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Effect of temperature on texture formation of 6061 aluminum sheet in equal-channel angular pressing
Materials Science and Engineering A 408 (2005) 79–84
Effect of temperature on texture formation of 6061 aluminum sheet in equal-channel angular pressing Qin Jining a,1 , Zhang Di a,1 , Zhang Guoding a,2 , Jae-Chul Lee b,∗ a
The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China b Division of Materials Science and Engineering, Korea University, Anam, Seoul 136-701, Korea Received in revised form 15 July 2005
Abstract A novel equal-channel angular pressing (ECAP) procedure was conducted on 6061 aluminum sheet at temperature up to 513 K to investigate temperature effect on microstructure and texture formation. The deformed microstructures were examined by transmission electron microscopy and the macrotexture evolution was investigated by orientation distribution functions (ODFs). The results shown that when ECAP at temperature below 433 K the microstructures exhibit high dislocation density within subgrains with thick and waved non-equilibrium boundaries, and the textures are characterized by the formation of Rotated cube component or a pair of textures components near Rotated cube component, such as {0 3 1}5 1 3 and {0 1 3}5 1 3or {0 4 1}6 1 4 and {0 1 4}6 4 1. Whereas under certain procedure condition, i.e. at 513 K, the ECAP procedure is capable of producing microstructure with low density dislocations subgrains separated by regular boundaries, and a preferable 1 1 1//ND texture fiber is formed with the R-value as higher as 1.5. The texture results show that the ECAP procedure could be expected to be a potential application to enhance the formability of aluminum sheet. © 2005 Elsevier B.V. All rights reserved. Keywords: ECAP; Texture; Aluminum alloys; Microstructure; Formability
1. Introduction In last few years there are numerous reports on the development of techniques for applying severe plastic deformation (SPD) to ductile metals and alloys, using methods such as equalchannel angular pressing (ECAP) [1]. The ECAP process, which leads to the creation of a submicron or nano-size grain structure after imposing high levels of strain [2–4], has provided a simple method to get advantages mechanical properties by refining microstructures without the major compositional modification. On the other hand, in automotive industry, the application of sheet aluminum alloys has led to an increasing interest as the demands of fuel efficiency and vehicle emission [5] increase. The conventional ECAP methods could only process the bulk materials, i.e. with circular or square cross section. In ∗ 1 2
Corresponding author. Tel.: +82 2 3290 3283; fax: +82 2 928 3584. E-mail address: [email protected] (J.-C. Lee). Tel.: +86 21 62933106; fax: +86 21 62822012. Tel.: +86 21 62932049; fax: +86 21 62822012.
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.128
present work a novel ECAP-base process [6,7], which can deal with sheet metals by continuous manner, were introduced to induce simple shear strain to aluminum sheet and to investigate the effectiveness of ECAP procedure in promoting sheet formability. The ECAP process parameters, such as the amount of deformation strain, the rotation between each repetitive pressing, the speed and the temperature in process, were believed to affect the final microstructure and texture formation in material. Texture, which represents the distribution of all orientations in the polycrystalline assembly, controls the formability of sheet metal. So the understanding and control of the texture evolution is of great importance with respect to the optimization of the formability of sheet metals. Up to date, to authors’ knowledge, there is no report available on the effect of temperature on texture formation of aluminum sheet during ECAP process, especially in conjunction with the sheet formability. Accordingly, the purpose of the current experiments was to investigate the microstructure and texture evolution of 6061 aluminum sheet at different ECAP temperature. The types and the intensities of the textures formed at each ECAP procedures
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and the effects of those textures on sheet formability are of the importance in this study. The microstructures were investigated by transmission electron microscopy (TEM), and the macrotextures were analyzed by three dimensional orientation distribution functions (ODFs) which were determined by X-ray diffraction. The R-values were calculated from those ODFs to assess the sheet formability connected to each texture.
were cut at side plane from the center of strips with the samples normal perpendicular to both strips normal and longitudinal direction. Then the samples were ground mechanically to a thickness of 40 m followed by ion miller to perforate a hole at the middle of the samples. Microstructure observations were taken in Hitach-800 transmission electron microscope operating at 175 kV. Select area electron diffraction (SEAD) patterns were taken from the region of nearest 1 1 1 zone, having a diameter of about 4 m, as a quantitative assess of the misorientation between subgrains. The specimens used to evaluate texture were cut from ECAPed strips and thinned chemically to half the thickness in a NaOH solution at 323 K for avoiding any texture change induced to specimens surface by mechanical polish process. The {1 1 1}, {2 0 0} and {2 2 0} pole figures were measured on an automated X-ray goniometer (Phillips X’Pert) with Cu K␣ radiation by the Schultz reflection method [9]. The orientation distribution functions (ODFs) were calculated from these three incomplete pole figures according to the WIMV method. To evaluate the formability associated with each texture, the R-value, along 0◦ –90◦ at 15◦ interval with respect to the direction of ECAP, were calculated using popLA program.
2. Experimental procedure
3. Results and discussion
The schematic diagram of the novel ECAP process used in present study is shown in Fig. 1. The design principle is to introduce shear deformation into metallic strip in a continuous manner. It makes use of two rollers as feeding apparatus before the forming dies, which construct two ECAP channels—the inlet and outlet channel. The thicknesses of the two channels are dissimilar to each other such that the outlet channel (1.55 mm) is slight thicker than the inlet channel (1.45 mm) to enable the consecutive ECA pressings. To change the strain introduced in each ECAP process, the oblique angle (Φ) between two channels can be adjusted. In present investigation the oblique angle is 120◦ , so the equivalent effective strain introduced into the sheet metal in present ECAP process can be determined [7] as 0.58. The material used in present investigation was a commercial 5 mm thickness 6061 aluminum plates with the chemical composition of Al–1.0Mg–0.6Si–0.3Cu–0.2Cr, where the alloy compositions are given in weight percent. Warm rolling was carried out to this aluminum plates unidirectionally with each rolling reduction about 10–15% without any intermittent annealing until the final thickness, 1.55 mm, were obtained. Thus the starting material possessed a typical rolling texture–a main texture conponent of -fiber and a weak Goss orientation{1 1 0}[0 0 1] [8]. The maixmum texture intensity in -fiber and Goss orientation are of 6.4 and 3.7 times of the random distribution. The rolled strips were cut with demension of 20 mm × 200 mm × 1.55 mm and put in the furnace at required temperature. The temperatures chosen for present study were room temperature, 353, 433 and 513 K, respectively. After held in furnace for 30 min, the strips were feeded quicklly into the ECAP apperatus to ECAPed at speed of 40 m/s. Microstructures were examined after ECAPed at each different temperature. The transmission electron microscopy samples
3.1. Characterization of microstructure evolution
Fig. 1. Schematic diagram of the ECA pressing process used in the present study.
Fig. 2 is the series of the TEM bright field images showing the evolution of microstructures and corresponding SAED patterns of 6061 aluminum sheet subjected to ECA pressing at room temperature, 353, 413 and 513 K, respectively. It is shown form Fig. 2(a), ECAPed at room temperature, that the microstructure is characterized by the thick and waved non-equilibrium grain boundaries formed by tangling dislocations and the high dislocation density within cells and subgrains. Fig. 2(b) and (c), ECAPed at temperature of 353 K and 433 K, demonstrate basically similar microstructure as that ECAPed on room temperature. The elongated fine subgrains with 0.5 m in width and some non-equilibrium grain boundaries can be observed while the high dislocation density still retains in subgrains. It should be noted that Fig. 2(d) unambiguously shows some microstructure changes have taken place when ECAPed at temperature of 513 K. Thin and regular-shaped cell boundaries were formed while the dislocation density within the cells was obviously diminished as compared to those formed at lower temperatures. Subgrains consisted of piled up and polygonized dislocations wall are present as marked by black arrows in Fig. 2(d). These microstructure characters indicated some annihilation of dislocation in opposite sign and the rearrangement of dislocations in cell interior, which suggest that a dynamic recovery may occur at this pressing temperature because of the high stacking fault energy of aluminum, the large deformation strain and the high deformation temperature in present experiment. The low density dislocation subgrains separated low angle boundaries had been observed in other aluminum alloys when ECAP was carry out at elevated temperature [10,11],
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Fig. 2. Microstructures and associated SADE patterns for 6061 aluminum sheets after ECAPed at: (a) room temperature; (b) 353 K; (c) 433 K; and (d) 513 K, respectively.
and were referred to the high recovery rate of the aluminum alloys. It can be seen from Fig. 2 that the ECA pressing up to temperature of 513 K does not lead to a substantial growing in grain size, and according to the SEAD patterns the misorientation are also keep on the level of a few degree. Whereas, when the pressing temperature increased, the dislocation density within the subgrains decrease gradually and the subgrains boundaries become thinner with a relatively regular SEAD patterns.
(1 1 1//ND). As the samples experience the simple shear deformation in ECAP procedure, the range of reduced Euler space for f.c.c. is of 0◦ ≤ φ1 ≤ 180◦ , 0◦ ≤ φ, φ2 ≤ 90◦ to present the monoclinic sample symmetry.
3.2. Textures formed at different temperature In order to facilitate interpretation of the texture presented below, some important orientation components and texture fiber in f.c.c. metal are plotted in reduced Euler space in Fig. 3. It is comprised of ␥-fiber, {1 1 2}1 1 0 component and Rotated cube {0 0 1}1 1 0 which has deteriorated to a skeleton line as the strong distortion of Euler space near φ = 0◦ . The ␥-fiber, which defined as a line of φ = 54.7◦ , φ2 = 45◦ , φ1 = 0–180◦ in reduced Euler space, have orientations of {1 1 1}u v w with a common {1 1 1} normal parallel to sheet normal direction
Fig. 3. Keys to some important textures components and fiber at reduced Euler space.
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Fig. 4. The spatial representation of ODFs after ECAPed at different temperatures of: (a) room temperature; (b) 353 K; (c) 433 K; and (d) 513 K.
Fig. 4 shows the spatial representations of textures ECAPed at different temperature, which display the iso-intensity surfaces of orientation distribution functions with intensity of f(g) = 3. The orientation distribution intensity greater than 3 are distributed within these iso-intensity surfaces, and the crosssections between the iso-intensity surfaces and the boundaries of Euler space reveal these intensity contours. The orientation intensity f(g) = 3 was chosen to present the iso-intensity surface for this intensity can clearly demonstrate the concentration of each prefer orientation. Either higher or lower threshold value may cause the minor components invisible. Fig. 4(a–d) clearly demonstrates the texture evolution in 6061 aluminum sheets when ECAP operation temperature changed from room temperature to 353, 433 and 513 K, respectively. It should be noticed in Fig. 4 that there is no any trace of the rolling texture (the -fiber) in the ECAPed textures. In fact, the rolling texture in starting material will change completely even under the effective ECAP strain of 0.37 [8]. In Fig. 4(a), ECAPed at room temperature, the texture is characterized by the main texture component Rotated cube {0 0 1}1 1 0 and the minor {1 1 2}1 1 0 component. It is
shown in Fig. 4(b) and (c) that there is no significant texture change when ECA pressing was carried out at temperatures of 353 and 433 K, respectively. The textures are exhibited by a pair of symmetric components near to rotated cube with the low index of {0 3 1}5 1 3 and {0 1 3}5 3 1 in 353 K (Fig. 4(b)), and {0 4 1}6 1 4and {0 1 4}6 4 1 in 433 K (Fig. 4(c)). Fig. 4(d) clearly shows that the texture developed at ECAP temperature of 513 K was change remarkably. The maximum intensities are shifted to the position of {3 3 2}3 1 3 in low index expression with the maximum orientation distribution intensity of f(g) = 8. As each {3 3 2}3 1 3 component is sited close to ␥-fiber, in fact the maximum orientations site around the ␥-fiber with approximately 6◦ misorientation, and each {3 3 2}3 1 3 components is scattered along the 1 1 1 fiber direction, virtually the resulting texture develop into a ␥-fiber. There is not any other texture component appearing. In Fig. 4, the maximum orientation intensities in different ECAP temperatures are kept basically at the level of eight times of random intensity. It should be owed to the same ECAP shear strain induced to each of the aluminum sheet. On the other hand, however, a transition of the main preferential orientations
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The r values, in directions of 0◦ , 45◦ and 90◦ with respect to the longitudinal direction of ECAP sample, were calculated using popLA program. The Lankford parameter (R-value) is determined by averaging these three r values according to the following equation: r0 + 2r45 + r90 R= (1) 4
Fig. 5. Variation of calculated R-values as a function of the ECAP processing temperatures.
occurs when ECAP was carried out at different processing temperature. It is commonly accepted that slip in aluminum alloys usually occurs on the octahedral {1 1 1} planes along 1 1 0 directions but there have been many reports of slipping on other crystal planes at elevated temperatures. These studies indicated that the non-octahedral slip occurs on the {1 1 0} plane [12,13] and {1 0 0} planes [13,14] during high temperature deformation of aluminum alloys. The change on deformation texture in present study suggest that the mechanism of the formation of the 1 1 1//ND texture may be the slip activity on non-octahedral slip system (Fig. 5). 3.3. Impact of texture on sheet formability The R-value (Lankford parameter), which controls the strain between the strain in plane and the strain normal to the plane, is the critically value in appreciating the neck resistance in sheet forming and has been used as a measurement for judging the formability. In order to achieve optimum formability, larger Rvalue is required in sheet forming. For some b.c.c. metals, such as low carbon steel sheets, the favorable R-values are readily achieved by the formation of the well-known fiber recrystallization textures [15], i.e. the formation of ␥-fiber. But for f.c.c. metals, this optimum condition is difficult to obtain by most conventional metal forming processes and usually the R-value of aluminum was lower than 1 [16]. Recently some effort [17] has been made to obtain the {1 1 1}//ND texture by means of an additional warm rolling without lubrication in conventional rolling process, but the preferable texture components were formed only in the sheet surface layers and the orientation intensities were weak. There also have reported [18,19] that some 1 1 1//ND shear texture components can be introduced by an ECAP based process, but it failed to get satisfactory uniformed deformation texture through the sample thickness and adequate intensities of the shear components. In present study, an essentially pure ␥fiber was obtained, as can be seen in Fig. 3(d), and no any other texture components appeared. So a preferable R-value could be expected.
The R-values evaluated based on Eq. (1) are presented in Fig. 4. As seen in this figure, the R-values in samples ECAPed at room temperature, 353 and 433 K, i.e. the macrotexture are characterized by Rotated cube or components near to the Rotated cube, hold the values of 1 or below 1. However, the sample ECAPed at temperature of 513 K obviously demonstrate the higher R-value of 1.5 as its texture contained the 1 1 1//ND fiber. The experimental results obtained in this study indicate that ECAP can be used as a means for controlling the texture of the aluminum sheets to enhance the sheet formability. 4. Conclusions The ECA pressing at temperature up to 513 K does not lead to a substantial growing in subgrain size. The microstructure ECAPed at 513 K is characterized by thin and regular boundaries with low dislocation density within subgrains while the non-equilibrium boundaries and higher dislocation density will appear when ECAPed at temperature below 433 K. The ECA pressing temperature strongly affects the texture formation in 6061 aluminum sheet. When ECAPed at a certain temperature, 513 K in present study, the {3 3 2}3 1 3 texture components will emerge; and moreover this {3 3 2}3 1 3 components are scattered along 1 1 1 direction, it virtually form a 1 1 1//ND texture fiber. The aluminum sheet with 1 1 1//ND texture fiber exhibits an R-value as higher as 1.5. It seemed from present study that the ECAP process not only refine the microstructure of aluminum sheet, but also can be used as a potential means to control the texture formation, i.e. for enhancing the formability of aluminum sheet under proper process condition. Acknowledgements The work of this paper was sponsored partly by the Korea Institute of Science and Technology Evaluation and planning (KISTEP) under the Science and Technology Manpower Exchange Program and partly supported from the Major Fundamental Research Project, Shanghai Science and Technology Committee under Grant no. 04DZ14002. References [1] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metall. 1 (1981) 99. [2] D.H. Shin, B.C. Kim, K.T. Park, W.Y. Choo, Acta Mater. 48 (2000) 3245. [3] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317. [4] M. Murayama, Z. Horita, K. Hono, Acta Mater. 49 (2001) 21.
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[5] O. Engler, J. Hirsch, Mater. Sci. Eng. A336 (2002) 249. [6] J.C. Lee, H.K. Seok, J.Y. Suh, Acta Mater. 50 (2002) 4005. [7] J.C. Lee, H.K. Seok, J.Y. Suh, J.H. Han, Y.H. Chunng, Metall. Mater. Trans. 33A (2002) 665. [8] J. Qin, J.H. Han, G.D. Zhang, J.C. Lee, Scripta Mater. 51 (2004) 185. [9] L.G. Schultz, J. Appl. Phys. 20 (1949) 1030. [10] A. Yamashita, D. Yamaguchi, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A287 (2000) 100. [11] C. Pithan, T. Hashimoto, M. Kawazoe, J. Nagahora, K. Higashi, Mater. Sci. Eng. A280 (2000) 62.
[12] [13] [14] [15] [16]
R. Le Hazif, J.P. Poirier, Acta Metall. 23 (1975) 865. F. Perocheau, J.H. Driver, Int. J. Plasticity 18 (2002) 185. M. Carrard, J.L. Martin, Phil. Mag. A58 (1988) 491. P. Lequeu, J.J. Jonas, Metall. Trans. 19A (1988) 105. J.C. Lee, H.K. Seok, J.H. Han, Y.H. Chung, Mater. Res. Bull. 36 (2001) 997. [17] O. Engler, H.C. Kim, M.Y. Huh, Mater. Sci. Technol. 17 (2001) 75. [18] Y. Saito, H. Utsunomiya, H. Suzuki, T. Sakai, Scripta Mater. 42 (2000) 1139. [19] J.H. Han, H.K. Seok, Y.H. Chung, M.C. Shin, J.C. Lee, Mater. Sci. Eng. A323 (2002) 342.
Effect of temperature on texture formation of 6061 aluminum sheet in equal-channel angular pressing Qin Jining a,1 , Zhang Di a,1 , Zhang Guoding a,2 , Jae-Chul Lee b,∗ a
The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China b Division of Materials Science and Engineering, Korea University, Anam, Seoul 136-701, Korea Received in revised form 15 July 2005
Abstract A novel equal-channel angular pressing (ECAP) procedure was conducted on 6061 aluminum sheet at temperature up to 513 K to investigate temperature effect on microstructure and texture formation. The deformed microstructures were examined by transmission electron microscopy and the macrotexture evolution was investigated by orientation distribution functions (ODFs). The results shown that when ECAP at temperature below 433 K the microstructures exhibit high dislocation density within subgrains with thick and waved non-equilibrium boundaries, and the textures are characterized by the formation of Rotated cube component or a pair of textures components near Rotated cube component, such as {0 3 1}5 1 3 and {0 1 3}5 1 3or {0 4 1}6 1 4 and {0 1 4}6 4 1. Whereas under certain procedure condition, i.e. at 513 K, the ECAP procedure is capable of producing microstructure with low density dislocations subgrains separated by regular boundaries, and a preferable 1 1 1//ND texture fiber is formed with the R-value as higher as 1.5. The texture results show that the ECAP procedure could be expected to be a potential application to enhance the formability of aluminum sheet. © 2005 Elsevier B.V. All rights reserved. Keywords: ECAP; Texture; Aluminum alloys; Microstructure; Formability
1. Introduction In last few years there are numerous reports on the development of techniques for applying severe plastic deformation (SPD) to ductile metals and alloys, using methods such as equalchannel angular pressing (ECAP) [1]. The ECAP process, which leads to the creation of a submicron or nano-size grain structure after imposing high levels of strain [2–4], has provided a simple method to get advantages mechanical properties by refining microstructures without the major compositional modification. On the other hand, in automotive industry, the application of sheet aluminum alloys has led to an increasing interest as the demands of fuel efficiency and vehicle emission [5] increase. The conventional ECAP methods could only process the bulk materials, i.e. with circular or square cross section. In ∗ 1 2
Corresponding author. Tel.: +82 2 3290 3283; fax: +82 2 928 3584. E-mail address: [email protected] (J.-C. Lee). Tel.: +86 21 62933106; fax: +86 21 62822012. Tel.: +86 21 62932049; fax: +86 21 62822012.
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.128
present work a novel ECAP-base process [6,7], which can deal with sheet metals by continuous manner, were introduced to induce simple shear strain to aluminum sheet and to investigate the effectiveness of ECAP procedure in promoting sheet formability. The ECAP process parameters, such as the amount of deformation strain, the rotation between each repetitive pressing, the speed and the temperature in process, were believed to affect the final microstructure and texture formation in material. Texture, which represents the distribution of all orientations in the polycrystalline assembly, controls the formability of sheet metal. So the understanding and control of the texture evolution is of great importance with respect to the optimization of the formability of sheet metals. Up to date, to authors’ knowledge, there is no report available on the effect of temperature on texture formation of aluminum sheet during ECAP process, especially in conjunction with the sheet formability. Accordingly, the purpose of the current experiments was to investigate the microstructure and texture evolution of 6061 aluminum sheet at different ECAP temperature. The types and the intensities of the textures formed at each ECAP procedures
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Q. Jining et al. / Materials Science and Engineering A 408 (2005) 79–84
and the effects of those textures on sheet formability are of the importance in this study. The microstructures were investigated by transmission electron microscopy (TEM), and the macrotextures were analyzed by three dimensional orientation distribution functions (ODFs) which were determined by X-ray diffraction. The R-values were calculated from those ODFs to assess the sheet formability connected to each texture.
were cut at side plane from the center of strips with the samples normal perpendicular to both strips normal and longitudinal direction. Then the samples were ground mechanically to a thickness of 40 m followed by ion miller to perforate a hole at the middle of the samples. Microstructure observations were taken in Hitach-800 transmission electron microscope operating at 175 kV. Select area electron diffraction (SEAD) patterns were taken from the region of nearest 1 1 1 zone, having a diameter of about 4 m, as a quantitative assess of the misorientation between subgrains. The specimens used to evaluate texture were cut from ECAPed strips and thinned chemically to half the thickness in a NaOH solution at 323 K for avoiding any texture change induced to specimens surface by mechanical polish process. The {1 1 1}, {2 0 0} and {2 2 0} pole figures were measured on an automated X-ray goniometer (Phillips X’Pert) with Cu K␣ radiation by the Schultz reflection method [9]. The orientation distribution functions (ODFs) were calculated from these three incomplete pole figures according to the WIMV method. To evaluate the formability associated with each texture, the R-value, along 0◦ –90◦ at 15◦ interval with respect to the direction of ECAP, were calculated using popLA program.
2. Experimental procedure
3. Results and discussion
The schematic diagram of the novel ECAP process used in present study is shown in Fig. 1. The design principle is to introduce shear deformation into metallic strip in a continuous manner. It makes use of two rollers as feeding apparatus before the forming dies, which construct two ECAP channels—the inlet and outlet channel. The thicknesses of the two channels are dissimilar to each other such that the outlet channel (1.55 mm) is slight thicker than the inlet channel (1.45 mm) to enable the consecutive ECA pressings. To change the strain introduced in each ECAP process, the oblique angle (Φ) between two channels can be adjusted. In present investigation the oblique angle is 120◦ , so the equivalent effective strain introduced into the sheet metal in present ECAP process can be determined [7] as 0.58. The material used in present investigation was a commercial 5 mm thickness 6061 aluminum plates with the chemical composition of Al–1.0Mg–0.6Si–0.3Cu–0.2Cr, where the alloy compositions are given in weight percent. Warm rolling was carried out to this aluminum plates unidirectionally with each rolling reduction about 10–15% without any intermittent annealing until the final thickness, 1.55 mm, were obtained. Thus the starting material possessed a typical rolling texture–a main texture conponent of -fiber and a weak Goss orientation{1 1 0}[0 0 1] [8]. The maixmum texture intensity in -fiber and Goss orientation are of 6.4 and 3.7 times of the random distribution. The rolled strips were cut with demension of 20 mm × 200 mm × 1.55 mm and put in the furnace at required temperature. The temperatures chosen for present study were room temperature, 353, 433 and 513 K, respectively. After held in furnace for 30 min, the strips were feeded quicklly into the ECAP apperatus to ECAPed at speed of 40 m/s. Microstructures were examined after ECAPed at each different temperature. The transmission electron microscopy samples
3.1. Characterization of microstructure evolution
Fig. 1. Schematic diagram of the ECA pressing process used in the present study.
Fig. 2 is the series of the TEM bright field images showing the evolution of microstructures and corresponding SAED patterns of 6061 aluminum sheet subjected to ECA pressing at room temperature, 353, 413 and 513 K, respectively. It is shown form Fig. 2(a), ECAPed at room temperature, that the microstructure is characterized by the thick and waved non-equilibrium grain boundaries formed by tangling dislocations and the high dislocation density within cells and subgrains. Fig. 2(b) and (c), ECAPed at temperature of 353 K and 433 K, demonstrate basically similar microstructure as that ECAPed on room temperature. The elongated fine subgrains with 0.5 m in width and some non-equilibrium grain boundaries can be observed while the high dislocation density still retains in subgrains. It should be noted that Fig. 2(d) unambiguously shows some microstructure changes have taken place when ECAPed at temperature of 513 K. Thin and regular-shaped cell boundaries were formed while the dislocation density within the cells was obviously diminished as compared to those formed at lower temperatures. Subgrains consisted of piled up and polygonized dislocations wall are present as marked by black arrows in Fig. 2(d). These microstructure characters indicated some annihilation of dislocation in opposite sign and the rearrangement of dislocations in cell interior, which suggest that a dynamic recovery may occur at this pressing temperature because of the high stacking fault energy of aluminum, the large deformation strain and the high deformation temperature in present experiment. The low density dislocation subgrains separated low angle boundaries had been observed in other aluminum alloys when ECAP was carry out at elevated temperature [10,11],
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Fig. 2. Microstructures and associated SADE patterns for 6061 aluminum sheets after ECAPed at: (a) room temperature; (b) 353 K; (c) 433 K; and (d) 513 K, respectively.
and were referred to the high recovery rate of the aluminum alloys. It can be seen from Fig. 2 that the ECA pressing up to temperature of 513 K does not lead to a substantial growing in grain size, and according to the SEAD patterns the misorientation are also keep on the level of a few degree. Whereas, when the pressing temperature increased, the dislocation density within the subgrains decrease gradually and the subgrains boundaries become thinner with a relatively regular SEAD patterns.
(1 1 1//ND). As the samples experience the simple shear deformation in ECAP procedure, the range of reduced Euler space for f.c.c. is of 0◦ ≤ φ1 ≤ 180◦ , 0◦ ≤ φ, φ2 ≤ 90◦ to present the monoclinic sample symmetry.
3.2. Textures formed at different temperature In order to facilitate interpretation of the texture presented below, some important orientation components and texture fiber in f.c.c. metal are plotted in reduced Euler space in Fig. 3. It is comprised of ␥-fiber, {1 1 2}1 1 0 component and Rotated cube {0 0 1}1 1 0 which has deteriorated to a skeleton line as the strong distortion of Euler space near φ = 0◦ . The ␥-fiber, which defined as a line of φ = 54.7◦ , φ2 = 45◦ , φ1 = 0–180◦ in reduced Euler space, have orientations of {1 1 1}u v w with a common {1 1 1} normal parallel to sheet normal direction
Fig. 3. Keys to some important textures components and fiber at reduced Euler space.
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Fig. 4. The spatial representation of ODFs after ECAPed at different temperatures of: (a) room temperature; (b) 353 K; (c) 433 K; and (d) 513 K.
Fig. 4 shows the spatial representations of textures ECAPed at different temperature, which display the iso-intensity surfaces of orientation distribution functions with intensity of f(g) = 3. The orientation distribution intensity greater than 3 are distributed within these iso-intensity surfaces, and the crosssections between the iso-intensity surfaces and the boundaries of Euler space reveal these intensity contours. The orientation intensity f(g) = 3 was chosen to present the iso-intensity surface for this intensity can clearly demonstrate the concentration of each prefer orientation. Either higher or lower threshold value may cause the minor components invisible. Fig. 4(a–d) clearly demonstrates the texture evolution in 6061 aluminum sheets when ECAP operation temperature changed from room temperature to 353, 433 and 513 K, respectively. It should be noticed in Fig. 4 that there is no any trace of the rolling texture (the -fiber) in the ECAPed textures. In fact, the rolling texture in starting material will change completely even under the effective ECAP strain of 0.37 [8]. In Fig. 4(a), ECAPed at room temperature, the texture is characterized by the main texture component Rotated cube {0 0 1}1 1 0 and the minor {1 1 2}1 1 0 component. It is
shown in Fig. 4(b) and (c) that there is no significant texture change when ECA pressing was carried out at temperatures of 353 and 433 K, respectively. The textures are exhibited by a pair of symmetric components near to rotated cube with the low index of {0 3 1}5 1 3 and {0 1 3}5 3 1 in 353 K (Fig. 4(b)), and {0 4 1}6 1 4and {0 1 4}6 4 1 in 433 K (Fig. 4(c)). Fig. 4(d) clearly shows that the texture developed at ECAP temperature of 513 K was change remarkably. The maximum intensities are shifted to the position of {3 3 2}3 1 3 in low index expression with the maximum orientation distribution intensity of f(g) = 8. As each {3 3 2}3 1 3 component is sited close to ␥-fiber, in fact the maximum orientations site around the ␥-fiber with approximately 6◦ misorientation, and each {3 3 2}3 1 3 components is scattered along the 1 1 1 fiber direction, virtually the resulting texture develop into a ␥-fiber. There is not any other texture component appearing. In Fig. 4, the maximum orientation intensities in different ECAP temperatures are kept basically at the level of eight times of random intensity. It should be owed to the same ECAP shear strain induced to each of the aluminum sheet. On the other hand, however, a transition of the main preferential orientations
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The r values, in directions of 0◦ , 45◦ and 90◦ with respect to the longitudinal direction of ECAP sample, were calculated using popLA program. The Lankford parameter (R-value) is determined by averaging these three r values according to the following equation: r0 + 2r45 + r90 R= (1) 4
Fig. 5. Variation of calculated R-values as a function of the ECAP processing temperatures.
occurs when ECAP was carried out at different processing temperature. It is commonly accepted that slip in aluminum alloys usually occurs on the octahedral {1 1 1} planes along 1 1 0 directions but there have been many reports of slipping on other crystal planes at elevated temperatures. These studies indicated that the non-octahedral slip occurs on the {1 1 0} plane [12,13] and {1 0 0} planes [13,14] during high temperature deformation of aluminum alloys. The change on deformation texture in present study suggest that the mechanism of the formation of the 1 1 1//ND texture may be the slip activity on non-octahedral slip system (Fig. 5). 3.3. Impact of texture on sheet formability The R-value (Lankford parameter), which controls the strain between the strain in plane and the strain normal to the plane, is the critically value in appreciating the neck resistance in sheet forming and has been used as a measurement for judging the formability. In order to achieve optimum formability, larger Rvalue is required in sheet forming. For some b.c.c. metals, such as low carbon steel sheets, the favorable R-values are readily achieved by the formation of the well-known fiber recrystallization textures [15], i.e. the formation of ␥-fiber. But for f.c.c. metals, this optimum condition is difficult to obtain by most conventional metal forming processes and usually the R-value of aluminum was lower than 1 [16]. Recently some effort [17] has been made to obtain the {1 1 1}//ND texture by means of an additional warm rolling without lubrication in conventional rolling process, but the preferable texture components were formed only in the sheet surface layers and the orientation intensities were weak. There also have reported [18,19] that some 1 1 1//ND shear texture components can be introduced by an ECAP based process, but it failed to get satisfactory uniformed deformation texture through the sample thickness and adequate intensities of the shear components. In present study, an essentially pure ␥fiber was obtained, as can be seen in Fig. 3(d), and no any other texture components appeared. So a preferable R-value could be expected.
The R-values evaluated based on Eq. (1) are presented in Fig. 4. As seen in this figure, the R-values in samples ECAPed at room temperature, 353 and 433 K, i.e. the macrotexture are characterized by Rotated cube or components near to the Rotated cube, hold the values of 1 or below 1. However, the sample ECAPed at temperature of 513 K obviously demonstrate the higher R-value of 1.5 as its texture contained the 1 1 1//ND fiber. The experimental results obtained in this study indicate that ECAP can be used as a means for controlling the texture of the aluminum sheets to enhance the sheet formability. 4. Conclusions The ECA pressing at temperature up to 513 K does not lead to a substantial growing in subgrain size. The microstructure ECAPed at 513 K is characterized by thin and regular boundaries with low dislocation density within subgrains while the non-equilibrium boundaries and higher dislocation density will appear when ECAPed at temperature below 433 K. The ECA pressing temperature strongly affects the texture formation in 6061 aluminum sheet. When ECAPed at a certain temperature, 513 K in present study, the {3 3 2}3 1 3 texture components will emerge; and moreover this {3 3 2}3 1 3 components are scattered along 1 1 1 direction, it virtually form a 1 1 1//ND texture fiber. The aluminum sheet with 1 1 1//ND texture fiber exhibits an R-value as higher as 1.5. It seemed from present study that the ECAP process not only refine the microstructure of aluminum sheet, but also can be used as a potential means to control the texture formation, i.e. for enhancing the formability of aluminum sheet under proper process condition. Acknowledgements The work of this paper was sponsored partly by the Korea Institute of Science and Technology Evaluation and planning (KISTEP) under the Science and Technology Manpower Exchange Program and partly supported from the Major Fundamental Research Project, Shanghai Science and Technology Committee under Grant no. 04DZ14002. References [1] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metall. 1 (1981) 99. [2] D.H. Shin, B.C. Kim, K.T. Park, W.Y. Choo, Acta Mater. 48 (2000) 3245. [3] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317. [4] M. Murayama, Z. Horita, K. Hono, Acta Mater. 49 (2001) 21.
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