Rev.
Adv. Mater.
Sci.
31wires(2012)
Processing
sheetsandby5-11
continuous high-pressure torsion

5

PROCESSING SHEETS AND WIRES BY CONTINUOUS
HIGH-PRESSURE TORSION
Kaveh Edalati1,2 and Zenji Horita1,2
1Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University,
Fukuoka 819-0395, Japan
2
WPI, International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,
Fukuoka 819-0395, Japan
Received: April 20, 2012
Abstract. Continuous high-pressure torsion (HPT) method was applied to pure Al (99.99%) andO 4Zl) @Ul&( E OZ
Z
]g <
bW
aa
V]e b
VOb ] b
Wc]ca CF 6 CF O PScaSRT]aSdSS
plastic deformation and thus for production of ultrafine grained microstructure as the conventional HPT. Furthermore, CHPT provides a means to process sheet and wire of the materials ina continuous way so that it can be promising for industrial applications. It is confirmed that an Al3%Mg-0.2%Sc alloy exhibits superplastic elongation of ~500% at 573K after processing by the
CHPT.

1. INTRODUCTION
Processing bulk metallic materials through the application of severe plastic deformation (SPD) leads
to attainment of ultrafine-grained structures with ahigh strength and reasonable ductility [1-3]. Among
different SPD methods, high-pressure torsion (HPT),
in which a disc or ring is subjected to high pressure
and simultaneous torsional straining [4], has capability of continuously imposing extremely large shear
strains up to a steady-state (saturation) level [5,6].
So far, HPT has been successfully applied to awide range of materials such as metallic materials(e.g., [7-13]), semi-metals (e.g., [14,15]), intermetallics (e.g., [16,17]), amorphous materials (e.g.,[18,19]) and ceramics [20,21]. In addition to the grain
refinement and resultant physical and mechanicalproperties, HPT provides unique opportunity for consolidation of metallic powders [22,23], amorphousmaterials [18,19], ceramics [20,21] and machining
chips [24-26]. The method is also applicable forcontrolling the allotropic phase transformations inseveral materials such as Ti [27-29], Zr [30-32], andZrO2 [21] as well as for decomposition of supersaturated solid soluted alloys such as Al-Zn [33], Al-Mg[33], and Co-Cu [34].
Despite these merits of HPT, the technique is
not potential for industrial applications because of
two reasons: first, the method is used in a form of
disc [7-36] or ring [37,38] specimens which haverestricted applications in the industry when compared to sheets or wires, and second, the dimensions of samples are so small (maximum 40 mm
diameter for discs [6] and 100 mm diameter for rings[37]). Continuous HPT (CHPT), which is schematically illustrated in Fig. 1, was recently developed
for processing of long lengths of sheets [39] andwires [40] of pure metals. The method was also
] Rc b
SRca
WUa
VSSb
a]TO 4Zl) @Ul&( Ealloy [41] which is known to exhibit superplasticity
if the alloy is processed by SPD to refine the grain
size to the submicrometer level [42-44].
In this paper, thus, we review our recent studies
on the development of CHPT for practical use of
HPT to produce ultrafine-grained sheets and wires.

Corresponding author: Kaveh Edalati, e-mail: kaveh.edalati@zaiko6.zaiko.kyushu-u.ac.jp
j(& (4Rd
O SREbcRg6S bS 6] bR

6

K. Edalati and Z. Horita

Fig. 1. Schematic illustration of CHPT using (a) sheets [37] and (b) wires [38].

Summary of recent study is first given including
examination of hardness, tensile properties andmicrostructures. This is then followed by requirements for future developments of the continuous
processing of HPT.

2. EXPERIMENTAL PROCEDURES
The experiments were conducted using sheets of
VW
UV cW
b
g4Z//// O RO 4Zl) @Ul&( Ealloy (in wt.%). Sheets of the pure Al and alloy were
cut to U-shaped specimens having 1 mm thickness,
3 mm width, and 80 mm length including a half circle
with outer diameter of OD = 20 or 30 mm. Wires ofpure Al (99.99%) with 2 mm diameter were also used
for the experiments. The wires were bent to U-shapewith outer diameter of OD = 40 mm. The Al sheets
eSSO SOZ
SRT] V]c Ob--) b
VS4Zl) @Ul
0.2%Sc alloy was solution treated in air for 1 hour
at 873K before HPT processing, and the Al wires
were used in an as-drawn form. The grain size of Al
was 1050 m after annealing and the grain size ofbVS4Zl) @Ul&( E OZ
Z
]geOa && m after solution treatment.
The facility for the CHPT consists of two anvils.
The lower anvil, which is rotated during process,has a flat surface with a roughened ring-shaped areafor processing of sheets (Fig. 1a) and a roughened
ring-shaped groove with circular cross sections of 2
mm diameter, 0.9 mm depth and outer diameter of
OD = 40 mm for processing of wires (Fig. 1b). The
upper anvil, which is fixed during process, has ahalf ring-shaped groove on the surfaces with 0.5 mm
depth, 3 mm width and outer diameter of OD = 20
or 30 mm for processing of sheets and a half ringshaped groove with circular cross sections with 2
mm diameter, 0.9 mm depth and outer diameter of
OD = 40 mm for processing of wires.
A U-shaped specimen is placed on the lower
anvil and a pressure of 1 GPa for Al and 1.25 GPa
for the alloy is applied on the sample by raising thelower anvil up to a rigid contact with the upper anvil.
The lower anvil is then rotated with respect to theupper anvil for either 1/4, 1 or 2 revolutions. Accord-

Processing sheets and wires by continuous high-pressure torsion

7

the stress-strain curve was delineated for eachspecimen.
Third, discs with 3 mm in diameter and rectanUcZ
O VWaeW
b
VRW
[S a
W
] a]T
& o(o)[[3 wereprepared from the positions 5 mm away from theexit of the upper anvil. The discs and chips wereground mechanically to a thickness of 0.15 mm andfurther polished with an electro-chemical polisher
using the solution given in Refs. [39,40,44]. TEM
was performed at a voltage of 200 kV for microstructural observation and for recording selected-area
electron diffraction (SAED) patterns.

3. RESULTS
Fig. 2. Appearance of U-shaped Al sheet samples
with before and after CHPT for 1/4 and 2 revolutions.

ingly the material starts to flow in the rotation direction. Fig. 2 shows the appearance of Al sheet
samples before and after CHPT for 1/4 and 2 revolutions.
The equivalent strain produced by CHPT, , isgiven as [39]

1 s

R
3t

,

(1)

where s is the fraction of sample slippage as described in an earlier report [45], R is the mean radius of the U-shaped sample and t is the thicknessof sample. Since s is estimated in the range of 0 <s < 1/2 in earlier papers [39,40], here, the equivalent strain was estimated under the condition of s =%k %The samples subjected to CHPT were evaluatedin terms of Vickers microhardness, tensile properties and transmission electron microscopy (TEM).
First, the sheet and wire samples were polished
to a mirror-like surface and the Vickers
microhardness was measured at 5 mm away fromthe exit of the upper anvil. For each hardness measurement, a load of 50 g for Al and a load of 200 gT]4Zl) @Ul&( E eSSO Z
W
SRT
] a
S ] Raand the average of hardness values was calculated.
Second, miniature tensile specimens having 1
mm gauge length and 1mm gauge width were cut
T][ b
VS4Z
l) @Ul&( E a
O[ Z
SaW b
VSRWS
tion parallel to the longitudinal axis of the sheet using
a wire-cutting electric discharge machine. Each tensile specimen was mounted horizontally on grips
and pulled to failure using a tensile testing machine
eW
b
VO WW
b
W
OZ
a
bOW ObS]T))o &-3 s-1 at 573K and

The average of hardness values was taken for eachsample and plotted in Fig. 3 together with earlier
data obtained by conventional HPT of Al (99.99%)
using disc specimens [8]. It is apparent that thehardness values for the sheet and wire samples processed by CHPT lie well on the steady-state (saturated) level obtained for disc samples after processing with HPT, although the hardness level for wire
sample processed by CHPT is slightly higher than
the steady-state level. It is noted that the measurement error of the hardness is within the symbols forthe sheet and wire CHPT. Earlier papers found thatthe results of hardness measurements for Cu andFe after CHPT are consistent with those of conventional HPT using disc and ring specimens and hardness reaches the steady-state levels just after one
pass through the anvils [39,40].
TEM bright-field image, SAED pattern and darkfield image are shown in Figs. 4a-4c for the Al wires
subjected to CHPT. Note that the dark-field image
was taken with the diffracted beam indicated by thearrow in the SAED pattern. Inspection of Fig. 4areveals that few dislocations are visible in the grains
and that grain boundaries are straight and well defined with an average grain size of ~1.3 m. Thesemicrostructural features are consistent with earlier
observations using conventional HPT [37-40]. Moreover, several elongated grains are seen in Fig. 4c,and this is different from equiaxed grain features
observed earlier using the conventional HPT [37-40].
The configuration of the elongated grains may be
attributed to a strong directional flow of material
during CHPT or to the relatively thick and round crosssection for the wire sample. The earlier experiments
found that ultrafine-grained microstructures areachieved successfully in Cu and Fe after processing by CHPT [39].
FVSVOR Sa
aT
]b
VS4Z
l) @Ul&( E OZ
Z]gsheet after processing by CHPT is plotted in Fig. 5

8

K. Edalati and Z. Horita

Fig. 3. Hardness value for CHPT using sheet and wire samples plotted against equivalent strain in graphreported earlier using conventional HPT for disc specimens of Al [35].

Fig. 4. TEM (a) bright-field and (b) dark-field micrographs including (c) SAED pattern of an Al wire after
processing with CHPT.

Fig. 5. Hardness value for CHPT using sheet sample plotted against equivalent strain in graph reported
earlier using conventional HPT for disc and ring specimens of Al-3%Mg-0.2%Sc [40].

together with earlier data obtained by conventional
HPT using disc and ring specimens [43]. The present
results of CHPT are reasonably consistent withthose of HPT-processed disc and ring samples[42,43]. It is found that the hardness value for CHPT

is still in the level before reaching the saturation.
Since the equivalent strain is insufficient to reachthe steady state, this low level of hardness is reasonable.

Processing sheets and wires by continuous high-pressure torsion

9

Fig. 6. Nominal stress versus nominal strain curves of Al-3%Mg-0.2%Sc sheet before and after processing
by CHPT.

Fig. 7. Appearance of tensile specimens of Al3%Mg-0.2%Sc sheet after pulling to failure at 573K
before and after processing by CHPT. Undeformed
specimen is also included.

The stress-strain curves are shown in Fig. 6 fromtensile testing conducted at 573K at an initial strain
ObS]T))o &-3 s-1 for the alloy before and after processing with CHPT. Following the CHPT, the tensile strength decreases from 96 to 45 MPa and thetotal elongation to failure increases from ~40% to~500%. It is confirmed that a superplastic elongation of ~500% is obtained by processing with CHPT.
However, this elongation is insufficient when compared with 1510% reported earlier for processing
with conventional HPT [43]. The difference can be
attributed to the fact that less strain is introduced
as shown in Fig. 6 and thus full development of an
ultrafine-grained microstructure is not attained. The
appearance of the tensile specimens before and after
testing is shown in Fig. 7. It is demonstrated thatprocessing by CHPT leads to the advent of super-

plasticity at 573K. It is noted that serration appearson the sample before processing by CHPT but noton the sample after the CHPT. This can be attributed to the effect of solute atoms on dynamic strain
hardening behavior and C]bSd
Wl S6VOb
SZ
W
S Wa
b
O
bility as often observed in solid-solution hardening
alloys like Al-3%Mg [46]. For the CHPT-processed
aO[ Z
Sb
VSC]b
SdWl S6VOb
SZ
W
S Wab
OPW
Z
W
b
gPS
comes insignificant because of grain refinement asshown in Ref. [42-44]. The decomposition of supersaturated solid solution as reported in Ref. [33] may
OZ
a
]OT
T
Sb
b
VSC]b
Sd
Wl S6VOb
SZ
W
S Wa
b
OPW
Z
W
b
g

4. DISCUSSIONS
Several important points should be considered for
future developments of CHPT method.
First, although the slippage between the anvils
and the sample is undesired in conventional HPT[5,45], a continuous flow of the material in the rotation direction is enabled by the slippage in CHPT. In
order to optimize the slippage and have a continuous flow of the materials, the surface roughness ofthe groove on the upper anvil is intentionally reducedwith respect to the surface roughness of the grooveon the lower anvil in CHPT [39,40].
Second, one important advantage of CHPT whencompared to other continuous SPD methods is thata steady-state of hardness and a minimum grain
size is reached just after one pass through the anvils. However, other continuous SPD methods, theworkpiece should be subjected to several repetitive
passes to attain high strains and reach the steadylevels [47,48]. The sample state can be checkedeasily whether it is in the steady-state level or not10with reference to the plot of hardness versus equivalent strain as shown in Fig. 3 and Fig. 5.
Third, the strain imparted by HPT can be estimated with the three main parameters such as the
distance from the disc or ring center, the rotation
angle and the sample thickness for the HPT process. In association with this point, the diameter ofgroove on the anvils should be increased for processing thicker sheets or wires to impose a higher
level of strain (see Eq. 1). It is noted that both imposed strain and processing speed are increased
as the ring diameter increases and this can be met
more with a practical requirement.
Forth, although the samples of pure metals withlengths of 100 mm was processed by CHPT in thisstudy, this length can be increased easily by using
proper feeding guides.
Fifth, the sheet or wire should be drawn to slightly
an upward direction and pulled out in tension using
proper guides to keep the material flow continuous.
Sixth, the sheet and wire samples are twisted
during the process, and thus, subsequent shaping
processes may be required to modify the shape ofthe samples such as rolling for the sheets and wiredrawing or round-concave rolling for the wires.
Seventh, CHPT was operated properly in thisstudy despite the unsymmetrical geometry between
the upper and lower anvils as illustrated in Fig. 1.
However, they should be symmetrical to avoid any
unexpected breaking of the sheets and wires forcontinuous smooth operation. In order to make thepressure center coincident with the rotation axis of
the anvils and thus keep the alignment between theupper anvil and the lower anvil best, it is suggested
to have a supporter on the other half of the upper
anvil or to introduce a two-way feeding system aroundthe rotational axis.
8W
UVb
V] b
VS ] bOgb] cS[Sb
OZ
ab
VS4Z
l
) @Ul&( E a
VSSbaO[ ZSaP]YSOTb
S i%revolutions in this study. This can be due mostly toa low ductility of the alloy at room temperature after
processing by SPD. However, the processing forless ductile materials can be circumvented by increasing the operation temperature. HPT operation
at elevated temperature is feasible as the system
can easily accommodate the direct heating of thesample [49].

5. CONCLUSIONSContinuous high-pressure torsion (HPT) was applied
b
]a
VSSb
aO R%
] eWSa]TVW
UV cW
b
g4Z
O RO 4Zl) @Ul&( E OZZ
]g FVS Sa
cZ
b
a ]T
W[SRb
VOb
processing by continuous HPT (CHPT) is effective

K. Edalati and Z. Horita
in producing ultrafine grains and subsequent enVO S[S b]T
VOR Sa
aW[Sb
OZ
aO Rb
VS4Z
l) @Ul
0.2%Sc alloy. The advent of superplasticity in the
4Zl) @Ul&( E OZ
Z
]geOaOZa] ]TW[SROaPg
the conventional HPT. It is concluded that HPT canbe operated continuously to produce sheets andwires having ultrafine grained structures and enhanced mechanical properties and, therefore, the
CHPT introduced in this study provides a good potential for practical application of the HPT processing.

ACKNOWLEDGEMENTS
One of the authors (KE) would like to thank the Islamic Development Bank (IDB) for a doctoral scholarship and Japan Society for Promotion of Science(JSPS) for a postdoctoral scholarship. This work
was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grant-in-Aid
for Scientific Research from the MEXT, Japan, in
< ]d
Ob
W
d
S4SOam5cZ
YAO ]abc b
cSR@SbOZ
anO Rin part by Kyushu University Interdisciplinary Programs in Education and Projects in Research Development (P&P).

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