*3.5. Deformation Routes*

Symmetric rolling can be performed without any kind of rotation of the sheet between each pass. However, in asymmetric rolling, the purpose of imposing high levels of shear strains leads to the idea of rotating the sheet between each rolling pass, taking advantage of the difference in work roll speeds [77]. Several rotation routes can be performed, as shown in Figure 30 [78–81]. By the deformation route UD, the sheet is not rotated before and after each rolling pass, while in the other three cases it is rotated by 180◦ degrees. By the route RD, the rotation axis is the rolling direction, by the route TD it is the transverse direction and by the route ND it is the normal direction. From the observation of the possible reversal routes, it becomes clear that the UD and TD routes do not impose any reversal of strains on the sheet's surface, whereas RD and ND impose strain reversal. In the literature, the TD route is one of the most commonly used, but when studying the asymmetric rolling process, S.H. Lee and D.N. Lee [78] observed textures closer to the ideal "shear" components on specimens processed by the RD and ND reversing routes. Using FEM simulations, Kim and Lee [82] studied the influence of shear strain evolution during symmetric and asymmetric rolling. They found that in symmetric rolling, the sheet undergoes positive and negative shear strains, before and after the neutral point, respectively [82,83]. However, during asymmetric rolling, shear strains are positive. By using the shear strain history, determined for symmetric and asymmetric rolling, Kim and Lee calculated the resulting crystallographic textures. They found that the ideal shear textures were closely related to the shear strain reversal. In order to understand the influence of the deformation route, it is necessary to examine the shear patterns. These patterns are illustrated schematically in the form of the dominant directions of shear for routes UD, TD, RD and ND. It is apparent from Figure 30 that the shear patterns are significantly different between UD, TD and RD, ND deformation routes [84]. Routes UD and TD repeat shear on the same plane. By contrast, routes RD and ND have two shear planes intersecting at the shear angle. Y.G. Ko and K. Hamad [85], for AZ31 Mg alloy, experimentally showed that the RD route is the most effective in grain refinement, yielding equiaxed grains with an average size of 1.2 μm, while routes UD and TD yielded elongated grains with average sizes of 1.4 and 2.1 μm, respectively (Figure 31). Route ND, in turn, led to equiaxed fine grains (1.3 μm), but the trough thickness homogeneity achieved by the ND route was less than that achieved by the RD. The coarser-grained microstructures

obtained for the sheets deformed by the UD and TD routes led to lower yield strengths compared to those deformed by the RD and ND routes [85].

**Figure 30.** Schematic illustration of deformation routes and shear directions during asymmetric rolling. In UD rolling, rolling direction is not changed, whereas in TD, RD and ND rolling, the sheet is rotated through 180◦ about TD, RD and ND axes, respectively, for each pass. Reprinted from [84].

**Figure 31.** Grain size distributions and average grain sizes of the AZ31 Mg alloy samples deformed by differential speed rolling using the different deformation routes (thickness reduction from 4 mm to 1 mm by two passes, speed ratio *SR* = 4.0), reproduced from [85], with permission from Elsevier, 2018.

Asymmetric rolling can give rise to severe plastic shear strains and in turn shear deformation textures through the sheet thickness. The ideal shear deformation texture of FCC metals can be approximated by the <111>//ND and {001}<110> orientations, among which the former improves the deep drawability. The ideal shear texture could not be obtained by using unidirectional asymmetric rolling, but only by using RD or ND reversing deformation routes [82,83].

One of the most important parameters in asymmetric rolling is the thickness reduction per pass [77]. When combined with a difference in work roll speeds, an increase in thickness reduction per pass imposes higher levels of shear strain on the sheet surface and, under the appropriate conditions, it is essential to achieve the total propagation of those shear strains along the sheet's thickness. Kim and Lee [82] studied the influence of the reduction per pass on the shear texture formation. They found that high values of this parameter were necessary to achieve shear textures. Low thickness reductions per pass led to plane strain textures, similar to those found with conventional rolling. At the same time, the thickness reduction per pass has a nonlinear effect on grain refinement. Lee and Lee [79,86], from the experimental results, concluded that a 5% or 50% reduction per pass gave rise to finer grains with a higher fraction of high angle boundaries than reductions of 10, 20 and 30% per pass (Figure 32). Figure 33a shows EBSD mappings of AA1050 sheets asymmetrically rolled by 93%. Uniformly distributed high-angle boundary structures developed in the sheets rolled with 5% and 50% reductions per pass. It can be seen that the 5% and 50% sheets have higher fractions of high-angle boundaries over 15◦ and finer grain sizes, as shown in Figure 32. The grain size decreased from the initial size of about 100 μm to 1.4–1.8 μm [79].

**Figure 32.** Disorientation distributions and average grain sizes in AA1050 sheets asymmetrically rolled by 93% after annealing at 195 ◦C for 60 min (thickness reduction from 4.2 mm to 0.3 mm, speed ratio *SR* ≈ 2.0 due to different diameters of work rolls of 248 mm and 128 mm), reproduced from [79], with permission from Elsevier, 2008.

The equivalent strain *ε* in the surface layer of the rolled sheet is a function of friction coefficient *μ* and the geometric shape factor *L*/*tave* (where *L* = - *<sup>R</sup>*(*h*0 − *h*1) is the length of the deformation zone; *tave* = (*h*0 + *h*1)/2 is the average thickness of the sheet; *h*0 is the initial sheet thickness; *h*1 is the final sheet thickness) [79]. During rolling with high contact friction, the equivalent strain *ε* is the highest when the geometric shape factor *L*/*tave* is very small or, on the contrary, very large (Figure 33b) [79]. For a given thickness reduction, the equivalent strain *ε* is proportional to shear strains because the normal strains are the same, as can be seen in Equation (6). Therefore, the minimum equivalent strain *ε* in Figure 33b corresponds to the minimum shear strain at some value of geometric shape factor *L*/*tave*. The geometric shape factors *L*/*tave* at 5% and 50% thickness reductions per pass in [79,86] could be on the left and right sides of the minimum equivalent strain *ε* or the minimum shear strain, respectively. Thus, the reason why a 5% or 50% thickness reduction per pass

gave rise to finer grains with a higher fraction of high-angle boundaries than thickness reductions of 10, 20 and 30% per pass can be explained.

**Figure 33.** (**a**) EBSD mapping along rolling plane of AA1050 sheets asymmetrically rolled by 93% after annealing at 195 ◦C for 60 min (thickness reduction from 4.2 mm to 0.3 mm, speed ratio *SR* ≈ 2.0 due to different diameters of work rolls of 248 mm and 128 mm). (**b**) Schematic representation of equivalent strain *ε* in the surface layer against geometric shape factor *L*/*tave* at different friction coefficients *μ*, reproduced from [79], with permission from Elsevier, 2008.

#### **4. Asymmetric (Hot, Warm, Cold) Rolling of Mg Alloys**

Magnesium alloys (especially these containing Al and Zn additions—so-called AZ series) are in many fields superior to aluminum alloys (e.g., they possess a lower density and thus a better specific strength). However, their problematic formability requires consideration [62]. Over the last 20 years, a number of scientific works have been devoted to a fabrication of Mg alloy sheets with a good drawability. It was proposed that the main reason for the very poor cold formability and high mechanical anisotropy is an induction of a strong {0 0 0 1} basal texture in conventional plastic-forming processing due to a limited number of slip systems in the hexagonal close-packed (hcp) crystal structure [62]. The results of an extensive study on various Mg alloys (Tables 2–4 and Figure 34) showed that the (hot, warm, cold) differential speed rolling has a grea<sup>t</sup> impact on the intensity of the basal texture, grain size and plasticity of these materials. It was established that increasing the shear strain by raising the roll speed ratio *SR* leads to weakening of the basal texture through facilitating the activation of prismatic slip during deformation. The basal texture weakening effect at high speed ratios is attributed to extensive tension twinning that occurs in the basal-oriented matrix. Consequently, the asymmetrically rolled Mg alloy sheets are characterized not only by more isotropic properties but also by the enhanced plasticity combined with exceptionally high strength that is related to the simultaneous structure refinement. Therefore, the differential speed rolling process is considered to be one of the most efficient techniques for processing these materials.





**Table 4.** Experimental data related to asymmetric cold rolling of Mg alloys.

**Figure 34.** Different Mg alloys, which were processed by asymmetric (hot, warm, cold) rolling (based on data from Tables 2–4).

In 2007, Kim et al. [120] first reported about AZ61 sheet processed by severe plastic deformation using differential speed rolling with a high speed ratio (*SR* = 3) and the AZ61 sheets contained ultrafine grains 0.3-0.5 μm in size after single-pass rolling at 200 ◦C (70% thickness reduction). The sheets exhibited good low-temperature superplasticity at 200 and 250 ◦C, including a maximum elongation of 850% at 250 ◦C and 3 × 10−<sup>4</sup> *s*<sup>−</sup>1.

Based on the analysis of publications related to asymmetric (hot, warm, cold) rolling of Mg alloys, it was found that exceptionally high strength in a Mg-3Al-1Zn (AZ31) alloy was achieved by Kim et al. in 2011 [116]. After high-ratio differential speed rolling (*SR* = 2) at 150 ◦C followed by immediate water quenching, a grain size of 0.6 μm was obtained. Figure 34 shows the TEM micrograph of the AZ31 sheet. Some grain boundaries were sharp while others were not, indicating a mix of high- and low-angle boundaries. There were many fringes (Figure 35a) and network structures (Figure 35b) within the grains, which corresponded to a high density of dislocations arranged into cell and subgrain boundaries or dislocation networks in the grains. Few twins were spotted. Figure 35b shows many very small spherical or rod-shaped particles homogeneously dispersed over the matrices of AZ31. Energy-dispersive spectroscopy elemental mapping indicated that most of the particles were Al–Mn phase. The Al/Mn atomic ratios were between 1.6 and 1.8, implying that the particles were most likely to be Al8Mn5 [116]. As well as the Al–Mn particles, β-Mg17Al12 particles were also occasionally encountered. They had a spherical morphology, with sizes of 100–150 nm, and were located mainly on the grain boundaries (Figure 35c). The ultrafine-grained AZ31 exhibited a very high yield stress of 382 MPa, ultimate tensile strength of 401 MPa and total elongation of 6.8% [116].

In 2014, W.Y. Kim and W.J. Kim [117] presented the continuous high-ratio differential speed rolling (HRDSR) technique (Figure 36) for the continuous production of ultrafine-grained AZ31 alloy sheets with enhanced room temperature mechanical properties compared to commercial Mg sheets.

**Figure 35.** (**a**) TEM micrograph of AZ31 sheet obtained after high-ratio differential speed rolling (*SR* = 2) at 150 ◦C followed by immediate water quenching. (**b**) Al–Mn and (**c**) β-Mg17Al12 particles in the matrix, reproduced from [116], with permission from Elsevier, 2011.

**Figure 36.** (**a**) The layout of the continuous HRDSR mill. (**b**) The appearance of the HRDSR-processed sheets in coil form. (**c**) The appearance of the HRDSR-processed sheets. Snap shots of the sheets coming out of the rollers during the second pass under (**d**) HRDSR and (**e**) conventional rolling conditions, reproduced from [117], with permission from Elsevier, 2014.

#### **5. Asymmetric (Hot, Warm, Cold, Cryo) Rolling of Al Alloys**

Aluminum and aluminum alloys are well known for their high mechanical property anisotropy (a so-called earing behavior) upon a deep drawing process [62]. It was recognized that the main determinant of such behavior is a {100}<100> cubic crystallographic texture formed in a fully annealed state [62]. On the other hand, it was proposed by Lequeu and Jonas [123] that formation of the undesired {100}<100> recrystallization texture component may be prominently inhibited through an application of shear strain prior to a heat treatment. Therefore, a number of works have been devoted to the development of asymmetric rolling-based processing techniques that allow for the fabrication of aluminum alloy sheets with enhanced formability [62]. Engler et al. [124] reported that the most efficient method of formability improvement is to introduce {111} textures (composed of crystallographic orientations that are characterized by {111} crystallographic planes parallel to a rolling plane). Since these orientations are normally found in BCC metals and alloys (and are responsible for an excellent drawability of low-carbon steels), in the case of FCC metals, they may be produced only by shear strain. Jin and Lloyd [125] proved that the recrystallization texture of AA5754 aluminum alloy is randomized (the {001}<100>

component is prominently reduced), when a high-ratio differential speed rolling (*SR* = 1.5 and *SR* = 2) is applied before the annealing treatment. The {111} shear strain texture is maintained in the material after annealing that allows for the lowering the so-called planar anisotropy (that characterizes an alteration of mechanical properties in different directions lying in the rolling plane). Analogous results were also obtained by Sakai et al. [39], who showed that 5052 aluminum alloy cold deformed with a 75% thickness reduction in a twopass asymmetric rolling process followed by recrystallization annealing at a temperature of 310–460 ◦C exhibits almost perfectly isotropic mechanical behavior (values of the planar anisotropy coefficient were reduced to nearly zero) [62].

Results of an extensive study on various Al alloys (Tables 5–8 and Figure 37) showed that (hot, warm, cold, cryo) differential speed rolling has a grea<sup>t</sup> impact on the grain size and mechanical properties of these materials.

**Figure 37.** Different Al alloys, which were processed by asymmetric (hot, warm, cold) rolling (based on data from Tables 5–8).


**Table 5.** Experimental data related to asymmetric hot rolling of Al alloys.

AA6xxx 2.0/1.0 - 2.0 30%/2 250 - 134 261 37 [128]




In 2009, Jiang et al. [149] showed that ultrafine-grained pure Al with an average grain size of ~1 μm (Figure 38a) could be prepared by asymmetric cold rolling. The sheets were rolled with a total thickness reduction of 90% from 4 mm to 0.4 mm after 22 rolling passes. The grains of the material were nearly equiaxed and the microstructure was homogeneous. Moreover, the high-angle grain boundaries were predominant. The fraction of high-angle grain boundaries (>15◦) was about 50%. Before rolling, the original pure Al was of very low yield stress (~100 MPa) and high ductility (~17%). The yield strength of asymmetrically rolled Al increased to 250 MPa, but the corresponding elongation decreased to 4% [149].

**Figure 38.** (**a**) TEM micrograph of pure Al after asymmetric cold rolling, reproduced from [149], with permission from Elsevier, 2009; (**b**) TEM micrograph of Al 1050 after asymmetric cryorolling [24] (RD—rolling direction).

In 2012, Yu et al. [24] reported about nanostructural Al 1050 sheets with a grain size of 0.211 μm (Figure 38b). Sheets were manufactured using a novel method of asymmetric cryorolling. Asymmetric cryorolling (*SR* = 1.4) was performed by dipping the sheets into liquid nitrogen for at least 8 min before each rolling pass. The sheets were rolled with a total thickness reduction of 88.3% from 1.45 mm to 0.17 mm after seven rolling passes.

In 2009, Kim et al. [19] showed that ultrafine-grained Al-Mg-Si alloy (AA6061) sheets could be fabricated by SPD using high-ratio differential speed rolling (HRDSR) (*SR* = 3) and subsequent low-temperature aging. The asymmetric warm rolling was conducted on preheated specimens (120 ◦C) and the work roll surfaces were maintained at 140 ◦C throughout the process. Sheets with a width of 100 mm were rolled from 2 mm to 0.6 mm (70% thickness reduction) by a single pass. Processed sheets exhibited an ultra-high strength (yield stress: 455 MPa, ultimate tensile strength: 489 MPa, total elongation: 7.4%). The strengthening effect was impressive compared with the results obtained by using other SPD techniques. The high strength of the AA6061 could be attributed to a significant decrease in grain size (0.37 μm) and increased Hall-Petch constant. The additional strengthening gained after the low-temperature aging was due to the precipitation of nanosized *β* particles. Figure 39 shows the microstructures of the AA6061 observed by TEM [19].

In 2014, Loorentz and Y.G. Ko [134] reported about nanostructured AA5052 (with grains of 0.7 μm) manufactured by differential speed rolling (*SR* = 4). The sheets were rolled with a total thickness reduction of 75% from 4 mm to 1 mm after four rolling passes. The yield strength, ultimate tensile strength and total elongation of the initial sheets were 65 ± 5 MPa, 137 ± 10 MPa and 32 ± 2%, respectively. The yield strength and ultimate tensile strength of the nanostructured sheets were several times higher than that of the initial coarse counterpart: 380 ± 10 MPa and 390 ± 10 MPa, respectively. However, total elongation was decreased to 4.2 ± 0.5% [134].

**Figure 39.** TEM micrographs of HRDSR AA6061 after thickness reduction of 70%: (**a**) furnace-cooled (FC) HRDSR Al, (**b**) water-quenched (WQ) HRDSR Al and (**c**) WQ-HRDSR Al after aging at 100 ◦C for 48 h. (**d**) WQ-HRDSR Al after aging at 100 ◦C for 48 h (taken on RD–ND plane) (**e**) *β* or *β* precipitates in the aged WQ-HRDSR Al (**f**) SiO2 (>200 nm) and α-AlFeSi particles (∼50 nm) commonly detected in all the HRDSR 6061 Al, reproduced from [19], with permission from Elsevier, 2009.

#### **6. Asymmetric (Warm, Cold, Cryo) Rolling of Ti Alloys**

Results of an extensive study (Tables 9–11 and Figure 40) showed that (warm, cold, cryo) differential speed rolling has a grea<sup>t</sup> impact on the grain size and mechanical properties of pure Ti and Ti-6Al-4V alloy. In 2010, Kim et al. [20] reported that high-strength CP-Ti (ASTM grade 2) sheets with ultrafine grains of 0.1–0.3 μm and ultimate tensile strength of 895–915 MPa could be fabricated by single-pass high-ratio differential speed rolling (HRDSR) (*SR* = 3) at room temperature (thickness reduction of 63% from 2 mm to 0.74 mm).

**Figure 40.** Different Ti alloys, which were processed by asymmetric (warm, cold, cryo) rolling (based on data from Tables 9–11).


**Table 9.** Experimental data related to asymmetric warm rolling of Ti alloys.

The microstructure of the HRDSR-processed Ti was composed of two types of region: the "U" region, where shear bands were densely populated, and the "F" region, where some fragments of the original grains remained. The U region was clearly the dominant area fraction. The formation of nearly unidirectional shear bands is a characteristic of the HRDSR process, where large shear strain is imposed on the sheet. The interval of the lamellar boundaries in the U region was between 0.9 and 1.2 μm. Very fine and equiaxed grains resided in shear bands and their sizes were comparable to or smaller than those of the lamellar spacing, indicating that the lamellar structure was related to the formation mechanism of ultrafine grains. Figure 41 shows TEM micrographs and the selected area electron diffraction (SAED) pattern of the HRDSR-processed Ti, taken at two places showing different microstructural features. There are two regions distinguished in the U region. One region (Figure 41a) is composed of elongated or nearly equiaxed subgrains with thick cell walls (0.2–0.3 μm in size), inside of which the dislocation density is high. In the other region (Figure 41b), grains are more equiaxed and smaller. Comparison of the SAED pattern between the two regions indicates that the grains in the latter have higher angle grain boundaries. The SAED patterns in both regions can be indexed as the close-packed hexagonal structure of α-Ti, indicating that no phase transformation occurred during rolling. The "A" region marked in Figure 41a indicates that the ultrafine grains were formed via a dynamic recovery process, by which the high density of dislocations forms equiaxed subgrains with low angle boundaries between the lamellar boundaries, which were then converted into equiaxed crystallites with high-angle boundaries via dynamic continuous recrystallization. As the local temperature in the shear bands can be greatly increased due to intense shear banding, dynamic recovery and recrystallization processes will be effectively promoted in the bands, even though the material is deformed at room temperature.

**Figure 41.** TEM micrographs and the selected area electron diffraction (SAED) pattern of the HRDSRprocessed CP-Ti (ASTM grade 2). (**a**) Region with elongated or nearly equiaxed subgrains. (**b**) Region with equiaxed and smaller grains. The arrows indicate the rolling direction, reproduced from [20], with permission from Elsevier, 2010.

Extensive formation of high-density shear bands over the entire thickness of the section and their conversion to equiaxed crystallites with high-angle boundaries via dynamic recovery or dynamic continuous recrystallization were important in achieving ultrafine grains with a size of 0.1–0.3 μm [20]. The resultant mechanical properties of pure Ti (Figure 42) exceeded those reported from tests using multiple passes of ECAP at elevated temperatures [162] or room temperature [163] and multiple cycles of accumulative roll bonding at room temperature [164].

**Figure 42.** Stress–strain curves of the as-received and the HRDSR-processed Ti sheets, reproduced from [20], with permission from Elsevier, 2010.

A superior balance of strength (YS = 1231 MPa, UTS = 1365 MPa) and ductility (uniform elongation = 4.93%, total elongation = 22.8%) in Ti-6Al-4V alloy was achieved in 2016 by Chao et al. [157] through warm asymmetric rolling by a pair of rolls with different diameters with a ratio of 5:3 (*SR* = 1.67). The sheets were rolled with a total thickness reduction of 70% from 5.75 mm to 1.72 mm at 800 ◦C. The application of asymmetric rolling at 800 ◦C led to a higher strength–ductility balance, reaching the highest UTS × TE of 30,000 MPa%. The exceptional mechanical properties were ascribed to the formation of an ultrafine-grained structure (0.229 μm), texture characteristics and transformation of the *β* phase upon straining.
