluation results show that the power generated was 123 W, with an efficiency of 71%, at a flow rate of 0.04 m³/s. Although the additive manufacturing process can realise the accurate forming of various complicated profile screws, its production efficiency and economics need to be improved.

3. Summary of Common Screw Manufacturing Processes

The manufacturing processes of screw-shaped parts mainly include cutting (milling and grinding), solid plastic forming, casting, additive manufacturing forming, etc. Additionally, these processes can also be used in the production of long-axis complex profile parts such as worm gear and steering gear components, high-pressure common rail pipes, crankshafts, gear racks and gear shafts [57,58,59], which are shown in Figure 14. Among the above processes, the main manufacturing method adopted by many enterprises is the cutting process, which has the advantages of overall manufacturing and a short process flow, but it has the following obvious shortcomings:

(1)

In the cutting process, the material is greatly wasted. Moreover, the metal fibre is cut off, resulting in a reduction in length; thus, the mechanical property of the part reduces. The bending deformation of the workpiece is risky to form.

(2)

The actual production process of screw rotors needs multiple pieces of equipment, such as lathes, milling machines and grinding machines. Each process requires the replacement of tools and fixtures. The processing process is cumbersome.

(3)

The manufacturing cost of screw-cutting tools is high. A group of tools shall be replaced for each type of screw, where the corresponding cutter edge grinding equipment shall be provided. Consequently, the equipment investment is high.

The solid plastic forming process of screw-shaped parts mainly includes the WCR process, the MRRFF process, the FEF process, the RCEF process, the IDFF process, etc. The screw-shaped parts produced via the solid plastic forming process have high mechanical properties. However, it is challenging to ensure the accuracy of producing the screw profile in the forming process. There are numerous disadvantages, such as a large machining margin and high energy consumption. In addition, due to the large forming force and high requirements for equipment tonnage during forming, the service life of the mould is short.

Casting has high production efficiency, through which the screw profile is easy to be formed. However, casting defects such as shrinkage, shrinkage cavities and segregation are inevitable in the forming process, resulting in poor mechanical properties of the formed products [60]. Meanwhile, the high temperature of the molten metal in the forming process will shorten the die life. Additionally, the additive manufacturing process can achieve the precise forming of various complex-shaped screws, but it is necessary to improve the production efficiency. Moreover, there are significant limitations on the types of forming materials.

The merits and demerits of the common screw manufacturing processes from the above analyses are summarised in

Table 1. Comparison between common screw manufacturing processes.

Common Screw Manufacturing Processes	Classification	Merits	Demerits	Materials	Application Areas	Reference
Cutting process	Milling (including finger-type cutter, disc-type cutter and whirlwind milling)	(1) Overall manufacturing and short process flow; (2) Dimensional and shape accuracy is about ISO IT8 level.	(1) More than 60% of the materials will be wasted during milling, resulting in a low material utilization rate; (2) The design and selection of cutters and the accurate control of their motion tracks are still the difficulties faced by various enterprises; (3) The element made in this method is only a workpiece that further requires roughing and finishing.	Y40Mn (HB 190); GCr15; 42CrMo; 16MnCrS5.	Screw rotors; screw pumps; screw compressors, screw vacuum pumps; screw expanders	[15,16,17,18,19,20,21,22,23]
	Direct powerful integral grind machining	(1) Overall manufacturing and short process flow; (2) Dimensional and shape accuracy is about ISO IT6 level.	Low machining efficiency and high requirements for equipment performance			[10,24,25]
Solid plastic forming process	CWR process	(1) Almost no material is lost in the solid plastic forming process; (2) The production efficiency can be significantly improved; (3) The parts have high mechanical properties.	(1) A large machining margin; (2) High energy consumption; (3) Large forming force and high requirements for equipment tonnage during forming; (4) The service life of the mould is short due to the high forming load; (5) The element made in this method is only a workpiece that further requires roughing and finishing; (6) Dimensional and shape accuracy is low (at around ISO IT14–18 levels).	45 Mn BH; 45 steel; 42CrMo; AA6082; Ti6Al4V; GH4169; 25CrMo4; 6061 alloy; TC6; 1100H16; 7075 alloy; AZ31	Screw rotors; manufacturing of shaft parts; railway vehicle axles; producing stepped axles and shafts that are used in the automotive, machine-building and railway industries	[27,28,29,32]
	FACR process			SAE1141	Screw compressor rotors	[26]
	MRRFF method			45 steel; 4140 alloy; 304 stainless steel; 20G	Screw rotors; screw drilling; spiral tubes	[33,34]
	FEF process			100Cr6; 102Cr6;	Twist drills; screw rotors	[35,36]
	RCEF process			T2 copper; 6063 Alloy	Screw rods	[37]
	IDFF process			ANSI1045	Helical tubes	[38]
Casting process	Gravity casting	(1) Dimensional and shape accuracy is about ISO IT8–15 levels; (2) Great production efficiency.	(1) Casting defects such as shrinkage, shrinkage cavities and segregation are inevitable in the forming process; (2) Poor mechanical properties of the formed products; (3) The high temperature of the molten metal in the forming process will shorten the die life; (4) The element made in this method is only a workpiece that further requires roughing and finishing.	Al7075/A360; ductile iron;	Screw rotors	[40]
	Low-pressure casting					[41]
	High-pressure casting					[42]
	Continuous casting					[43]
	RTM method	(1) Dimensional and shape accuracy is about ISO IT8 level; (2) Low cost.	(1) The waste gas generated from the gasification of macromolecular materials is seriously pollutant; (2) Poor mechanical properties of the formed products.	Epoxy	Screw rotors	[49]
Additive manufacturing process	3D-printing	(1) Realising the accurate forming of various complicated-profile screws; (2) Dimensional and shape accuracy about ISO IT8 level.	(1) The mechanical properties of forming screws are poor; (2) The forming materials are limited significantly.	TC4 titanium alloy; polylactic acid (PLA)	Screw rotors	[54,55,56]

. To summarise, although cutting is the production mode adopted by most enterprises currently, more than 60% of the materials in the manufacturing process are removed by cutting, where the material utilization rate is low. Solid-state plastic forming, casting, additive manufacturing and other processes can effectively improve the material utilization rate. Nevertheless, their forming quality and size specifications are subject to certain restrictions. Therefore, it is necessary to research the new process of long-axis complex screw-shaped parts, such as female and male screw rotors. Only by carrying out in-depth research on the new process, structure and performance control technology of long-axis complex parts can the processing and manufacturing problems of the screw-shaped parts be effectively solved.

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. Flemings 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:

(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. 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. Similarities and differences between our novelty SSCEF process and the common screw manufacturing processes.

Manufacturing Processes		Near-Net Formation	Production Efficiency	Dimensional and Shape Accuracy	Service Life of Die and Tool	Mechanical Properties	Further Improved Mechanical Properties by Heat Treatment
Cutting process	Milling	No	Low	ISO IT8 level, and requires subsequent manufacturing steps including grinding and polishing.	Short	High	No
	Direct powerful integral grind machining	No	Low	ISO IT6 level, and requires subsequent manufacturing steps including polishing,	Short	High	No
Solid plastic forming process	CWR process	Yes	High	ISO IT14–18 levels, and requires subsequent manufacturing steps including cutting, grinding and polishing.	Short	High	Yes
	FACR process	Yes	High		Short	High	Yes
	MRRFF method	Yes	High		Short	High	Yes
	FEF process	Yes	High		Short	High	Yes
	RCEF process	Yes	High		Short	High	Yes
	IDFF process	Yes	High		Short	Very high	Yes
Casting process	Gravity casting	Yes	Low	ISO IT12–15 levels, and require subsequent manufacturing steps such as machining, grinding and polishing.	Long	data	No
	Low-pressure casting	Yes	Low		Short	data	No
	High-pressure casting	Yes	High	ISO IT8–12 level, and require subsequent manufacturing steps including grinding and polishing.	Short	data	No
	Continuous casting	Yes	High		Short	data	No
	RTM method	Yes	High		Short	data	No
Additive manufacturing process	3D-printing	Yes	Low	ISO IT8 level, and require subsequent manufacturing steps including grinding and polishing.	Long	Low	No
Our process	SSCEF process	Yes	High		Long	Very high	Yes

. 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 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.

(5)

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.

5. Conclusions

• The manufacturing technology of screw-shaped parts mainly includes a cutting process (including milling and grinding), solid plastic forming, casting and additive manufacturing. Among of these processes, the main manufacturing method adopted by many enterprises is the cutting process due to the advantages of overall manufacturing and short process flow, and whirlwind milling is one of the most widely used cutting processes. Moreover, grinding has gradually become the main method for the fine machining or integral machining of complex screw profile parts because the element made in whirlwind milling is only a workpiece that further requires roughing and finishing.
• Compared with the traditional cutting process with the disadvantages of serious material waste, complex process passage and high equipment cost, solid plastic forming has almost no material loss, and can also significantly improve the production efficiency. The solid plastic forming process mainly includes the WCR process, MRRFF process, FEF process, RCEF process, IDFF process, etc.
• The screw-shaped parts produced via the solid plastic forming process have high mechanical properties. However, it is challenging to ensure the accuracy of producing the screw profile in the forming process. There are numerous disadvantages such as a large machining margin and high energy consumption. In addition, the die service life is short due to the large forming force and high requirements for equipment tonnage during forming.
• The casting process of screw rotors has high production efficiency; however, the defects of casting, such as shrinkage porosity, shrinkage cavities and segregation, lead to the poor mechanical properties of forming parts, and the die life is short due to the filling process’s high temperatures. The additive manufacturing process can achieve the precise forming of various complex-shaped screw rotors, but there are disadvantages in additive manufacturing, such as the low production efficiency and large limitations on the forming material types.
• The SSCEF process of screw-shaped parts is proposed in this work, in which the semi-solid spherical grain is firstly prepared by the RFSIMA process, which includes radial forging and the isothermal procedure, for long-shaft raw materials. Then, the screw rotor can be fabricated by the high-efficiency closed extrusion process. The new process has the advantages of high efficiency, near-net forming, good mechanical property, high density, etc., which may help to solve the problem of “efficient, accurate and high-performance forming of long-axis complex metal components”.