Accumulative Roll-Bonding

eeq ¼ � ffiffiffi 3 p ln t0 ¼ � ffiffiffi 3 p ln ðl � rÞ ð1Þ where t0, t, and r are initial thickness of the stacked sheets, the thickness after roll-bonding, and the re- duction in thickness per cycle, respectively. Geo- metrical changes in the materials in the ARB process using 50% reduction per cycle (r¼ 0.5 in Eqn. (1)) are summarized in Table 1. An equivalent strain of 4 can be achieved in 5 ARB cycles in this case, and after 10 ARB cycles, the 1-mm-thick product sheet involves 1024 pieces of original sheets. A similar process had been used before for bulk mechanical alloying of dif- ferent metals or for fabricating multilayered materials (Atzmon et al. 1985, Yasuna et al. 1997). However, roll-bonding was not used in such attempts, but dif- fusion bonding at elevated temperatures was carried out between each pressing or rolling. The concept of the ARB process is rather similar to the traditional Japanese sward production. Bulk mechanical alloying by the ARB process is of course possible, and none- quilibrium structures including amorphous phases have been fabricated (Ohsaki et al. 2007). It has been found that roll-bonding of metals is not very difficult, though the surface treatment is critical to achieve good bonding. The ARB process has been applied to almost all kinds of metals and alloys 1 1. ARB Process ARB was developed by Saito et al. in 1998. The principle of the ARB process is schematically illus- trated in Fig. 1. The ARB is an SPD process using rolling deformation. Rolling is the most advan- tageous metal working process for continuous pro- duction of bulky materials with shapes of plates, sheets, bars, and so on. However, it is substantially impossible to realize ultrahigh plastic strain above equivalent strain of 4–5 in conventional rolling, be- cause the dimension of the materials (like thickness of sheets) decreases with increasing total plastic strain applied. In the ARB process, for example, a 2-mm- thick sheet is firstly rolled by 50% reduction in thickness. The rolled sheets with a thickness of 1 mm are cut into two, stacked to reach the initial dimen- sions before the first rolling, and then rolled again. To obtain one-body solid material, the rolling in the ARB process is not only a deformation process but also a bonding process, which is known as ‘‘roll- bonding’’ used for the production of clad sheets. To achieve good bonding, the contact surfaces of the sheets were typically treated by degreasing and wire- brushing. The roll-bonding is sometimes carried out at elevated temperatures less than the recrystalliza- tion temperature of the material, in order to make the bonding better and to reduce the rolling force. By Accumulative Roll-Bonding Accumulative roll-bonding (ARB) is a kind of severe plastic deformation (SPD) processes for fabricating bulk nanostructured metals (Saito et al. 1999, Tsuji et al. 2003). It has been clarified since around 1990 that bulk nanostructured metals and alloys, which are composed of ultrafine grains with mean grain size of several hundreds of nanometers or nanocrystals with mean grain size of several tens of nanometers, can be fabricated by the plastic deformation up to very high strain (above logarithmic equivalent strain of 4–5), which is often called SPD. Various kinds of unique SPD processes, such as equal-channel angular extrusion, high-pressure torsion, and cyclic extrusion and compression have been developed for realizing bulk nanostructured metals (Altan 2006, Zehetbauer and Zhu 2009). Among the SPD processes, the ARB is advantageous for continuous production of sheet materials, because it uses rolling deformation in principle. On laboratory scale, the ARB has been applied to various kinds of metals and alloys and has succeeded in producing bulky sheets having nanos- tructures. A number of unique properties and nanostructures have been found through the studies using the ARB process. It has been also applied to practical application to fabricate thin strips of ultra- fine grained (UFG) stainless steels. repeating the procedure, one can apply very high plastic strain to the sheet material without changes of dimensions. The Von Mises equivalent strain (eeq) after n cycles of the ARB can be expressed as 2n t 2n Surface treatment Degreasing wire-brushing Cutting Roll-bonding Stacking 2 + + 1 Figure 1 Schematic illustration showing the principle of the accumulative roll-bonding (ARB) process. Reproduced with permission from Huang X, Tsuji N, Hansen N, Minamino Y 2003 Microstructural evolution during accumulative roll-bonding of commercial purity aluminum. Mater. Sci. Eng. A 340, 265–71. Copyright Elsevier. Table 1 Geometrical changes in the material during the ARB where two pieces of the 1-mm-thick sheets are stacked and roll-bonded by 50% reduction per cycle. No. of cycles 1 2 3 4 5 6 7 8 9 10 y n No. of layers 2 4 8 16 32 64 128 256 512 1024 y 2n No. of bonded boundaries 1 3 7 15 31 63 127 255 511 1023 2n� 1 Layer interval (mm) 500 250 125 62.5 31.3 15.6 7.8 3.9 1.9 0.96 1000/2n Total reduction (%) 50 75 87.5 93.8 96.9 98.4 99.2 99.6 99.8 99.9 (1� 1/2n)� 100 Equivalent strain 0.8 1.6 2.4 3.2 4 4.8 5.6 6.4 7.2 8 0.8n A ccu m u la tive R o ll-B o n d in g 2 deformable in rolling, and in such materials, UFG or nanocrystalline structures have been formed. A critical rolling reduction to realize bonding exists, depending on the material, processing temperature, and roll geometry (Tylecote 1968). Fracture of the sheet is a rather serious issue in the ARB process. Typical ap- pearance of the sheets after successful and un- successful ARB is shown in Fig. 2(a) and (b), respectively. In certain kinds of materials, edge cracking of the sheet frequently happens and the cracks propagate into the center of the sheet (Fig. 2(b)). Once such cracks appear, it becomes dif- ficult to repeat the ARB cycle again. Various minor techniques to avoid cracking have been considered, so that sound bulky sheets with nanostructures can be obtained after appropriate processing procedures. 1100-Al, 5 cycles 5083-Al, 2 cycles (a) (b) TD 50 mm 50 mm Accumulative Roll-Bonding RD Figure 2 Typical appearance of the ARB processed sheets. (a) 1100-Al ARB processed by five cycles at room temperature. Successful example. (b) 5083-Al ARB processed by two cycles at room temperature. Unsuccessful example with cracks. Reproduced with permission from Altan B S, Miskioglu I, Purcek G, Mulyukov R R, Artan R (eds.) 2006 Production of bulk nanostructured metals by accumulative roll bonding (ARB) process. In: Severe Plastic Deformation: Towards Bulk Production of Nanostructured Materials. NOVA Science Publishers, New York, Chap. 7.4, pp. 543–64. Copyright NOVA. 2. Nanostructures Obtained by ARB Figure 3(a) shows a typical UFG structure in ARB processed pure aluminum taken by transmission electron microscopy (TEM). The microstructure was observed from the transverse direction (TD) of the sheet. Fine grains or lamellar structures elongated to the rolling direction (RD) are seen. This is a typical UFG structure obtained by the ARB process (Huang et al. 2003). The mean thickness of the elongated grains (or the mean interval of the lamellar bound- aries) in Fig. 3 is 200 nm. In this TEM observation, a Kikuchi-line diffraction pattern was taken from each local region to determine the crystallographic orien- tation precisely, and the misorientation between ad- jacent regions was calculated. Figure 3(b) is a boundary map of areas identical to those in Fig. 3(a) representing the misorientation results. The bound- ary map clearly shows that each elongated region has quite large misorientations with the neighboring re- gions. The boundaries between adjacent regions seem to be quite sharp. It can be concluded, therefore, that the elongated regions are certainly ‘‘grains’’ from the viewpoint of misorientation. At the same time, however, the morphology of the grains is elongated along the principle deformation direction and the microstructure involves many low-angle boundaries and dislocations. This means that the nanostructures fabricated by the ARB process (and by other SPD processes) are essentially deformation micro- structures. The characteristics of the deformation microstructures significantly affect the properties of bulk nanostructured metals fabricated by SPD. Microstructure evolution during the ARB is understood as a function of primarily applied strain (Kamikawa et al. 2007). Figure 4 shows the effect of equivalent strain applied by the ARB on the average spacing of high-angle grain boundaries (HAGBs) and the fraction of HAGBs in an ultralow carbon interstitial free (IF) steel. When the ARB process is carried out under unlubricated condition, a large amount of redundant shear strain caused by large friction between the sheet and rolls is introduced in the subsurface regions of the sheets and this significantly affects the microstructure evolution. In Fig. 4, the microstructural parameters are plotted versus the total strain taking account of both rolling strain and redundant shear strain. At the early stage of the ARB, the HAGB spacing decreases and the fraction of HAGBs increases monotonously with increasing strain, and then both parameters nearly saturate. This means that a homogeneous UFG structure is achieved only above a total equivalent strain of 5. The tendencies are almost the same in most ARB processed materials. The grain size eventually obtained depends on the kind of the material as well as on the deformation conditions. As the purity of the metal is higher, the obtained UFGs become coarser, and even in the as-processed 3 Accumulative Roll-Bonding state equiaxed grains, equivalent to conventionally recrystallized grains, can be obtained in high-purity aluminum (Kamikawa et al. 2006). This is because recovery and grain boundary migration (or grain High-angle boundary (�≥15°) Low-angle boundary (�<15°) Not measured RD N D 2.5 (a) (b) 27.6 35.5 37.3 40.5 49.5 45.1 43.8 39.7 3.1 59.2 58.6 40.5 35.6 44.0 59.8 49.4 51.3 38.5 1.0 17.0 50.8 50.6 38.6 58.2 4.0 0.7 55.8 55.3 56.7 27.2 60.4 60.1 55.8 0.5 31.3 56.6 39.2 39.0 1.5 51.6 17.7 17.5 0.5 39.4 1.6 39.5 39.5 0.4 1.2 20.4 9.6 9.6 0.6 39.3 51.2 46.74 48.8 39.1 20.1 50 53.3 36. 3 60 1.1 7.3 0.3 1.7 1.6 42.7 43.7 40.3 28.9 28.6 Figure 3 TEMmicrograph (a) and corresponding boundary misorientat ARB processed by six cycles at 200 1C. Observed from the tr indicated in (b) were calculated from the precise orientation o Reproduced with permission from Huang X, Tsuji N, Hanse accumulative roll-bonding of commercial purity aluminum. M 4 growth) happens during the process. Therefore, roughly speaking, finer UFGs are obtained in the alloys including larger amount of alloying elements or by processing at lower homologous temperatures. 8.1 2.0 44.1 40.1 5.9 8.8 3.5 .9 50.9 0.4 9 34.6 3.9 .3 60.5 2.3 0.7 34.4 54.2 40.7 58.8 6.8 4.7 4.83.2 33.9 13.3 31.430.2 29.1 27.1 0.7 33.733.1 5.9 43.11.8 58.457.4 41.5 12.1 48.1 26.5 56.5 61.2 29.7 51.8 51.2 24.90.9 11.025.6 56.7 54.753.1 54.4 42.232.5 32.3 28.215.8 27.5 2.1 27.3 1 μm 1.3 1.3 2.8 40.0 31.9 7.3 10.4 11.2 5.410.6 10.2 ion map (b) of the commercial purity aluminum (1100-Al) ansverse direction. The misorientation angles (degrees) f each region measured by TEM/Kikuchi-line analysis. n N, Minamino Y 2003 Microstructural evolution during ater. Sci. Eng. A 340, 265–71. Copyright Elsevier. 10 15 200 5 total rai the nt s ake Accumulative Roll-Bonding 20 10 5 2 1 0.5 *Dotted marks = Reverse sheared regions No. of cycles 1 2 3 4 5 6 7 Without lub.* With lub. Av e ra ge s pa cin g of H AG B, d t H AG B (μm ) 0.2 0.1 10 15 20 Total equivalent strain, �eq 0 5 (a) total Figure 4 (a) Relationship between average spacing of high-angle g and (b) relationship between the fraction of HAGBs and 500 1C with or without lubrication. In the total equivale caused by large friction between the sheet and rolls are t The nanostructures fabricated by the ARB process can be changed by subsequent heat treatments. Fig- ure 5 shows TEM micrographs of a 99% commercial purity aluminum that was ARB processed and then annealed at various temperatures for 1.8 ks (Tsuji et al. 2002). The microstructures were observed from TD. Upon annealing at low temperatures, elongated morphology of UFGs is kept, recovery diminishes dislocations inside grains, and the grain size slightly increases. After annealing at 225 1C, nearly equiaxed grains free from inside dislocations are observed (Fig. 5(d)), which are difficult to be distinguished from conventionally recrystallized grains. However, the mean grain size is still very fine (approximately 1 mm). Similar changes in microstructures have been observed in an ARB processed IF steel (Tsuji et al. 2002). The microstructural changes in these materials are fairly continuous and uniform, so it is sometimes called ‘‘continuous recrystallization’’ (Humphreys et al. 2001), distinguished from conventional re- crystallization characterized by nucleation and growth (discontinuous recrystallization). However, such a continuous change in microstructure is rather ex- ceptional and observed only in materials that show enhanced recovery, like face centered cubic (fcc) aluminum having high stacking fault energy and determined through electron back-scattering diffraction (EB with a field-emission-type gun (FE-SEM) Reproduced with Effect of redundant shear strain on microstructure and textur carbon IF steel. Acta Mater. 55(17), 5873–88. Copyright El Total equivalent strain, �eq(b) n boundaries (HAGBs) and the total equivalent strain, total equivalent strain, in the IF steel ARB processed at train, both rolling strain and redundant shear strain n into account. The microstructural parameters were *Dotted marks = Reverse sheared regions No. of cycles 1 2 3 4 5 6 7 Without lub.* With lub. 100 80 60 40 Fr a ct io n of H AG B, f H AG B (% ) 20 0 body centered cubic (bcc) iron. In many kinds of ARB processed materials, microstructural changes similar to discontinuous recrystallization occur during subsequent annealing, even if they have uniform UFG structures with high density of HAGBs after the ARB. 3. Mechanical Properties of ARB Processed Materials Figure 6 shows the change in stress–strain curves during the ARB process of a commercial purity alu- minum. The strength of the specimen significantly increases after one-cycle ARB, whereas the elonga- tion, especially the uniform elongation, of the ma- terial strongly decreases. This is a typical mechanical property of strain-hardened metals. With increasing number of ARB cycles, i.e., applied strain, the strength monotonously increases and the tensile strength reaches 330 MPa after seven ARB cycles, which is surprisingly more than four times higher than that of the starting specimen having a coarse recrystallized microstructure. Tensile elongation of the ARB processed aluminum is low: the uniform elongation is a few percent and the total elongation is SD) analysis in a scanning electron microscope equipped permission from Kamikawa N, Sakai T, Tsuji N 2007 e evolution during accumulative roll-bonding in ultralow sevier. 5 Accumulative Roll-Bonding (a) 2 μm (b) 2 μm less than 10%. These are typical changes in mech- anical properties during ARB of metals and alloys. There have been many arguments on whether oxides formed at the bonded interfaces affect the mechanical properties of the ARB processed sheets or not. There must be a certain amount of oxides at the bonded interfaces. However, even after six ARB cycles that can produce a homogeneous UFG structure (Fig. 4), the number of the bonded interfaces within the 1-mm sheet is only 63 and the interval of the bonded interfaces (the thickness of the original sheet) is 15.6 mm (Table 1), which is much larger than the mean grain size. The total volume of oxides would thus be relatively small. Therefore, it is believed that the effect of oxides on the mechanical properties of the ARB processed sheets is not significant below about 10 cycles, which is generally used for fabri- cating UFG structures. Stress–strain curves of a commercial purity alu- minum that was ARB processed and then annealed (c) N D 2 μm RD Figure 5 TEM microstructures of the commercial purity aluminum ( then annealed at various temperatures for 1.8 ks. Observed (b) 150 1C, (c) 200 1C, (d) 225 1C, (e) 250 1C, and (f) 300 1C Saito Y, Minamino Y 2002 Strength and ductility of ultrafi annealing. Scr. Mater. 47, 893–9. Copyright Elsevier. 6 (d) (e) 2 μm 2 μm are shown in Fig. 7. The change in microstructures during annealing has been already shown in Figure 5. The flow stress of the material decreases with in- creasing annealing temperature, but the tensile elongation, especially the uniform elongation, re- covers only after the mean grain size becomes larger than 1 mm. Quite similar changes in mechanical properties have been reported in an IF steel that was ARB processed and annealed. The limited uniform elongation of the UFG materials is understood in terms of early plastic instability (Tsuji et al. 2002). The plastic instability condition, i.e., necking criteria in tensile test, can be simply expressed by means of the following equation: s � ds de ð2Þ Here, s and e are true stress and true strain, respectively. Grain refinement much increases the (f) 2 μm 1100-Al) ARB processed by six cycles at 200 1C and from the transverse direction: (a) annealed at 100 1C, . Reproduced with permission from Tsuji N, Ito Y, ne grained aluminum and iron produced by ARB and Accumulative Roll-Bonding 500 400 7 cycles flow stress, especially the yield strength of the ma- terial. However, strain-hardening is not enhanced by grain refinement. As a result, plastic instability ex- pressed in Eqn. (2) is easily achieved in UFG 300 200 N om in al s tre ss , s (M Pa ) 100 0 0 10 20 0 c 1 cycle 2 cycles 3 cycles 5 cycles Nominal st Figure 6 Nominal stress–strain curves of the commercial purity alumi temperature without lubrication. 300 As ARBed (dt = 0.27 μm TA = 150 °C ( 200 °C (dt = 22 300 ° 400 °C (dt = 10 μm) 200 250 150 100 N om in al s tre ss , s (M Pa ) 50 0 0 10 20 Nominal Figure 7 Nominal stress–strain curves of the commercial purity alum lubrication and then annealed at various temperatures for 1 indicated together with the annealing temperature. 1100-AI materials. Therefore, it is necessary to enhance strain- hardening ability of the matrix in order to manage both high strength and ductility in nanostructured metals. Actually, a good balance between strength ycle 4030 50 60 rain, e (%) num (1100-Al) ARB processed by various cycles at room ) 1100-Al dt = 0.40 μm) 0.66 μm) 5 °C (dt = 1.2 μm) 250 °C (dt = 2.0 μm) C (dt = 5.4 μm) 4030 50 strain, e (%) inum (1100-Al) ARB processed by six cycles without .8 ks. Mean grain thickness (dt) of each specimen is 7 4. Conclusion The ARB process is an only-SPD process applicable to continuous production of large bulky materials, whereas other batch-type SPD processes are dis- See also: Joining of Metals; Metal Working: Cold Rolling Bibliography State Mater. Sci. 5, 15–21 Kamikawa N, Tsuji N, Huang X, Hansen N 2006 Quantifi- cation of annealed microstructures in ARB processed alu- minum. Acta Mater. 54, 3055–66 Kamikawa N, Sakai T, Tsuji N 2007 Effect of redundant shear strain on microstructure and texture evolution during accumulative roll-bonding in ultralow carbon IF steel. Acta Mater. 55 (17), 5873–88 rep m lis Accumulative Roll-Bonding Copyright r 2011 Elsevier Ltd. All rights reserved. No part of this publication may be in any form or by any means: electronic, electrostatic, otherwise, without permission in writing from the pub Altan B S, Miskioglu I, Purcek G, Mulyukov R R, Artan R (eds.) 2006 Production of bulk nan