Severe Plastic Deformation for Industrial Applications

CHAPTER 5

Severe Plastic Deformation for Industrial Applications 5.1 INTRODUCTION As mentioned earlier, one of the major challenges encountered in severe plastic deformation (SPD) is the industrial production of ultrafine-grained (UFG) and nanograined (NG) metals [1]. This chapter reviews SPD processes that have been developed for the industrial-scale production of UFG metals. However, some processes, such as accumulative roll bonding, continuous frictional angular extrusion, asymmetric rolling, continuous repetitive corrugation, and straightening, which are suitable for industrial production of sheet samples, were discussed in chapter 3. Other SPD methods suitable for industrial production of UFG metals are discussed in this chapter. It is worth mentioning that most of the developed industrial methods may not be considered as SPD processes because they are a combination of conventional metal forming methods and modern SPD processes. In this combined approach, the cross-section of the samples changes during processing.

5.2 INTEGRATED EXTRUSION AND EQUAL CHANNEL ANGULAR PRESSING The main obstacle to a broader use of equal channel angular pressing (ECAP) in industrial manufacturing is low productivity, because the process is performed in batches rather than a continuous process [2]. ECAP usually involves many steps and is not easily adaptable from a laboratory scale to an industrial manufacturing environment. The possibility of producing long UFG bars with excellent mechanical properties by a “semicontinuous” process is promising for the transfer of SPD processing to industrial manufacturing scale [3]. Orlov et al. demonstrated an integrated process that combines ECAP and conventional extrusion into a single processing step [4]. The as-received bars are extruded through a die shown schematically in Fig. 5.1. It consists of two sections: (1) a conical section (where conventional extrusion takes place) and (2) a section with Severe Plastic Deformation DOI: https://doi.org/10.1016/B978-0-12-813518-1.00005-9

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Conical section PC section

Φ1

d0 d1

Φ2

K

Extrusion ECAP

Figure 5.1 Schematic illustration of the integrated extrusion and ECAP process.

two parallel channels—the PC section (where two consecutive ECAP events take place). The main parameters of the die geometry, which influence both the flow pattern and the stress and strain state of the ECAPrelated part of the process, are the distance, K, between the axes of the two parallel channels and the angle, ϕ, at which they intersect the connecting channel [4].

5.3 ECAP
CONFORM The conform extrusion process was developed in the 1970s for the continuous extrusion of wire products [5,6], but, in 2004, it was conveniently combined with ECAP and named the ECAP
conform process [7]. In this process, the principle used to generate the frictional force to push a workpiece through an ECAP die is similar to the conform process [5], but a modified ECAP die design is used so that the workpiece can be repetitively processed to produce UFG structures [8]. The design and ECAP
conform setup are schematically illustrated in Fig. 5.2. As shown in the figure, a rotating shaft in the center consists of a groove, and the workpiece is fed into this groove. The workpiece is driven forward by frictional forces at the three contact interfaces with the groove so that the workpiece rotates with the shaft. However, the workpiece is constrained within the groove by a stationary constraint die, which also stops the workpiece and forces it to turn at an angle by shear as in a regular ECAP process. In the current set-up, the angle is close to 90 degrees, which is the most commonly used channel intersection angle in ECAP. This setup effectively makes the ECAP process continuous. Other ECAP parameters, such as the die angle and the strain rate, can also be incorporated into the facility [7,8].

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Stationary constraint die

Rotating die

Out

Sample In

Figure 5.2 A schematic illustration of an ECAP
conform set-up: the arrow marks the transition to a rectangular cross-section.

When a wire sample enters the ECAP
conform die, the rectangular cross-section is formed shortly after the wire enters the groove (marked by an arrow in Fig. 5.2). This change in cross-section is driven by the frictional force between the groove wall and the workpiece. Thus, the frictional force pushes the wire forward and deforms the wire to the groove shape. After changing cross-section of the wire into a square shape, the frictional force per unit of wire length becomes larger because of the larger contact area between the groove and the wire. The total frictional force pushes the wire into the stationary die channel that intersects the groove at an angle of 90 degrees. This latter part of the straining process is therefore similar to the conventional ECAP process [7,8].

5.4 EQUAL CHANNEL ANGULAR DRAWING Equal channel angular drawing (ECAD), potentially suitable for continuous SPD processing, has been introduced for the processing of UFG samples [9,10]. Subsequent experiments and calculations revealed that ECAD reduces the cross-sectional area of the sample by .15% and hence it cannot be used effectively for multipass processing [8,11]. However, low hydrostatic stresses may influence the structure and properties of the final product [12,13]. This process involves pulling the workpiece through the die. The ECAD die consists of two square channels intersecting at an angle of 135 degrees as shown in Fig. 5.3. The overall forming force will be reduced during the drawing operation by eliminating friction between the workpiece and the entrance channel of the die. However, several

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Load Gripper

Sample

Die

Figure 5.3 Principal scheme of processing by ECAD. Plunger φ

Die

Figure 5.4 Principal scheme of sheet processing by ECADS.

limitations arose in the drawing process. First, the applied stress for the drawing of material should not exceed the strength of the drawn sample. Second, tensile stress may initiate damage, nanocracks, and defects leading to lower mechanical properties [14,15]. Equal channel angular drawing of sheet metals (ECADS) was proposed by Zisman and colleagues (Fig. 5.4). The principal deformation mode is simple shear supplemented by some elongation along the drawing direction and consequently thickness reduction. The plunger position can be adjusted to the desired sheet thickness by special screws. ECADS has been used for pure Al at room temperature on a strip of 40 mm width and 1 mm thickness [16,17].

5.5 ECAP WITH ROLLS Some facilities based on the ECAP method have been schematically suggested to achieve the bulk UFG and NS material for industrial

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(A)

(B)

169

(C)

Figure 5.5 Schematic of different design for the use of rotating tools: (A) single large work roller, (B) small work roller with a cluster of support rolls, and (C) single row of work rollers in the conventional planetary arrangement.

applications. The first of these is a variation of ECAP in which a portion of the conventional fixed die is substituted by rotating tools [18]. The rotating tool can be a single roller or a series of smaller rollers. The rolling tool is positioned so that the thickness of the final product is (nominally) the same as the initial thickness. In the first design, the rotating tool is a single large-diameter roller as shown in Fig. 5.5A. The sample is fed down toward the large roller which coerces the sample to change direction, and the shear stress is applied to the material. In the second version, the single roller is replaced by a cluster of small work rolls as shown in Fig. 5.5B. In the other case, a single layer of small rolls is arranged around the central support roll in a conventional planetary arrangement (Fig. 5.5C) [18]. These designs for the ECAP method have not been experimentally performed, but only suggested to the researcher to enhance greater volumes of UFG and NS materials with respect to conventional ECAP. The designer mentioned many potential benefits of these designs such as possible industrial applications, including alternate and larger workpiece geometries, lower tooling loads, ease of lubrication, automated or reduced part handling, and, in some cases, potentially continuous operation [18]. The “push
pull” arrangement for a continuous severe plastic deformation (CSPD) is shown schematically in Fig. 5.6. Rollers are shown for illustrative purposes, though several other mechanisms could be used for both pushing and pulling. Furthermore, several such stages can be sequenced to cause deformation on different shear planes to achieve any of the processing routes used in standard ECAP [19].

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From previous stage

Draw rolls Push rolls

To next stage

ECAP die Draw rolls

Push rolls

Figure 5.6 Schematic of a “push
pull” arrangement in the CSPD process.

5.6 INCREMENTAL ECAP The industrial production of long UFG billets is a major challenge. The friction increases dramatically with the length of a billet and the possibility of punch buckling increases in the conventional ECAP process. Rosochowski and Olejnik proposed a solution to this problem by separating the stages of feeding and plastic deformation [20,21]. Incremental ECAP (I-ECAP) was presented in 2007 to reduce forces and process relatively large billets or plates. Unlike conventional ECAP, I-ECAP is performed in small steps in which the deformation and feeding are affiliated with two different tools acting synchronously [22]. A schematic of I-ECAP is shown in Fig. 5.7. The fixed die and holder establish the input channel, while the fixed die and the moving punch play as the output channel. The moving punch has the duty of applying deformation, which actuates cyclically at an appropriate angle to the billet. By doing that, it periodically comes into contact with the billet and deforms it. Feeding of the billet takes place during withdrawal of the moving punch when there is no contact. When the billet contacts the moving punch and becomes fixed in position, the moving punch advances and deforms the billet plastically in the “shaded” zone (Fig. 5.7). This operation is carried out continuously on the billet, and consecutive shear zones result in a uniform strain distribution along the billet. The mode of deformation is simple shear, and the feeding stroke (distance “a” in Fig. 5.7) is not excessive. Separation of the feeding and deformation

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Moving punch

a

Holder Fixed die

Material feeding

Figure 5.7 Schematic illustration of I-ECAP.

Moving punch

a

Fixed die

Fixed die

Material feeding

Figure 5.8 Schematics of double-billet I-ECAP.

stages reduces or eliminates friction during feeding, and this enables the processing of long billets [23,24]. A double-billet version of I-ECAP was developed in 2008 by the same presenters of I-ECAP [23]. The process configuration is illustrated in Fig. 5.8. Compared to I-ECAP, the holder is replaced with another fixed die. The moving punch is positioned in the top of the fixed dies and creates two output channels. According to this configuration, with feeding the material, two zones of simple shear are established in the shaded mode as shown in Fig. 5.8. Compared to the I-ECAP method, the shear zones increase from one to two, increasing the process force [23].

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Bearing die

ECAP channel container

Tube P-ECAP die

Initial billet

Ram

Extrusion direction

Figure 5.9 Schematic of the P-ECAP process.

5.7 PORTHOLE-EQUAL CHANNEL ANGULAR PRESSING In recent years, several techniques have been developed based on a combination of conventional and SPD methods, some of which will be discussed here. The method of porthole-equal channel angular pressing (P-ECAP) with four portholes has been introduced as a combination of extrusion and quad-channel ECAP to produce UFG tubes with four-seam welding [25]. The P-ECAP method is schematically shown in Fig. 5.9. The production of UFG tubes by the P-ECAP process consists of four stages: (1) dividing stage: the initial billet is split into four parts, (2) ECAP stage: divided parts are driven into four channels with the same cross-sectional area, (3) welding stage: divided parts are driven to the welding die to weld the connection lines at high pressure and solid state, and (4) production stage: UFG tube is exited from the bearing part of the die. The tube that was fabricated using a P-ECAP die showed significant refinement in microstructure with improved mechanical properties outside the seam joint portion. In conventional extrusion, the dimension of the billet is greater than that of the product. However, the P-ECAP was applied to manufacture a tube with a diameter greater than the initial billet. Thus, the extrusion loads using porthole die are lower than the load in the conventional extrusion because of the reduced billet dimension or extrusion ratio [25].

5.8 CONTINUOUS CONFINED STRIP SHEARING A metal forming technique called the continuous confined strip shearing (C2S2) process based on ECAP was introduced by Lee et al. [26] in 2001. The characteristics of ECAP can be utilized as a basis for introducing a forming technique through to producing UFG and NS materials with tailored properties [27]. This process is also referred to as dissimilar

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channel angular pressing (DCAP) in some works [28
30] or equalchannel angular rolling (ECAR) [31]. The concept of the developed process that was designed to introduce the shear deformation into metallic strips in a continuous manner is shown in Fig. 5.10. A specially designed feeding roll and a guide roll are used as a feeding apparatus. Nulls were machined on the surface of the feeding roll so that it can deliver the power required to feed the metal strip through the ECAP channel. Before feeding, a commercial synthetic oil was applied to the surface of the strip to reduce the friction between the die wall and the strip during forming. The feeding speed is dependent on the dimensions of the workpiece and the angle of the channel, but it is normally in the range from 5 to 50 m/min. The forming die is equipped with two channels; whose thicknesses are dissimilar to each other such that the thickness of the outlet channel is slightly larger than that of the inlet channel. The oblique angle (Φ) of the channel, which is the intersecting angle of the inlet and the outlet channel, can be adjusted from 100 to 140˚ with a fixed curvature angle (ψ) of 0˚. When the strip with the larger initial thickness (t0 ) is fed through the feeding rolls, the strip thickness is reduced (t) upon escaping the roll gap and proceeds along the die gap toward the forming zone. Once the strip passes through the forming zones, where the inlet and the outlet channels intersect, and exits through the outlet channel, it retains its initial thickness (t0 ). The fact that the thickness of the metal strip retains its initial thickness upon exiting the die makes the multipass operation possible in a continuous manner [27]. However, a specially designed feeding roller with grooves is used in the C2S2 process that causes surface defects on the workpiece. Eq. (5.1) gives the effective strain attainable from DCAP, which was introduced for the C2S2 process [27]. The equation is expressed in terms

Figure 5.10 The concept of the C2S2 process for continuous confined strip shearing with the details of the ECAP channel.

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of the oblique angle (Φ), the passage ðN Þ, and thickness ratio ðK Þ, which is defined as the inlet thickness divided by the outlet thickness [27,30].   2N 2 Φ ε 5 pffiffiffi K cot (5.1) 2 3

5.9 CONSHEARING The conshearing method was proposed for use with metal strips in 1997 [32
34]. This process employs a continuous rolling mill and is schematically illustrated in Fig. 5.11. In this procedure, the material is fed into the mill between satellite rollers and a large central roller, and all of these rollers rotate at the same peripheral speed to generate a large extrusion force. The strip passes between the rollers and ultimately passes from the mill through an abutment where it is displaced through an angle φ. Detailed experiments with commercial-purity aluminum strips showed that optimum conditions for ECAP were achieved when the angle within the abutment was given by φ 5 65 degrees [34]. This process is not only productive but also applicable to coiled materials. Given that simple shear deformation is continuously imposed on coiled strips, the process can be used as a texture-control method. This process was successfully applied to an aluminum alloy [35], and the formation of shear textures was reported [34]. However, the conshearing process uses a large number of rollers to impose a high-friction force on the workpiece which is one of disadvantages of this method. Guide shoe Satellite roll Cover φ ECA-die θ Abutment Strip material

Central roll

Figure 5.11 Schematic illustration of the conshearing process.

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Flattening roll

175

Bending roll

Material feeding

Advance direction

Figure 5.12 Roll-driven CCB machine.

5.10 CONTINUOUS CYCLIC BENDING Continuous cyclic bending (CCB) has been proposed as a straining technique for sheet materials that can produce high strain on the surface and low strain on the inside [36]. This cyclic deformation straining technique leads to a definite difference in stored strain energy between the surface and central layers. Thus, the CCB process and the subsequent annealing make it possible to produce the gradient microstructure with the coarsegrained surface layer and the fine-grained central layer. For the aluminum alloy sheet consisting of such layers, an improvement in the fatigue properties was reported in 2004 [37]. Takayama et al. used roll-driven CBC, illustrated in Fig. 5.12, where true strain on the surface after a single pass is calculated as 0.05463 [38]. The lower strain, lower hydrostatic pressure, lower shear strain, and high possibility of cracking at multipass processing are the main disadvantages of this process.

5.11 CALIBER ROLLING Recently, the multipass caliber rolling process has attracted particular attention due to its ability to produce bulk UFG rods in large quantities [3]. In contrast to the conventional flat rolling, the rolls in this process have several calibers with various diameters to fabricate long metallic rods. A large strain can be accumulated by gradually reducing the diameter of the metal using a repetitive deformation [39]. An ideal illustration of caliber rolling (CAROL) is schematically shown in Fig. 5.13. The upper roll and the counterpart lower roll have channels of identical dimensions. However, the roll diameter is smaller at the bottom portion than the top portion of the caliber roll. Therefore, the outer rolling speed is slower at the bottom (Vbtm ) than at the top

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Figure 5.13 The scheme of the CAROL process: (A) first pass and (B) subsequent pass.

(Vtop ). When a billet is subjected to rolling, the material becomes deformed plastically for a certain duration. The cross-sectional area of the billet is reduced at a fixed ratio, while the discrepancy of Vtop and Vbtm creates a certain shear strain after passing the roll, as illustrated in Fig. 5.13A. After the first rolling pass, the rolled billet is rotated 90˚ clockwise around the rolling axis to enhance more uniform strain. The billet is then subjected to rolling in the same way as the former pass. The cross-section is reduced again with the same reduction ratio; however, the shear direction is opposite to that of the former pass, as illustrated in Fig. 5.13B. The actual deformation may be more complicated due to factors such as lateral extension during the processing [40]. Long UFG carbon steel bars of over several kilometers are manufactured by the warm continuous caliber rolling and cooling process, from which micro bolts are manufactured [41].

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5.12 RING HIGH-PRESSURE TORSION Three main limitations of the HPT process are the sample shape (disk), the sample size (small), and an inhomogeneous distribution of microstructure across the diameter [42]. To overcome the sample shape and inhomogeneity limitations, the HPT using a ring shape, which is more appropriate for industrial applications than a disk shape, was introduced by Harai et al. [43] in 2008. Using ring HPT, it is possible not only to eliminate a less strained and coarse-grained center part [43
45] but also to scale up the sample, e.g., up to 100 mm in diameter [46,47]. A schematic illustration of the HPT with a ring sample is given in Fig. 5.14. Two advantages of ring HPT were explored with respect to disk HPT. First, the ring diameter is increased by the amount corresponding to the hollow inner area. Second, the whole area of the ring sample can be usable because of the homogeneous strain and thus of homogeneous microstructure throughout the ring [46].

5.13 HIGH-PRESSURE SLIDING As another attempt to enhance the opportunity for scaling-up the HPT process, high-pressure sliding (HPS) was developed for producing metallic sheet materials with 0.8 mm thickness, 5 mm width, and 100 mm length [48]. As schematically illustrated in Fig. 5.15, the HPS process consists of one plunger and two U-shaped anvils. The upper and lower surfaces of the plunger are grooved. Also, each of the lower and upper U-shaped anvils is grooved on the inner bottom surface. The sheet samples are then placed Upper anvil Load Ring sample

Load Lower anvil Rotation

Figure 5.14 Schematic illustration of ring HPT.

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Pressure

Upper anvil Plunger Sample

Lower anvil

Pressure

Figure 5.15 Schematic illustration of HPS.

into the grooves, and a load is applied to the anvils. The plunger is then pressed, and shear strain is applied to the sheet under high pressure from the U-shaped anvils. It is different from HPT such that the HPS no longer requires rotation of the anvils but sliding between the anvils. The equivalent plastic strain applied in this process, ε, is given by Eq. (5.2) [48]: x ε 5 pffiffiffi (5.2) 3t where t is the sample thickness, and x is the sliding length.

5.14 CONTINUOUS HIGH-PRESSURE TORSION In both HPS and ring HPT, the size of the sample is still limited because the pressure is sacrificed with the increase in size of the sample. For example, the HPT processing of a ring with 3 mm width and 500 mm outer diameter under a pressure of 2 GPa requires a compression load of 9400 KN. For HPS processing of a sheet with 5 mm width and 500 mm length under a pressure of 2 GPa, the compression load should be 5000 KN. Therefore, an alternative SPD process was developed in 2010 for processing of metallic sheets with HPT in a continuous manner, which Edalati et al. called continuous high-pressure torsion (CHPT) [47]. The facility for CHPT, schematically illustrated in Fig. 5.16, consists of two anvils: upper and lower anvils. The lower anvil has a flat surface with a roughened ring-shaped area which is rotated during the process, and the upper anvil has a half ring-shaped groove on the surfaces.

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Sample Upper anvil

In

Load

Groove

Roughened Out

Load Lower anvil

Upper anvil (Fix)

Lower anvil (Rotate)

Rotation

Figure 5.16 Schematic illustration of CHPT [47].

To induce continuous flow of the material due to the difference in slippage, the surface roughness of the upper anvil is reduced with respect to the surface roughness of the lower anvil. A U-shaped sample is placed between the anvils, and pressure is applied to the sample by raising the lower anvil to make a rigid contact with the upper anvil. The lower anvil is then rotated while the upper anvil no longer rotates. A shear strain is applied to the sample under high pressure, and the material starts to flow in the direction of rotation [47]. The equivalent strain induced by CHPT, ε, is calculated as follows [47]: πR ε 5 ð1-sÞ pffiffiffi 3t

(5.3)

where s is the fraction of sample slippage, and R is the mean radius of the U-shaped sample. The slippage was evaluated by measuring the discrepancy of the markers made on both surfaces of the sample after rotating by a quarter revolution (90 degrees). It has been found that the slippage was negligibly small for the first quarter revolution for all of Al, Cu, and Fe. However, the revolution by 180 degrees led to the disappearance of the markers, indicating that slippage occurred during another 90 degrees revolution [47].

5.15 SEVERE TORSION STRAINING The principles of the severe torsion straining (STS) process developed by Nakamura et al. are represented schematically in Fig. 5.17 [49]. The process consists of producing a locally heated zone and creating torsional strain in the zone by rotating one end. The rod-shaped sample is moved along the longitudinal axis while creating the local straining. The torsion

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Induction coil for local heating

Water spray for cooling

Rod

Shearing

Rotation

Continuous movement

Figure 5.17 Schematic illustration of STSP.

straining (TS) zone is localized by making the zone softer than the other two portions by local heating and cooling. Therefore, a severe plastic strain is induced continuously throughout the rod. To create a torsional strain efficiently, the rotation of the rod should be fast with respect to the movement of the rod, and the locally heated zone should be narrow. Moreover, the cooling system is modified so that the heated zone is more localized to create torsion strain [41]. The STS method is different from the conventional torsion testing procedure for measurement of mechanical properties [50] as STS consists of the creation of a localized soft zone with respect to the other portions of the rod and movement of the zone along the longitudinal direction of the rod. Similar to other SPD processes like ECAP, HPT, and ARB, an important feature of STS is that the cross-section of the rod remains unchanged during straining. Also, unlike the other processes, STSP requires no die (may be considered as a dieless SPD process) and imparts severe strain to samples continuously. It is expected that STSP will be a potential process for the continuous processing of tubes and possibly wires [49]. However, heating may enhance the grain growth mechanism and consequently increase the grain size, leading to lower mechanical properties [51,52]. It seems that it would be hard to achieve UFG and NG microstructures using STSP.

5.16 INTEGRATING FORWARD EXTRUSION AND TORSION DEFORMATION Lu et al., in 2014, demonstrated that applying the integrating forward extrusion and torsion deformation process would greatly improve the efficiency of processing of Mg alloys and promote the up-scaling processing

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Figure 5.18 Sketch of the integrated extrusion die illustrating the die structure and extrusion fashion, and definition of external orientations [53].

[53]. Fig. 5.18 shows a sketch of the integrated extrusion dies. The particular zones of the extrusion deformation were labeled I, II, III, and IV, which represent the conical part, the forward extruded rod, the torsional part, and the eventual forming stage, respectively. The integrated extrusion die consists of two equal sectional dies that were clamped by screws, each of which had a half torsion structure after forward extrusion. The torsional shearing strain can thereby be imposed on forward-extruded materials to further modify the microstructure and texture [53].

5.17 KOBO PROCESS Korbel and Bochniak suggested one more method of SPD for industrial applications, called the KoBo process [54,55]. As shown in Fig. 5.19, the method consists of a cyclic change of the deformation path in combined torsion and extrusion, forging, rolling, or drawing processes. The principal aim of the KoBo method is to take advantage of strain-induced softening that results from the change of deformation path. The change of the deformation path favors especially the mechanism of localized plastic flow in slip bands for monocrystalline materials in which shear bands for polycrystalline aggregates, at the expense of the multisystem crystallographic slip decreasing the work hardening of the material [56,57].

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Reversibly rotating die

Reversibly rotating die

Sample

Sample

(B)

(A)

Reversibly rotating die Roll

Die Wire

Sample Gearbox

Reversing shifting roll

(C)

(D) Motor

Figure 5.19 Schematic representation of some technical solutions of KoBo metal forming: (A) extrusion, (B) forging, (C) rolling, and (D) drawing.

The KoBo method resembles HPT (pressure is implied simultaneously with torsion), with a significant difference: torsion is oscillating with a frequency of about several Hertz and amplitude about 5
7 degrees. Such a complex method of plastic straining causes the highly heterogeneous flow of metals in the strongly shortened zone of deformation and as a result of radial flow in the direct vicinity of the die. An associated drastic decrease in the extrusion force, which depends on the frequency and amplitude of the die rotations, deforms the metal with very large strains at low temperature, which make the method unique. During the KoBo extrusion process, the metal billet undergoes reversible plastic twisting just before entering the cross-section reducing die. The reversible metal twist does not affect the geometry of the billet directly [58].

5.18 CRYO-ROLLING Cryo-rolling is another technique that can be used to produce continuously long product as compared to other SPD processes [59]. In cryorolling, the material is dipped in liquid nitrogen (
190˚C) and held there

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for a distinct time (depending on the requirements) and then rolled between two rollers. Moreover, the cryo-rolling process offers other advantages, such as lower required plastic deformations and simple processing.

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