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Development of microstructure and texture during high temperature equal channel angular extrusion of aluminium
Journal of Materials Processing Technology 117 (2001) 169±177
Development of microstructure and texture during high temperature equal channel angular extrusion of aluminium Uday Chakkingala,*, P.F. Thomsonb a
Department of Metallurgical Engineering, Indian Institute of Technology, Madras, Chennai 600 036, India b Department of Materials Engineering, Monash University, Clayton, VIC. 3168, Australia Received 16 January 2001; accepted 24 August 2001
Abstract The feasibility of using the equal channel angular extrusion (ECAE) process for the breakdown of cast structures in commercial purity aluminium has been investigated. Billets of an as-cast coarse grained Al (99.7% pure) were subjected to ECAE at 5008C using a die that imparts an equivalent plastic strain of 0.67 per pass. Multiple extrusion passes were performed using two different processing routes: Route I in which the billet orientation was kept constant from pass to pass, and Route II in which the billet was rotated by 1808 about its longitudinal axis from one pass to the next resulting alternately in shear in planes oriented at 1208 to each other. Processing by Route I through ®ve passes produced elongated grains showing evidence of dynamic recovery. Processing by Route II through ®ve passes however produced largely equiaxed grains with a dynamically recovered substructure due to the intervention of static recrystallization during the reheating to extrusion temperature between passes. Both processing routes produced a dynamically recovered structure with equiaxed subgrains approximately 6 mm in size. Sharp deformation textures were not obtained. The relevance of this technique to industrial processing is discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Equal channel angular extrusion; Aluminium extrusion
1. Introduction Various plastic deformation processes have been proposed for processing materials using simple shear. One key technique for subjecting materials to large shear strains is the process of equal channel angular extrusion (ECAE) originally proposed by Segal [1,2]. A schematic diagram of the ECAE process is shown in Fig. 1. The die used for ECAE consists of two channels of equal cross-section intersecting at an angle, usually between 908 and 1358. The billet is inserted into the top channel and extruded into the intersecting channel by a punch. The most important advantage of ECAE is that materials can be deformed to very high strains without any decrease in cross-sectional area. Materials can therefore be processed to give improved mechanical properties, extremely ®ne grain sizes, and varying textures. In addition, by changing the orientation of the billet from one pass to the next and thereby changing the strain path to which the billet is subjected, different microstructures and mechanical properties can be produced. The ECAE process has been studied by various * Corresponding author. Tel.: 91-44-445-8610; fax: 91-44-235-0509. E-mail address: [email protected] (U. Chakkingal).
investigators [3±7]. Most of the previous work has been on materials subjected to ECAE in order to obtain ultra®ne grained material with submicron or nanometer-sized grains. One of the important objectives of the current study is to study the effect of ECAE in breaking down the coarse as-cast structure since large deformations can be imparted to the material without any resultant decrease in cross-sectional area. This is in contrast to processing operations like rolling which are typically used to break down coarse as-cast structures. Previous research has shown that aluminium does not undergo dynamic recrystallization during hot working [8± 11]. Instead it undergoes extensive dynamic recovery because of its high stacking fault energy. Typical microstructures obtained after hot deformation of Al by torsion or rolling show dynamically recovered subgrains. Generally, dynamic recovery results in the formation of equiaxed subgrains which rapidly reach an equilibrium size with increasing strain. In this study, a commercially pure aluminium (composition 99.7% Al) was subjected to multiple ECAE passes at 5008C using two different processing routes. The microstructures of the resultant samples were studied using optical and transmission electron microscopy (TEM). Texture
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Fig. 1. Schematic diagram of the ECAE process.
measurement was also carried out. The results of microstructural observations and studies of texture are analysed and the viability of the process as a step in industrial processing is discussed. 2. Experimental procedure The die used for ECAE consisted of two rectangular channels of cross-sectional area 18 mm 10 mm intersecting at an angle of 1208 as shown in Fig. 1. Specimens 18 mm 10 mm in cross-section and 70 mm long were cut from the billet obtained in the as-cast condition and used as workpieces for subsequent extrusion. The as-cast billet had extremely coarse elongated grains (approximately 2000 mm 500 mm in size). Both the dies and the specimens were coated with a molybdenum disulphide dry ®lm lubricant to minimize friction and eliminate sticking of the billet to the die during extrusion. All the extrusions were carried out at 5008C. The die was initially placed in the furnace, and once the temperature reached 5008C, the workpiece and punch were inserted into the die. The workpiece was held at 5008C for 45 min, after which the whole setup was removed from the furnace and the specimen extruded immediately using a hydraulic press. The maximum pressure required for extrusion was noted. The extruded specimens were allowed to cool in air. Some degree of temperature drop is to be expected due to heat loss by radiation. However, this is compensated to some extent by deformation heating during extrusion. Yamaguchi et al. [12] have experimentally measured temperature changes due to adiabatic heating during room temperature ECAE of pure aluminium and report temperature increase of about 258C at high pressing speeds and negligible increase at low pressing speeds. Therefore, current deformation conditions can be assumed to be isothermal. The workpieces were given the same heat treatment before any subsequent pass. The workpieces were subjected to ®ve extrusion passes using two different processing routes as described below: Route I. The orientation of the workpiece was kept the same from one pass to the next so that the planes of macroscopic shear remained the same.
Route II. The workpiece was rotated by 1808 about its longitudinal axis from one pass to the next, resulting in shear alternately on planes oriented at 1208 to each other in successive passes. Vickers hardness tests were carried out on longitudinal sections using 5 kg load. Optical electron microscopy and TEM were used to study the microstructural evolution. All microscopy was conducted on longitudinal sections of the extruded billet. For optical microscopy, metallographically polished samples were electrolytically etched with a 5% aqueous solution of HBF4 and observed under polarized light. Specimens for TEM were prepared by electropolishing mechanically thinned samples (using a 33% nitric acid±67% methanol solution) and observations were carried out at 100 kV. Texture measurements were conducted on transverse sections perpendicular to the extrusion axis using Co Ka radiation and the data was analysed using the popLA [13] software for texture analysis. 3. Results After each pass, the von Mises equivalent plastic strain imparted to the billet by shear through the die in plain strain is given by Segal [1] as 2 De p cotan f 3 where 2f is the angle between the intersecting channels of the die (2f 120 for the present die configuration). The equivalent plastic strain after one pass is therefore 0.67. The maximum extrusion pressure required for each pass is shown in Fig. 2. The maximum extrusion pressure required decreases from pass to pass for both processing routes (from a value of 5.5 MPa for the ®rst pass to 3.8 and 3.6 MPa, respectively, for the ®fth pass of Routes I and II). 3.1. Hardness testing Hardness testing was performed to obtain an approximation of the mechanical properties resulting from multiple
Fig. 2. Variation of the maximum extrusion pressure with number of passes.
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Fig. 3. Hardness resulting from various extrusion passes.
passes of ECAE. The Vickers hardness numbers resulting from the various extrusion passes are shown in Fig. 3 (the hardness values reported are the average of 10 measurements). The values for both processing routes are very similar to that obtained from the as-cast material, indicating the absence of any strengthening. The variation in the hardness values within a sample is much greater in the case of workpieces processed by Route I compared to Route II. This is probably because rotation of the workpiece by 1808 between passes in Route II evens out any inhomogeneities in deformation between different regions of the workpiece. Processing by Route II also resulted in slightly lower hardness compared to processing by Route I.
Fig. 5. Microstructure obtained after the second pass through Route I.
The as-cast billet had an elongated grain structure with an extremely coarse grain size (approximately 500 mm 2000 mm). For workpieces processed by Route I, grains which are more elongated than the initial grains are observed after the ®rst and second passes (Figs. 4 and 5, respectively). There is no evidence of a subgrain structure within the grains. As the number of passes increases, a structure typical
of dynamic recovery is observed. The original grain boundaries are highly serrated and a subgrain structure is clearly visible within the original grains. A typical microstructure after ®ve passes exhibiting this structure is shown in Fig. 6. Dynamic recrystallization during extrusion or static recrystallization during heating to extrusion temperature prior to extrusion did not occur. This is evident from the similar microstructures obtained after the ®rst and second passes (Figs. 4 and 5) in which the original grain boundaries are clearly distinguished. For extrusions conducted through processing Route II, the results are somewhat different. Figs. 7 and 8 show the microstructure resulting from the third and ®fth pass, respectively. Elongated subgrain structures are visible within the original grains after the third pass. After the fourth and ®fth pass, the original elongated grains are no longer visible. Instead, approximately equiaxed grains with a subgrain structure are observed. This implies that static recrystallization occurs between passes during reheating and holding at extrusion temperature after suf®cient strain has accumulated.
Fig. 4. Microstructure obtained after the first pass through Route I.
Fig. 6. Microstructure obtained after the fifth pass through Route I.
3.2. Optical microscopy
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Fig. 9. Subgrain sizes resulting from various extrusion passes processed through Routes I and II. Fig. 7. Microstructure resulting from three passes through Route II.
Fig. 10. TEM micrograph showing dynamically recovered subgrains obtained after one pass through processing Route I. Fig. 8. Microstructure resulting from five passes through Route II.
3.3. Transmission electron microscopy (TEM) TEM observations reveal a microstructure consisting of equiaxed subgrains with a network of dislocations which is characteristic of aluminium that has undergone dynamic recovery during hot working. The variation in average subgrain size with number of passes for the two processing routes is shown in Fig. 9. A slight decrease in the average subgrain size is observed between the ®rst and ®fth pass in material processed by Route I. There is no signi®cant difference in subgrain structure resulting from various passes and from the two different processing routes. The subgrains are approximately 6 mm in size. The microstructures after the ®rst pass processed through Route I and the third pass processed through Route II, which are typical of the microstructures obtained, are shown in Figs. 10 and 11, respectively.
Fig. 11. TEM micrograph showing dynamically recovered subgrains obtained after three passes through processing Route II.
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Fig. 12. {1 1 1} pole figure showing the initial texture of the billet. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, 3, and 3.5 times random.)
Fig. 13. {1 1 1} pole figure showing texture resulting from four passes through processing Route I. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 3 times random.)
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Fig. 14. {1 1 1} pole figure showing texture resulting from two passes through processing Route II. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 3 times random.)
Fig. 15. {1 1 1} pole figure showing texture resulting from five passes through processing Route II. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 2.5 times random.)
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3.4. Crystallographic texture The (1 1 1) pole ®gure of the as-cast billet before extrusion is shown in Fig. 12. The texture is not random; three peaks of {1 1 1} poles are clearly observed. Fig. 13 shows the {1 1 1} pole ®gure for a workpiece subjected to four passes through processing Route I which is typical of textures resulting from this route. The texture is somewhat similar to the initial texture but with a much larger spread from the initial positions, leading to an attenuation of maximum pole densities. The initial texture is not completely destroyed con®rming that dynamic recrystallization during hot working or static recrystallization during cooling and reheating to the extrusion temperature does not occur. This tendency to progressively reduce the sharpness of the original texture increases as the number of passes is increased. An exception to this trend was observed in the workpiece processed to two passes. This is probably due to slightly different initial texture of the billet processed to two passes. For workpieces processed by Route II, in which the workpiece is rotated by 1808 between passes, the results are similar for the ®rst two passes. The {1 1 1} pole ®gure from the second pass is shown in Fig. 14. However, once some amount of static recrystallization takes place, the texture is markedly different and more random. Fig. 15 shows the {1 1 1} pole ®gure after the ®fth pass. The variation from the initial texture in all cases is greater than that of the material processed by Route I. An exception was again found in the case of the workpiece processed to four passes where the resultant texture was somewhat similar to the texture from the second pass. 4. Discussion The maximum pressure required for extrusion decreases continuously from one pass to the next for both the processing routes studied. This decrease is similar to the decrease in drawing stress observed in a similar process of equal channel angular drawing (ECAD) in which a bar was drawn through the two channels at room temperature [14]. For room temperature deformation, this was attributed to extensive micro shear banding that occurs during deformation. Korbel and Bochniak [15] and Bochniak and Korbel [16] have shown that the work required for plastic deformation is lowered when transgranular shear occurs as in a micro band. It is possible that a similar mechanism operates at large plastic strains even at hot working temperatures. Distinct shear offsets at grain boundaries resulting from intersection of micro bands are not observed after high temperature deformation because of the in¯uence of dynamic recovery [17,18]. The measured hardness values are very similar after every pass and very close to the hardness of the initial as-cast billet. Tensile strengths were not measured during the current study.
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All the samples exhibit dynamic recovery (DRV). Between passes, the billet was reheated to the extrusion temperature of 5008C and held at that temperature prior to extrusion. The total time involved in reheating and holding at extrusion temperature was 45 min. No static recrystallization (SRX) occurred during processing through Route I. This was con®rmed by subjecting some of the extruded samples to the reheating and holding schedule at 5008C to simulate the thermal cycle between passes. SRX did not occur, although some recrystallized grains were observed near the edges of the billet. The absence of SRX is due to the extensive dynamic recovery that took place during the extrusion. The driving force for SRX is also reduced by the static recovery that occurs during cooling from and reheating to the extrusion temperature [19,20]. With an increasing number of passes (especially in workpieces processed by Route I), the initial grain boundaries are extremely serrated. The presence of grain boundary serrations after hot working has been reported extensively in aluminium and has been attributed to local migrations of the grain boundaries to absorb dislocation walls [9,11,21]. TEM reveals a dynamically recovered structure consisting of equiaxed subgrains approximately 6 mm in size after all the passes. However, optical microscopy using polarized light did not always reveal a subgrain structure. This is probably due to the subgrain misorientations being too low after the ®rst and second passes for the subgrains to be resolved under polarized light. A ®ne grained microstructure with high angle grain boundaries did not develop because the total strain imparted to the samples were not suf®cient for the formation of high angle boundaries. For example, in room temperature studies, Chang et al. [22] have reported that an equivalent strain of approximately 8 is required to develop a microstructure with largely high angle grain boundaries. In this study, the total accumulated strain is only around 3.35 after ®ve passes. Gholinia et al. [23], Shan et al. [24], DeLo and Semiatin [25] and Semiatin et al. [26] have shown that the deformation is not completely homogenous and the sample does not experience a uniform strain across its cross-section. A dead zone develops in the outer section of the intersecting channels where the extent of strain is lower than the calculated value. The degree of non-homogeneity in the deformation is in¯uenced by factors like frictional conditions at the workpiece±die interface, strength of the extruded material, die design, etc. Higher friction conditions suppress dead zone formation at the die corner and enables more homogenous deformation to be obtained [23]. FEM simulations of the process by Semiatin et al. [26] have shown that some strain hardening and a small to medium amount of strain rate hardening would increase the tendency for strain uniformity. In the present experiments, frictional conditions are higher than in room temperature processes as it is a high temperature process with more severe die±workpiece interactions. Therefore, it can be assumed that the deformation is nearly homogenous.
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A considerable amount of static recrystallization occurs during processing by Route II after three to four passes. This difference is probably due to the variation in strain paths to reach the same value of total strain between Routes I and II. Since the workpiece is rotated by 1808 between passes in Route II, the strain path is different from that in Route I. During ECA extrusion through the die with an angle of 1208 between the two intersecting channels, intense shear occurs on planes that make an angle of 308 with the transverse plane of the billet. For workpieces processed by Route I, shear occurs on the same plane from pass to pass. However, for workpieces processed by Route II in which the workpiece is rotated by 1808 between passes, shear takes place alternately on two intersecting macroscopic shear planes making an angle of 1208 with each other. Therefore, in practice, rotating the sample by 1808 between passes ensures more uniform strain distribution in addition to changing the strain path. It is possible that this accelerates SRX, especially after three to four passes when there is suf®cient accumulated strain. Therefore, for the fourth and ®fth pass, the starting material is essentially an SRX microstructure consisting of approximately equiaxed grains undergoing dynamic recovery during extrusion. Hence the grain boundaries are not serrated (and the grains are not elongated) and the subgrain formation as revealed by optical microscopy is not as extensive as the fourth or ®fth pass in billets processed by Route I. The textures resulting from the two processing routes are different, although no sharp deformation textures are produced. Billets processed by Route I have textures similar to the original texture although the texture gradually becomes more and more random as the number of passes increases. Since SRX intervenes after about three passes in Route II, the original texture is destroyed much more rapidly. The microstructures as observed by TEM do not show any signi®cant difference and both routes produce equiaxed subgrains of similar size. This is in contrast to substructures produced by the similar ECAD process at room temperature in which Route I produced elongated subgrains and Route II produced equiaxed subgrains [14]. This similarity in microstructures can be attributed to enhanced recovery mechanisms resulting from greater dislocation mobility at elevated temperatures which tend to produce similar recovered substructures. An equivalent strain of 0.67 resulting from one ECA extrusion pass can be achieved by a 49% reduction in area during hot rolling. Since no static or dynamic recrystallization occurs, the strains resulting from multiple passes can be considered to be approximately cumulative. This implies that a workpiece subjected to ®ve ECAE passes has undergone an equivalent strain of about 3.35 which is the equivalent strain achieved by a 96.5% reduction in area during hot rolling. Hot rolling to these strains usually results in a distinct deformation texture [27,28]. However, the same strain produced in hot ECA extrusion (in ®ve passes) does not produce a well-de®ned deformation texture. The absence
of a deformation texture in ECA extrusion is similar to that in ECA drawing at room temperature reported previously [14]. The extremely large grain size and the nature of the shear deformation result in subgrains misoriented from each other by very small angles. There is also no reduction in cross-sectional area as in the case of hot rolling. This results in a ®nal texture similar to the initial texture but with a tendency to reduce the intensities of the original components as the number of passes increases except in the case of billets processed through Route II where recrystallization takes place and the original texture is destroyed. The die angle between the intersecting channels through which the billet is extruded has also been shown to in¯uence microstructural evolution at room temperature even when the billets are processed to the same equivalent strain [29]. Ren and Morris [28] have reported that rolling commercial purity Al to an equivalent strain of 2.66 in one pass at 5108C resulted in a dynamically recrystallized equiaxed grain structure, whereas lower reductions resulted in a dynamically recovered structure. A die angle of 908 would result in an equivalent strain of 1.15 per pass. Therefore, it is possible that changing the die angle to 908 would result in a different (possibly dynamically recrystallized) microstructure. Traditional hot rolling to break down the cast structure of aluminium and its alloys involves rolling at temperatures around 5008C with reductions of 10±20% per pass. The deformation is generally inhomogenous and several passes are required to ensure that the centre of the billet is hot worked [30]. In contrast, ECAE processing especially through Route II results in largely homogenous deformation and problems like alligatoring or edge cracking that can occur during rolling do not occur. Multiple extrusion passes through ECAE by processing Route II, in which the billet is rotated 1808 between passes, can possibly be used as a step in industrial processing to break down the coarse as-cast structure of commercial purity aluminium. 5. Conclusions During the ECA extrusion of commercial purity aluminium, the maximum extrusion pressure required decreases with increasing number of passes. There is no signi®cant difference in hardness, implying that there is no signi®cant change in mechanical properties as a result of ECA extrusion. Multiple passes through Route I, in which the workpiece orientation is kept the same between passes, results in elongated grains with serrated grain boundaries showing evidence of extensive dynamic recovery. No static recrystallization occurs between passes. There is no sharp deformation texture; instead there is a steady progression away from the original texture to a more random texture as the number of passes increases. Processing by Route II, in which the billet is rotated by 1808 between passes, produces similar results after the ®rst three passes. However after three passes, static recrystallization occurs during reheating and
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holding at the extrusion temperature. This results in largely equiaxed grains, showing evidence of dynamic recovery after the fourth and ®fth passes. The texture is also more random. In all cases, the dynamically recovered subgrains were equiaxed and approximately 6 mm in size. Multiple ECA extrusion passes through processing Route II can possibly be used as a step in industrial processing to break down a coarse as-cast structure. Acknowledgements This work was largely supported by the Australian Department of Education, Employment, Training and Youth Affairs (DEETYA) through the Australian Research Council Collaborative Grant AM 9280142 and an ARC Small Grant. The work was conducted at Monash University. References [1] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metall. 1 (1981) 99±105. [2] V.M. Segal, Mater. Sci. Eng. A 197A (1995) 157±164. [3] D.H. Shin, C.S. Seo, J. Kim, K.T. Park, W.Y. Choo, Scripta Mater. 42 (2000) 695±699. [4] R.S. Mishra, R.Z. Valiev, S.X. McFadden, R.K. Islamgaliev, A.K. Mukherjee, Scripta Mater. 40 (1999) 1151±1155. [5] S.M.L. Sastry, R.N. Mahapatra, D.F. Hasson, Scripta Mater. 42 (2000) 731±736. [6] S. Ferasse, V.M. Segal, K.T. Hartwig, R.E. Goforth, Metall. Trans. A 28A (1997) 1047±1057. [7] R. Valiev, D.A. Salimonenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scripta Mater. 37 (1997) 1945±1950. [8] W. Blum, Q. Zhu, R. Merkel, H.J. McQueen, Z. Metallkde. 87 (1996) 14±23. [9] M.E. Kassner, M.M. Myshlyaev, H.J. McQueen, Mater. Sci. Eng. A 108A (1989) 45±61.
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[10] H.J. McQueen, Metall. Trans. A 8A (1977) 807±824. [11] H.J. McQueen, E. Evangelista, M.E. Kassner, Z. Metallkde. 82 (1991) 336±345. [12] D. Yamaguchi, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 41 (1999) 791±796. [13] U.F. Kocks, J.S. Kallend, H.R. Wenk, A.D. Rollett, S.I. Wright, popLA Ð Preferred Orientation Package Ð Los Alamos, Los Alamos National Laboratory, Los Alamos, NM, USA, 1994. [14] U. Chakkingal, A.B. Suriadi, P.F. Thomson, Mater. Sci. Eng. A 266A (1999) 241±249. [15] A. Korbel, W. Bochniak, J. Mater. Process. Technol. 53 (1995) 229±237. [16] W. Bochniak, A. Korbel, in: E.A. Starke Jr., T.H. Sanders Jr. (Eds.), Proceedings of the Fourth International Conference on Aluminum Alloys, GA, USA, 1994, pp. 283±289. [17] W. Bochniak, A. Korbel, in: Hosoi, Yoshina, Oikawa (Eds.), Proceedings of the International Symposium on Aspects of High Temperature Deformation, Nagoya, Japan, 1993, pp. 425±431. [18] W. Bochniak, A. Korbel, in: Oikawa, et al. (Eds.), Strength of Materials, Japan Institute of Metals, Japan, 1994, pp. 579±582. [19] H.J. McQueen, N. Ryum, Scand. J. Metall. 14 (1985) 183±194. [20] H.J. McQueen, O.C. Celliers, Can. Metall. Quart. 35 (1996) 305±319. [21] H.J. McQueen, N.D. Ryan, E.V. Konopleva, X. Xia, Can. Metall. Quart. 34 (1995) 219±229. [22] C.P. Chang, P.L. Sun, P.W. Kao, Acta Mater. 48 (2000) 3377±3385. [23] A. Gholinia, J.R. Bowen, P.B. Prangnell, F.J. Humphreys, in: T. Sato, S. Kumai, T. Kobayashi, Y. Murakami (Eds.), Proceedings of the Sixth International Conference on Al Alloys, ICAA 6, Toyohashi, Japan, 1998, pp. 577±582. [24] A. Shan, I.G. Moon, H.S. Ko, J.W. Park, Scripta Mater. 41 (1999) 353±357. [25] D.P. DeLo, S.L. Semiatin, Metall. Trans. A 30A (1999) 1391±1402. [26] S.L. Semiatin, D.P. Lelo, E.B. Shell, Acta Mater. 48 (2000) 1841±1851. [27] J.R. Hirsch, in: T.G. Langdon, H.D. Merchant, J.G. Morris, M.A. Zaidi (Eds.), Hot Deformation of Aluminum Alloys, TMS, Warrendale, PA, USA, 1991, pp. 379±388. [28] B. Ren, J.G. Morris, Metall. Trans. A 26A (1995) 31±40. [29] K. Nakashima, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 1589±1599. [30] T.E. Scott, in: E.A. Starke Jr., T.H. Sanders Jr. (Eds.), Aluminum Alloys Ð Their Physical and Mechanical Properties, Vol. III, EMAS Ltd., UK, 1986, pp. 1359±1440.
Development of microstructure and texture during high temperature equal channel angular extrusion of aluminium Uday Chakkingala,*, P.F. Thomsonb a
Department of Metallurgical Engineering, Indian Institute of Technology, Madras, Chennai 600 036, India b Department of Materials Engineering, Monash University, Clayton, VIC. 3168, Australia Received 16 January 2001; accepted 24 August 2001
Abstract The feasibility of using the equal channel angular extrusion (ECAE) process for the breakdown of cast structures in commercial purity aluminium has been investigated. Billets of an as-cast coarse grained Al (99.7% pure) were subjected to ECAE at 5008C using a die that imparts an equivalent plastic strain of 0.67 per pass. Multiple extrusion passes were performed using two different processing routes: Route I in which the billet orientation was kept constant from pass to pass, and Route II in which the billet was rotated by 1808 about its longitudinal axis from one pass to the next resulting alternately in shear in planes oriented at 1208 to each other. Processing by Route I through ®ve passes produced elongated grains showing evidence of dynamic recovery. Processing by Route II through ®ve passes however produced largely equiaxed grains with a dynamically recovered substructure due to the intervention of static recrystallization during the reheating to extrusion temperature between passes. Both processing routes produced a dynamically recovered structure with equiaxed subgrains approximately 6 mm in size. Sharp deformation textures were not obtained. The relevance of this technique to industrial processing is discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Equal channel angular extrusion; Aluminium extrusion
1. Introduction Various plastic deformation processes have been proposed for processing materials using simple shear. One key technique for subjecting materials to large shear strains is the process of equal channel angular extrusion (ECAE) originally proposed by Segal [1,2]. A schematic diagram of the ECAE process is shown in Fig. 1. The die used for ECAE consists of two channels of equal cross-section intersecting at an angle, usually between 908 and 1358. The billet is inserted into the top channel and extruded into the intersecting channel by a punch. The most important advantage of ECAE is that materials can be deformed to very high strains without any decrease in cross-sectional area. Materials can therefore be processed to give improved mechanical properties, extremely ®ne grain sizes, and varying textures. In addition, by changing the orientation of the billet from one pass to the next and thereby changing the strain path to which the billet is subjected, different microstructures and mechanical properties can be produced. The ECAE process has been studied by various * Corresponding author. Tel.: 91-44-445-8610; fax: 91-44-235-0509. E-mail address: [email protected] (U. Chakkingal).
investigators [3±7]. Most of the previous work has been on materials subjected to ECAE in order to obtain ultra®ne grained material with submicron or nanometer-sized grains. One of the important objectives of the current study is to study the effect of ECAE in breaking down the coarse as-cast structure since large deformations can be imparted to the material without any resultant decrease in cross-sectional area. This is in contrast to processing operations like rolling which are typically used to break down coarse as-cast structures. Previous research has shown that aluminium does not undergo dynamic recrystallization during hot working [8± 11]. Instead it undergoes extensive dynamic recovery because of its high stacking fault energy. Typical microstructures obtained after hot deformation of Al by torsion or rolling show dynamically recovered subgrains. Generally, dynamic recovery results in the formation of equiaxed subgrains which rapidly reach an equilibrium size with increasing strain. In this study, a commercially pure aluminium (composition 99.7% Al) was subjected to multiple ECAE passes at 5008C using two different processing routes. The microstructures of the resultant samples were studied using optical and transmission electron microscopy (TEM). Texture
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Fig. 1. Schematic diagram of the ECAE process.
measurement was also carried out. The results of microstructural observations and studies of texture are analysed and the viability of the process as a step in industrial processing is discussed. 2. Experimental procedure The die used for ECAE consisted of two rectangular channels of cross-sectional area 18 mm 10 mm intersecting at an angle of 1208 as shown in Fig. 1. Specimens 18 mm 10 mm in cross-section and 70 mm long were cut from the billet obtained in the as-cast condition and used as workpieces for subsequent extrusion. The as-cast billet had extremely coarse elongated grains (approximately 2000 mm 500 mm in size). Both the dies and the specimens were coated with a molybdenum disulphide dry ®lm lubricant to minimize friction and eliminate sticking of the billet to the die during extrusion. All the extrusions were carried out at 5008C. The die was initially placed in the furnace, and once the temperature reached 5008C, the workpiece and punch were inserted into the die. The workpiece was held at 5008C for 45 min, after which the whole setup was removed from the furnace and the specimen extruded immediately using a hydraulic press. The maximum pressure required for extrusion was noted. The extruded specimens were allowed to cool in air. Some degree of temperature drop is to be expected due to heat loss by radiation. However, this is compensated to some extent by deformation heating during extrusion. Yamaguchi et al. [12] have experimentally measured temperature changes due to adiabatic heating during room temperature ECAE of pure aluminium and report temperature increase of about 258C at high pressing speeds and negligible increase at low pressing speeds. Therefore, current deformation conditions can be assumed to be isothermal. The workpieces were given the same heat treatment before any subsequent pass. The workpieces were subjected to ®ve extrusion passes using two different processing routes as described below: Route I. The orientation of the workpiece was kept the same from one pass to the next so that the planes of macroscopic shear remained the same.
Route II. The workpiece was rotated by 1808 about its longitudinal axis from one pass to the next, resulting in shear alternately on planes oriented at 1208 to each other in successive passes. Vickers hardness tests were carried out on longitudinal sections using 5 kg load. Optical electron microscopy and TEM were used to study the microstructural evolution. All microscopy was conducted on longitudinal sections of the extruded billet. For optical microscopy, metallographically polished samples were electrolytically etched with a 5% aqueous solution of HBF4 and observed under polarized light. Specimens for TEM were prepared by electropolishing mechanically thinned samples (using a 33% nitric acid±67% methanol solution) and observations were carried out at 100 kV. Texture measurements were conducted on transverse sections perpendicular to the extrusion axis using Co Ka radiation and the data was analysed using the popLA [13] software for texture analysis. 3. Results After each pass, the von Mises equivalent plastic strain imparted to the billet by shear through the die in plain strain is given by Segal [1] as 2 De p cotan f 3 where 2f is the angle between the intersecting channels of the die (2f 120 for the present die configuration). The equivalent plastic strain after one pass is therefore 0.67. The maximum extrusion pressure required for each pass is shown in Fig. 2. The maximum extrusion pressure required decreases from pass to pass for both processing routes (from a value of 5.5 MPa for the ®rst pass to 3.8 and 3.6 MPa, respectively, for the ®fth pass of Routes I and II). 3.1. Hardness testing Hardness testing was performed to obtain an approximation of the mechanical properties resulting from multiple
Fig. 2. Variation of the maximum extrusion pressure with number of passes.
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Fig. 3. Hardness resulting from various extrusion passes.
passes of ECAE. The Vickers hardness numbers resulting from the various extrusion passes are shown in Fig. 3 (the hardness values reported are the average of 10 measurements). The values for both processing routes are very similar to that obtained from the as-cast material, indicating the absence of any strengthening. The variation in the hardness values within a sample is much greater in the case of workpieces processed by Route I compared to Route II. This is probably because rotation of the workpiece by 1808 between passes in Route II evens out any inhomogeneities in deformation between different regions of the workpiece. Processing by Route II also resulted in slightly lower hardness compared to processing by Route I.
Fig. 5. Microstructure obtained after the second pass through Route I.
The as-cast billet had an elongated grain structure with an extremely coarse grain size (approximately 500 mm 2000 mm). For workpieces processed by Route I, grains which are more elongated than the initial grains are observed after the ®rst and second passes (Figs. 4 and 5, respectively). There is no evidence of a subgrain structure within the grains. As the number of passes increases, a structure typical
of dynamic recovery is observed. The original grain boundaries are highly serrated and a subgrain structure is clearly visible within the original grains. A typical microstructure after ®ve passes exhibiting this structure is shown in Fig. 6. Dynamic recrystallization during extrusion or static recrystallization during heating to extrusion temperature prior to extrusion did not occur. This is evident from the similar microstructures obtained after the ®rst and second passes (Figs. 4 and 5) in which the original grain boundaries are clearly distinguished. For extrusions conducted through processing Route II, the results are somewhat different. Figs. 7 and 8 show the microstructure resulting from the third and ®fth pass, respectively. Elongated subgrain structures are visible within the original grains after the third pass. After the fourth and ®fth pass, the original elongated grains are no longer visible. Instead, approximately equiaxed grains with a subgrain structure are observed. This implies that static recrystallization occurs between passes during reheating and holding at extrusion temperature after suf®cient strain has accumulated.
Fig. 4. Microstructure obtained after the first pass through Route I.
Fig. 6. Microstructure obtained after the fifth pass through Route I.
3.2. Optical microscopy
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Fig. 9. Subgrain sizes resulting from various extrusion passes processed through Routes I and II. Fig. 7. Microstructure resulting from three passes through Route II.
Fig. 10. TEM micrograph showing dynamically recovered subgrains obtained after one pass through processing Route I. Fig. 8. Microstructure resulting from five passes through Route II.
3.3. Transmission electron microscopy (TEM) TEM observations reveal a microstructure consisting of equiaxed subgrains with a network of dislocations which is characteristic of aluminium that has undergone dynamic recovery during hot working. The variation in average subgrain size with number of passes for the two processing routes is shown in Fig. 9. A slight decrease in the average subgrain size is observed between the ®rst and ®fth pass in material processed by Route I. There is no signi®cant difference in subgrain structure resulting from various passes and from the two different processing routes. The subgrains are approximately 6 mm in size. The microstructures after the ®rst pass processed through Route I and the third pass processed through Route II, which are typical of the microstructures obtained, are shown in Figs. 10 and 11, respectively.
Fig. 11. TEM micrograph showing dynamically recovered subgrains obtained after three passes through processing Route II.
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Fig. 12. {1 1 1} pole figure showing the initial texture of the billet. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, 3, and 3.5 times random.)
Fig. 13. {1 1 1} pole figure showing texture resulting from four passes through processing Route I. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 3 times random.)
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Fig. 14. {1 1 1} pole figure showing texture resulting from two passes through processing Route II. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 3 times random.)
Fig. 15. {1 1 1} pole figure showing texture resulting from five passes through processing Route II. (The extrusion direction, which is along the longitudinal axis of the billet, coincides with the centre of the pole figure; pole densities shown correspond to 0.5, 1, 2, and 2.5 times random.)
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3.4. Crystallographic texture The (1 1 1) pole ®gure of the as-cast billet before extrusion is shown in Fig. 12. The texture is not random; three peaks of {1 1 1} poles are clearly observed. Fig. 13 shows the {1 1 1} pole ®gure for a workpiece subjected to four passes through processing Route I which is typical of textures resulting from this route. The texture is somewhat similar to the initial texture but with a much larger spread from the initial positions, leading to an attenuation of maximum pole densities. The initial texture is not completely destroyed con®rming that dynamic recrystallization during hot working or static recrystallization during cooling and reheating to the extrusion temperature does not occur. This tendency to progressively reduce the sharpness of the original texture increases as the number of passes is increased. An exception to this trend was observed in the workpiece processed to two passes. This is probably due to slightly different initial texture of the billet processed to two passes. For workpieces processed by Route II, in which the workpiece is rotated by 1808 between passes, the results are similar for the ®rst two passes. The {1 1 1} pole ®gure from the second pass is shown in Fig. 14. However, once some amount of static recrystallization takes place, the texture is markedly different and more random. Fig. 15 shows the {1 1 1} pole ®gure after the ®fth pass. The variation from the initial texture in all cases is greater than that of the material processed by Route I. An exception was again found in the case of the workpiece processed to four passes where the resultant texture was somewhat similar to the texture from the second pass. 4. Discussion The maximum pressure required for extrusion decreases continuously from one pass to the next for both the processing routes studied. This decrease is similar to the decrease in drawing stress observed in a similar process of equal channel angular drawing (ECAD) in which a bar was drawn through the two channels at room temperature [14]. For room temperature deformation, this was attributed to extensive micro shear banding that occurs during deformation. Korbel and Bochniak [15] and Bochniak and Korbel [16] have shown that the work required for plastic deformation is lowered when transgranular shear occurs as in a micro band. It is possible that a similar mechanism operates at large plastic strains even at hot working temperatures. Distinct shear offsets at grain boundaries resulting from intersection of micro bands are not observed after high temperature deformation because of the in¯uence of dynamic recovery [17,18]. The measured hardness values are very similar after every pass and very close to the hardness of the initial as-cast billet. Tensile strengths were not measured during the current study.
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All the samples exhibit dynamic recovery (DRV). Between passes, the billet was reheated to the extrusion temperature of 5008C and held at that temperature prior to extrusion. The total time involved in reheating and holding at extrusion temperature was 45 min. No static recrystallization (SRX) occurred during processing through Route I. This was con®rmed by subjecting some of the extruded samples to the reheating and holding schedule at 5008C to simulate the thermal cycle between passes. SRX did not occur, although some recrystallized grains were observed near the edges of the billet. The absence of SRX is due to the extensive dynamic recovery that took place during the extrusion. The driving force for SRX is also reduced by the static recovery that occurs during cooling from and reheating to the extrusion temperature [19,20]. With an increasing number of passes (especially in workpieces processed by Route I), the initial grain boundaries are extremely serrated. The presence of grain boundary serrations after hot working has been reported extensively in aluminium and has been attributed to local migrations of the grain boundaries to absorb dislocation walls [9,11,21]. TEM reveals a dynamically recovered structure consisting of equiaxed subgrains approximately 6 mm in size after all the passes. However, optical microscopy using polarized light did not always reveal a subgrain structure. This is probably due to the subgrain misorientations being too low after the ®rst and second passes for the subgrains to be resolved under polarized light. A ®ne grained microstructure with high angle grain boundaries did not develop because the total strain imparted to the samples were not suf®cient for the formation of high angle boundaries. For example, in room temperature studies, Chang et al. [22] have reported that an equivalent strain of approximately 8 is required to develop a microstructure with largely high angle grain boundaries. In this study, the total accumulated strain is only around 3.35 after ®ve passes. Gholinia et al. [23], Shan et al. [24], DeLo and Semiatin [25] and Semiatin et al. [26] have shown that the deformation is not completely homogenous and the sample does not experience a uniform strain across its cross-section. A dead zone develops in the outer section of the intersecting channels where the extent of strain is lower than the calculated value. The degree of non-homogeneity in the deformation is in¯uenced by factors like frictional conditions at the workpiece±die interface, strength of the extruded material, die design, etc. Higher friction conditions suppress dead zone formation at the die corner and enables more homogenous deformation to be obtained [23]. FEM simulations of the process by Semiatin et al. [26] have shown that some strain hardening and a small to medium amount of strain rate hardening would increase the tendency for strain uniformity. In the present experiments, frictional conditions are higher than in room temperature processes as it is a high temperature process with more severe die±workpiece interactions. Therefore, it can be assumed that the deformation is nearly homogenous.
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A considerable amount of static recrystallization occurs during processing by Route II after three to four passes. This difference is probably due to the variation in strain paths to reach the same value of total strain between Routes I and II. Since the workpiece is rotated by 1808 between passes in Route II, the strain path is different from that in Route I. During ECA extrusion through the die with an angle of 1208 between the two intersecting channels, intense shear occurs on planes that make an angle of 308 with the transverse plane of the billet. For workpieces processed by Route I, shear occurs on the same plane from pass to pass. However, for workpieces processed by Route II in which the workpiece is rotated by 1808 between passes, shear takes place alternately on two intersecting macroscopic shear planes making an angle of 1208 with each other. Therefore, in practice, rotating the sample by 1808 between passes ensures more uniform strain distribution in addition to changing the strain path. It is possible that this accelerates SRX, especially after three to four passes when there is suf®cient accumulated strain. Therefore, for the fourth and ®fth pass, the starting material is essentially an SRX microstructure consisting of approximately equiaxed grains undergoing dynamic recovery during extrusion. Hence the grain boundaries are not serrated (and the grains are not elongated) and the subgrain formation as revealed by optical microscopy is not as extensive as the fourth or ®fth pass in billets processed by Route I. The textures resulting from the two processing routes are different, although no sharp deformation textures are produced. Billets processed by Route I have textures similar to the original texture although the texture gradually becomes more and more random as the number of passes increases. Since SRX intervenes after about three passes in Route II, the original texture is destroyed much more rapidly. The microstructures as observed by TEM do not show any signi®cant difference and both routes produce equiaxed subgrains of similar size. This is in contrast to substructures produced by the similar ECAD process at room temperature in which Route I produced elongated subgrains and Route II produced equiaxed subgrains [14]. This similarity in microstructures can be attributed to enhanced recovery mechanisms resulting from greater dislocation mobility at elevated temperatures which tend to produce similar recovered substructures. An equivalent strain of 0.67 resulting from one ECA extrusion pass can be achieved by a 49% reduction in area during hot rolling. Since no static or dynamic recrystallization occurs, the strains resulting from multiple passes can be considered to be approximately cumulative. This implies that a workpiece subjected to ®ve ECAE passes has undergone an equivalent strain of about 3.35 which is the equivalent strain achieved by a 96.5% reduction in area during hot rolling. Hot rolling to these strains usually results in a distinct deformation texture [27,28]. However, the same strain produced in hot ECA extrusion (in ®ve passes) does not produce a well-de®ned deformation texture. The absence
of a deformation texture in ECA extrusion is similar to that in ECA drawing at room temperature reported previously [14]. The extremely large grain size and the nature of the shear deformation result in subgrains misoriented from each other by very small angles. There is also no reduction in cross-sectional area as in the case of hot rolling. This results in a ®nal texture similar to the initial texture but with a tendency to reduce the intensities of the original components as the number of passes increases except in the case of billets processed through Route II where recrystallization takes place and the original texture is destroyed. The die angle between the intersecting channels through which the billet is extruded has also been shown to in¯uence microstructural evolution at room temperature even when the billets are processed to the same equivalent strain [29]. Ren and Morris [28] have reported that rolling commercial purity Al to an equivalent strain of 2.66 in one pass at 5108C resulted in a dynamically recrystallized equiaxed grain structure, whereas lower reductions resulted in a dynamically recovered structure. A die angle of 908 would result in an equivalent strain of 1.15 per pass. Therefore, it is possible that changing the die angle to 908 would result in a different (possibly dynamically recrystallized) microstructure. Traditional hot rolling to break down the cast structure of aluminium and its alloys involves rolling at temperatures around 5008C with reductions of 10±20% per pass. The deformation is generally inhomogenous and several passes are required to ensure that the centre of the billet is hot worked [30]. In contrast, ECAE processing especially through Route II results in largely homogenous deformation and problems like alligatoring or edge cracking that can occur during rolling do not occur. Multiple extrusion passes through ECAE by processing Route II, in which the billet is rotated 1808 between passes, can possibly be used as a step in industrial processing to break down the coarse as-cast structure of commercial purity aluminium. 5. Conclusions During the ECA extrusion of commercial purity aluminium, the maximum extrusion pressure required decreases with increasing number of passes. There is no signi®cant difference in hardness, implying that there is no signi®cant change in mechanical properties as a result of ECA extrusion. Multiple passes through Route I, in which the workpiece orientation is kept the same between passes, results in elongated grains with serrated grain boundaries showing evidence of extensive dynamic recovery. No static recrystallization occurs between passes. There is no sharp deformation texture; instead there is a steady progression away from the original texture to a more random texture as the number of passes increases. Processing by Route II, in which the billet is rotated by 1808 between passes, produces similar results after the ®rst three passes. However after three passes, static recrystallization occurs during reheating and
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holding at the extrusion temperature. This results in largely equiaxed grains, showing evidence of dynamic recovery after the fourth and ®fth passes. The texture is also more random. In all cases, the dynamically recovered subgrains were equiaxed and approximately 6 mm in size. Multiple ECA extrusion passes through processing Route II can possibly be used as a step in industrial processing to break down a coarse as-cast structure. Acknowledgements This work was largely supported by the Australian Department of Education, Employment, Training and Youth Affairs (DEETYA) through the Australian Research Council Collaborative Grant AM 9280142 and an ARC Small Grant. The work was conducted at Monash University. References [1] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Russ. Metall. 1 (1981) 99±105. [2] V.M. Segal, Mater. Sci. Eng. A 197A (1995) 157±164. [3] D.H. Shin, C.S. Seo, J. Kim, K.T. Park, W.Y. Choo, Scripta Mater. 42 (2000) 695±699. [4] R.S. Mishra, R.Z. Valiev, S.X. McFadden, R.K. Islamgaliev, A.K. Mukherjee, Scripta Mater. 40 (1999) 1151±1155. [5] S.M.L. Sastry, R.N. Mahapatra, D.F. Hasson, Scripta Mater. 42 (2000) 731±736. [6] S. Ferasse, V.M. Segal, K.T. Hartwig, R.E. Goforth, Metall. Trans. A 28A (1997) 1047±1057. [7] R. Valiev, D.A. Salimonenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scripta Mater. 37 (1997) 1945±1950. [8] W. Blum, Q. Zhu, R. Merkel, H.J. McQueen, Z. Metallkde. 87 (1996) 14±23. [9] M.E. Kassner, M.M. Myshlyaev, H.J. McQueen, Mater. Sci. Eng. A 108A (1989) 45±61.
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