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Page 1: Severe plastic deformation (SPD) processes for metalseng.uok.ac.ir/A.Hasani/pdf/Severe plastic deformation (SPD... · Severe plastic deformation (SPD) processes for metals ... Graduate

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, Japanb Institute of Metal Forming, RWTH Aachen University, Aachen, Germanyc Department of Materials Science and Engineering, Helsinki University of Technology, Espoo, Finlandd Department of Mechanical Engineering, KAIST, Deajeon, Republic of Koreae Department of Manufacturing and Management Engineering, University of Palermo, Palermo, Italyf Timken Research, The Timken Company, Canton, OH, USAg Institute for Production Engineering and Forming Machines, University of Technology, Darmstadt, Germanyh Institute of industrial Science, The University of Tokyo, Tokyo, Japani Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, Osaka, Japanj Department of Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, United Kingdom

CIRP Annals - Manufacturing Technology 57 (2008) 716–735

A R T I C L E I N F O

Keywords:

Forming

Metal

Strain

A B S T R A C T

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 roll-

bonding (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.

� 2008 CIRP.

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http://ees.elsevier.com/cirp/default.asp

1. Introduction

Processes with severe plastic deformation (SPD) may be definedas metal forming processes in which an ultra-large plastic strain isintroduced into a bulk metal in order to create ultra-fine grainedmetals [1–7]. The main objective of a SPD process is to producehigh 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 lessthan about 2.0. When multi-pass rolling, drawing and extrusionare carried out up to a plastic strain of greater than 2.0, thethickness and the diameter become very thin and are not suitableto be used for structural parts. In order to impose an extremelylarge strain on the bulk metal without changing the shape, manySPD processes have been developed.

Various SPD processes such as equal channel angular pressing(ECAP) [8–11], accumulative roll-bonding (ARB) [12–14], highpressure torsion (HPT) [15,16], repetitive corrugation and straigh-tening (RCS) [17], cyclic extrusion compression (CEC) [18], torsionextrusion [19], severe torsion straining (STS) [20], cyclic closed-dieforging (CCDF) [21], super short multi-pass rolling (SSMR) [22]have been developed.

The major SPD processes are summarized in Table 1 withschematic configurations and the attainable plastic strain. ECAP,ARB and HPT processes are well-investigated for producing ultra-

* Corresponding author.

0007-8506/$ – see front matter � 2008 CIRP.

doi:10.1016/j.cirp.2008.09.005

fine grained metals. It is known that the metals produced by theseprocesses 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 processesexhibit high strength [23–25], and thus they may be used as ultra-high strength metals with environmental harmony. The yieldstress of polycrystalline metals is related to the grain diameter d bythe following Hall–Petch equation:

sY ¼ s0 þ Ad�1=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 ahigher tensile strength without reducing the toughness, whichdiffers from other strengthening methods such as heat treatment.

The relationship between proof stress and grain size of pureiron is shown in Fig. 1 [6]. The proof stress changes inversely withthe square root of the grain size, following the Hall–Petchrelationship. It is seen that the proof stress of the ultra-finegrained irons, with sub-micrometer grains, is five times greaterthan commercially pure iron. Thus, the conventional structuralmetals with ultra-fine grains are lighter due to their high strength.Since pure iron does not contain harmful elements, it is in harmonywith a clean environment. Moreover, the improvements of thesuperplasticity, corrosion and fatigue properties of metalsprocessed by SPD are expected. On the other hand, the ultra-finegrained metals are available only for micro-parts [26,27].

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Table 1Summary of major SPD processes

Process name Schematic representation Equivalent plastic strain

Equal channel angular extrusion (ECAE) (Segal, 1977) e ¼ n 2ffiffi3p cotð’Þ

High-pressure torsion (HPT) (Valiev et al., 1989) e ¼ gðrÞffiffi3p , gðrÞ ¼ n 2pr

t

Accumulative roll-bonding (ARB) (Saito, Tsuji, Utsunomiya, Sakai, 1998) e ¼ n 2ffiffi3p ln t0

t

� �

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 717

In Fig. 2 [28], the mechanical properties of a wire specimenmade by SPD is plotted against the ratio of wire diameter D to thegrain size d, D/d. The proof stress decreases with decreasing D/dwhen D/d is less than 100. In particular, when D/d is less than 5, theproof stress decreases abruptly with decreasing D/d. From theseobservations, the ratio of D/d must be greater than 100 in order toguarantee the safety and the reliability of metals for micro-parts.

This paper reviews the severe plastic deformation processesto create metals with ultra-fine grains. In the following, thefabrications of the SPD processes are shown in Section 2. Then, the

Fig. 2. Material behavior during forming processes of micro-parts of a wire

specimen with diameter to grain size D/d [28].

Fig. 1. Relationship between proof stress and grain size of pure iron [6].

properties of metals processed by SPD processes are shownin Section 3, the combined processes developed recently areexplained in Section 4, and the applications of the ultra-finegrained 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 extrusionprocesses, which are a kind of double axis extrusion or sideextrusion [29]. Fig. 3(d and e) indicates the process in which pureshear deformation can be repeatedly imposed on materials so thatan intense plastic strain is produced with the materials withoutany change in the cross-sectional dimensions of the workpiece.These processes are named as ECAE (Equal channel angularextrusion) or ECAP.

Segal [8,30] proposed this process in 1977 in order to create anultra-fine grained material. Although ECAP is generally applied tosolid metals, it may also be used for consolidation of metallicpowder. Kudo and coworkers [31] employed repetitive sideextrusion with back pressure to consolidate a pure aluminumpowder. In the 1990s, developments of ultra-fine grained materialswere carried out with this method by Valiev et al. [9,10,32], Horitaand coworkers [33–45] and Azushima et al. [46–48] and others[49–51].

The schematic representation of the ECAE process is shown inFig. 4. The specimen is side extruded through the sheardeformation zone with the dead zone in the outer corner of thechannel. When the workpiece is side extruded through thechannel, the total strain is

e ¼ 1ffiffiffi3p 2cot

f2þc

2

� �þc cosec

f2þc

2

� �� �(2)

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.

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].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735718

where F is the angle of intersection of two channels and C is theangle subtended by the arc of curvature at the point of intersection.When F = 908 and C = 08, the total strain from the above equationis 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 thespecimen is inserted within the channel and it can be pressedthrough the die using a punch. There are four basic processingroutes in ECAP. In route A, the specimen is pressed withoutrotation, in route BA the specimen is rotated by 908 in an alternatedirection between consecutive passes, in route BC the specimen isrotated 908 counterclockwise between each pass, and in route C thespecimen is rotated by 1808 between passes.

From these macroscopic distortions shown in Fig. 5, theinfluence of the processing route on the development of anultra-fine grained microstructure can be considered [33,36]. Horitaand coworkers [42] reported that the ultra-fine grained micro-structure of pure aluminum after 10 passes in route A was the sameas that after 4 passes in route BC.

2.1.2. Developed ECAP processes

Azushima et al. [46–48] proposed the repetitive side extrusionprocess with back pressure. It is a process in which a high backpressure is applied in the process as shown in Fig. 6, in order toproduce uniform shear deformation and prevent defects in theworkpiece. The specimen is side-extruded between the punches Aand B, while the punches C and D are fixed. In this process, the totalstrain becomes 1.15 after one pass. The punch A, controlled by thefunction generator, moves at a constant speed and the punch Bgenerates a constant back pressure. Recently, ECAP die-sets havebeen developed to conduct the ECAP process with a back pressurewhich is controlled by computers [46,47].

Nishida et al. [52–57] developed a rotary-die ECAP shown inFig. 7, which consists of a die containing two channels with thesame cross-sections intersecting at the center with a right angle inorder to remove the limitation in the conventional ECAP, i.e. thesample must be removed from the die and reinserted again in eachstep. At first, the sample is inserted into the die with the plunger asshown in Fig. 7(a), and after pressing the sample as Fig. 7(b), the dieis 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 thepunch A with a back pressure from the punch B, similarly to thatshown in Fig. 6. Repetitive pressings may be carried out with therotary ECAP. This process is equivalent to route A in Fig. 5.

In the same way as the repetitive ECAP process, a method toreduce the repetitive number by increasing the number of channelturns in the die [58–61] was developed. Using the two-turnchannels the strain in one pass becomes double and theproductivity of the ECAP process increases. A counterpunch for

Fig. 6. Schematic representation of repetitive side extrusion process with the back

pressure [46].

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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].

Fig. 7. The ECAP process using a rotary-die: (a) initial state, (b) after one pass and (c)

after 908 die rotation [52].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 719

providing additional pressure as shown in Fig. 8 may become aviable option available on common hydraulic presses. Presses withtwo opposite and equally powerful rams could be used for a cyclicprocess. In this process, the total strain becomes 2.3 after one pass.

These ECAP processes have been used only in the laboratorybecause of their low productivity. For mass production, continuousprocessing techniques must be developed. First, in order toproduce 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. Inthe ECAD process, the material in the form of a bar is drawnthrough the two channels. The rods are preformed by bendingthem 1358 to fit to the die, and are drawn through the ECAP dieusing as Instron tensile testing machine.

The principle of the con-shearing process is representedschematically in Fig. 10 [63–65]. An equal-channel die with achannel angle is located at the exit of a satellite mill. Satellite rollsand a central roll are used as feed rolls. All the rolls are driven at anequal peripheral speed to generate large extrusion forces and thestrip is extruded through the die continuously. This process usesfriction between rolls to push the workpiece through an ECAP die.In this process, the shear deformation is given to the stripcontinuously, and the total strain after one pass is given by Eq. (2).

Recently, equal channel angular rolling (ECAR) [66–68] andECAP conform [69] were developed. The principle of the ECAR

Fig. 9. Schematic of the equal channel angular drawing process (ECAD) [62].

process is represented schematically in Fig. 11. The strip is fedbetween two rolls and extruded to reduce the thickness of thestrip. Then, the strip flows into the outlet channel. The principle ofthe ECAP conform process is represented schematically in Fig. 12.The workpiece is driven forward by frictional forces on the threecontact interfaces with the groove. The workpiece is constrained tothe groove by the stationary constraint die, which restricts theworkpiece and forces it to turn by shear deformation similarly tothe ECAP process.

Fig. 13 shows an Al workpiece at every stage of the ECAPconform process, from the initial feeding stock with a round cross-section to the rectangular rod after the first ECAP pass. For theECAR process and the ECAE conform process, the total strain afterone pass operation is given by Eq. (2), and the accumulated totalstrain is n � e after n passes.

The Incremental ECAP (I-ECAP) was developed by Rosochowskiet al. [70–72]. Fig. 14 explains the principle of this process; it isbased on incremental feeding of the billet by a distance ‘‘b’’and using a reciprocating die ‘‘C’’ whose movement is synchro-nized with feeding. This enables feeding to take place duringthe withdrawal phase of die ‘‘C’’. When the billet stops at a pre-determined position, die ‘‘C’’ approaches it and deforms a small

Fig. 12. Schematic illustration of the ECAP– Conform process [69].

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Fig. 14. Schematic representation of I-ECAP [70].

Fig. 13. Al workpiece undergoing processing by ECAP–Conform: the arrow marks

the transition to a rectangular cross-section [69].

Table 2Summarizes the geometrical changes of the specimen during the ARB process

where roll-bonded by 50% reduction per cycle [81]

Number of Cycles, n 1 2 3 5 10

Number of layers, m 2 4 8 32 1024

Total reduction, r (%) 50 75 87.5 96.9 99.9

Equivalent strain, e 0.80 1.60 2.40 4.00 8.00

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735720

volume of the billet. The mode of deformation is that of simpleshear and, provided the feeding stroke is not excessive,consecutive shear zones overlap resulting in a uniform straindistribution along the billet. Separation of the feeding anddeformation 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 weredeveloped in order to produce long bulk bars with square cross-sections [73,74]. The principle of this process is schematicallyrepresented in Fig. 15 [73]. By moving the tool, the friction forcesover the three contacting interfaces become zero and the extrusionload decreases.

The ECAP process may be used for the consolidation of metallicpowder [31,75–79]. An aluminum powder and a steel powder atroom temperature was pressed using the ECAP facility as shown inFig. 4.

2.2. Accumulative roll-bonding (ARB) process

The ARB process was first developed by Saito et al. [80–83]. Theprinciple of the ARB process is represented systematically in Fig. 16

Fig. 15. The principle of friction-reduced ECAP processes [73].

[81,82]. Stacking of sheets and conventional roll-bonding arerepeated in the process. First, a strip is neatly placed on top ofanother strip. The interfaces of the two strips are surface-treated inadvance in order to enhance the bonding strength. The two layersare joined together by rolling, as in the conventional roll-bondingprocess. Then, the length of the rolled material is sectioned intotwo halves. The sectioned strips are again surface-treated, stackedand roll-bonded. These procedures can be repeated limitlessly inprinciple, so that very large plastic strain can be applied to thematerial.

The strain after n cycles of the ARB process can be expressed as,

e ¼ffiffiffi3p

2lnðrÞ; r ¼ 1� t

t0¼ 1� 1

2n (3)

where t0 is the initial thickness of the stacked sheets, t thethickness after roll-bonding and r the reduction in thickness percycle. Table 2 summarizes the geometrical changes of thespecimen thick sheets are stacked and roll-bonded by a 50%reduction per cycle. The number of the initial sheets included in thespecimen processed by n cycles of ARB becomes 2n. After 10 cyclesof the ARB process, the number of layers becomes 1024 so that themean thickness of the initial sheet is smaller than 1 mm.

Optical micrographs of the ARB processed IF steel are shown inFig. 17. In the material processed by two cycles ARB (Fig. 17(c)), theinterface introduced in the second cycle is seen clearly. However, itis difficult to find the interface of the first pass at a quarter of thethickness. After five cycles, the whole thickness is covered by verythin and elongated grains. This process has been used by manyresearchers 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 hisexperiments, attention was not paid to the microstructure changetaking place in severely deformed metals. Another implementationof HPT was carried out by Erbel [90]. The specimen was a short ringwith conical faces whose virtual extensions met at the axis of theapparatus as shown in Fig. 18. The conical matching faces of the

Fig. 16. Diagrammatic representation of the accumulative roll-bonding (ARB)

process [81].

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Fig. 18. Schematic diagram of ring tension device and dimensions of ring

specimens [90].

Fig. 20. Schematic illustration of the bulk-HPT process [94].

Fig. 17. Longitudinal cross-section of initial and ARB processed IF steel strips [82].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 721

punches have radial teeth to facilitate the application of torque.The ring specimens were constrained from all directions whichcreated a condition closer to hydrostatic pressure.

Recently, Valiev et al. conducted the HPT process using devicesunder high pressure as shown in Fig. 19 [15,16,89,91–104]. Thedesign is a further development of the Bridgman anvil type device.In this device, a very thin disk is compressed in a closed die by avery high pressure. The torque is provided by the punch withcontact friction at the interface between the punch and disk. Thestrain in torsion is given by

gðrÞ ¼ 2pnr

l(4)

where r is the distance from the axis of the disk sample, n the numberof rotation and l the thickness of the sample. The equivalent strainaccording to the von Mises yield criterion is given by

eðrÞ ¼ gðrÞffiffiffi3p ; (5)

This method has the disadvantage that it utilizes specimens inthe form of relatively small discs and is not available for the

Fig. 19. Schematic illustration of the thin disc-HPT process.

production of large bulk materials. Another disadvantage is thatthe microstructures produced are dependent on the appliedpressure and the location within the disc. In order to solve theproblem, Horita and coworkers developed an HPT process for useof the bulk sample as shown in Fig. 20 [94]. This process isdesignated as Bulk-HPT for comparisons with conventional Disk-HPT [95–104].

The severe plastic torsion straining (SPTS) process can be usedfor the consolidation powders using a similar apparatus as shownin Fig. 19 [105–107]. By using this process at room temperature,the disk type samples with a high density close to 100% weredeveloped. The SPTS consolidation of powders is an effectivetechnique for fabricating metal–ceramic nano-composites with ahigh density, ultra-fine grain size and high strength.

2.4. Other processes

The principle of the cyclic extrusion compression (CEC) processdeveloped by Korbel et al. is represented schematically in Fig. 21[18,108–111]. In the CEC process, a sample is contained within achamber and then extruded repeatedly backwards and forwards.This process was invented to allow arbitrarily large straindeformation of a sample with preservation of the original sampleshape after n passes. The accumulated equivalent strain isapproximately given by

e ¼ 4nlnD

d

� �(6)

where D is the chamber diameter, d the channel diameter and n thenumber of deformation cycles. Since the billet in the CEC process iscompressed from the both ends, a high hydrostatic pressure isimposed. The extrusion–compression load becomes high so thatthe special pre-stressed tools are required, otherwise the tool lifewill be short. This process is better suited for processing softmaterial such as aluminum alloys. However, the strain introducedin the forward extrusion may be cancelled by the strain introducedon the backward extrusion.

The principle of the cyclic closed-die forging (CCDF) processdeveloped by Ghosh et al. is represented schematically in Fig. 22[21,112,113]. A billet is first compressed in the vertical directionand then in the horizontal direction. The equivalent strain peroperation is given by

e ¼ 2lnðH=WÞffiffiffi

3p (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. 21. Schematic of cyclic extrusion compression (CEC).

Fig. 24. Principle of linear flow splitting [114].

Fig. 25. Principle of spin extrusion [115].Fig. 22. Schematic of cyclic closed-die forging (CCDF).

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735722

tically in Fig. 23 [17]. The technique consists of bending a straightbillet with corrugated tools and then restoring the straight shape ofthe billet with flat tools. The equivalent strain per one operation isgiven by

e ¼ 4ln½ðr þ tÞ=ðr þ 0:5tÞ�ffiffiffi

3p (8)

where t is the thickness of sample and r is the curvature of bentzone. By repeating these processes in a cyclic manner, high strainscan be introduced in the workpiece.

Linear flow splitting developed by Groche et al. is anotherpossibility to obtain ultra-fine grained metal [114]. The principle ofthis process is shown in Fig. 24. A sheet metal is compressedbetween the splitting roll and the supporting rolls. Under this stateof stress two flanges are formed into the gap between the splittingand the supporting rolls. The material flow is mainly associated bya surface enlargement of the band edge. Several hundred percent ofplastic strain occur. As a consequence, the outer surface areas ofthe flanges consist of ultra-fine grained metal. The properties of themetal in this state can be used for an increase of load bearingcapability, e.g. bearings for rollers.

The applicability of incremental bulk forming processes withhigh deformation for grain refinement in the sub-micrometerrange was investigated by Neugebauer et al. [115]. A specific aspect

Fig. 23. Principle of repetitive corrugating and straightening [17].

of this approach is the opportunity to create a changed structure inthe surface region, keeping the lower region or core unchanged.The incremental forming method of the spin extrusion as shown inFig. 25 is used to create cup shaped or tube shaped parts from solidbillets. The hollow shape is created by the concurrent partialpressure of three rolls on the surface of the workpiece and thepressure of the forming mandrel acting in the axial direction. Thematerial flows axially and a cup wall is created between theforming tools [116].

The principle of the severe torsion straining (STS) processdeveloped by Nakamura et al. is represented schematically inFig. 26 [20]. The process consists of producing a locally heated zoneand creating torsion strain in the zone by rotating one end with theother. The rod is moved along the longitudinal axis while creatingthe local straining. Therefore, a severe plastic strain is producedcontinuously throughout the rod. In order to create the torsionstrain efficiently, the locally heated zone should be narrow and therotation of the rod should be fast with respect to the moving speedof the rod. Moreover, a modification is made for the cooling systemso that the heated zone is more localized to create torsion strain.

The principle of the torsion extrusion process developed byMizunuma et al. is represented schematically in Fig. 27. Thisprocess is characterized by rotation of a die or a container duringan extrusion process for introducing a very large strain in to themetal. As high hydrostatic pressure involved in the extrusion raisesthe ductility of the metals, a very large torsion straining can beintroduced to the workpiece. The mean value of representative

Fig. 26. Principle of the severe torsion straining (STS) process [20].

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Fig. 27. Principle of the torsion extrusion process [19].

Fig. 29. TEM Micrograph of ultra-low carbon steel after ECAPed 10 passes by route

A [46].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 723

strain on a cross-section of a column can be calculated as below.

e ¼ 4pRN

3ffiffiffi3p

H(9)

where R is radius of column, H is the height of the column, N is thenumber of rotation.

Fig. 28 [19] shows a magnified view of the longitudinal sectionof the etched aluminum specimen after the torsion extrusionprocess, compared with that of the conventional extrusion. Thetorsion extruded part of the specimen is clearly observed to bemore severely strained than that of the conventional extrusion.

3. Properties of metals processed by SPD

The SPD-processed metals normally have ultra-fine grainedstructures that cannot be obtained through conventional thermo-mechanical processing. As a result, the SPD metals exhibit uniqueand excellent properties such as high strength, compared with theconventional materials having a coarse grain size of over severaltens of micrometers.

In the optical microstructure of metals over 5 passes of the ECAPprocesses, it is observed that the strong filamentary microstructureis developed with an increasing number of passes. In theseconditions, observed microstructure must use a TEM analyzer.From the TEM microstructure, it is confirmed that many metalswith an ultra-fine grain size (under 1 mm) are developed by ECAPprocesses.

The ultra-fine grains of sub-micron size were created by ECAPprocesses 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 of7 at 160 8C [2]. Aluminum and aluminum alloys with a sub-microngrain 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 of0.5 mm and a minor axis length of 0.2 mm was developed by 10passes of repetitive side extrusion at room temperature as shownin Fig. 29 [46]. At the same time, they showed the relationshipbetween the area fraction and the mis-orientation angle by theEBSP analysis [46]. They reported that most of the boundaries arehigh-angle grains, so that the processed steel is considered to be akind of ultra-fine grain structured metal.

In the ARB processes, it was noted that the evolution ofmicrostructure and the increase in mis-orientation of boundarieswere much faster than those when using conventional rolling[117,118]. A typical TEM micrograph of the ultra-fine structure inthe interstitial free (IF) steel ARB processed by 7 cycles at 500 8C isshown in Fig. 30. From the crystallographic analysis by Kikuchi-line analysis, they reported that most of the boundaries were at ahigh angle.

From these TEM microstructures, it is expected that thehardness and the tensile strength of metals with ultra-fine grainsbecome higher. A number of studies have been conducted on thestrength and ductility of various kinds of metallic materialsprocessed by various SPD processes. The SPD-processed materialsgenerally have very high strength compared with conventionalmetals. Fig. 31 illustrates a general tendency of the change instrength and ductility during SPD. The strength of the materialscontinuously increases with increasing the applied strain and thengradually saturates. On the other hand, the ductility drops greatlywith a relatively small strain, and then keeps a nearly constantvalue 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]. Thetensile strength increases with increasing pass number. The tensilestrength is increased from 300 to 750 MPa after one pass. Thetensile strength is increased by a factor of 2 after one pass incomparison with the specimen before the ECAP process, andincreases with increasing pass number up to 8 passes. The tensilestrength is higher than 800 MPa after 8 passes. On the other hand,the elongation decreases from 20% for the specimen before theECAP 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].

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Fig. 31. Illustration showing the general tendency of the change in strength and

ductility during SPD. Fig. 34. Relationship between total elongation and pass number for carbon steels

based on ref. [121].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735724

Horita et al. [33–35] reported the same results for thealuminum alloys, and Azushima et al. [46–48,119–121] and Shinet al. [122–129] also reported for the steels. In particular, Aoki andAzushima [121] reported the relationship between nominal stressand nominal strain of specimens of ultra-low carbon steel, 0.15%Csteel, 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 thatthe as-received material exhibits a stress–strain curve thatindicates normal strain hardening, while the specimens afterECAP do not exhibit strain hardening. The stress for each specimenincreases rapidly with increasing strain and reaches a maximum atlower strain.

Fig. 33 shows the relationship between the tensile strength andthe pass number for the carbon steels. The tensile strengthincreases with increasing number of passes of ECAP. The tensilestrength of ultra-low carbon steel after 10 passes was greater than1000 MPa and was increased by a factor of 3 in comparison withthe as-received material. The experimental data of the specimen

Fig. 33. Relationship between tensile strength and pass number for the carbon

steels based on ref. [121].

Fig. 32. Relationship between tensile strength, elongation and pass number of ECAP

for Armco steel [30].

after 10 passes are plotted in the Hall–Petch relationship of theyield stress against the root grain size as shown in Fig. 2. In thisfigure, the results for these specimens show good agreement withthe standard Hall–Petch relationship of iron obtained by Takakiand coworkers [130].

Fig. 34 shows the relationship between total elongation and thepass number for the carbon steels. For the low carbon steel, theelongation decreases to 20% after 3 passes, and for the other carbonsteel, 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 elevatedtemperatures as shown in Fig. 35. The tensile strength decreaseswith increasing processing temperature of ECAP and the totalelongation increases.

In the ARB process, Saito, Tsuji et al. [131–142] reported themechanical properties of many metals processed by ARB. Therelationship between the tensile strength, elongation and cycles ofa commercially pure aluminum (JIS-1100) SPD processed by theARB process is shown in Fig. 36 [132]. The tensile strength of the1100-Al greatly increases to 185 MPa while the total elongationdrops down to 13% by the 1 ARB cycle (equivalent strain of 0.8). Asthe number of the ARB cycles (strain) further increases, the flowstress continuously increases and reaches 340 MPa, which is fourtimes higher than that of the starting material having aconventionally recrystallized microstructure.

On the other hand, the elongation of the 1100-Al does notchange as much after the second ARB cycle. As was illustrated inFig. 29, this is the typical change in the mechanical propertiesduring SPD, which seems to occur regardless of the kind of SPDprocess and material.

The decrease in ductility is a general feature of strain-hardenedmetallic materials. Thus, it is not surprising that the SPD-processedmaterials, i.e., ultra-high strained materials show limited tensileductility. It can be expected that the ductility can be recovered bysubsequent heat treatment, as is the case with deformed andannealed 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].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 725

Fig. 37 shows the stress–strain curves of the 1100-Al and ultra-low-carbon interstitial free steel SPD processed by the ARB andthen annealed at various temperatures for 1.8 ks [132]. In thefigures, true stress and true strain are indicated by assuminguniform elongation. Also the mean grain size of the specimensmeasured from the microstructure observations are superimposedin the figures. The strength of the materials decreases withincreasing grain size, i.e., with increasing annealing temperature.However, large elongation can be obtained only after the strengthdecreases. In particular, the curves clearly show that the flow stressreaches its maximum at an early stage of tensile test and is thennecked down to fracture in the UFG specimens.

The limited tensile ductility of the ultra-fine grained materials isunderstood in terms of early plastic instability. As is well-known, theplastic instability condition (i.e., necking condition in tensile test) forstrain-rate insensitive materials, for example, is expressed as

s� dsde (10)

where s and e are true stress and strain, respectively. Ultra-grainrefinement greatly increases the strength, especially yieldstrength, of the materials. When the strain-hardening ratecoincides with the flow stress, plastic instability, in other wordsnecking, starts in the tensile test, which demonstrates a uniformelongation.

The mechanical properties of the metals with ultra-fine grainprocessed by SPD have been investigated [143–149]. Cherukuriet al. reported a comparison of the properties of SPD-processed AA-6061 by ECAP, CCDF and ARB as shown in Fig. 38 [149].Commercially available AA-6061 in the annealed condition wassubjected to severe plastic deformation processing by ECAP, CCDFand ARB at room temperature to approximately the sameaccumulated strain (�4). From Fig. 38, it is understood that theSPD technique used did not show much effect on the flow behaviorof 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].

Besides the mechanical properties, the fatigue property[120,150–154] and superplasticity property [133,155–167] wereinvestigated by many researchers.

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 tointroduce crystalline defects and refine the grains. Recently, twocombined processes, the ECAP process and cold rolling, and theECAP process and cold extrusion were developed.

The principle of the combined process of ECAP and cold rollingis represented schematically in Fig. 39. Azushima et al. carried outexperiments in which the specimens of ultra-low carbon steelwere processed by ECAP in route A at room temperature and thenthe specimens processed by ECAP were rolled repetitively at roomtemperature in order to increase the strength. Fig. 40 shows thetensile strength after the combined process [168]. After 10 passesof ECAP, the tensile strength of ultra-low carbon steel is 1000 MPaand after cold rolling with a reduction in thickness of 95% itbecomes 1300 MPa.

Next, warm ECAP process was first used to refine the grain sizeof commercially pure Ti billets and the billets were furtherprocessed by repetitive cold rolling. The properties of the pure Tiprocessed by the two-step method are summarized in Table 3[169]. ECAP increased the yield and tensile even strength to 640and 710 MPa, respectively. After a cold reduction of 35%, the yieldand tensile strengths increased to 940 and 1040 MPa which arehigher than those for the Ti–6Al–4V alloy. Further cold rolling to areduction of 55% resulted in even higher yield and tensilestrengths.

The principle of the combined process of ECAP and coldextrusion is represented schematically in Fig. 41. Stolyarov et al.[170] carried out experiments in which the billet of commerciallypure Ti were first processed by ECAP in route BC at about 400 8C andthen the billets processed by ECAP were further processed by coldextrusion 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. 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. 40. Transition of tensile strength after combined process [168].

Fig. 41. Principle of the combined process of ECAP process and conventional

extrusion.

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735726

Ti processed by warm ECAP and cold extrusion are summarized inTable 4. After cold extrusion to 47% reduction in cross-section, theyield and tensile strengths were increased to 910 and 930 MParespectively, which are higher than those of Ti–6Al–4V. Furthercold extrusion in cross-section of 75% yielded even higher yield andtensile strengths. Next, in order to increase the strength ofaluminum alloy (AA-6101) this combined process was conducted.The experimental results show that improved properties after coldextrusion are heavily dependent upon the prior ECAP processingroutes.

On the other hand, in order to improve the superplasticproperties of metals processed by ECAP, cold deformation can becombined 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

Table 3Properties of pure Ti processed by two-step [169]

Processing state s0.2 (MPa) su (MPa) d (%)

Coarse grain (10 mm) 380 460 27

ECAP(8)a 640 710 14

ECAP(8) + CR(35%) 940 1040 7

ECAP(8) + CR(55%) 1020 1050 6

ECAP(12) + CR(35%) 920 955 15

CR(35%)b 660 670 16

a ECAP route BC was used for all samples.b The value inside parentheses is cross-section reduction.

Table 4Properties of pure Ti processed by warm ECAP and cold extrusion [170]

Processing state s0.2 (MPa) su (MPa) d (%) RA (%)

Coarse grain 380 460 27 69

ECAP(8)a 640 710 14 61

ECAP(8) + Cold extrusion(47%)b 910 930 – 55

ECAP(8) + Cold extrusion(75%) 1020 1050 6 42

Ti–6Al–4Vc 920 955 10 25

a ECAP route BC was used for all samples.b Reduction in cross-section area from cold extrusion.c From ASTM F 136-96.

a reduction in thickness of 70%. The comparison of the dependenceof elongation on strain rat e between ECAP and ECAP + cold rolling(70%) samples is shown in Fig. 42. The elongations of theECAP + cold rolling samples were higher than that of the ECAPsample at all strain rates. The maximum elongation was 812% at5 � 10�3 s�1 and it was much higher than that of the eight passesECAPed sample (595%). For the purpose of comparison, anappearance of the ECAP and ECAP + cold rolling (70%) sampleselongated to failure is shown in Fig. 43. Similarly, the super-plastic properties of 7075 aluminum alloy processed by ECAPof one pass and isothermal rolling at 250 8C were examined anda the alloy processed exhibited a maximum elongation of 820%at a temperature of 450 8C and an initial strain rate of 5.6 �10�3 s�1.

4.2. SPD process and annealing

The high strength of metal processed by SPD is obtained, but theductility of the metals after SPD becomes very low. In order toimprove the ductility of the metal processed by SPD, the metalswere annealed after SPD process.

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. 44. Relationship between tensile strength and hardness of specimens of ultra-

low 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].

Fig. 45. Tensile strength and elongation of ECA pressed low carbon steel annealed at

480 8C for various times [173].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 727

Aoki and Azushima [121] carried out experiments in which thecarbon steel samples were first processed by ECAP of 3 passes inroute A at room temperature and then the samples processed byECAP were further processed by annealing at a temperature of600 8C and changing annealing times. Fig. 44 shows the relation-ship between nominal stress and nominal strain of specimens ofultra-low carbon steel, 0.15%C steel, 0.25%C steel and 0.50%C steelafter the ECAP of 3 passes and after annealing. The tensile strengthsbecome lower and the total elongations increase with decreasingtensile strength. The uniform elongations increase with decreasingtensile strength for all carbon steel samples. For example, a 0.5%Csteel sample with a tensile strength 900 MPa and a total elongationof over 20% is obtained.

Shin et al. [129,173,174] investigated static annealing afterwarm ECAP with a view to thermal stability. The low carbon steel:0.15C–0.25Si–1.1Mn (in wt.%) (hereafter CS steel) was used. ECAPwas carried out on the samples at 350 8C up to 4 passes by Route C,then samples for subsequent annealing were encapsulated in aglass tube with Ar atmosphere in order to minimize the possibledecarburization. The static annealing treatment was conducted at480 8C up to 72 h. Stress–strain curves of the as-received, as-pressed and annealed samples are shown in Fig. 45. The as-pressedand annealed samples exhibited no strain hardening behavior. It isof interest to note that stress–strain curves of the samplesannealed for 24 and 72 h were almost identical. This observationimplies that the sample annealed for 24 h was mechanically stablealthough the microstructural examination revealed that recoverywas in progress after 24 h annealing.

Fig. 46. Super short interval m

In order to improve the ductility of metals processed by ARB, anannealing process was conducted. Tsuji et al. carried outexperiments in which the aluminum and iron samples were firstprocessed by ARB at a warm temperature and then the samplesprocessed by ARB were further processed by annealing for 600 s or1.8 ks from 200 to 800 8C. Fig. 37 shows the stress–strain curve ofcommercially pure aluminum (1100-Al) and ultra-low carboninterstitial free steel specimens processed by various annealingconditions. The mean grain size is also indicated in this figure.

The flow stress of both metals increases with decreasing meangrain size. Once the mean grain size becomes smaller than 1 mm,elongation of both Al and Fe suddenly reduced, though the strengthstill increased with decreasing grain size. On the other hand, as thegrain size became larger than 1 mm, typical strain-hardening wasobserved and the elongation increased with increasing grain size.

Stolyarov et al. [169] carried out experiments in which thecommercially pure Ti billets were first processed by warm ECAPand repetitive cold rolling and further the billets processed byannealing at temperatures of 200 and 300 8C. The properties of thepure Ti billets processed are summarized in Table 5. Annealingpure Ti processed by SPD at temperatures below 300 8C generallyimproves 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 finegrained plain carbon steel with a ferrite grain size of 3 mm [175].TMCP and micro-alloying technology is widely used to manufac-ture precipitation-hardened high-strength steel sheets. The sameprocess may be used in the rolling of strip, but precipitates are noteasily controllable during rolling because strip rolling mills arearranged in tandem to gain higher productivity and constantquality rather than flexibility.

To manufacture fine-grained steel strips, the rolling tempera-ture must lie just above the transformation temperature or in thesupercooled austenite state to accelerate transformation in therun-out table. This type of combined process can be used tomanufacture plain carbon fine-grained steel sheets since the

ulti-pass rolling process.

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Table 5Properties of pure Ti processed by warm ECAP and cold rolling with subsequent annealing [169]

Processing state s0.2 (MPa) su (MPa) d (%)

Coarse grain 380 460 27

ECAP(8)a 640 710 14

ECAP(8) + Cold rolling(35%)b + Annealing 200 8C, 0.5 h 985 990 8

ECAP(8) + Cold rolling(73%) + Annealing 300 8C, 1 h 942 1037 12.5

ECAP(12) + Cold rolling(35%) + Annealing 300 8C, 0.5 h 920 1000 14

Cold rolling(35%) 660 670 16

a ECAP route BC was used for all samples.b Reduction in cross-section area by cold rolling.

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735728

micro-alloying technology is difficult to be applied to thisprocess. The temperature of the strip is controlled throughoutthe combined process to accumulate the dislocations in the grainsbefore 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–Mnsteel with a grain size of 1 mm [175]. A strip with a width of300 mm was successfully produced by this process. Fig. 47 showsan example of ultra-fine grained C–Si–Mn steel obtained by hotextrusion. Fig. 48 shows the yield strength of the steel sheetproduced by the SSMR process as a function of ferrite grain size[176]. Some previous studies and Hall–Petch equation arealso shown in the figure as a comparison. The yield strength is

Fig. 47. Ultra-fine grained steels obtained by SPD process.

Fig. 48. Yield strength as a function of ferrite grain size [176].

increased from 350 to over 700 MPa with in decreasing grain size4.5–1 mm, which is in good agreement with the Hall–Petchrelationship. It is also confirmed that the uniform elongationdeceases.

Another example of the combined process for producing theultra-fine grained steel is warm rolling and cooling, which usesferrite recrystallization during warm rolling [177–183]. Torizukaet al. [177,180–183] carried out multi-pass warm caliber rolling oftwo low carbon steel (SM490) specimens with a microstructure offerrite and Pearlite.

The specimen of the square bar with a side width of 80 mm wasused. The warm caliber rolling schedule is summarized in Fig. 49.The caliber rolling at 500 8C was conducted in five stages to obtainspecimens of different cumulative strains for different micro-structure and mechanical properties. The cumulative reductionand the cumulative strain at each stage of rolling are also shown inFig. 49.

Fig. 50 shows the relationship between nominal stress andnominal strain of specimens subjected to different cumulativestrains. The yield and tensile strengths of the caliber rolledspecimen increase monotonically with increasing cumulativestrain. There is a reduction in the elongation to failure of thecaliber rolled specimens compared to the undeformed specimen,but there is almost no change among the specimens with differentaccumulative strains.

Fig. 49. Caliber rolling schedule with cumulative reduction and cumulative strain at

each stage [180].

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].

Fig. 54. View of article of ‘‘Piston’’ type fabricated from nanostructured Al1420 [2].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 729

5. Applications

The properties of the metals processed by SPD exhibit highstrength, ductility and fatigue characteristics. UFG metals are usedas a structural material due to these properties. Bolts are alsomanufactured with titanium alloys processed by ECAP as shown inFig. 51 [184] and are widely used in the automobile and aircraftindustries. Micro bolts using the UFG carbon steel processed bycold ECAP have also been manufactured as shown in Fig. 52 [185].

Long carbon steel bars, of over several kilometers, with ultra-fine grains are manufactured by the warm continuous caliberrolling and cooling process, from which the micro bolts aremanufactured. Recently, in a Japanese National Project, sheets oflow carbon steel of 2 mm thickness with ultra-fine grains weremanufactured by the TMCP process. The deep drawing ratio of eachsheet was 1.9 and the parts were used in sheet metal forming asshown in Fig. 53 [186].

It is well known [187,188] that superplastic forming is a highlyefficient method of processing complex shape articles. An exampleof a possible practical application for nanostructured Al alloys isshown in Fig. 54 [2]. It presents a complex shape article of ‘Piston’type which was fabricated from the nanostructured Al1420 alloyby superplastic forming using the high strain rate superplasticity.

In practice, despite a range of improved mechanical andphysical properties of bulk UFG metals produced by SPD, theuptake of these materials by industry has been very slow so far.There are several reasons for this; one is the lack of industrialawareness of UFG metals. This is despite a large number ofacademics being engaged in research on SPD and UFG metals.Another reason is the scarcity of appropriately sized UFG samplesfor industrial trials; those produced by laboratories are usually toosmall because they are intended for metallurgical observations orbasic mechanical testing. Finally, it is still not clear which of the

Fig. 53. Examples of ultra-fine-grained C-Mn steel sheet forming [186].

Fig. 52. Overview and cross-section of micro bolts manufactured UFG Carbon steel

processed by cold ECAP [185].

numerous laboratory-based SPD methods will emerge as the mostappropriate for industrial implementation.

As a result, potential producers of UFG metals hesitate tocommit themselves to any particular method. They are alsoconcerned about the commercial viability of UFG metals, whichdepends on the demand from potential markets and the cost ofproduction. Both of them are difficult to assess because of the lowavailability of UFG metals and uncertainty regarding the SPDtechnology. There is also a lack of knowledge regarding post-SPDprocessing or shaping of UFG metals.

Nevertheless, there are some applications which, with a highdegree of probability, will be leading the introduction of UFGmetals into commercial markets. Initially, those applications arelikely to be in the niche markets producing low volume specialtyproducts (e.g. sputtering targets). The next step will be the mediumvolume markets with the emphasis put on product’s performancerather than price (medical implants, defense applications, aero-space components, sports equipment). Eventually, the massproduction of components may be undertaken by the automotiveand construction industries.

With the exception of sputtering targets, the examplespresented below refer to potential applications rather than thecurrent ones. Despite the focus of this paper on SPD-produced UFGmetals, applications using UFG consolidated powders and nanos-tructured electrodeposited metals will also be considered as theseare indicative of what can be achieved with all types of UFG metals.

The first commercial application of bulk UFG metals was insputtering targets for physical vapour deposition (Fig. 55).Honeywell Electronic Materials, a division of Honeywell Interna-tional Inc., offers UFG Al and Cu sputtering targets up to 300 mm indiameter which are produced from plates by ECAP [189,190]. Theyare used for metallization of silicone wafers in the production ofsemiconductor devices. The main advantages of UFG sputteringtargets, compared to their coarse grained (CG) counterparts, are:(1) the life span increased by 30% due to stronger material whichallows the use of monolithic targets and (2) a more uniformdeposited coating which results from reduced arcing. Anothercompany offering UFG Cu targets is Praxair Electronics, whichclaims better sputter performance and 75% reduction in theownership 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].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735730

The next highly anticipated application is in the area of medicalimplants. These include hip, knee and dental implants as well asvarious screws, plates (Fig. 56) and meshes used in orthopaedicapplications. Popular materials usually used in these applicationsare cobalt-chrome alloys, stainless steel and titanium alloys.Titanium alloys are used for implants because of their strength, lowmodulus of elasticity (better matching that of bones), corrosionresistance and good biocompatibility. Commercial pure (CP)titanium has better compatibility than titanium alloys but it isnot used for load bearing implants because it is not strong enough.

However, when nanostructured by SPD and subjected to furtherthermo-mechanical treatment, CP titanium can be strengthened toachieve the yield stress of 1100 MPa, which is comparable with theyield strength of titanium alloys [191]. Traditional titaniumimplants do not perform well with respect to wear resistanceand 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 arebeing already tried.

The defense industry could benefit from two large scaleapplications of UFG metals, which are armor plates and armorpenetrators. Lighter armor for military vehicles (Fig. 57) is crucial forthe reduction of fuel consumption, higher speed, better maneuver-ability, longer operation range and air-transport of vehicles toremote locations. At the same time the ballistic performancemust not be reduced. This can be achieved by the nanostructuringof aluminium or titanium alloys traditionally used for lightarmored vehicles. A good example is a UFG Al 5083 plate, whichwas 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 massreduction compared to the standard plate.

Improvements in ballistic performance are also reported for theelectrodeposited nanocrystalline nickel-iron alloys produced byIntegran Technologies. Armor structures are usually fabricated bywelding of plates. However, traditional welding based on meltingis destructive to the UFG material. An alternative technique is asolid state process of friction stir welding, which has the ability torefine grain structure. This results in the weld hardness being onlymarginally reduced compared to the initial hardness of a UFGmaterial [194].

Health issues surrounding the use of depleted uranium forarmor penetrators resulted in a search for alternative materialswith similar performance characteristics. One of those character-istics is an inherent ability of depleted uranium to self-sharpen onimpact which is due to the generation of adiabatic shear bands.Tungsten, sometimes considered as a replacement for depleteduranium because of its high density, does not have this ability; thuspenetrators made of tungsten undergo mushrooming on impact,which results in less penetration. UFG metals are known to havereduced strain hardening capacity, which promotes localizedplastic deformation; at high deformation rates this leads toadiabatic shear banding. This was confirmed by producing UFGtungsten (by ECAP with a die angle of 1208 at 1100–1000 8C andsubsequent rolling at 600–700 8C), which exhibited adiabatic shearbanding when subjected to a dynamic load [195].

The aerospace industry values even small weight reductionswhich might be achieved by the introduction of new materials ortechnologies. However, this industry is very cautious because ofthe safety concerns, and slow in implementing any changes.Introducing a new material may take 10–20 years, which resultsfrom the requirements of the well established technology and afully developed supply chain. UFG metals are not chemicallydifferent from their CG precursors, so there should be nofundamental obstacles to their use.

On the other hand the new properties of UFG metals have to bewell documented with respect to aerospace applications and theSPD and post-SPD processes have to be commercially available. Allthese requirements mean that we will have to wait a few moreyears for the first aerospace applications. These, most likely, will beassociated with light UFG metals used for structural components ofthe fuselage and wings. Regarding the engines, some externalelements (e.g. shields) and less thermally demanding internalelements (e.g. titanium blades for the low pressure compressorsection) can also be considered. There has only been limitedinformation published so far on the potential use of UFG metals bythe aerospace industry; Boeing, filed a few patents on friction stirwelding used as a means of nanostructuring metals for fastenersand other parts [196] while EADS is interested in UFG aluminumsheets.

Users of sports equipment will also benefit from UFG metals,particularly where high strength and low weight is required. UFGmetals could find applications in high performance bicycles, sailingequipment, mountaineering equipment, golf, tennis, hockey, etc.One example is NanoDynamics high performance (NDMX) golfballs, 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).

Fig. 61. SEM pictures of a micro-bulged sheet made of CG and UFG Al 1070 [200].

A. Azushima et al. / CIRP Annals - Manufacturing Technology 57 (2008) 716–735 731

technology licensed from Purdue University. Another example ofusing UFG metals in sporting goods is the commercial activity ofPowerMetal Technologies, a company with an exclusive license touse 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 carbonfibres and nanocrystalline metal.

UFG metals can be beneficial to some products throughimprovements in their manufacturing processes. The most promis-ing one is superplastic forming which is currently confined to a lowvolume production because of a very low process speed, necessarywhen forming classical superplastic metals. UFG metals possessbetter superplastic properties, which allow a tenfold increase of theforming speed, and some temperature reduction [197]. SuperplasticUFG metals exhibit higher ductility which makes them suitable forforming more complex components. Despite large volume ofresearch on SPF of UFG metals, practical applications are still amatter for the future. One possible application has been presentedby the Institute for Metals Superplasticity Problems, Ufa, Russia.They made models of hollow blades by diffusion bonding (DB) andSPF using UFG Ti–6Al–4V sheets (Fig. 60). By using sheets with thegrain size down to 0.2 mm they were able to decrease thetemperature of the process from 900 to 800 8C for DB and to700 8C for SPF [198]. The temperature reductions observed willimprove technical feasibility and the economics of the process.

Among many interesting properties of UFG metals is theirability to flow easier and at lower temperatures when forged intocomplex shapes. It is claimed that energy savings up to 30% couldbe achieved due to: lower forging temperature, shorter heat-uptime, smaller forging stock size, fewer number of hits and lowerforging load [199]. A very small grain size can be a virtue of its own.This is the case with metal micro-parts having geometrical sizescomparable with coarse grains of classical materials. Using UFGmetals in microforming allows micro-billets to behave aspolycrystalline billets. This refers to both the inner body and thesurface of the billet. The latter is illustrated in Fig. 61 as a

Fig. 60. Models of hollow blades made of UFG Ti–6Al–4V sheet (image courtesy of

Institute for Metals Superplasticity) [198].

substantial reduction of the orange peel effect [200]. Anotheradvantage of using UFG metals is better surface finish resultingfrom micro-milling [201], micro-EDM [202] and diamond turning[203].

The above applications of UFG metals are only a fraction of thepossible uses. Since the SPD technology can convert all CG metalsinto UFG metals, it is only a matter of time when new, sometimesunexpected, applications will be discovered. For this to happen,information dissemination among industrial engineers, transfer ofreliable SPD technologies to industry and commercialization effortis required [204].

6. Conclusion

Processes of severe plastic deformation, defined as metalforming processes in which an ultra-large plastic strain wasimposed on a bulk material in order to make ultra-fine grainedmetals, were reviewed in this keynote paper. As processes used forthis purpose, various methods such as, ARB, HPT, RCS, CEC, STS,CCDF, etc. were developed, and combined SPD processes withconventional processes were also proposed.

The properties of the metals processed by SPD are alsoreviewed. The SPD-processed metals have very high strength,and in order to increase the strength further, conventional coldforming processes are combined with SPD processes. Since theductility of metals is reduced by relatively low strain, the heattreatment of annealing is conducted after the SPD process in orderto improve the ductility. The properties of the metals processed bythe SPD processes exhibit high strength and ductility that lead togood fatigue characteristics.

The UFG metals could be used as structural materials due tothese properties, but the area of application is limited at themoment because the available size of billet is small. Since SPDtechnology can convert all metals into UFG metals, it is expectedthat new methods of producing larger billets will enlarge the areaof applications.

Acknowledgment

The authors wish to thank Prof. K. Osakada and Dr. J. Allwood forchecking the manuscript of keynote paper.

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