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Effect of ECAP on microstructure and mechanical properties of a commercial 6061 Al alloy produced by powder metallurgy
Journal of Alloys and Compounds 354 (2003) 216–220
L
www.elsevier.com / locate / jallcom
Effect of ECAP on microstructure and mechanical properties of a commercial 6061 Al alloy produced by powder metallurgy a, b c b Si-Young Chang *, Ki-Seung Lee , Seung-Hoe Choi , Dong Hyuk Shin a
Department of Materials Engineering, Hankuk Aviation University, 200 -1 Hwajon-dong, Koyang-shi, Kyunggi-do 412 -791, South Korea b Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-do 425 -791, South Korea c Department of General Studies, Hankuk Aviation University, Koyang-shi, Kyunggi-do 412 -791, South Korea Received 15 January 2002; received in revised form 7 May 2002; accepted 29 October 2002
Abstract The 6061 (Al–1.01 wt% Mg–1.07 wt% Si) Al alloy was fabricated by powder metallurgy, and then subjected to equal channel angular pressing. The microstructure and mechanical properties such as microhardness and tensile properties of the equal channel angular pressed P/ M 6061 Al alloy were investigated. The P/ M 6061 Al alloy had an initial grain size of approximately 20 mm. After two pressings at 373 K using route A, the sample revealed microstructure of subgrain bands with a length of |0.8 mm and a width of |0.3 mm. The subgrain bands became larger above 1 mm in length and width after two pressings at 573 K. An equiaxed ultra-fine grained structure with the mean grain size of |0.5 mm was obtained after four repetitive equal channel angular pressings at 473 K using route A and C. The microhardness of P/ M 6061 Al alloys was drastically increased from about 40 to 80 Hv by two repetitive pressings at 373 K. However, the microhardness decreased with increasing the pressing temperature. The tensile strength of 6061Al alloy before the equal channel angular pressing was 95 MPa, whereas it increased to both 248 MPa after two pressings at 373 K and 130 MPa after four pressings at 473 K, which was superior to that of a commercial 6061-O Al alloy. 2003 Elsevier Science B.V. All rights reserved. Keywords: Ultra-fine grained Al–Mg–Si alloy; Powder metallurgy; Equal channel angular pressing; Mechanical properties
1. Introduction According to a number of recent research studies, the equal channel angular pressing (ECAP) [1–5] has received much attention as an attractive method to obtain a submicrometer ultra-fine grained (UFG) structure in a variety of bulk metallic materials without residual porosity: for example, Armco iron [6,7], Al alloys and their composites [8–12], Cu [13–16], Mg alloys [8], pure Ni [6], low carbon steel [4,5,17–20], Ti alloys [21–23], and Zn–22% Al [24]. Among Al alloys, a commercial 6061 Al alloy which is one of Al–Mg–Si system alloys, has been widely used as structural material for construction and transportation such as vessels and automobile, because it has medial strength
*Corresponding author. Tel.: 182-2-300-0168; fax: 182-2-3158-3770. E-mail address: [email protected] (S.-Y. Chang).
and excellent corrosion resistance, weldability, and fatigue strength [25]. However, this alloy is basically used in the type of as-cast, except only a portion used as wrought material due to its poor workability. In order to improve the poor workability, the powder metallurgy can be adopted in the step of processing because of its several merits that are possible to reduce the compounds occurring during solidification and to obtain the fine grained structure compared with as cast materials [26]. In addition, following the powder metallurgy, the subsequent processes such as rolling, extrusion and forging are generally conducted to improve the mechanical properties. Accordingly, it is very much of interest to apply the ECAP as the subsequent process of a commercial 6061 Al alloy produced by powder metallurgy. In this study, therefore, a commercial 6061 Al alloy was prepared by the powder metallurgy and then the microstructure and mechanical properties at room temperature after obtaining the UFG structure of P/ M 6061 Al alloy by ECAP was evaluated.
0925-8388 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00008-2
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
2. Experimental procedure
3. Results
Air atomized 6061 Al alloy (Al–1.01% Mg–1.07% Si–0.35% Cu–0.25% Fe–0.05% Mn–0.12% Cr (in wt%)) powders with a size below 30 mm were prepared. The powders were hot-pressed at 773 K with a pressure of 100 MPa under a vacuum atmosphere of approximately 1 Pa. The hot-pressed samples with a dimension of [32380 mm were machined to cylindrical samples of [10380 mm as pre-materials for ECAP. The ECAP was carried out at 373–573 K. The present ECAP die was designed to yield an effective strain of |1 by a single pass: the inner angle and the arc of curvature at the outer point of contact between channels of the die were 90 and 208, respectively [27,28]. During the ECAP, the samples were pressed repetitively using two processing routes: route A in which the sample is repetitively pressed without any rotation and route C in which the sample is rotated 1808 around its longitudinal axis between individual pressings [11]. Tensile tests were performed using an Instron machine on the tensile specimens with 10 mm in gauge length at the initial strain rate of 1.00310 23 s 21 and at room temperature. Microhardness was measured using a Vickers microhardness tester at a load of 0.05 kg for 15 s. Microstructure examination of samples before / after ECAP was carried out using a field emission scanning electron microscope (FE-SEM, JSM6330F, Jeol, Japan) and a transmission electron microscope (TEM, Jeol 2010, Japan) operated at 200 keV. The microstructure of x-plane and y-plane in samples before / after ECAP was observed; where the x-plane is the plane perpendicular to the longitudinal axis of the sample and the y-plane denotes the side-viewed plane parallel to the longitudinal axis of the sample.
3.1. Microstructural characteristics
217
Fig. 1 shows microstructure and EDS analysis of P/ M 6061 Al alloys before ECAP. The grains have a size of approximately 20 mm. The particles which existed in the grain boundary were identified as Si by EDS analysis. In this study, however, the compounds such as Mg 2 Si and AlFeSi that are generally included in cast 6061 Al alloy [29] were not detected. The microstructure of the ECA pressed P/ M 6061 Al alloy are shown in Fig. 2. After ECAP, the initial grain boundaries were hardly identified, and the Si particles were uniformly distributed. The sample pressed at relatively low temperature of 373 K showed that the Si particles were slightly aligned to flow direction. However, the phenomenon disappeared with increasing pressing temperature. This is because the magnitude of shear deformation in metallic materials during ECAP is closely related to the pressing temperature [30,31]. In addition, it is apparent that the particles are well-distributed with increasing pressing temperature. On the other hand, there is no microstructural difference in the samples pressed using route A and route C (Fig. 2c and d). Fig. 3 represents TEM micrographs of ECA pressed P/ M 6061 Al alloy. After two pressings at 373 K using route A, the elongated grains with a length of |0.8 mm and a width of |0.3 mm were observed. The SAED pattern showed the appearance of the diffused spots and the extra spots, indicating the formation of a high angle boundary. In contrast, the grains after two pressings at relatively high temperature of 573 K became larger above 1 mm in length and width. The corresponding SAED pattern was char-
Fig. 1. (a) Microstructure of as hot-pressed 6061 Al alloy before ECAP and (b) EDS analysis of a particle at a grain boundary.
218
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
Fig. 2. Microstructure of 6061 Al alloys after ECAP: (a) and (b) two pressings at 373 and 573 K, respectively, using route A; (c) and (d): four pressings at 473 K using route A and C, respectively.
acterized by relatively clear spots. This implies that most of the boundaries in the grains formed by two pressings would be low-angled. Near equiaxed ultra-fine grains approximately 0.5 mm in grain size were obtained by four repetitive ECAPs. However, no difference in microstructure according to the route was observed. In addition, the number of rings in the SAED pattern increased and the spots became more diffused compared with samples after two ECAPs at 373 and 573 K.
3.2. Mechanical properties The microhardness of ECA pressed P/ M 6061 Al alloy is shown in Table 1. The microhardness revealed a substantial increase after the ECAP. In particular, there was a significant change in the microhardness after the pressing at 373 K. The increase of the pressing temperature resulted in the small increase of microhardness. However, there was no change according to the difference of route. In addition, the microhardness between the x-plane and the y-plane had same tendency to increase after the pressing. The drastic increase of microhardness after the pressing is because of the work hardening that is caused by the formation of sub-micrometer ordered grains and the density increase of dislocation occurring with the shear
deformation in the initial grain interior. Additionally, the reason why the microhardness decreases with increasing the pressing temperature is deduced to be due to the dynamic recovery occurring during the pressing [28] at higher temperatures. Table 2 shows the variation of the tensile strength and elongation of P/ M 6061 Al alloy after the ECAP, together with the commercial 6061 Al alloys. The tensile strength of hot-pressed 6061 Al alloy drastically increased from 95 to 248 MPa after two pressings at 373 K. In contrast, after four repetitive ECAPs at 473 K, the tensile strength increases to 130 MPa and the elongation is 25%. Additionally, the P/ M 6061 Al alloy pressed repetitively at 573 K revealed low tensile strength and elongation. These results are well compatible with both the microstructural characteristics shown in Fig. 3 and the recent report [30,31]. From the above results, it is apparent that the improvement of tensile strength is attributable to the grain refining by the ECAP process. On the other hand, there was no difference in tensile properties with the route. Additionally, Table 2 compares the tensile strength and elongation of the ECA pressed P/ M 6061 Al alloy to the same values for both the commercial fully-annealed 6061O and T4 treated 6061 available [32]. The tensile strength of two repetitively ECA pressed P/ M 6061 Al alloy at 373
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
219
Fig. 3. TEM micrographs of the ECA pressed P/ M 6061 Al alloys: (a) and (b) two pressings at 373 and 573 K, respectively, using route A; (c) and (d) four pressings at 473 K using route A and C, respectively.
Table 1 Microhardness (Hv) of the ECA pressed P/ M 6061 Al alloys (Hv)
As-received Two pressings at 373 K (route A) Two pressings at 573 K (route A) Four pressings at 473 K (route A) Four pressings at 473 K (route C)
x-plane
y-plane
39 82 54 65 67
46 79 55 65 65
K is nearly two times higher than that of the fully annealed 6061-O alloy and was comparable to that of the T4 treated 6061 T4 alloy, whereas the elongation decreases. In contrast, there was a slight increase in tensile strength of P/ M 6061 Al alloy after four repetitive pressings at 473 K
over the fully annealed 6061-O alloy without decrease in elongation. According to recent reports, it has a close relation to the annihilation kinetics of extrinsic grain boundary dislocation introduced by severe plastic deformation and the number of dislocations necessary to deform the ultra-fine grain [13,33,34].
4. Summary 1. The ECAP was successfully conducted at various temperatures and two different routes on the same sample up to a total of four pressings through the die such that the sample was not rotated (route A) and
Table 2 Tensile properties of the ECA pressed P/ M 6061 Al alloys and the commercial 6061 Al alloys
As-received P/ M 6061 Two ECA pressed P/ M 6061 (373 K, route A) Two ECA pressed P/ M 6061 (573 K, route A) Four ECA pressed P/ M 6061 (473 K, route A) Four ECA pressed P/ M 6061 (473 K, route C) 6061-O 6061-T4
Tensile strength (MPa)
Elongation (%)
95 248 90 130 130 123 235
8 10 18 25 25 25 22
220
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
rotated 1808 (route C) around its longitudinal axis between pressings. 2. After two ECAPs at 373 K using route A, the sample revealed microstructure of subgrain bands with a length of |0.8 mm and a width of |0.3 mm. The subgrain bands became larger above 1 mm in length and width after two ECAPs at 573 K. An equiaxed ultra-fine grained (UFG) structure with the mean grain sizes of |0.5 mm was obtained after four repetitive ECAPs using route A and C. 3. The microhardness and tensile strength increased with decreasing the pressing temperature. The microhardness of sample after four repetitive ECAPs at 473 K was approximately 20 Hv higher in microhardness than that of as-received one. The tensile strength of P/ M 6061 Al alloy increased from 95 MPa to 248 MPa after two ECAPs at 373 K and 130 MPa after four pressings at 473 K, which was superior to that of a fully annealed 6061-O Al alloy. 4. The ECAP technique can be effectively applied as the subsequent process of a commercial 6061 Al alloy produced by powder metallurgy. In particular, many repetitive pressings at low temperatures (,1 / 2 T m ) are sufficient to give the ultrafine grained (UFG) structure resulting in the highly increased microhardness and tensile strength.
Acknowledgements This work was supported by the ‘National Research Laboratory Program’ of Korea Ministry of Science and Technology.
References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [2] D.H. Shin, Y.S. Kim, E.J. Lavernia, Acta Mater. 49 (2001) 1. [3] D.H. Shin, I.Y. Kim, J. Kim, K.T. Park, Acta Mater. 49 (2001) 1285. [4] D.H. Shin, B.C. Kim, K.T. Park, W.Y. Choo, Acta Mater. 48 (2000) 3245. [5] D.H. Shin, B.C. Kim, Y.S. Kim, K.T. Park, Acta Mater. 48 (2000) 2247.
[6] V.M. Segal, Mater. Sci. Eng. A 197 (1995) 157. [7] R.Z. Valiev, Yu.V. Ivanisenko, E.F. Rauch, B. Baudelet, Acta Mater. 44 (1996) 4705. [8] M.V. Markshev, C.C. Bampton, M.Yu. Murashkin, D.A. Hardwick, Mater. Sci. Eng. A 234–236 (1997) 927. [9] R.Z. Valiev, A.V. Korznikov, R.R. Mulyukov, Mater. Sci. Eng. A 168 (1993) 141. [10] P.B. Berbon, N.K. Tsenev, R.Z. Valiev, M. Furukawa, Z. Horita, M. Nemoto, T.G. Landon, Metall. Mater. Trans. A 29 (1998) 2237. [11] Y. Iwahashi, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29 (1998) 2245. [12] R.Z. Valiev, R.K. Islamgaliev, N.F. Kuzmina, Y. Li, T.G. Langdon, Scripta Mater. 40 (1999) 117. [13] R.Z. Valiev, E.V. Kozlov, Yu.F. Ivanov, J. Lian, A.A. Nazarov, B. Baudelet, Acta Metall. Mater. 42 (1994) 2467. [14] S. Ferrasse, V.M. Segal, K.T. Hartwig, R.E. Goforth, Metall. Mater. Trans. A 28 (1997) 1047. [15] R.Z. Valiev, I.V. Alexander, Nanostruct. Mater. 12 (1999) 35. [16] I.V. Alexander, R.Z. Valiev, Nanostruct. Mater. 12 (1999) 709. [17] D.H. Shin, K.H. Oh, W.J. Kim, S.W. Lee, W.Y. Choo, J. Korean Inst. Metall. Mater. 37 (1999) 1048. [18] D.H. Shin, W.J. Kim, W.Y. Choo, Scripta Mater. 41 (1999) 259. [19] D.H. Shin, C.W. Seo, J.R. Kim, K.T. Park, W.Y. Choo, Scripta Mater. 42 (2000) 695. [20] K.T. Park, Y.S. Kim, J.K. Lee, D.H. Shin, Mater. Sci. Eng. A 293 (2000) 165. [21] S.L. Semitan, V.M. Segal, R.E. Goforth, N.D. Frey, D.P. DeLo, Metall. Mater. Trans. A 30 (1999) 1425. [22] D.P. DeLo, S.L. Semitan, Metall. Mater. Trans. A 30 (1999) 2473. [23] S.L. Semitan, D.P. DeLo, Mater. Design 21 (2000) 311. [24] M. Furukawa, Y. Ma, Z. Horita, M. Nemoto, R.Z. Valiev, T.G. Langdon, Mater. Sci. Eng. A 241 (1998) 122. [25] Japan Institute of Light Metals, Microstructure and Properties of Aluminum Alloys, 1991, pp. 290–291. [26] R.M. German, Powder Metallurgy of Iron and Steel, Wiley, New York, 1998, pp. 380–385. [27] K. Nakashima, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 1589. [28] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [29] Japan Institute of Light Metals, Microstructure and Properties of Aluminum Alloys, 1991, pp. 279. [30] D.H. Shin, Y.K. Kim, K.T. Park, Y.S. Kim, Mater. Sci. Eng. (2001) in press. [31] A. Yamashita, D. Yamaguchi, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 287 (2000) 100. [32] Japan Institute of Light Metals, Microstructure and Properties of Aluminum Alloys, 1991, p. 478. [33] J. Lian, B. Baudelet, A.A. Nazarov, Mater. Sci. Eng. A 172 (1993) 23. [34] W. Lojkowski, Acta Metall. Mater. 39 (1991) 1891.
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Effect of ECAP on microstructure and mechanical properties of a commercial 6061 Al alloy produced by powder metallurgy a, b c b Si-Young Chang *, Ki-Seung Lee , Seung-Hoe Choi , Dong Hyuk Shin a
Department of Materials Engineering, Hankuk Aviation University, 200 -1 Hwajon-dong, Koyang-shi, Kyunggi-do 412 -791, South Korea b Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-do 425 -791, South Korea c Department of General Studies, Hankuk Aviation University, Koyang-shi, Kyunggi-do 412 -791, South Korea Received 15 January 2002; received in revised form 7 May 2002; accepted 29 October 2002
Abstract The 6061 (Al–1.01 wt% Mg–1.07 wt% Si) Al alloy was fabricated by powder metallurgy, and then subjected to equal channel angular pressing. The microstructure and mechanical properties such as microhardness and tensile properties of the equal channel angular pressed P/ M 6061 Al alloy were investigated. The P/ M 6061 Al alloy had an initial grain size of approximately 20 mm. After two pressings at 373 K using route A, the sample revealed microstructure of subgrain bands with a length of |0.8 mm and a width of |0.3 mm. The subgrain bands became larger above 1 mm in length and width after two pressings at 573 K. An equiaxed ultra-fine grained structure with the mean grain size of |0.5 mm was obtained after four repetitive equal channel angular pressings at 473 K using route A and C. The microhardness of P/ M 6061 Al alloys was drastically increased from about 40 to 80 Hv by two repetitive pressings at 373 K. However, the microhardness decreased with increasing the pressing temperature. The tensile strength of 6061Al alloy before the equal channel angular pressing was 95 MPa, whereas it increased to both 248 MPa after two pressings at 373 K and 130 MPa after four pressings at 473 K, which was superior to that of a commercial 6061-O Al alloy. 2003 Elsevier Science B.V. All rights reserved. Keywords: Ultra-fine grained Al–Mg–Si alloy; Powder metallurgy; Equal channel angular pressing; Mechanical properties
1. Introduction According to a number of recent research studies, the equal channel angular pressing (ECAP) [1–5] has received much attention as an attractive method to obtain a submicrometer ultra-fine grained (UFG) structure in a variety of bulk metallic materials without residual porosity: for example, Armco iron [6,7], Al alloys and their composites [8–12], Cu [13–16], Mg alloys [8], pure Ni [6], low carbon steel [4,5,17–20], Ti alloys [21–23], and Zn–22% Al [24]. Among Al alloys, a commercial 6061 Al alloy which is one of Al–Mg–Si system alloys, has been widely used as structural material for construction and transportation such as vessels and automobile, because it has medial strength
*Corresponding author. Tel.: 182-2-300-0168; fax: 182-2-3158-3770. E-mail address: [email protected] (S.-Y. Chang).
and excellent corrosion resistance, weldability, and fatigue strength [25]. However, this alloy is basically used in the type of as-cast, except only a portion used as wrought material due to its poor workability. In order to improve the poor workability, the powder metallurgy can be adopted in the step of processing because of its several merits that are possible to reduce the compounds occurring during solidification and to obtain the fine grained structure compared with as cast materials [26]. In addition, following the powder metallurgy, the subsequent processes such as rolling, extrusion and forging are generally conducted to improve the mechanical properties. Accordingly, it is very much of interest to apply the ECAP as the subsequent process of a commercial 6061 Al alloy produced by powder metallurgy. In this study, therefore, a commercial 6061 Al alloy was prepared by the powder metallurgy and then the microstructure and mechanical properties at room temperature after obtaining the UFG structure of P/ M 6061 Al alloy by ECAP was evaluated.
0925-8388 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00008-2
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
2. Experimental procedure
3. Results
Air atomized 6061 Al alloy (Al–1.01% Mg–1.07% Si–0.35% Cu–0.25% Fe–0.05% Mn–0.12% Cr (in wt%)) powders with a size below 30 mm were prepared. The powders were hot-pressed at 773 K with a pressure of 100 MPa under a vacuum atmosphere of approximately 1 Pa. The hot-pressed samples with a dimension of [32380 mm were machined to cylindrical samples of [10380 mm as pre-materials for ECAP. The ECAP was carried out at 373–573 K. The present ECAP die was designed to yield an effective strain of |1 by a single pass: the inner angle and the arc of curvature at the outer point of contact between channels of the die were 90 and 208, respectively [27,28]. During the ECAP, the samples were pressed repetitively using two processing routes: route A in which the sample is repetitively pressed without any rotation and route C in which the sample is rotated 1808 around its longitudinal axis between individual pressings [11]. Tensile tests were performed using an Instron machine on the tensile specimens with 10 mm in gauge length at the initial strain rate of 1.00310 23 s 21 and at room temperature. Microhardness was measured using a Vickers microhardness tester at a load of 0.05 kg for 15 s. Microstructure examination of samples before / after ECAP was carried out using a field emission scanning electron microscope (FE-SEM, JSM6330F, Jeol, Japan) and a transmission electron microscope (TEM, Jeol 2010, Japan) operated at 200 keV. The microstructure of x-plane and y-plane in samples before / after ECAP was observed; where the x-plane is the plane perpendicular to the longitudinal axis of the sample and the y-plane denotes the side-viewed plane parallel to the longitudinal axis of the sample.
3.1. Microstructural characteristics
217
Fig. 1 shows microstructure and EDS analysis of P/ M 6061 Al alloys before ECAP. The grains have a size of approximately 20 mm. The particles which existed in the grain boundary were identified as Si by EDS analysis. In this study, however, the compounds such as Mg 2 Si and AlFeSi that are generally included in cast 6061 Al alloy [29] were not detected. The microstructure of the ECA pressed P/ M 6061 Al alloy are shown in Fig. 2. After ECAP, the initial grain boundaries were hardly identified, and the Si particles were uniformly distributed. The sample pressed at relatively low temperature of 373 K showed that the Si particles were slightly aligned to flow direction. However, the phenomenon disappeared with increasing pressing temperature. This is because the magnitude of shear deformation in metallic materials during ECAP is closely related to the pressing temperature [30,31]. In addition, it is apparent that the particles are well-distributed with increasing pressing temperature. On the other hand, there is no microstructural difference in the samples pressed using route A and route C (Fig. 2c and d). Fig. 3 represents TEM micrographs of ECA pressed P/ M 6061 Al alloy. After two pressings at 373 K using route A, the elongated grains with a length of |0.8 mm and a width of |0.3 mm were observed. The SAED pattern showed the appearance of the diffused spots and the extra spots, indicating the formation of a high angle boundary. In contrast, the grains after two pressings at relatively high temperature of 573 K became larger above 1 mm in length and width. The corresponding SAED pattern was char-
Fig. 1. (a) Microstructure of as hot-pressed 6061 Al alloy before ECAP and (b) EDS analysis of a particle at a grain boundary.
218
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
Fig. 2. Microstructure of 6061 Al alloys after ECAP: (a) and (b) two pressings at 373 and 573 K, respectively, using route A; (c) and (d): four pressings at 473 K using route A and C, respectively.
acterized by relatively clear spots. This implies that most of the boundaries in the grains formed by two pressings would be low-angled. Near equiaxed ultra-fine grains approximately 0.5 mm in grain size were obtained by four repetitive ECAPs. However, no difference in microstructure according to the route was observed. In addition, the number of rings in the SAED pattern increased and the spots became more diffused compared with samples after two ECAPs at 373 and 573 K.
3.2. Mechanical properties The microhardness of ECA pressed P/ M 6061 Al alloy is shown in Table 1. The microhardness revealed a substantial increase after the ECAP. In particular, there was a significant change in the microhardness after the pressing at 373 K. The increase of the pressing temperature resulted in the small increase of microhardness. However, there was no change according to the difference of route. In addition, the microhardness between the x-plane and the y-plane had same tendency to increase after the pressing. The drastic increase of microhardness after the pressing is because of the work hardening that is caused by the formation of sub-micrometer ordered grains and the density increase of dislocation occurring with the shear
deformation in the initial grain interior. Additionally, the reason why the microhardness decreases with increasing the pressing temperature is deduced to be due to the dynamic recovery occurring during the pressing [28] at higher temperatures. Table 2 shows the variation of the tensile strength and elongation of P/ M 6061 Al alloy after the ECAP, together with the commercial 6061 Al alloys. The tensile strength of hot-pressed 6061 Al alloy drastically increased from 95 to 248 MPa after two pressings at 373 K. In contrast, after four repetitive ECAPs at 473 K, the tensile strength increases to 130 MPa and the elongation is 25%. Additionally, the P/ M 6061 Al alloy pressed repetitively at 573 K revealed low tensile strength and elongation. These results are well compatible with both the microstructural characteristics shown in Fig. 3 and the recent report [30,31]. From the above results, it is apparent that the improvement of tensile strength is attributable to the grain refining by the ECAP process. On the other hand, there was no difference in tensile properties with the route. Additionally, Table 2 compares the tensile strength and elongation of the ECA pressed P/ M 6061 Al alloy to the same values for both the commercial fully-annealed 6061O and T4 treated 6061 available [32]. The tensile strength of two repetitively ECA pressed P/ M 6061 Al alloy at 373
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
219
Fig. 3. TEM micrographs of the ECA pressed P/ M 6061 Al alloys: (a) and (b) two pressings at 373 and 573 K, respectively, using route A; (c) and (d) four pressings at 473 K using route A and C, respectively.
Table 1 Microhardness (Hv) of the ECA pressed P/ M 6061 Al alloys (Hv)
As-received Two pressings at 373 K (route A) Two pressings at 573 K (route A) Four pressings at 473 K (route A) Four pressings at 473 K (route C)
x-plane
y-plane
39 82 54 65 67
46 79 55 65 65
K is nearly two times higher than that of the fully annealed 6061-O alloy and was comparable to that of the T4 treated 6061 T4 alloy, whereas the elongation decreases. In contrast, there was a slight increase in tensile strength of P/ M 6061 Al alloy after four repetitive pressings at 473 K
over the fully annealed 6061-O alloy without decrease in elongation. According to recent reports, it has a close relation to the annihilation kinetics of extrinsic grain boundary dislocation introduced by severe plastic deformation and the number of dislocations necessary to deform the ultra-fine grain [13,33,34].
4. Summary 1. The ECAP was successfully conducted at various temperatures and two different routes on the same sample up to a total of four pressings through the die such that the sample was not rotated (route A) and
Table 2 Tensile properties of the ECA pressed P/ M 6061 Al alloys and the commercial 6061 Al alloys
As-received P/ M 6061 Two ECA pressed P/ M 6061 (373 K, route A) Two ECA pressed P/ M 6061 (573 K, route A) Four ECA pressed P/ M 6061 (473 K, route A) Four ECA pressed P/ M 6061 (473 K, route C) 6061-O 6061-T4
Tensile strength (MPa)
Elongation (%)
95 248 90 130 130 123 235
8 10 18 25 25 25 22
220
S.-Y. Chang et al. / Journal of Alloys and Compounds 354 (2003) 216–220
rotated 1808 (route C) around its longitudinal axis between pressings. 2. After two ECAPs at 373 K using route A, the sample revealed microstructure of subgrain bands with a length of |0.8 mm and a width of |0.3 mm. The subgrain bands became larger above 1 mm in length and width after two ECAPs at 573 K. An equiaxed ultra-fine grained (UFG) structure with the mean grain sizes of |0.5 mm was obtained after four repetitive ECAPs using route A and C. 3. The microhardness and tensile strength increased with decreasing the pressing temperature. The microhardness of sample after four repetitive ECAPs at 473 K was approximately 20 Hv higher in microhardness than that of as-received one. The tensile strength of P/ M 6061 Al alloy increased from 95 MPa to 248 MPa after two ECAPs at 373 K and 130 MPa after four pressings at 473 K, which was superior to that of a fully annealed 6061-O Al alloy. 4. The ECAP technique can be effectively applied as the subsequent process of a commercial 6061 Al alloy produced by powder metallurgy. In particular, many repetitive pressings at low temperatures (,1 / 2 T m ) are sufficient to give the ultrafine grained (UFG) structure resulting in the highly increased microhardness and tensile strength.
Acknowledgements This work was supported by the ‘National Research Laboratory Program’ of Korea Ministry of Science and Technology.
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