Severe plastic deformation (SPD) processes for metals CIRP Annals

Transcription

CIRP Annals - Manufacturing Technology 57 (2008) 716–735
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CIRP Annals - Manufacturing Technology
journal homepage: http://ees.elsevier.com/cirp/default.asp
Severe plastic deformation (SPD) processes for metals
A. Azushima (1)a,*, R. Kopp (1)b, A. Korhonen (1)c, D.Y. Yang (1)d, F. Micari (1)e, G.D. Lahoti (1)f,
P. Groche (2)g, J. Yanagimoto (2)h, N. Tsuji i, A. Rosochowski j, A. Yanagida a
a
Department of Mechanical Engineering, Graduate School of Engineering, Yokohama National University, Yokohama, Japan
Institute of Metal Forming, RWTH Aachen University, Aachen, Germany
Department of Materials Science and Engineering, Helsinki University of Technology, Espoo, Finland
d
Department of Mechanical Engineering, KAIST, Deajeon, Republic of Korea
e
Department of Manufacturing and Management Engineering, University of Palermo, Palermo, Italy
f
Timken Research, The Timken Company, Canton, OH, USA
g
Institute for Production Engineering and Forming Machines, University of Technology, Darmstadt, Germany
h
Institute of industrial Science, The University of Tokyo, Tokyo, Japan
i
Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, Osaka, Japan
j
Department of Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, United Kingdom
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Forming
Metal
Strain
Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very
large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal. The
objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by
using high strength metal for the safety and reliability of micro-parts and for environmental harmony. In
this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative rollbonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals
processed by the SPD processes are shown. Moreover, the combined processes developed recently are
also explained. Finally, the applications of the ultra-fine grained (UFG) metals are discussed.
ß 2008 CIRP.
1. Introduction
Processes with severe plastic deformation (SPD) may be defined
as metal forming processes in which an ultra-large plastic strain is
introduced into a bulk metal in order to create ultra-fine grained
metals [1–7]. The main objective of a SPD process is to produce
high strength and lightweight parts with environmental harmony.
In the conventional metal forming processes such as rolling,
forging and extrusion, the imposed plastic strain is generally less
than about 2.0. When multi-pass rolling, drawing and extrusion
are carried out up to a plastic strain of greater than 2.0, the
thickness and the diameter become very thin and are not suitable
to be used for structural parts. In order to impose an extremely
large strain on the bulk metal without changing the shape, many
SPD processes have been developed.
Various SPD processes such as equal channel angular pressing
(ECAP) [8–11], accumulative roll-bonding (ARB) [12–14], high
pressure torsion (HPT) [15,16], repetitive corrugation and straightening (RCS) [17], cyclic extrusion compression (CEC) [18], torsion
extrusion [19], severe torsion straining (STS) [20], cyclic closed-die
forging (CCDF) [21], super short multi-pass rolling (SSMR) [22]
have been developed.
The major SPD processes are summarized in Table 1 with
schematic configurations and the attainable plastic strain. ECAP,
ARB and HPT processes are well-investigated for producing ultra-
* Corresponding author.
0007-8506/$ – see front matter ß 2008 CIRP.
doi:10.1016/j.cirp.2008.09.005
fine grained metals. It is known that the metals produced by these
processes have very small average grain sizes of less than 1 mm,
with grain boundaries of mostly high angle mis-orientation.
The ultra-fine grained metals created by the SPD processes
exhibit high strength [23–25], and thus they may be used as ultrahigh strength metals with environmental harmony. The yield
stress of polycrystalline metals is related to the grain diameter d by
the following Hall–Petch equation:
s Y ¼ s 0 þ Ad1=2
(1)
where s0 is the friction stress and A is a constant.
Eq. (1) means that the yield stress increases with decreasing
square root of the grain size. The decrease of grain size leads to a
higher tensile strength without reducing the toughness, which
differs from other strengthening methods such as heat treatment.
The relationship between proof stress and grain size of pure
iron is shown in Fig. 1 [6]. The proof stress changes inversely with
the square root of the grain size, following the Hall–Petch
relationship. It is seen that the proof stress of the ultra-fine
grained irons, with sub-micrometer grains, is five times greater
than commercially pure iron. Thus, the conventional structural
metals with ultra-fine grains are lighter due to their high strength.
Since pure iron does not contain harmful elements, it is in harmony
with a clean environment. Moreover, the improvements of the
superplasticity, corrosion and fatigue properties of metals
processed by SPD are expected. On the other hand, the ultra-fine
grained metals are available only for micro-parts [26,27].
A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735
717
Table 1
Summary of major SPD processes
Process name
Schematic representation
Equivalent plastic strain
Equal channel angular extrusion (ECAE) (Segal, 1977)
e ¼ n p2ffiffi3 cotð’Þ
High-pressure torsion (HPT) (Valiev et al., 1989)
e ¼ gpðrÞffiffi3 , g ðrÞ ¼ n 2pt r
Accumulative roll-bonding (ARB) (Saito, Tsuji, Utsunomiya, Sakai, 1998)
e ¼ n p2ffiffi3 ln
In Fig. 2 [28], the mechanical properties of a wire specimen
made by SPD is plotted against the ratio of wire diameter D to the
grain size d, D/d. The proof stress decreases with decreasing D/d
when D/d is less than 100. In particular, when D/d is less than 5, the
proof stress decreases abruptly with decreasing D/d. From these
observations, the ratio of D/d must be greater than 100 in order to
guarantee the safety and the reliability of metals for micro-parts.
This paper reviews the severe plastic deformation processes
to create metals with ultra-fine grains. In the following, the
fabrications of the SPD processes are shown in Section 2. Then, the
Fig. 1. Relationship between proof stress and grain size of pure iron [6].
Fig. 2. Material behavior during forming processes of micro-parts of a wire
specimen with diameter to grain size D/d [28].
t0
t
properties of metals processed by SPD processes are shown
in Section 3, the combined processes developed recently are
explained in Section 4, and the applications of the ultra-fine
grained metals are discussed in Section 5.
2. SPD processes
2.1. Equal channel angular press (ECAP) process
2.1.1. Conventional ECAP processes
Fig. 3 shows the schematic representation of side extrusion
processes, which are a kind of double axis extrusion or side
extrusion [29]. Fig. 3(d and e) indicates the process in which pure
shear deformation can be repeatedly imposed on materials so that
an intense plastic strain is produced with the materials without
any change in the cross-sectional dimensions of the workpiece.
These processes are named as ECAE (Equal channel angular
extrusion) or ECAP.
Segal [8,30] proposed this process in 1977 in order to create an
ultra-fine grained material. Although ECAP is generally applied to
solid metals, it may also be used for consolidation of metallic
powder. Kudo and coworkers [31] employed repetitive side
extrusion with back pressure to consolidate a pure aluminum
powder. In the 1990s, developments of ultra-fine grained materials
were carried out with this method by Valiev et al. [9,10,32], Horita
and coworkers [33–45] and Azushima et al. [46–48] and others
[49–51].
The schematic representation of the ECAE process is shown in
Fig. 4. The specimen is side extruded through the shear
deformation zone with the dead zone in the outer corner of the
channel. When the workpiece is side extruded through the
channel, the total strain is
1
f c
f c
þ c cosec
þ
(2)
e ¼ pffiffiffi 2cot þ
2 2
2 2
3
Fig. 3. Schematic illustration of side extrusion process, which are a kind of double
axis extrusion or side extrusion [29].
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Fig. 4. Schematic representation of ECAE process.
where F is the angle of intersection of two channels and C is the
angle subtended by the arc of curvature at the point of intersection.
When F = 908 and C = 08, the total strain from the above equation
is e = 1.15. After n passes, it becomes n e.
Fig. 5 shows the fundamental process of metal flow during ECAP
[6]. The channel is bent through an angle equal to 908 and the
specimen is inserted within the channel and it can be pressed
through the die using a punch. There are four basic processing
routes in ECAP. In route A, the specimen is pressed without
rotation, in route BA the specimen is rotated by 908 in an alternate
direction between consecutive passes, in route BC the specimen is
rotated 908 counterclockwise between each pass, and in route C the
specimen is rotated by 1808 between passes.
From these macroscopic distortions shown in Fig. 5, the
influence of the processing route on the development of an
ultra-fine grained microstructure can be considered [33,36]. Horita
and coworkers [42] reported that the ultra-fine grained microstructure of pure aluminum after 10 passes in route A was the same
as that after 4 passes in route BC.
2.1.2. Developed ECAP processes
Azushima et al. [46–48] proposed the repetitive side extrusion
process with back pressure. It is a process in which a high back
pressure is applied in the process as shown in Fig. 6, in order to
produce uniform shear deformation and prevent defects in the
workpiece. The specimen is side-extruded between the punches A
and B, while the punches C and D are fixed. In this process, the total
strain becomes 1.15 after one pass. The punch A, controlled by the
function generator, moves at a constant speed and the punch B
generates a constant back pressure. Recently, ECAP die-sets have
been developed to conduct the ECAP process with a back pressure
which is controlled by computers [46,47].
Nishida et al. [52–57] developed a rotary-die ECAP shown in
Fig. 7, which consists of a die containing two channels with the
same cross-sections intersecting at the center with a right angle in
order to remove the limitation in the conventional ECAP, i.e. the
sample must be removed from the die and reinserted again in each
step. At first, the sample is inserted into the die with the plunger as
shown in Fig. 7(a), and after pressing the sample as Fig. 7(b), the die
is then rotated by 908, and the sample is pressed again as Fig. 7(c).
By using this ECAP apparatus, a sample can be pressed by the
punch A with a back pressure from the punch B, similarly to that
shown in Fig. 6. Repetitive pressings may be carried out with the
rotary ECAP. This process is equivalent to route A in Fig. 5.
In the same way as the repetitive ECAP process, a method to
reduce the repetitive number by increasing the number of channel
turns in the die [58–61] was developed. Using the two-turn
channels the strain in one pass becomes double and the
productivity of the ECAP process increases. A counterpunch for
Fig. 5. Fundamental process of metal flow during ECAP. (a) The deformation of a
cubic element on a single pass [33]. (b) Shearing characteristics for four different
processing routes [36].
Fig. 6. Schematic representation of repetitive side extrusion process with the back
pressure [46].
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Fig. 7. The ECAP process using a rotary-die: (a) initial state, (b) after one pass and (c)
after 908 die rotation [52].
Fig. 10. The principle of the con-shearing process [64].
Fig. 11. The principle of the ECAR process for use in continuous production [68].
Fig. 8. Schematic representation of 2 turn ECAP [61].
providing additional pressure as shown in Fig. 8 may become a
viable option available on common hydraulic presses. Presses with
two opposite and equally powerful rams could be used for a cyclic
process. In this process, the total strain becomes 2.3 after one pass.
These ECAP processes have been used only in the laboratory
because of their low productivity. For mass production, continuous
processing techniques must be developed. First, in order to
produce long metal bars and strips, equal channel angular drawing
(ECAD) [62] and con-shearing were developed.
The principle of ECAD is represented schematically in Fig. 9. In
the ECAD process, the material in the form of a bar is drawn
through the two channels. The rods are preformed by bending
them 1358 to fit to the die, and are drawn through the ECAP die
using as Instron tensile testing machine.
The principle of the con-shearing process is represented
schematically in Fig. 10 [63–65]. An equal-channel die with a
channel angle is located at the exit of a satellite mill. Satellite rolls
and a central roll are used as feed rolls. All the rolls are driven at an
equal peripheral speed to generate large extrusion forces and the
strip is extruded through the die continuously. This process uses
friction between rolls to push the workpiece through an ECAP die.
In this process, the shear deformation is given to the strip
continuously, and the total strain after one pass is given by Eq. (2).
Recently, equal channel angular rolling (ECAR) [66–68] and
ECAP conform [69] were developed. The principle of the ECAR
process is represented schematically in Fig. 11. The strip is fed
between two rolls and extruded to reduce the thickness of the
strip. Then, the strip flows into the outlet channel. The principle of
the ECAP conform process is represented schematically in Fig. 12.
The workpiece is driven forward by frictional forces on the three
contact interfaces with the groove. The workpiece is constrained to
the groove by the stationary constraint die, which restricts the
workpiece and forces it to turn by shear deformation similarly to
the ECAP process.
Fig. 13 shows an Al workpiece at every stage of the ECAP
conform process, from the initial feeding stock with a round crosssection to the rectangular rod after the first ECAP pass. For the
ECAR process and the ECAE conform process, the total strain after
one pass operation is given by Eq. (2), and the accumulated total
strain is n e after n passes.
The Incremental ECAP (I-ECAP) was developed by Rosochowski
et al. [70–72]. Fig. 14 explains the principle of this process; it is
based on incremental feeding of the billet by a distance ‘‘b’’
and using a reciprocating die ‘‘C’’ whose movement is synchronized with feeding. This enables feeding to take place during
the withdrawal phase of die ‘‘C’’. When the billet stops at a predetermined position, die ‘‘C’’ approaches it and deforms a small
Fig. 9. Schematic of the equal channel angular drawing process (ECAD) [62].
Fig. 12. Schematic illustration of the ECAP– Conform process [69].
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Table 2
Summarizes the geometrical changes of the specimen during the ARB process
where roll-bonded by 50% reduction per cycle [81]
Number of Cycles, n
Number of layers, m
Total reduction, r (%)
Equivalent strain, e
Fig. 13. Al workpiece undergoing processing by ECAP–Conform: the arrow marks
the transition to a rectangular cross-section [69].
volume of the billet. The mode of deformation is that of simple
shear and, provided the feeding stroke is not excessive,
consecutive shear zones overlap resulting in a uniform strain
distribution along the billet. Separation of the feeding and
deformation stages reduces or eliminates friction during feeding;
this enables processing of infinite billets, both bars and plates/
sheets.
On the other hand, the friction-reduced ECAP processes were
developed in order to produce long bulk bars with square crosssections [73,74]. The principle of this process is schematically
represented in Fig. 15 [73]. By moving the tool, the friction forces
over the three contacting interfaces become zero and the extrusion
load decreases.
The ECAP process may be used for the consolidation of metallic
powder [31,75–79]. An aluminum powder and a steel powder at
room temperature was pressed using the ECAP facility as shown in
Fig. 4.
2
4
75
1.60
3
8
87.5
2.40
5
32
96.9
4.00
10
1024
99.9
8.00
[81,82]. Stacking of sheets and conventional roll-bonding are
repeated in the process. First, a strip is neatly placed on top of
another strip. The interfaces of the two strips are surface-treated in
advance in order to enhance the bonding strength. The two layers
are joined together by rolling, as in the conventional roll-bonding
process. Then, the length of the rolled material is sectioned into
two halves. The sectioned strips are again surface-treated, stacked
and roll-bonded. These procedures can be repeated limitlessly in
principle, so that very large plastic strain can be applied to the
material.
The strain after n cycles of the ARB process can be expressed as,
e¼
Fig. 14. Schematic representation of I-ECAP [70].
1
2
50
0.80
pffiffiffi
3
t
1
lnðrÞ; r ¼ 1 ¼ 1 n
2
t0
2
(3)
where t0 is the initial thickness of the stacked sheets, t the
thickness after roll-bonding and r the reduction in thickness per
cycle. Table 2 summarizes the geometrical changes of the
specimen thick sheets are stacked and roll-bonded by a 50%
reduction per cycle. The number of the initial sheets included in the
specimen processed by n cycles of ARB becomes 2n. After 10 cycles
of the ARB process, the number of layers becomes 1024 so that the
mean thickness of the initial sheet is smaller than 1 mm.
Optical micrographs of the ARB processed IF steel are shown in
Fig. 17. In the material processed by two cycles ARB (Fig. 17(c)), the
interface introduced in the second cycle is seen clearly. However, it
is difficult to find the interface of the first pass at a quarter of the
thickness. After five cycles, the whole thickness is covered by very
thin and elongated grains. This process has been used by many
researchers in order to create ultra-fine grained metals [84–88].
2.3. High Pressure Torsion (HPT) Process
The HPT process was first investigated by Bridgman [89]. In his
experiments, attention was not paid to the microstructure change
taking place in severely deformed metals. Another implementation
of HPT was carried out by Erbel [90]. The specimen was a short ring
with conical faces whose virtual extensions met at the axis of the
apparatus as shown in Fig. 18. The conical matching faces of the
2.2. Accumulative roll-bonding (ARB) process
The ARB process was first developed by Saito et al. [80–83]. The
principle of the ARB process is represented systematically in Fig. 16
Fig. 15. The principle of friction-reduced ECAP processes [73].
Fig. 16. Diagrammatic representation of the accumulative roll-bonding (ARB)
process [81].
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Fig. 17. Longitudinal cross-section of initial and ARB processed IF steel strips [82].
Fig. 20. Schematic illustration of the bulk-HPT process [94].
Fig. 18. Schematic diagram of ring tension device and dimensions of ring
specimens [90].
punches have radial teeth to facilitate the application of torque.
The ring specimens were constrained from all directions which
created a condition closer to hydrostatic pressure.
Recently, Valiev et al. conducted the HPT process using devices
under high pressure as shown in Fig. 19 [15,16,89,91–104]. The
design is a further development of the Bridgman anvil type device.
In this device, a very thin disk is compressed in a closed die by a
very high pressure. The torque is provided by the punch with
contact friction at the interface between the punch and disk. The
strain in torsion is given by
g ðrÞ ¼
2pnr
l
(4)
where r is the distance from the axis of the disk sample, n the number
of rotation and l the thickness of the sample. The equivalent strain
according to the von Mises yield criterion is given by
g ðrÞ
eðrÞ ¼ pffiffiffi ;
3
(5)
This method has the disadvantage that it utilizes specimens in
the form of relatively small discs and is not available for the
production of large bulk materials. Another disadvantage is that
the microstructures produced are dependent on the applied
pressure and the location within the disc. In order to solve the
problem, Horita and coworkers developed an HPT process for use
of the bulk sample as shown in Fig. 20 [94]. This process is
designated as Bulk-HPT for comparisons with conventional DiskHPT [95–104].
The severe plastic torsion straining (SPTS) process can be used
for the consolidation powders using a similar apparatus as shown
in Fig. 19 [105–107]. By using this process at room temperature,
the disk type samples with a high density close to 100% were
developed. The SPTS consolidation of powders is an effective
technique for fabricating metal–ceramic nano-composites with a
high density, ultra-fine grain size and high strength.
2.4. Other processes
The principle of the cyclic extrusion compression (CEC) process
developed by Korbel et al. is represented schematically in Fig. 21
[18,108–111]. In the CEC process, a sample is contained within a
chamber and then extruded repeatedly backwards and forwards.
This process was invented to allow arbitrarily large strain
deformation of a sample with preservation of the original sample
shape after n passes. The accumulated equivalent strain is
approximately given by
D
d
e ¼ 4nln
where D is the chamber diameter, d the channel diameter and n the
number of deformation cycles. Since the billet in the CEC process is
compressed from the both ends, a high hydrostatic pressure is
imposed. The extrusion–compression load becomes high so that
the special pre-stressed tools are required, otherwise the tool life
will be short. This process is better suited for processing soft
material such as aluminum alloys. However, the strain introduced
in the forward extrusion may be cancelled by the strain introduced
on the backward extrusion.
The principle of the cyclic closed-die forging (CCDF) process
developed by Ghosh et al. is represented schematically in Fig. 22
[21,112,113]. A billet is first compressed in the vertical direction
and then in the horizontal direction. The equivalent strain per
operation is given by
e¼2
Fig. 19. Schematic illustration of the thin disc-HPT process.
(6)
lnðH=WÞ
pffiffiffi
3
(7)
where W is the width of specimen and H the height of specimen.
The strain distribution is not uniform after strain accumulation.
The principle of the repetitive corrugated and straightening
(RCS) process developed by Huang et al. is represented schema-
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Fig. 24. Principle of linear flow splitting [114].
Fig. 21. Schematic of cyclic extrusion compression (CEC).
Fig. 22. Schematic of cyclic closed-die forging (CCDF).
Fig. 25. Principle of spin extrusion [115].
tically in Fig. 23 [17]. The technique consists of bending a straight
billet with corrugated tools and then restoring the straight shape of
the billet with flat tools. The equivalent strain per one operation is
given by
of this approach is the opportunity to create a changed structure in
the surface region, keeping the lower region or core unchanged.
The incremental forming method of the spin extrusion as shown in
Fig. 25 is used to create cup shaped or tube shaped parts from solid
billets. The hollow shape is created by the concurrent partial
pressure of three rolls on the surface of the workpiece and the
pressure of the forming mandrel acting in the axial direction. The
material flows axially and a cup wall is created between the
forming tools [116].
The principle of the severe torsion straining (STS) process
developed by Nakamura et al. is represented schematically in
Fig. 26 [20]. The process consists of producing a locally heated zone
and creating torsion strain in the zone by rotating one end with the
other. The rod is moved along the longitudinal axis while creating
the local straining. Therefore, a severe plastic strain is produced
continuously throughout the rod. In order to create the torsion
strain efficiently, the locally heated zone should be narrow and the
rotation of the rod should be fast with respect to the moving speed
of the rod. Moreover, a modification is made for the cooling system
so that the heated zone is more localized to create torsion strain.
The principle of the torsion extrusion process developed by
Mizunuma et al. is represented schematically in Fig. 27. This
process is characterized by rotation of a die or a container during
an extrusion process for introducing a very large strain in to the
metal. As high hydrostatic pressure involved in the extrusion raises
the ductility of the metals, a very large torsion straining can be
introduced to the workpiece. The mean value of representative
e ¼ 4ln
½ðr þ tÞ=ðr þ 0:5tÞ
pffiffiffi
3
(8)
where t is the thickness of sample and r is the curvature of bent
zone. By repeating these processes in a cyclic manner, high strains
can be introduced in the workpiece.
Linear flow splitting developed by Groche et al. is another
possibility to obtain ultra-fine grained metal [114]. The principle of
this process is shown in Fig. 24. A sheet metal is compressed
between the splitting roll and the supporting rolls. Under this state
of stress two flanges are formed into the gap between the splitting
and the supporting rolls. The material flow is mainly associated by
a surface enlargement of the band edge. Several hundred percent of
plastic strain occur. As a consequence, the outer surface areas of
the flanges consist of ultra-fine grained metal. The properties of the
metal in this state can be used for an increase of load bearing
capability, e.g. bearings for rollers.
The applicability of incremental bulk forming processes with
high deformation for grain refinement in the sub-micrometer
range was investigated by Neugebauer et al. [115]. A specific aspect
Fig. 23. Principle of repetitive corrugating and straightening [17].
Fig. 26. Principle of the severe torsion straining (STS) process [20].
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Fig. 29. TEM Micrograph of ultra-low carbon steel after ECAPed 10 passes by route
A [46].
Fig. 27. Principle of the torsion extrusion process [19].
strain on a cross-section of a column can be calculated as below.
4pRN
e ¼ pffiffiffi
3 3H
(9)
where R is radius of column, H is the height of the column, N is the
number of rotation.
Fig. 28 [19] shows a magnified view of the longitudinal section
of the etched aluminum specimen after the torsion extrusion
process, compared with that of the conventional extrusion. The
torsion extruded part of the specimen is clearly observed to be
more severely strained than that of the conventional extrusion.
3. Properties of metals processed by SPD
The SPD-processed metals normally have ultra-fine grained
structures that cannot be obtained through conventional thermomechanical processing. As a result, the SPD metals exhibit unique
and excellent properties such as high strength, compared with the
conventional materials having a coarse grain size of over several
tens of micrometers.
In the optical microstructure of metals over 5 passes of the ECAP
processes, it is observed that the strong filamentary microstructure
is developed with an increasing number of passes. In these
conditions, observed microstructure must use a TEM analyzer.
From the TEM microstructure, it is confirmed that many metals
with an ultra-fine grain size (under 1 mm) are developed by ECAP
processes.
The ultra-fine grains of sub-micron size were created by ECAP
processes in many of the metals and the grain size of the Al–4%Cu–
0.5%Zr alloy became about 200 nm by ECAP with a plastic strain of
7 at 160 8C [2]. Aluminum and aluminum alloys with a sub-micron
grain size were developed by ECAP processes [45]. For the ultra-
Fig. 28. Magnified view of a longitudinal section of the etched aluminum specimen
[19].
low carbon steel an ultra-fine grain size with a major axis length of
0.5 mm and a minor axis length of 0.2 mm was developed by 10
passes of repetitive side extrusion at room temperature as shown
in Fig. 29 [46]. At the same time, they showed the relationship
between the area fraction and the mis-orientation angle by the
EBSP analysis [46]. They reported that most of the boundaries are
high-angle grains, so that the processed steel is considered to be a
kind of ultra-fine grain structured metal.
In the ARB processes, it was noted that the evolution of
microstructure and the increase in mis-orientation of boundaries
were much faster than those when using conventional rolling
[117,118]. A typical TEM micrograph of the ultra-fine structure in
the interstitial free (IF) steel ARB processed by 7 cycles at 500 8C is
shown in Fig. 30. From the crystallographic analysis by Kikuchiline analysis, they reported that most of the boundaries were at a
high angle.
From these TEM microstructures, it is expected that the
hardness and the tensile strength of metals with ultra-fine grains
become higher. A number of studies have been conducted on the
strength and ductility of various kinds of metallic materials
processed by various SPD processes. The SPD-processed materials
generally have very high strength compared with conventional
metals. Fig. 31 illustrates a general tendency of the change in
strength and ductility during SPD. The strength of the materials
continuously increases with increasing the applied strain and then
gradually saturates. On the other hand, the ductility drops greatly
with a relatively small strain, and then keeps a nearly constant
value or slightly decreases as the strain increases.
Fig. 32 shows the relationship between the tensile strength,
elongation and number of passes in ECAP for Armco steel [30]. The
tensile strength increases with increasing pass number. The tensile
strength is increased from 300 to 750 MPa after one pass. The
tensile strength is increased by a factor of 2 after one pass in
comparison with the specimen before the ECAP process, and
increases with increasing pass number up to 8 passes. The tensile
strength is higher than 800 MPa after 8 passes. On the other hand,
the elongation decreases from 20% for the specimen before the
ECAP process to several percents after 8 passes.
Fig. 30. TEM microstructure of the IF steel ARB processed by 7 cycles (e = 5.6) at
500˚ C [118].
724
Fig. 31. Illustration showing the general tendency of the change in strength and
ductility during SPD.
Horita et al. [33–35] reported the same results for the
aluminum alloys, and Azushima et al. [46–48,119–121] and Shin
et al. [122–129] also reported for the steels. In particular, Aoki and
Azushima [121] reported the relationship between nominal stress
and nominal strain of specimens of ultra-low carbon steel, 0.15%C
steel, 0.25%C steel and 0.50%C steel processed by ECAP of 1, 2, 3, 5,
and 10 passes in route A at room temperature. They reported that
the as-received material exhibits a stress–strain curve that
indicates normal strain hardening, while the specimens after
ECAP do not exhibit strain hardening. The stress for each specimen
increases rapidly with increasing strain and reaches a maximum at
lower strain.
Fig. 33 shows the relationship between the tensile strength and
the pass number for the carbon steels. The tensile strength
increases with increasing number of passes of ECAP. The tensile
strength of ultra-low carbon steel after 10 passes was greater than
1000 MPa and was increased by a factor of 3 in comparison with
the as-received material. The experimental data of the specimen
Fig. 32. Relationship between tensile strength, elongation and pass number of ECAP
for Armco steel [30].
Fig. 33. Relationship between tensile strength and pass number for the carbon
steels based on ref. [121].
Fig. 34. Relationship between total elongation and pass number for carbon steels
based on ref. [121].
after 10 passes are plotted in the Hall–Petch relationship of the
yield stress against the root grain size as shown in Fig. 2. In this
figure, the results for these specimens show good agreement with
the standard Hall–Petch relationship of iron obtained by Takaki
and coworkers [130].
Fig. 34 shows the relationship between total elongation and the
pass number for the carbon steels. For the low carbon steel, the
elongation decreases to 20% after 3 passes, and for the other carbon
steel, it decreases to 10% after 3 passes.
Moreover, Shin and coworkers [128] also reported the stress–
strain curve of low carbon steel processed by ECAP at elevated
temperatures as shown in Fig. 35. The tensile strength decreases
with increasing processing temperature of ECAP and the total
elongation increases.
In the ARB process, Saito, Tsuji et al. [131–142] reported the
mechanical properties of many metals processed by ARB. The
relationship between the tensile strength, elongation and cycles of
a commercially pure aluminum (JIS-1100) SPD processed by the
ARB process is shown in Fig. 36 [132]. The tensile strength of the
1100-Al greatly increases to 185 MPa while the total elongation
drops down to 13% by the 1 ARB cycle (equivalent strain of 0.8). As
the number of the ARB cycles (strain) further increases, the flow
stress continuously increases and reaches 340 MPa, which is four
times higher than that of the starting material having a
conventionally recrystallized microstructure.
On the other hand, the elongation of the 1100-Al does not
change as much after the second ARB cycle. As was illustrated in
Fig. 29, this is the typical change in the mechanical properties
during SPD, which seems to occur regardless of the kind of SPD
process and material.
The decrease in ductility is a general feature of strain-hardened
metallic materials. Thus, it is not surprising that the SPD-processed
materials, i.e., ultra-high strained materials show limited tensile
ductility. It can be expected that the ductility can be recovered by
subsequent heat treatment, as is the case with deformed and
annealed materials. However, this has proven to be not so simple.
Fig. 35. Stress–strain curves of the CS steel after ECA pressing at 350, 480, 540 and
600 8C [128].
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Fig. 36. Tensile strength and elongation of the 1100 commercially pure aluminum
ARB processed by various cycles at RT [131].
Fig. 38. Yield strength and UTS vs. accumulated strain for AA-6061 SPD processed
by ECAP, MAC/F and ARB at room temperature [149].
Fig. 37 shows the stress–strain curves of the 1100-Al and ultralow-carbon interstitial free steel SPD processed by the ARB and
then annealed at various temperatures for 1.8 ks [132]. In the
figures, true stress and true strain are indicated by assuming
uniform elongation. Also the mean grain size of the specimens
measured from the microstructure observations are superimposed
in the figures. The strength of the materials decreases with
increasing grain size, i.e., with increasing annealing temperature.
However, large elongation can be obtained only after the strength
decreases. In particular, the curves clearly show that the flow stress
reaches its maximum at an early stage of tensile test and is then
necked down to fracture in the UFG specimens.
The limited tensile ductility of the ultra-fine grained materials is
understood in terms of early plastic instability. As is well-known, the
plastic instability condition (i.e., necking condition in tensile test) for
strain-rate insensitive materials, for example, is expressed as
Besides the mechanical properties, the fatigue property
[120,150–154] and superplasticity property [133,155–167] were
investigated by many researchers.
s
ds
de
(10)
where s and e are true stress and strain, respectively. Ultra-grain
refinement greatly increases the strength, especially yield
strength, of the materials. When the strain-hardening rate
coincides with the flow stress, plastic instability, in other words
necking, starts in the tensile test, which demonstrates a uniform
elongation.
The mechanical properties of the metals with ultra-fine grain
processed by SPD have been investigated [143–149]. Cherukuri
et al. reported a comparison of the properties of SPD-processed AA6061 by ECAP, CCDF and ARB as shown in Fig. 38 [149].
Commercially available AA-6061 in the annealed condition was
subjected to severe plastic deformation processing by ECAP, CCDF
and ARB at room temperature to approximately the same
accumulated strain (4). From Fig. 38, it is understood that the
SPD technique used did not show much effect on the flow behavior
of AA-6061.
Fig. 37. True stress–strain curves of (a) the 1100-Al ARB processed by 6 cycles at
200 8C and then annealed at various temperatures ranging from 100 to 400 8C for
1.8 ks and (b) IF steel ARB processed by 5 cycles at 500 8C and then annealed at
various temperatures from 200 to 800 8C for 1.8 ks [132].
4. Combined process and properties
4.1. SPD process and conventional process
In order to improve the strength of the ECAP processed metals,
cold deformation can be combined with the ECAP process to
introduce crystalline defects and refine the grains. Recently, two
combined processes, the ECAP process and cold rolling, and the
ECAP process and cold extrusion were developed.
The principle of the combined process of ECAP and cold rolling
is represented schematically in Fig. 39. Azushima et al. carried out
experiments in which the specimens of ultra-low carbon steel
were processed by ECAP in route A at room temperature and then
the specimens processed by ECAP were rolled repetitively at room
temperature in order to increase the strength. Fig. 40 shows the
tensile strength after the combined process [168]. After 10 passes
of ECAP, the tensile strength of ultra-low carbon steel is 1000 MPa
and after cold rolling with a reduction in thickness of 95% it
becomes 1300 MPa.
Next, warm ECAP process was first used to refine the grain size
of commercially pure Ti billets and the billets were further
processed by repetitive cold rolling. The properties of the pure Ti
processed by the two-step method are summarized in Table 3
[169]. ECAP increased the yield and tensile even strength to 640
and 710 MPa, respectively. After a cold reduction of 35%, the yield
and tensile strengths increased to 940 and 1040 MPa which are
higher than those for the Ti–6Al–4V alloy. Further cold rolling to a
reduction of 55% resulted in even higher yield and tensile
strengths.
The principle of the combined process of ECAP and cold
extrusion is represented schematically in Fig. 41. Stolyarov et al.
[170] carried out experiments in which the billet of commercially
pure Ti were first processed by ECAP in route BC at about 400 8C and
then the billets processed by ECAP were further processed by cold
extrusion to the accumulative reduction. The properties of the pure
Fig. 39. Principle of the combined process of ECAP process and cold rolling.
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Fig. 40. Transition of tensile strength after combined process [168].
Fig. 42. Elongation to failure of the ECAP (4 passes) and ECAP (4 passes) + CR(70%)
samples as a function of initial strain rate at 450 8C [172].
Fig. 41. Principle of the combined process of ECAP process and conventional
extrusion.
Ti processed by warm ECAP and cold extrusion are summarized in
Table 4. After cold extrusion to 47% reduction in cross-section, the
yield and tensile strengths were increased to 910 and 930 MPa
respectively, which are higher than those of Ti–6Al–4V. Further
cold extrusion in cross-section of 75% yielded even higher yield and
tensile strengths. Next, in order to increase the strength of
aluminum alloy (AA-6101) this combined process was conducted.
The experimental results show that improved properties after cold
extrusion are heavily dependent upon the prior ECAP processing
routes.
On the other hand, in order to improve the superplastic
properties of metals processed by ECAP, cold deformation can be
combined with ECAP to refine the grains. Park et al. [171,172]
examined the superplastic properties at 450 8C of Al–Mg alloy
(A5154) processed by ECAP to 4 passes at 200 8C and cold rolling at
a reduction in thickness of 70%. The comparison of the dependence
of elongation on strain rat e between ECAP and ECAP + cold rolling
(70%) samples is shown in Fig. 42. The elongations of the
ECAP + cold rolling samples were higher than that of the ECAP
sample at all strain rates. The maximum elongation was 812% at
5 103 s1 and it was much higher than that of the eight passes
ECAPed sample (595%). For the purpose of comparison, an
appearance of the ECAP and ECAP + cold rolling (70%) samples
elongated to failure is shown in Fig. 43. Similarly, the superplastic properties of 7075 aluminum alloy processed by ECAP
of one pass and isothermal rolling at 250 8C were examined and
a the alloy processed exhibited a maximum elongation of 820%
at a temperature of 450 8C and an initial strain rate of 5.6 103 s1.
4.2. SPD process and annealing
The high strength of metal processed by SPD is obtained, but the
ductility of the metals after SPD becomes very low. In order to
improve the ductility of the metal processed by SPD, the metals
were annealed after SPD process.
Table 3
Properties of pure Ti processed by two-step [169]
Processing state
s0.2 (MPa)
su (MPa)
d (%)
Coarse grain (10 mm)
ECAP(8) a
ECAP(8) + CR(35%)
ECAP(8) + CR(55%)
ECAP(12) + CR(35%)
CR(35%) b
380
640
940
1020
920
660
460
710
1040
1050
955
670
27
14
7
6
15
16
a
b
ECAP route BC was used for all samples.
The value inside parentheses is cross-section reduction.
Table 4
Properties of pure Ti processed by warm ECAP and cold extrusion [170]
Processing state
s0.2 (MPa)
su (MPa)
d (%)
RA (%)
Coarse grain
ECAP(8) a
ECAP(8) + Cold extrusion(47%)b
ECAP(8) + Cold extrusion(75%)
Ti–6Al–4Vc
380
640
910
1020
920
460
710
930
1050
955
27
14
–
6
10
69
61
55
42
25
a
b
c
Reduction in cross-section area from cold extrusion.
From ASTM F 136-96.
Fig. 43. Appearance of (a) ECAP sample and (b) ECAP + CR(70%) sample tested up to
failure at 450 8C [172].
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Fig. 45. Tensile strength and elongation of ECA pressed low carbon steel annealed at
480 8C for various times [173].
Fig. 44. Relationship between tensile strength and hardness of specimens of ultralow carbon steel, 0.15%C, 0.25%C and 0.50%C steel after ECAP of 3 passes and then
heat treatments of annealing [121].
Aoki and Azushima [121] carried out experiments in which the
carbon steel samples were first processed by ECAP of 3 passes in
route A at room temperature and then the samples processed by
ECAP were further processed by annealing at a temperature of
600 8C and changing annealing times. Fig. 44 shows the relationship between nominal stress and nominal strain of specimens of
ultra-low carbon steel, 0.15%C steel, 0.25%C steel and 0.50%C steel
after the ECAP of 3 passes and after annealing. The tensile strengths
become lower and the total elongations increase with decreasing
tensile strength. The uniform elongations increase with decreasing
tensile strength for all carbon steel samples. For example, a 0.5%C
steel sample with a tensile strength 900 MPa and a total elongation
of over 20% is obtained.
Shin et al. [129,173,174] investigated static annealing after
warm ECAP with a view to thermal stability. The low carbon steel:
0.15C–0.25Si–1.1Mn (in wt.%) (hereafter CS steel) was used. ECAP
was carried out on the samples at 350 8C up to 4 passes by Route C,
then samples for subsequent annealing were encapsulated in a
glass tube with Ar atmosphere in order to minimize the possible
decarburization. The static annealing treatment was conducted at
480 8C up to 72 h. Stress–strain curves of the as-received, aspressed and annealed samples are shown in Fig. 45. The as-pressed
and annealed samples exhibited no strain hardening behavior. It is
of interest to note that stress–strain curves of the samples
annealed for 24 and 72 h were almost identical. This observation
implies that the sample annealed for 24 h was mechanically stable
although the microstructural examination revealed that recovery
was in progress after 24 h annealing.
In order to improve the ductility of metals processed by ARB, an
annealing process was conducted. Tsuji et al. carried out
experiments in which the aluminum and iron samples were first
processed by ARB at a warm temperature and then the samples
processed by ARB were further processed by annealing for 600 s or
1.8 ks from 200 to 800 8C. Fig. 37 shows the stress–strain curve of
commercially pure aluminum (1100-Al) and ultra-low carbon
interstitial free steel specimens processed by various annealing
conditions. The mean grain size is also indicated in this figure.
The flow stress of both metals increases with decreasing mean
grain size. Once the mean grain size becomes smaller than 1 mm,
elongation of both Al and Fe suddenly reduced, though the strength
still increased with decreasing grain size. On the other hand, as the
grain size became larger than 1 mm, typical strain-hardening was
observed and the elongation increased with increasing grain size.
Stolyarov et al. [169] carried out experiments in which the
commercially pure Ti billets were first processed by warm ECAP
and repetitive cold rolling and further the billets processed by
annealing at temperatures of 200 and 300 8C. The properties of the
pure Ti billets processed are summarized in Table 5. Annealing
pure Ti processed by SPD at temperatures below 300 8C generally
improves the ductility without decreasing the strength.
4.3. SPD process and cooling
Fig. 46 shows the strip thermo-mechanical control process
(TMCP), or combined strip fabrication process, to manufacture fine
grained plain carbon steel with a ferrite grain size of 3 mm [175].
TMCP and micro-alloying technology is widely used to manufacture precipitation-hardened high-strength steel sheets. The same
process may be used in the rolling of strip, but precipitates are not
easily controllable during rolling because strip rolling mills are
arranged in tandem to gain higher productivity and constant
quality rather than flexibility.
To manufacture fine-grained steel strips, the rolling temperature must lie just above the transformation temperature or in the
supercooled austenite state to accelerate transformation in the
run-out table. This type of combined process can be used to
manufacture plain carbon fine-grained steel sheets since the
Fig. 46. Super short interval multi-pass rolling process.
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Table 5
Properties of pure Ti processed by warm ECAP and cold rolling with subsequent annealing [169]
Processing state
s0.2 (MPa)
su (MPa)
d (%)
Coarse grain
ECAP(8) a
ECAP(8) + Cold rolling(35%)b + Annealing 200 8C, 0.5 h
ECAP(8) + Cold rolling(73%) + Annealing 300 8C, 1 h
ECAP(12) + Cold rolling(35%) + Annealing 300 8C, 0.5 h
Cold rolling(35%)
380
640
985
942
920
660
460
710
990
1037
1000
670
27
14
8
12.5
14
16
a
b
Reduction in cross-section area by cold rolling.
micro-alloying technology is difficult to be applied to this
process. The temperature of the strip is controlled throughout
the combined process to accumulate the dislocations in the grains
before accelerated transformation.
An example of such combined processes is shown in Fig. 46.
This process can be used to produce ultra-fine grained C–Si–Mn
steel with a grain size of 1 mm [175]. A strip with a width of
300 mm was successfully produced by this process. Fig. 47 shows
an example of ultra-fine grained C–Si–Mn steel obtained by hot
extrusion. Fig. 48 shows the yield strength of the steel sheet
produced by the SSMR process as a function of ferrite grain size
[176]. Some previous studies and Hall–Petch equation are
also shown in the figure as a comparison. The yield strength is
increased from 350 to over 700 MPa with in decreasing grain size
4.5–1 mm, which is in good agreement with the Hall–Petch
relationship. It is also confirmed that the uniform elongation
deceases.
Another example of the combined process for producing the
ultra-fine grained steel is warm rolling and cooling, which uses
ferrite recrystallization during warm rolling [177–183]. Torizuka
et al. [177,180–183] carried out multi-pass warm caliber rolling of
two low carbon steel (SM490) specimens with a microstructure of
ferrite and Pearlite.
The specimen of the square bar with a side width of 80 mm was
used. The warm caliber rolling schedule is summarized in Fig. 49.
The caliber rolling at 500 8C was conducted in five stages to obtain
specimens of different cumulative strains for different microstructure and mechanical properties. The cumulative reduction
and the cumulative strain at each stage of rolling are also shown in
Fig. 49.
Fig. 50 shows the relationship between nominal stress and
nominal strain of specimens subjected to different cumulative
strains. The yield and tensile strengths of the caliber rolled
specimen increase monotonically with increasing cumulative
strain. There is a reduction in the elongation to failure of the
caliber rolled specimens compared to the undeformed specimen,
but there is almost no change among the specimens with different
accumulative strains.
Fig. 47. Ultra-fine grained steels obtained by SPD process.
Fig. 49. Caliber rolling schedule with cumulative reduction and cumulative strain at
each stage [180].
Fig. 48. Yield strength as a function of ferrite grain size [176].
Fig. 50. Nominal stress–strain curves of undeformed specimen (e = 0) as well as
caliber rolled specimens to various cumulative plastic strains (e = 0.7–3.8) [180].
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Fig. 51. High strength thread articles out of SPD Ti alloy [184].
5. Applications
The properties of the metals processed by SPD exhibit high
strength, ductility and fatigue characteristics. UFG metals are used
as a structural material due to these properties. Bolts are also
manufactured with titanium alloys processed by ECAP as shown in
Fig. 51 [184] and are widely used in the automobile and aircraft
industries. Micro bolts using the UFG carbon steel processed by
cold ECAP have also been manufactured as shown in Fig. 52 [185].
Long carbon steel bars, of over several kilometers, with ultrafine grains are manufactured by the warm continuous caliber
rolling and cooling process, from which the micro bolts are
manufactured. Recently, in a Japanese National Project, sheets of
low carbon steel of 2 mm thickness with ultra-fine grains were
manufactured by the TMCP process. The deep drawing ratio of each
sheet was 1.9 and the parts were used in sheet metal forming as
shown in Fig. 53 [186].
It is well known [187,188] that superplastic forming is a highly
efficient method of processing complex shape articles. An example
of a possible practical application for nanostructured Al alloys is
shown in Fig. 54 [2]. It presents a complex shape article of ‘Piston’
type which was fabricated from the nanostructured Al1420 alloy
by superplastic forming using the high strain rate superplasticity.
In practice, despite a range of improved mechanical and
physical properties of bulk UFG metals produced by SPD, the
uptake of these materials by industry has been very slow so far.
There are several reasons for this; one is the lack of industrial
awareness of UFG metals. This is despite a large number of
academics being engaged in research on SPD and UFG metals.
Another reason is the scarcity of appropriately sized UFG samples
for industrial trials; those produced by laboratories are usually too
small because they are intended for metallurgical observations or
basic mechanical testing. Finally, it is still not clear which of the
Fig. 52. Overview and cross-section of micro bolts manufactured UFG Carbon steel
processed by cold ECAP [185].
Fig. 53. Examples of ultra-fine-grained C-Mn steel sheet forming [186].
Fig. 54. View of article of ‘‘Piston’’ type fabricated from nanostructured Al1420 [2].
numerous laboratory-based SPD methods will emerge as the most
appropriate for industrial implementation.
As a result, potential producers of UFG metals hesitate to
commit themselves to any particular method. They are also
concerned about the commercial viability of UFG metals, which
depends on the demand from potential markets and the cost of
production. Both of them are difficult to assess because of the low
availability of UFG metals and uncertainty regarding the SPD
technology. There is also a lack of knowledge regarding post-SPD
processing or shaping of UFG metals.
Nevertheless, there are some applications which, with a high
degree of probability, will be leading the introduction of UFG
metals into commercial markets. Initially, those applications are
likely to be in the niche markets producing low volume specialty
products (e.g. sputtering targets). The next step will be the medium
volume markets with the emphasis put on product’s performance
rather than price (medical implants, defense applications, aerospace components, sports equipment). Eventually, the mass
production of components may be undertaken by the automotive
and construction industries.
With the exception of sputtering targets, the examples
presented below refer to potential applications rather than the
current ones. Despite the focus of this paper on SPD-produced UFG
metals, applications using UFG consolidated powders and nanostructured electrodeposited metals will also be considered as these
are indicative of what can be achieved with all types of UFG metals.
The first commercial application of bulk UFG metals was in
sputtering targets for physical vapour deposition (Fig. 55).
Honeywell Electronic Materials, a division of Honeywell International Inc., offers UFG Al and Cu sputtering targets up to 300 mm in
diameter which are produced from plates by ECAP [189,190]. They
are used for metallization of silicone wafers in the production of
semiconductor devices. The main advantages of UFG sputtering
targets, compared to their coarse grained (CG) counterparts, are:
(1) the life span increased by 30% due to stronger material which
allows the use of monolithic targets and (2) a more uniform
deposited coating which results from reduced arcing. Another
company offering UFG Cu targets is Praxair Electronics, which
claims better sputter performance and 75% reduction in the
ownership cost of such targets.
Fig. 55. Worn out UFG 300 mm sputtering target [189].
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Fig. 56. Plate implants made of nanostructured titanium [192].
The next highly anticipated application is in the area of medical
implants. These include hip, knee and dental implants as well as
various screws, plates (Fig. 56) and meshes used in orthopaedic
applications. Popular materials usually used in these applications
are cobalt-chrome alloys, stainless steel and titanium alloys.
Titanium alloys are used for implants because of their strength, low
modulus of elasticity (better matching that of bones), corrosion
resistance and good biocompatibility. Commercial pure (CP)
titanium has better compatibility than titanium alloys but it is
not used for load bearing implants because it is not strong enough.
However, when nanostructured by SPD and subjected to further
thermo-mechanical treatment, CP titanium can be strengthened to
achieve the yield stress of 1100 MPa, which is comparable with the
yield strength of titanium alloys [191]. Traditional titanium
implants do not perform well with respect to wear resistance
and fatigue life. Therefore, improvements in these properties,
reported for UFG titanium, will be appreciated. Some Russian [192]
and USA laboratories report that the UFG CP titanium implants are
being already tried.
The defense industry could benefit from two large scale
applications of UFG metals, which are armor plates and armor
penetrators. Lighter armor for military vehicles (Fig. 57) is crucial for
the reduction of fuel consumption, higher speed, better maneuverability, longer operation range and air-transport of vehicles to
remote locations. At the same time the ballistic performance
must not be reduced. This can be achieved by the nanostructuring
of aluminium or titanium alloys traditionally used for light
armored vehicles. A good example is a UFG Al 5083 plate, which
was obtained by cryogenic ball milling, consolidation by HIP,
forging or extrusion and finally rolling [193]. With the yield strength
Fig. 57. AAV7A1 Amphibious Assault Vehicle (image courtesy of BAE Systems).
of 600–700 MPa and elongation of 11%, the material exhibited a 33%
improvement in the ballistic performance or a similar mass
reduction compared to the standard plate.
Improvements in ballistic performance are also reported for the
electrodeposited nanocrystalline nickel-iron alloys produced by
Integran Technologies. Armor structures are usually fabricated by
welding of plates. However, traditional welding based on melting
is destructive to the UFG material. An alternative technique is a
solid state process of friction stir welding, which has the ability to
refine grain structure. This results in the weld hardness being only
marginally reduced compared to the initial hardness of a UFG
material [194].
Health issues surrounding the use of depleted uranium for
armor penetrators resulted in a search for alternative materials
with similar performance characteristics. One of those characteristics is an inherent ability of depleted uranium to self-sharpen on
impact which is due to the generation of adiabatic shear bands.
Tungsten, sometimes considered as a replacement for depleted
uranium because of its high density, does not have this ability; thus
penetrators made of tungsten undergo mushrooming on impact,
which results in less penetration. UFG metals are known to have
reduced strain hardening capacity, which promotes localized
plastic deformation; at high deformation rates this leads to
adiabatic shear banding. This was confirmed by producing UFG
tungsten (by ECAP with a die angle of 1208 at 1100–1000 8C and
subsequent rolling at 600–700 8C), which exhibited adiabatic shear
banding when subjected to a dynamic load [195].
The aerospace industry values even small weight reductions
which might be achieved by the introduction of new materials or
technologies. However, this industry is very cautious because of
the safety concerns, and slow in implementing any changes.
Introducing a new material may take 10–20 years, which results
from the requirements of the well established technology and a
fully developed supply chain. UFG metals are not chemically
different from their CG precursors, so there should be no
fundamental obstacles to their use.
On the other hand the new properties of UFG metals have to be
well documented with respect to aerospace applications and the
SPD and post-SPD processes have to be commercially available. All
these requirements mean that we will have to wait a few more
years for the first aerospace applications. These, most likely, will be
associated with light UFG metals used for structural components of
the fuselage and wings. Regarding the engines, some external
elements (e.g. shields) and less thermally demanding internal
elements (e.g. titanium blades for the low pressure compressor
section) can also be considered. There has only been limited
information published so far on the potential use of UFG metals by
the aerospace industry; Boeing, filed a few patents on friction stir
welding used as a means of nanostructuring metals for fasteners
and other parts [196] while EADS is interested in UFG aluminum
sheets.
Users of sports equipment will also benefit from UFG metals,
particularly where high strength and low weight is required. UFG
metals could find applications in high performance bicycles, sailing
equipment, mountaineering equipment, golf, tennis, hockey, etc.
One example is NanoDynamics high performance (NDMX) golf
balls, which have a hollow nanostructured titanium core (Fig. 58).
The core material is manufactured using the UFG chip machining
Fig. 58. NDMX golf ball (image courtesy of NanoDynamics).
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Fig. 59. Metallix racquet (image courtesy of HEAD).
technology licensed from Purdue University. Another example of
using UFG metals in sporting goods is the commercial activity of
PowerMetal Technologies, a company with an exclusive license to
use Integran’s electrodeposition technology in consumer products.
They cooperate with HEAD in the production of their new Metallix
(Fig. 59) and Airflow racquets, which use a composite of carbon
fibres and nanocrystalline metal.
UFG metals can be beneficial to some products through
improvements in their manufacturing processes. The most promising one is superplastic forming which is currently confined to a low
volume production because of a very low process speed, necessary
when forming classical superplastic metals. UFG metals possess
better superplastic properties, which allow a tenfold increase of the
forming speed, and some temperature reduction [197]. Superplastic
UFG metals exhibit higher ductility which makes them suitable for
forming more complex components. Despite large volume of
research on SPF of UFG metals, practical applications are still a
matter for the future. One possible application has been presented
by the Institute for Metals Superplasticity Problems, Ufa, Russia.
They made models of hollow blades by diffusion bonding (DB) and
SPF using UFG Ti–6Al–4V sheets (Fig. 60). By using sheets with the
grain size down to 0.2 mm they were able to decrease the
temperature of the process from 900 to 800 8C for DB and to
700 8C for SPF [198]. The temperature reductions observed will
improve technical feasibility and the economics of the process.
Among many interesting properties of UFG metals is their
ability to flow easier and at lower temperatures when forged into
complex shapes. It is claimed that energy savings up to 30% could
be achieved due to: lower forging temperature, shorter heat-up
time, smaller forging stock size, fewer number of hits and lower
forging load [199]. A very small grain size can be a virtue of its own.
This is the case with metal micro-parts having geometrical sizes
comparable with coarse grains of classical materials. Using UFG
metals in microforming allows micro-billets to behave as
polycrystalline billets. This refers to both the inner body and the
surface of the billet. The latter is illustrated in Fig. 61 as a
Fig. 61. SEM pictures of a micro-bulged sheet made of CG and UFG Al 1070 [200].
substantial reduction of the orange peel effect [200]. Another
advantage of using UFG metals is better surface finish resulting
from micro-milling [201], micro-EDM [202] and diamond turning
[203].
The above applications of UFG metals are only a fraction of the
possible uses. Since the SPD technology can convert all CG metals
into UFG metals, it is only a matter of time when new, sometimes
unexpected, applications will be discovered. For this to happen,
information dissemination among industrial engineers, transfer of
reliable SPD technologies to industry and commercialization effort
is required [204].
6. Conclusion
Processes of severe plastic deformation, defined as metal
forming processes in which an ultra-large plastic strain was
imposed on a bulk material in order to make ultra-fine grained
metals, were reviewed in this keynote paper. As processes used for
this purpose, various methods such as, ARB, HPT, RCS, CEC, STS,
CCDF, etc. were developed, and combined SPD processes with
conventional processes were also proposed.
The properties of the metals processed by SPD are also
reviewed. The SPD-processed metals have very high strength,
and in order to increase the strength further, conventional cold
forming processes are combined with SPD processes. Since the
ductility of metals is reduced by relatively low strain, the heat
treatment of annealing is conducted after the SPD process in order
to improve the ductility. The properties of the metals processed by
the SPD processes exhibit high strength and ductility that lead to
good fatigue characteristics.
The UFG metals could be used as structural materials due to
these properties, but the area of application is limited at the
moment because the available size of billet is small. Since SPD
technology can convert all metals into UFG metals, it is expected
that new methods of producing larger billets will enlarge the area
of applications.
Acknowledgment
The authors wish to thank Prof. K. Osakada and Dr. J. Allwood for
checking the manuscript of keynote paper.
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Severe plastic deformation (SPD) processes for metals CIRP Annals

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