Review of Metal Screw Extrusion: State of the Art and Beyond

Abstract

Metal screw extrusion (MSE) is a continuous, solid-state forming method utilizing an inherently high degree of deformation to consolidate fragmented input materials into a solid bulk by breaking their oxide skins. Severe plastic deformation with equivalent strain in the range of 10–20 can be achieved depending on set process parameters. Rigorous mixing can be employed to form sophisticated materials like bulk composites, nanocomposites, particle-reinforced metals, and fine-grained materials. Furthermore, the inherent solid-state processing is well suited for recovery of difficult-to-recycle materials. A range of non-ferrous materials has been manufactured by MSE and further characterized in terms of microstructural evolution and mechanical and functional properties. Furthermore, MSE has been studied in terms of flow, accumulated strain, and environmental impact. The following review aims to critically highlight the existing work performed on MSE, compare it to existing and emerging technologies as well as explore future development and possible applications. MSE has the potential to be utilized for numerous commercial applications. To realize industrial use of MSE, key aspects of the process and the influence of processing parameters on the resulting product must be understood.

1. Introduction to Metal Screw Extrusion

The principle of displacing matter by screws dates back to ancient Egypt, where it was utilized for pumping water to a higher elevation [1]. The principle is based on a helical screw covered by a hollow tube or liner. When rotating the screw shaft (or the entire cylinder), the matter of interest is transported from one side to another. Pressure is built if the outlet side of the cylinder is narrower than the inlet side; hence, an extrusion principle can be utilized. Today, screw extrusion is extensively used in the processing of polymer materials and edibles [2,3]. Most processes using screw extrusion share a common property of using relatively low-viscosity materials, which make the flow of material relatively easy.

Solid state screw extrusion of materials with high viscosity, such as metals, is commercially non-existent. The idea of metal screw extrusion (MSE) was born at the Norwegian University of Science and Technology (NTNU) in collaboration with the aluminium company Norsk Hydro in the early 2000s. Following an initial concept study, a breakthrough was achieved in 2006 when the first successful trial using a laboratory scale screw extruder to process aluminium was performed. A patent was filed by the stakeholders shortly after, being published in 2008 [4]. The technology became more publicly and academically open with secured intellectual property rights, and a publicly funded research project lifted the technology readiness level (TRL) from the concept stage (TRL2) to an operational laboratory extruder (TRL4). Subsequently, over twenty student works on the Master’s and Doctoral level have been performed since 2008 related to MSE. At present, two versions of a laboratory-scale extruder and one pilot factory have been built to explore MSE technology. As the technology is now emerging on an industrial scale, this article aims to highlight the main points of the accumulated knowledge. This will include the current understanding of the metal screw extrusion process itself, as well as the characteristics of materials that have been manufactured.

Metal screw extrusion resembles polymer screw extrusion using a counter-clockwise rotating Archimedes screw. A conical extrusion chamber and a die tool are situated in front of the screw. A schematic of the MSE assembly presently used at the lab scale is shown in Figure 1. The feeding of material at the rear of the screw is shown by pellets falling through a feeding funnel. The material is initially transported forward by the screw until it is compacted into a solid mass, forming a billet in the conical extrusion chamber. When pressure from oncoming material exceeds the force required to overcome the flow stress of the material, extrusion of a profile takes place at the die tool outlet. Unlike polymer screw extrusion, where the fed material becomes liquid, the material in MSE is in the solid state during the whole process. The metal oxide skin of the fed material is broken, and solid-state welding takes place, further outlined in Section 2. Solid-state flow of metal implies high plastic deformation of the metal, causing severe strain on the final product, as described in Section 3. The accumulation of strain is outlined in Section 4.

The forward flow of material is ensured by sticking friction between the metal and the surrounding tool steel. To ensure favorable sticking friction conditions, the MSE system is kept within certain temperature ranges by combining active cooling and controlled heat generation [5]. Temperature control is hence considered one of the main factors governing successful extrusion. Depending on materials, die temperatures ranging from 350–650 °C have been used. Further, keeping temperatures low towards the feed hole is necessary to avoid sticking friction in this region.

Frictional conditions in MSE contrast to conventional ram extrusion, where friction against the container surface is an unwanted effect as it increases the ram force. Generally, friction occurs when you have relative motion between two bodies. Typically, in the modeling of conventional extrusion, a number of frictional models exist [6]. Sticking friction is used as a boundary condition for most surfaces based on empirical data. In contrast to MSE, the temperatures typically do not make it possible to avoid this. In cases where container friction is a limiting factor for direct extrusion, indirect (backwards) or hydrostatic extrusion is applied since it mainly entails friction through the die package. Another aspect of friction is its role in die design for extrusion of metals, where friction against the bearing surface is used to balance profile flow. This is also similar for MSE.

The potential industrial impacts of MSE are vast. The continuous nature of MSE enables extrusion without interruptions, in contrast to semi-continuous ram extrusion. This would reduce cut-offs and process scrap from extrusion butts, which imply material loss reductions by 5–15% [7]. MSE can hence improve the profitability and environmental footprint of conventional profile extrusion. Further, solid-state recycling is considered an energy-efficient and low-carbon footprint alternative to conventional remelting of aluminium scrap. Duflou et al. compared the environmental impact of different liquid- and solid-state recycling methods, including MSE [8]. The assessment showed that MSE has an 80–90% lower environmental impact compared to scrap remelting due to lower energy consumption and insignificant material loss. Case studies of materials from MSE are reviewed in Section 5, and a comparison between MSE and other emerging solid-state forming technologies is reviewed in Section 6. Future prospects for MSE and beyond the state of the art are outlined in Section 7.

2. Material Flow in MSE

Material flow through the screw extruder is complex and yet not fully understood. Solid-state flow was systematically studied by Widerøe and Welo [9] and is summarized in the following section. They divided the process into six sections, as shown in Figure 2. Each section was studied individually and included sliding and merging of input material, screw channel flow, backflow around the screw tip, forward flow in the extrusion chamber, and final extrusion through the die.

Generally, granular material is fed into the rotating screw, where it falls to the bottom of the container due to gravity and is transported forward, being pushed by the screw flight (Zone 1, Figure 2). Later in the process, the material will interact with material from the previous screw revolution (Zone 2). Here, it is starting to merge into a solid mass, where the already compacted material is at an elevated temperature. The (partially) consolidated material is further compacted and pushed forward in the screw channel (Zone 3). Sticking friction conditions now start to arise between the material and surrounding surfaces. The material is thus fully consolidated to a compact material and further formed into a billet in front of the screw (Zone 4). The incoming material is continuously smeared onto this billet, layer by layer, as the screw flight tip rotates. The process can be visualized by imagining spreading butter on a piece of bread. In this situation, you will notice that some of the butter is curling up on the other side of the knife (i.e., on the low-pressure side). In MSE, such a backflow is also observed (Zone 5). As the high-pressure side of the screw flight pushes the material forward, a concurrent low-pressure side is developed at the opposite side of the screw flight tip, as illustrated in Figure 3b. Thus, a part of the on-smeared material is flowing backwards towards the low-pressure side in the wake behind the screw flight tip. Here, the material mixes with the screw channel flow. As oncoming material ensure forward-acting pressure on the billet in front of the screw, it finally extrudes through the die orifice (Zone 6).

The material flow in the billet in front of the screw entails several flow regimes. Dead zones are observed due to sticking friction with the container wall and the screw tip and at the corners of the extrusion chamber, see Figure 3a. Further, a zone of fast flow is developed at the screw tip where the material from the center of the screw channel is smeared onto the billet. Between these two extrema is an intermediate slow-flow zone. Visual observation of these zones was achieved by Widerøe and Welo by feeding different materials that provide a contrast after etching to two individual screw flights in a double-flighted screw [9]. This visualized the flow of material entering from each screw channel, as shown in Figure 4. Such samples were made and studied in many orientations and positions, forming the basis for the understanding of the flow in MSE. Such a method of using contrast material is commonly used when studying flow in various forming methods [10,11]. The contrast material method is a two-dimensional representation of the flow situation in the extrusion billet; hence, it is unable to provide the whole picture of the flow regime. By combining different sections and computational representation, a better understanding of flow can be deduced in future studies.

The material flow in MSE is very different from that of conventional ram extrusion. This results in large shear deformations, which allow oxide layers on the granular material surfaces to be completely broken and mixed with the rest of the material. Torsional motion in the solid plug, as well as in the screw channel, contributes to this deformation, in addition to the flow around the screw tip. The various flow speeds seen in conjunction with the complex strain path often lead to recrystallization (similar to what one can observe around seam welds or towards surfaces in conventional extrusion due to the variation in strain rates). Using correct processing parameters in combination with improved geometrical designs can (in contrast to ram extrusion) be used to control the various flow stages, modify the material path towards the die, and optimize texture and grain size distribution of screw extruded materials.

One effect of optimizing parameters is to manipulate the frictional conditions along the process length (screw/extrusion axis). This requires a fundamental understanding of the pressure-generating device (i.e., the screw), which is of paramount importance to ensure a stable material flow [12]. Analogue to polymer extrusion, where Rauwendaal showed that a small change in the frictional conditions of a solid plug caused large variations in the transportation rate [13], friction in metal screw extrusion varies stochastically in relation to input material orientation. By better controlling the pressure generation (i.e., by controlling the length of the screw used to generate this pressure), one can improve process stability. Seen in conjunction with further work on screw geometry design, as well as feeding procedures, this is a topic of research that can lead to significant improvements towards a more stable operation of the process. A stable process with even extrusion speed is a prerequisite for the extrusion of more complex profile geometries and to achieve better geometrical tolerances.

Thus far, mainly circular profiles ranging in diameter from Ø40 mm down to Ø1.2 mm have been produced by a laboratory MSE equipment. Furthermore, tests extruding other solid profiles, such as flat bars and L-shaped angle profiles, have been proven feasible. Even extrusion of “seamless pipes” using the screw tip as a mandril was performed. However, a challenge arises from the abovementioned flow, where it is twisting around its own axis as the extrusion butt is partially rotating with the screw. This encourages further research to generate an expanded understanding of how to correctly utilize friction and (screw and pre-chamber) geometry to achieve preferred material flow through the die orifice.

3. Pressure and Strain-Driving Mechanism

As mentioned in Section 1, temperature control is considered a main factor for successful screw extrusion. It influences the flow stress of the processed material and sticking friction conditions. Sticking friction between the tool steel and the processed material at the correct locations in the extruder is imperative to high extrusion efficiency. The relation between temperature and local pressure for a given material was investigated in relation to the screw extruder and described by Widerøe and Welo [5]. By comparing the normal contact pressure q with the characteristic shear strength k of an aluminium alloy, $q/k>0.6$ yielded sticking friction for temperatures above 300 °C, while a $q/k$ ratio of 0.7 was required for lower temperatures.

In the MSE process, the (normal) pressure on the material from the screw flight and container wall is difficult to evaluate. Further, it is likely to vary depending on time as the screw flight position differs relative to the free flowing material in the transport zone (i.e., screw flights aligned vertically or horizontally). However, a general understanding of how temperatures at different positions can be used to achieve more optimized operation has been found. This relates to the statement in the original MSE patent that compacted material is limited to a length corresponding to “up to 540° of the screw flight length” [4]. This can be achieved by ensuring that the temperature in this length exceeds the temperature needed to achieve sticking friction, while the temperature outside of this length is maintained at the condition of sliding friction.

Thus, when establishing a process window for MSE, flow stress, sticking friction conditions, and the global melting temperature for the processed material must be taken into consideration. For some materials, the processing window may become narrow. At the same time as the temperature dependency of sticking friction and flow stress imply using a higher temperature, low melting phases set an upper limit. As shown in several MSE lab trials, composites may prove especially challenging in that regard, as one must consider the phase with the lowest melting point in the composite system.

Understanding frictional conditions is the key to understanding the MSE process. Sticking friction is beneficial where it is needed to generate forward pressure and also control the total imposed plastic deformation. However, it is a challenge to restrict sticking friction where it is redundant: in the zone of material transport and around the screw tip. For the former, sticking friction can onset earlier compaction, which will eventually stall the process. For the latter, reduced sticking friction results in less torsional movement in the pre-chamber, which will amount to a more efficient screw. However, torsional movement can be beneficial for cases where one wants to impose more plastic deformation, e.g., to refine composite materials. Because full sticking is achieved between the processed material and the tool steel parts, relative material movement results in material plastic deformation, similar to other SPD methods utilizing the same principle.

4. Accumulation of Strain

Skorpen et al. published the first approximation to predict the accumulated strain in MSE by adapting established formulas for other plastic deformation processes [14]. MSE was divided into several strain-contributing regions, each of which was compared to individual deformation processes.

One of the motivations for this approach was the work on high-pressure torsion extrusion (HPTE) by Ivanisenko et al. [15]. Here, a linear moving material was subjected to strain in a rotational “die”. This is, in many ways, equivalent to what happens in parts of MSE, where a rotational material is moving forward to a stationary die. Working from these ideas, the knowledge of the MSE process accumulated through a wide range of experimental work was broken down to build an analytical model of the accumulated strain.

The segment in front of the screw (i.e., the extrusion chamber) is similar to an inverse HPTE process. The analysis of material left inside the MSE chamber at abrupt stops indicates spiralling flow, where part of the rotation of the screw leads to torsional deformation in that region. Hence, a formula describing this twisting motion was established. In addition, as the segment is conical, a contribution from change in the cross-section was included. To handle both contributions, the segment was subdivided into slices, and the strain of the segment was calculated in a loop around these subdivisions. The subdivision converged upon approaching 1000 slices, at least in the face of the uncertainty in the assumptions described below.

Secondly, at the screw tip, a layer of material is smeared onto the solid billet in front of the screw (recall the butter-on-bread analogy). One can imagine this as a reverse-shearing event. For machining, the shearing of chips or turnings was evaluated [16]. The formulas for that process were adapted to the geometry in the smearing region.

Third, the screw channel was evaluated. Here, the situation is somewhat more complex. The deformation in this region mainly depends on several processing parameters. In internal reports describing early conceptual tests [unpublished], efforts were made to correlate the length of compacted material in the screw channel region to processing parameters (such as feed rate, temperatures in various positions, and screw rotation speed). No significant relations were found, although a tendency that these parameters affect the compacted length indirectly by affecting the onset of sticking friction was seen (as also discussed in Section 3). Hence, the length defined by the patent was used in the evaluation, equivalent to 540° rotation [4]. From there, it was a matter of correlation with torsional deformation to further decompose the screw channel flow in a similar fashion that is well established in the simulation of polymer screw extrusion [13]. As the deformation in this segment is dependent on compacted length, which again is highly dependent on processing parameters, the estimation of total strain is challenging. The further development and understanding of these relations will be crucial for the development of a more precise strain estimation of the MSE process.

Fourth, since there is an abrupt change in diameter at the outlet die, one can add a simple extrusion step for this MSE segment.

Adding together these four segments will then result in an accumulated strain in MSE, from material compaction to an extruded product. This is accumulated through torsion in the screw channel, smearing at the screw tip, torsion and cross-sectional reduction in front of the screw, and cross-sectional reduction in the die. From this, the accumulated strain in MSE defines the process as a severe plastic deformation process. Typically, total strain is in the range of 10–20, as seen in Figure 5. For comparison, one pass of equal channel angular pressing (ECAP) typically has an equivalent strain of 1, showing the severe plastic deformation in MSE [17,18].

5. Materials and Applications

The main body of research related to metal screw extruded materials has been devoted to aluminium and its alloys. This is a natural starting point, given the formability of aluminium and the relatively low melting point. Extruded material geometries range from solid rods, bars, and wires with circular cross-sections, to flat and angle profiles and tubes. Most trials have extruded profiles with a symmetric (round) cross-section due to the simplicity of post-extrusion metallurgical characterization. Furthermore, most of the work has focused on 6xxx series alloys, but aluminium from nearly all wrought-alloy series have been tested, such as 1xxx, 4xxx, 5xxx, and 7xxx alloys. Even foundry alloys (3xx type alloys) have been processed by MSE. In the following, some of this work is discussed.

Several comparative studies between conventional ram extrusion and MSE have been performed. As the pressure-generating devices are different (i.e., ram or screw), the material flow, deformation path, and temperature development during extrusion is different. Kristiansen highlighted microstructural and mechanical responses for three 6xxx Al–Mg–Si alloys subjected to ram extrusion and MSE [19]. The effect of air and water cooling in the two processes was studied. Ram extrusion was performed with a reduction ratio (RR) of 9.5, while MSE had a RR equal to 3.1. Similar mechanical properties after solution heat treatment and peak aging (T6) were found irrespective of the extrusion method, as shown in

Table 1. Mechanical properties of ram and screw extrusion of three different Al–Mg–Si alloys after peak aging. Reduction ratio (RR) of ram extrusion was 9.5, and RR of MSE was 3.1. Reproduced with permission from Kristiansen, published by NTNU Open, 2020 [19].

Alloy	6082				6005				6060
Extrusion	Screw		Ram		Screw		Ram		Screw		Ram
Cooling media	Air	Water	Air	Water	Air	Water	Air	Water	Air	Water	Air	Water
Rp0.2 [MPa]	345	330	380	355	310	310	305	310	225	220	230	225
UTS [MPa]	360	355	390	365	345	340	338	315	250	250	248	245
Elongation at break [%]	15	17	13	15	13	12	18	18	16	17	17	17

.

Some differences were observed in the quench-sensitive 6082 alloy, where the water cooling implemented in MSE was likely too soft (positioned a bit far from the die), which resulted in 8% lower ultimate tensile strength compared to water-cooled ram extruded profiles. It should be noted that only tensile properties were examined. For future studies, a comparison of fatigue performance is of high interest.

Despite similar mechanical properties, the microstructural appearance was significantly different between the ram and screw extruded 6xxx alloys. The screw extruded materials have an apparently higher recrystallization resistance, exemplified by the 6005.40 alloy in Figure 6 after extrusion and solution heat treatment at 550 °C for 10 min. Here, the ram extruded variant is completely recrystallized after the solution heat treatment, while the screw extruded counterpart remains a fibrous structure apparently aligned with the extrusion direction when seen in the longitudinal section. The microstructural evolution during metal screw extrusion was further elaborated by Sveen for 6060 and 6082 alloys [20]. The higher resistance to recrystallization in metal screw extruded materials was attributed to the lower strain rates and higher extrusion temperatures in MSE compared to ram extrusion. The lower extrusion ratio for MSE may also have played a role. Consequently, the driving force for recrystallization is lower for MSE materials.

Another effect contributing to the grain structure in MSE, which influences recrystallization, is pinning particles in the MSE structure. Dispersed oxide particles, formed by the breaking of surface oxides on the input material, can interact and decrease grain boundary migration through Zener pinning [21]. Leading evidence is the observation of MSE materials with partial recrystallization, where recrystallization happens inside discrete bands. An example of a 6xxx alloy is shown in Figure 7 [22]. Here, one can imagine that the band is defined by the original input material particle size and that there exists a dispersion of oxides in a line where input materials have been merged. The dependency on processing parameters on the final microstructure is also prominent in the presented micrographs. In-depth TEM studies of second-phase particles in MSE materials are of high interest to systematically study the effect of oxides at the nanoscale.

Commercial pure aluminium alloys find their applications as food packaging and electrical conductors. The material is soft and, therefore, has no structural applications. They have a relatively high melting point and seemingly high sticking friction with the tool steel during metal screw extrusion and appear relatively easily extrudable. Langelandsvik examined the microstructural evolution of electrical grade 1370 after metal screw extrusion [23]. The study focused on the importance of precipitating intermetallic iron phases, as trace iron in solid solution is known to be harmful to electrical properties. An extrusion temperature of 450 °C coincided with the optimum precipitation kinetics of iron aluminides, creating a material with a very high electrical conductivity. Værnes compared the electrical conductivity of screw extruded super pure aluminium (99.99 wt.% Al) with a commercial reference, showing no degradation of the electrical properties after MSE [24].

Furthermore, 4xxx series alloys have silicon as their main alloying element and are mainly utilized as a joining material, e.g., as welding wires, brazing wires, and brazing plates. Langelandsvik et al. compared the performance of the common welding wire alloy 4043 in a commercial state and as a screw extruded product [25]. Here, the welding wire was directly metal screw extruded into an Ø1.2 $\mathrm{m}\mathrm{m}$ wire, i.e., with an extrusion reduction ratio of 25. In fact, this is at present the highest reduction ratio used in MSE. Wires for welding and brazing are normally supplied with excellent surface quality to ensure a stable arc and to avoid porosity. A poor surface appearance of the wire produced by MSE resulted in a slightly elevated porosity content in a welded joint but with little impact on mechanical properties. The extrudability of 4xxx alloys with high silicon content is relatively poor, and this also holds true for MSE.

Difficult-to-extrude or non-extrudable aluminium alloys have also been successfully manufactured by MSE. Amundsen demonstrated the recycling of an automotive engine block material based on a silicon- and copper-rich 3xx foundry alloy to a fully dense bar [26]. A range of different intermetallic particles containing Al, Fe, Cu, Ni, Si, and Mn were observed, confirming the large range of alloying elements present in the alloy. The original cast structure having primary $\alpha$ (or Chinese script) intermetallic phase structures was broken up and refined during processing due to severe plastic deformation. This is shown in Figure 8, which compares the sizes of intermetallic particles. The recycled engine block was further demonstrated as concrete reinforcement with acceptable corrosion, adhesion, and mechanical properties [27]. Motor engines are projected to accumulate in the value chain due to the rapid transition to electric vehicle transportation [26]. New recycling routes and new applications to keep such “dirty” alloys in the value chain are sought, where MSE can play a vital role. Furthermore, high-strength 7xxx alloys have poor formability and are thus difficult to extrude. These alloys are also hard to recycle due to excessive oxidation and loss of volatile elements such as Zn and Mg. Samuelsen demonstrated the applicability of MSE for forming the Al–Zn–Mg–Zr alloy 7108 using MSE with excellent surface quality and good mechanical properties [28].

Apart from processing of commercial aluminium alloys, MSE has also been explored for in situ alloying and composite manufacturing. Al–Mg macro composites with up to 10 wt.% Mg were produced by mixing aluminium 6063 and commercial pure magnesium in the granular form [29,30]. Al–Mg alloys are prone to forming brittle and corrosion-susceptible Al₃Mg₂ intermetallics with increasing Mg content. Liquid-state manufacturing of high-magnesium aluminium alloys is thus impractical due to this challenge. Solid-state low-temperature processing of Al–Mg composites has shown promising results with combined good mechanical and corrosion properties [31,32]. Skorpen et al. successfully created an Al-10%Mg profile with tensile strength close to 350 MPa and acceptable ductility [33]. However, islands consisting of the Al₃Mg₂ phase surrounded by $\gamma$-Mg₁₇Al₁₂ were observed, being likely fracture initiation sites. Finer additions of the magnesium feedstock could possibly give a finer dispersion of magnesium in the structure. In principle, infinite combinations of binary- or multi-metal mixtures can be manufactured by MSE.

Another application where MSE has potential is towards aluminium mixed with micro- or nanosized non-metallic constituents, which are termed aluminium matrix composites (AMC). AMCs combine the light weight and formability of aluminium with strength and stiffness from a (usually) ceramic compound and find application in a range of industries [34]. Nano-AMCs are challenging to produce with casting due to settlement, agglomeration, and phase transformations of the particle phase. Solid-state severe plastic deformation is a promising alternative due to less reactivity between the constituents and the possibility of a finely dispersed nanophase. Bulk processing by SPD has, however, been limited. MSE can, in this respect, play a vital role in nanocomposite manufacturing given its continuous nature. Several notable examples of MSE AMCs have been demonstrated in recent years. Langelandsvik et al. mixed the Al–Si alloy 4043 with 1 wt.% 40–60 nm TiC nanoparticles [25]. A fine dispersion of nanoparticles was found in the final extrudate and the remaining extrusion butt, showing the potential to finely distribute the ceramic phase. Similar findings have later been reported for higher concentrations of TiC nanoparticles [28,35] and TiB₂ microparticles [36,37]. Edvardsen compared the dispersion of Al₂O₃ particles as a function of particle size (0.5–50 μm), particle concentration (5–10 wt.%), and material feed rate [38]. Polarized optical microscopy in the cross-section of the extrudates in Figure 9 shows that particles tend to follow the spiralling flow lines of the extrudate. Moreover, particles tend to concentrate in the center of profiles under certain feeding schemes. Thus, optimum flow conditions are necessary to obtain a sound and uniform dispersion of a secondary phase during MSE. The principle of metal–metal and metal–ceramic mixing in MSE has endless potential candidates and combinations. However, the boundary conditions related to sticking friction, the influence of hard materials on screw and liner wear, and secondary phase flow have yet to be systematically studied.

Metal screw extrusion is not limited to aluminium alloys. In principle, all metallic materials can be processed by MSE, given sufficient sticking friction for forward flow without causing excessive wear on the extrusion equipment. Meling utilized MSE as a compaction method of titanium sponge into Ø30 mm bars [39]. Profiles were not fully dense as internal flaws caused premature failure in tensile testing. In addition, embrittlement by elevated nitrogen levels due to air exposure during extrusion caused inferior mechanical properties. The high melting point of titanium (1668 °C) combined with high hardness induced excessive wear on the extrusion equipment. The tool steel screw flights were excessively worn post-extrusion, and special equipment must be developed to accommodate MSE towards such applications.

Metal screw extrusion was originally invented as a low-energy recycling method of post-consumer aluminium scrap. Aluminium is well-known as one of the easiest materials to recycle, as 75% of all aluminium ever produced is still in circulation [40]. Although remelting of aluminium requires only 5% of the original energy to produce virgin metal, solid-state recycling can improve this figure down to 0.5% [8]. Furthermore, metal screw extrusion can increase the metal yield, as dross formation during remelting is almost avoided [41]. Three recycling strategies have been proposed for MSE, outlined in the following paragraphs.

The pre-compaction of scrap aluminium by MSE prior to remelting has been proposed to increase the metal yield. Thin aluminium scraps such as turnings and chips have low yields as they tend to float to the top of the recycling melt and create a thick oxide layer. Fully dense bars or briquettes sink into the melt bulk, avoiding excessive dross formation [42]. The melt will consequently become cleaner with less oxide bi-films. Post-consumer scraps such as turnings, incinerated bottom ash, coffee capsules, and household aluminium foil have been proposed as a potential feedstock for MSE [24,43,44]. The solution of MSE as a preparation stage before remelting is promising to increase metal recovery but does not fully utilize its environmental potential as the metal still must be remelted before being turned into useful products.

Production of pre-alloying materials for casthouses (i.e., master alloys) by MSE has been proposed as an industrial application of the technology. When manufacturing foundry aluminium alloys with a high silicon content, primary silicon normally serves as an alloying addition. Primary silicon comes at a high cost and is produced with high energy consumption. Substituting this with silicon waste sources has both an economic and environmental benefit. In fact, there are ongoing commercial efforts to utilize silicon dust in aluminium scrap [45,46], showing great potential to lift landfill silicon sawdust into a useful product.

MSE, as a direct recycling route, omitting remelting, is a solution with substantial potential to decrease the carbon footprint of aluminium. As stated by Duflou et al. [8], the environmental impact can be reduced by 90% due to low energy use and close-to-no metal losses. However, recycling of post-consumer scrap is a complex task. Accumulation of alloying elements in recycled alloys can deter the mechanical, corrosion, and functional properties [47]. This accumulation can be mitigated by diluting out-of-spec alloys with virgin aluminium. However, this solution is mainly reserved for remelting. Metal screw extrusion can therefore be seen as most suitable for closed-loop recycling of end-of-life products into a similar product.

Langelandsvik et al. examined the electrical properties of aluminium used for high-voltage transmission lines after MSE processing [23]. Direct recycling into a conductor wire resulted in a material with excellent electrical properties but lower tensile strength. Microstructural analysis revealed a partly recrystallized structure due to repeated deformation and thermal annihilation of dislocations during extrusion. As most aluminium electrical conductors are severely cold-worked by drawing, the material state was not directly comparable to industrial standards. Studies are currently underway, where MSE of commercially pure aluminium combined with wire cold drawing is applied to assess the potential of MSE as a closed-loop recycling method. Further, closed-loop recycling requires control of material streams to avoid alloy mixing and contamination. MSE can contribute to this by decentralizing the recycling process. It can be envisaged that in-process scrap (e.g., chips, cut-offs, and extrusion butts) is recycled on-site by table-top extruders, simplifying collection and avoiding mixing of material classes. Based on the electrical conductor example, cable wire manufacturers could directly take care of process scrap and end-of-life transmission lines and transform them into new, similar products [48].

The major difference between solid-state recycling by MSE and liquid-state remelting is the possibility of partitioning the metal and non-metallic contaminants. In remelting with rotary furnaces, oil residues, lacquer, and plastics are burnt off, and oxide skins are separated as dross [49]. Metal screw extrusion does not possess this possibility and has stricter demands on input feedstock. Volatile substances such as lubrication oils evaporate at elevated temperatures in the extruder system. The gaseous phase either escapes backwards through the feedhole, where it can obstruct the forward flow of material or is trapped in the extrusion chamber and is released through the die opening. The latter can result in blistering of the extrudate or a sharkskin surface tearing effect, which are common issues seen in polymer extrusion [50]. Examples of blistering and sharkskin observed during MSE of Ø10 $\mathrm{m}\mathrm{m}$ circular profiles are shown in Figure 10.

To omit the possibility of resulting flaws in MSE, a prior cleaning step can be utilized. For commercial application, such dressing can be carried out inline as part of a production line, in conjunction with other dressing methods, e.g., sorting. For organic compounds, chemical and thermal treatments are commonly employed [52]. Ketone-based solvents such as acetone are highly effective when used to degrease metals and dry quickly in dry air at room temperature. However, health, safety, and environment (HSE) considerations must be employed due to the highly flammable nature and the long exposure hazard to human soft tissue during inhalation. Thermal degreasing involves high capacity with lower cost and reduced health-related issues. Cui studied the thermal degreasing of aluminium turnings and gas evolution at different temperatures [42]. The temperature must be sufficient to evaporate the non-metallic substances but low enough to avoid excessive oxidation of the material. A baking temperature of 350 °C was shown to be a good compromise in the evaporation of oil and grease, as illustrated in Figure 11 [53].

The effect of oxides from metal screw extruded materials is an interesting issue that has been scarcely investigated. The dense oxide layer on the feedstock is seemingly broken up and baked into the matrix during solid-state flow throughout the process. This contrasts to liquid-state processing, where oxides are either separated as dross or is present as bi-films. The oxide skins on the input materials are at a thickness of nominally tens of nanometers. Their size and distribution in screw extruded materials have not been systematically studied, although Skorpen examined the distribution of particles in an Al–Mg alloy by TEM [54]. It should be noted that the alloys studied were binary alloys with a relatively high Mg content (up to 10 wt.%), and the effect of this on the input materials’ surface oxide thickness is not known. However, oxides were observed as arrays aligned along grain boundaries, as shown in Figure 12a.

At lower magnification, such grain boundaries can be seen to follow a pattern related to the solid-state flow. As mentioned earlier, they also tend to pin the grain structure. This alignment implies a relation to the original location of surface oxides. Such pinning lines have a relatively large distance relative to the area examined by TEM. Thus, grain boundary oxide arrays are statistically difficult to observe in TEM. Moreover, sporadic distribution of intragranular oxides was also observed by Skorpen, Figure 12b. Oxide fragments may work as fracture initiation points for microcracks and voids. This would affect the ductility and fatigue life of the material, a topic for future research.

Aluminium alloys with high magnesium content should be given special care under high-temperature extrusion conditions. Unlike the dense aluminium oxide layer, MgO is porous and enables further oxidation at elevated temperatures. This drives magnesium out of the solid solution, decreasing the mechanical properties. Magnesium oxides on the metal surface can also lead to galvanic corrosion in seawater, where Al-Mg 5xxx alloys are commonly used. Skorpen found dense populations of oxides in metal screw extruded Al-10Mg alloys, with oxides aligned along the grain boundaries [54]. Such oxidation should be avoided, especially due to the risk of galvanic corrosion. An increased partial pressure of CO₂ during processing has been shown to restrict the oxidation of magnesium in aluminium alloys [55]. Ragnvaldsen [35] compared the mechanical response of metal screw extruded 5183 Al-5%Mg alloys with and without CO₂ cover gas in the extrusion chamber. As magnesium atoms in a solid solution with aluminium highly influence the strain hardening response, assessing the related strain hardening exponent is an indirect measure of magnesium content. A 14% increase in the strain hardening exponent was observed when CO₂ was employed, showing the potency of magnesium oxidation in MSE. Langelandsvik et al. showed that hydrogen uptake was 88% higher in screw extruded 5183 without CO₂ gas coverage due to the hydration of magnesium oxide into Mg(OH)₂ [56]. The use of cover gases in a closed extruder system can become an efficient approach to avoid oxidation of volatile elements such as Zn and Mg.

6. Emerging Solid-State Manufacturing Technologies

The field of severe plastic deformation goes back to the 1930s and the pioneering work of Bridgman [57]. Later, the invention of equal channel angular extrusion (ECAE, often referred to as equal channel angular pressing, ECAP) by Segal [58,59] and high-pressure torsion (HPT), among others, have been creating bulk nanostructured materials with extreme properties [60,61,62,63]. In the latter years, techniques applying large strains have also been developed aimed at the solid-state processing of granulated materials. One driver for the development of such processing methods is the increased need for efficient, low-energy, high-yield methods for recycling waste into bulk materials. Emerging solutions are based on different basic principles, e.g., torsional movement of tooling or specific geometrical solutions to the internal workings of the process. This section seeks to highlight relevant comparisons to MSE, with the aim of providing a context for the MSE field.

An interesting method utilizing torsion for solid-state flow is high-pressure torsion extrusion (HPTE). Here, a rotating die is utilized to add shear deformation to a billet as it is extruded [15]. This can be considered analogous to screw extrusion in the sense that the solid billet in front of the screw rotates relative to a stationary surrounding die. A large deformation is achieved in both cases by the twisting of material, which sticks to the surrounding inner surfaces and rotates relative to the processed material [64]. Work is ongoing to develop this technique through modeling tools and experimental work [65,66]. HPTE is a semi-continuous processing method, as a billet must be loaded for every punch, similar to conventional ram extrusion. Hu et al. proposed to overcome discontinuity by a combination of HPTE with Conform™ to enable continuous extrusion with a high degree of shear on the processed material [67].

Materials processed by HPTE generally exhibit a coarser grain structure in the centre, which becomes finer towards the outer edge [68]. Such a systematic trend is seldom observed in materials processed by metal screw extrusion. This reflects the more complex material flow and/or contribution from other process parts to the accumulated strain in MSE. Further, by using granulated material as input to the process, oxide particles seem to pin the microstructure, which complicates the picture. HPTE applied to granulated input material has not been described in the literature, and the authors muse on its applicability and why it should not be feasible given that one can establish a counter pressure in front of the torsional die. In particular, it seems especially applicable for such an application in the aforementioned constellation as placed in conjunction with the Conform™ method, where one can presumably also achieve strain paths in several directions.

The already mentioned Conform™ technology is in itself an interesting processing approach [69]. The principle relies on sticking friction to a rotating wheel to pull material onto a die placed at an angle to this wheel. The basic principles are, hence, similar to MSE. Although the die-channel angle introduces strain as the material flows towards the die orifice, it is limited to strain levels similar to that of ECAP and can be considered a low-strain method compared to MSE. Several passes in Conform™ would be needed to equal the accumulated strain in MSE. Like metal screw extrusion, the capacity is dependent on how big pressure can be generated by sticking friction between the rotating wheel and the input material. This implies a limit in reduction ratio, capacity, or die-channel angle. In 1978, Conform™ was already depicted as having a “potential for direct recycling of scrap non-ferrous metal in various forms” [70]. The authors are aware of ongoing research aimed at recycling low alloyed aluminium scraps but are not familiar with the maturity of the process in this regard and the challenges they face viz the aforementioned limitations. The development of this process has continued, and the applicability towards granulated material has been conceptually shown. A notable example is the recovery of titanium powder waste from powder-based additive manufacturing into wire feedstock [71]. Since Conform™ extrusion relies on friction to drive the material forward, this indicates that recycling Ti powders could also be feasible using MSE. Also, a processing route labeled as the friction-induced recycling process (FIRP) seems to apply Conform™ for aluminium chip recycling [72].

Another process utilizing torsion, and where they indeed try to apply it to compact granulated material, is the shear-assisted processing and extrusion (ShAPE™) method. Ongoing work on commercialization of the process for solid-state recycling of aluminium is reported [73]. The principle is based on a rotating tool that is plunged into a billet like in reverse extrusion. The apt comparison between friction stir extrusion and friction stir back extrusion is carried out by the underlying authors [74]. A similar principle for direct recycling of chips is friction stir extrusion, where efforts are currently ongoing [75]. The authors’ view is, however, that such processes might meet challenges when upscaling, as heat generation naturally increases as size increases. To what scale the commercialization is seeking is unknown, although it should have a considerable container diameter to enable such batch processes to meet industrial capacity demands and make it applicable for scrap upcycling.

Reports also indicate efforts to make friction stir extrusion continuous, e.g., by adding something similar to screw extrusion as a feeding system [76]. In the cited work, the screw extruder part of the continuous friction stir extrusion (CFSE), in principle, resembles early work on the metal screw extruder. Although CFSE seems to operate at much higher screw rotation speeds, the process is likely governed by other factors. At the reported levels, it can be questioned how long the process can be operational before the system is overheated, especially if considering a scaled-up version.

A manufacturing principle comparable to MSE is SolidStir. This is an extrusion method that uses two plungers in conjunction with a step spiral tool to drive the material through a die, causing a friction-stir-like deformation path [77]. Its reported application towards combining graphite and aluminium into a conductive wire can be mentioned in particular [78], showcasing the potential of composite manufacturing. Similar materials have been exhibited by MSE, further outlined in Section 5.

Research on (continuous) processes applying (severe) plastic deformation seems to have gained traction in the later years. It seems to be applied both as a solution to the search for efficient routes for recycling and as a method to generate materials with new, tailored properties. Several attempts to pilot and upscale technologies to assess the commercial value are reported, a necessary measure to develop sustainable, low-energy recycling routes for metallic materials. Although all of the reviewed processes are different in nature, they all rely on friction-driven solid-state forming of metals. The authors see a potential for knowledge transfer and prosperous collaborations between research groups to further accelerate this class of emerging technologies into the commercial phase.

7. Future Development of MSE

Until now, the application of MSE has mainly focused on aluminium alloys and aluminium matrix composites with the addition of secondary metallic or ceramic constituents. The understanding of screw extrusion of other ductile materials such as titanium, copper, magnesium, or brass is scarce or non-existent [39]. Even for applications within aluminium materials, the maturity of the process is difficult to define, as it depends on the end use of the product. For recycling purposes to make a master alloy or to pre-compact difficult scrap for successive remelting, the technology readiness level (TRL) can be considered 6–7, i.e., the technology is demonstrated in a relevant environment with an upscaled pilot unit [46]. For other applications, more understanding and knowledge need to be developed. It is self-explanatory that high-quality end uses (where the extruded part has a design criteria) are more challenging to manufacture than a remelt ingot. Semi-finished products for further manufacturing steps, e.g., by wire drawing or forging, are between these manufacturing extrema. Here, geometrical inhomogeneities could be remedied at a later process step. This section will not delve into various applications but will try to explore some of the more general knowledge gaps that could lead to improvements either in the near future or in a longer time frame.

As indicated in the above paragraph, one can generally assume that different applications need different technological approaches. Design principles of the screw and other parts of the MSE system might be altered when high deformation is needed compared to high throughput, when one has ductile input materials compared to a more brittle constituent, when extrudate geometry is important, and so on. The machine size and screw geometry are other design factors related to capacity and specific applications. As an example, the screw channel to screw stem ratio can imply major differences related to extrusion capacity, total deformation, and consequently resulting material properties. Further, when considering the commercialization aspect, the size of the machine is also a factor: when would one large machine be more beneficial than two or several smaller machines in parallel? A key driver for the latter would be process control automatization and process flexibility, which in itself is a topic for a later publication. However, there is still a need to increase a more generic understanding of the effect of both design alterations and process variations on the physical properties of the resulting materials. Even though some work has been done, this has been limited to a narrow range of process parameter variations applied to a specific material on a laboratory-scale metal screw extruder [22].

The metallurgical understanding of aluminium processed by MSE has been well documented, as shown in Section 5. However, the understanding of the relationship between the process, microstructure, and properties of MSE materials can be further explored. Of primary interest is the effect of secondary phases in the final profile. As the feedstock to screw extrusion is fragmented material, there will always be a merging of individual interfaces to a solid bulk. In the case of most commercial metals, these interfaces will be decorated by oxides. Further investigations of the effect of oxides on physical and electrochemical properties are needed. Examples include mechanical properties, such as fatigue and fracture mechanisms, and functional properties, such as electrical conductivity, recrystallization kinetics, and corrosion resistance.

The accumulation of unwanted impurities may deteriorate the effectiveness of aluminium recycling [47], such as motor engine blocks and household aluminium waste [79]. Utilizing MSE to refine secondary phases might open up recycling to highly polluted waste not commercially relevant to dilute or conventional recycling. The inherent severe plastic deformation of MSE can thus generate a new class of materials like severe plastic deformed high-entropy alloys [80]. In a similar way, in-baked oxides can be seen as something contributing positively to physical properties, i.e., as grain pinning particles or strengthening nanoparticles.

Similarly, optimum design and sets of process parameters need to be developed to achieve the geometrical tolerances of an MSE profile. As the billet in MSE is pressed forward by oncoming material, the volumetric flow velocity depends on the amount of material being pushed into the billet. This has been observed to vary even at constant running parameters. Increased understanding of how pressure is generated and the effect from different design approaches would significantly improve the operability of MSE. A valuable tool to work on such optimization is numerical modeling, e.g., finite element modeling or computational fluid dynamics. With the development of more extrusion data and precision sensors from the extrusion machine, artificial intelligence may be used to relate process parameters to output results such as extrusion speed and pressure generation.

Initial work has been performed for MSE based on commercial finite element method tools for extrusion and forming [unpublished]. However, a basic understanding and mathematical base for the complex flow in MSE is currently lacking. A rich literature catalogue can be found within numerical modeling of polymer screw extrusion [13], which could serve as a starting point for MSE. However, the highly different boundary conditions related to the viscosity and friction of the fed material may imply the need for a new set of boundary conditions and mathematical solutions. Furthermore, numerical assessments of local heat generation, local pressure conditions, and material interactions between the screw, container, and material are all paramount for tailoring new extrusion designs for different applications. Predictive models can be developed for process optimization depending on materials, applications, and thermophysical conditions.

Metal screw extrusion is now on the verge of leaping from the pilot stage to full commercialization for selected applications. The inherent properties of solid-state severe plastic deformation with a considerable shear component should open up new opportunities in low-emission materials processing and the development of new classes of materials. The scientific community has a special responsibility to push the boundaries for new applications and materials in MSE while simultaneously further developing the generic understanding of the MSE process. The full utilization of the MSE technology is only within reach with an interdisciplinary research approach combining materials science, mechanical engineering, numerical modeling, physics, and mathematics.

8. Concluding Remarks

The state of the art of metal screw extrusion has been summarized in this review. MSE is a continuous extrusion principle utilizing a rotating screw as a pressure-generating device. The material is always in solid state, with a complex flow and high strain. MSE is projected to become useful for applications that include recycling of post-consumer scrap, profile extrusion, and composite manufacturing. MSE possess the properties of rigorous mixing and a high degree of deformation. This is utilized to efficiently break surface oxides and disperse particles to refine the final microstructure. Although significant academic work has been devoted to MSE over the years, the process is not fully understood. Of special importance is a deeper knowledge of the connection between process parameters to material flow and the final microstructure of the extruded profile. A physics–mathematical understanding and representation of the complex flow during extrusion is lacking, which is an essential part to be solved to numerically model the process with fair accuracy. The industrial train of MSE will soon leave the platform, and the process is projected to experience significant growth in the years to come.