metals
Review
Review of Metal Screw Extrusion: State of the Art and Beyond
Geir Kvam-Langelandsvik 1, * , Kristian Grøtta Skorpen 1 , Jens Christofer Werenskiold 2
and Hans Jørgen Roven 2,†
1
2
*
†
SINTEF Industry, Richard Birkelands veg 2B, 7034 Trondheim, Norway; kristian.g.skorpen@sintef.no
Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU),
7034 Trondheim, Norway; jens.c.werenskiold@ntnu.no (J.C.W.)
Correspondence: geir.langelandsvik@sintef.no
Deceased author.
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.
Keywords: extrusion; screw extrusion; severe plastic deformation; forming; recycling; SPD
Citation: Kvam-Langelandsvik, G.;
Skorpen, K.G.; Werenskiold, J.C.;
Roven, H.J. Review of Metal Screw
Extrusion: State of the Art and
Beyond. Metals 2024, 14, 1117.
https://doi.org/10.3390/met14101117
Academic Editors: José Valdemar
Fernandes and Ulrich Prahl
Received: 22 July 2024
Revised: 11 September 2024
Accepted: 27 September 2024
Published: 1 October 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
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.
Metals 2024, 14, 1117. https://doi.org/10.3390/met14101117
https://www.mdpi.com/journal/metals
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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.
Figure 1. Conceptual sketch of a metal screw extrusion setup. Material feeding of granules through a
funnel are shown, and extrusion of a bar profile at the right-hand side.
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
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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
Metals 2024, 14, 1117
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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.
Figure 2. Material transport zones in metal screw extrusion. Reproduced with permission from
Widerøe and Welo, Journal of Materials Processing Technology, published by Elsevier, 2013 [9].
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
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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.
Figure 3. (a) Material flow channels in the compacted extrusion billet. (b) The rise of backflow in the
wake of the passive flight. Reproduced with permission from Widerøe and Welo, Journal of Materials
Processing Technology, published by Elsevier, 2013 [9].
Figure 4. Using contrast material to reveal the flow lines in metal screw extrusion. The different
flow zones are indicated. Reproduced with permission from Widerøe and Welo, Journal of Materials
Processing Technology, published by Elsevier, 2013 [9].
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.
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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 addi-
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tion, 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].
Figure 5. Map of equivalent strain in MSE as a function of material feed rate and screw rotation
speed [14].
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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.
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
Extrusion
6082
6005
Screw
Ram
6060
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
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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.
Figure 6. Microstructure of (a) MSE and (b) ram-extruded 6005.40 aluminium alloys after solution
heat treatment at 550 °C for 10 min. Reproduced with permission from Kristiansen, published by
NTNU Open, 2020 [19].
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
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was directly metal screw extruded into an Ø1.2 mm 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.
Figure 7. Flow patterns in metal screw extruded 6xxx bars in cross section. Note the partly recrystallized structure in discrete bands. The final microstructure is highly dependent on operational
conditions during extrusion. Reproduced with permission from Skorpen, published by NTNU Open,
2011 [22].
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 α (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-
Metals 2024, 14, 1117
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susceptible Al3 Mg2 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 Al3 Mg2 phase surrounded by γ-Mg17 Al12 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.
Figure 8. (a) Refined particle structure in a 3xx alloy engine block material recycled by MSE. (b) Original cast microstructure with Chinese script particles. Note the different scale bars of the micrographs.
Reproduced with permission from Amundsen, published by NTNU Open, 2018 [26].
Another application where MSE has potential is towards aluminium mixed with microor 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 Al2 O3
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.
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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.
Figure 9. Flow patterns in metal screw extruded Al–Al2 O3 composites in cross section. The flow
of non-metallic particles is highly dependent upon processing conditions. (a,b,g,h) 0.5 µm Al2 O3 ;
(c,d,i,j) 5 µm Al2 O3 ; (e,f,k,l) 50 µm Al2 O3 . Reproduced with permission from Edvardsen, published
by NTNU Open, 2021 [38].
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
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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 highvoltage 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 mm circular profiles are shown in Figure 10.
Figure 10. Potential flaws in metal screw extruded Ø10 mm bars due to contamination of the
feedstock (a) blistering and (b) sharkskin. Reproduced with permission from Ebbesen and Amundsen,
published by NTNU Open, 2016 and 2021 [26,51].
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
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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.
Figure 11. Evaporation of volatile substances on aluminium turnings as a function of temperature.
Copyright 2010 The Minerals, Metals and Materials Society. Used with permission from [53].
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
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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.
Figure 12. TEM dark field macrograph of metal screw extruded Al-10%Mg alloy. (a) Grain boundary
oxide array. (b) Intragranular oxides. Reproduced with permission from Skorpen, published by
NTNU Open, 2018 [54].
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 [60–63]. 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,
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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.
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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
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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
Metals 2024, 14, 1117
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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.
Author Contributions: Conceptualization, G.K.-L., K.G.S., J.C.W. and H.J.R.; validation, G.K.-L.,
K.G.S. and J.C.W.; investigation, G.K.-L. and K.G.S.; visualization, K.G.S.; writing—original draft
preparation, G.K.-L., K.G.S. and J.C.W.; writing—review and editing, G.K.-L. and K.G.S.; supervision,
H.J.R.; funding acquisition, H.J.R. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was supported by the Research Council of Norway through grant number 309584.
Acknowledgments: This publication was funded by SFI PhysMet (Centre for Research-based Innovation, 309584). The authors gratefully acknowledge financial support from the Research Council of
Norway and the partners of the SFI PhysMet.
Conflicts of Interest: Jens C. Werenskiold reports a relationship with Nuvosil AS that includes the
following: consultancy, patent applications, and equity and stocks. Jens C. Werenskiold has a patent
#WO2008/063076, EP 2086 697 B1, issued to Norsk Hydro ASA. Kristian Grøtta Skorpen has a patent,
NO347874 B1, WO2024061854 A1, issued to Norsk Hydro ASA. The other authors declare that they
have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
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