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The potential for scaling ECAP: effect of sample size on grain refinement and mechanical properties
Materials Science and Engineering A318 (2001) 34 – 41 www.elsevier.com/locate/msea
The potential for scaling ECAP: effect of sample size on grain refinement and mechanical properties Zenji Horita a, Takayoshi Fujinami a , Terence G. Langdon b,* a
b
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Uni6ersity, Fukuoka 812 -8581, Japan Departments of Aerospace and Mechanical Engineering and Materials Science, Uni6ersity of Southern California, Los Angeles, CA 90089 -1453, USA Received 3 January 2001; received in revised form 13 March 2001
Abstract The potential for scaling equal-channel angular pressing (ECAP) for use with large samples was investigated by conducting tests on an aluminum alloy using cylinders having diameters from 6 – 40 mm. The results show the refinement of the microstructure and the subsequent mechanical properties after pressing are independent of the initial size of the sample and, for the largest sample with a diameter of 40 mm, independent of the location within the sample at least to a distance of 5 mm from the sample edge. By making direct measurements of the imposed load during ECAP, it is shown that the applied load is determined by the sample strength rather than frictional effects between the sample and the die walls. The results demonstrate the feasibility of scaling ECAP to large sizes for use in industrial applications. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloys; Equal-channel angular pressing; Mechanical properties; Severe plastic deformation; Ultrafine grains sizes
1. Introduction Equal-channel angular pressing (ECAP) is a processing procedure in which a material is subjected to severe plastic deformation through simple shear [1]. This procedure differs from more conventional metal-working processes, such as rolling and extrusion. because it has the potential for introducing an exceptionally high plastic strain without any concomitant change in the crosssectional dimensions of the sample. It is now well-established that ECAP is capable of producing considerable grain refinement in metals. typically to the submicrometer level [2,3], and these very small grains provide a potential for achieving extremely high tensile ductilities. with elongations up to \ 1000%. at strain rates at and above 10 − 2 s − 1 [4,5]. Thus, there is a possibility of using ECAP as a tool for processing metals for subsequent superplastic forming at rapid strain rates. It was demonstrated recently that samples subjected to ECAP may be rolled into sheets after * Tel.: +1-213-7400491 fax: +1-213-7408071. E-mail address: [email protected] (T.G. Langdon).
pressing without any diminution in the superplastic properties [6] thereby confirming the potential of this procedure for the production of superplastic sheet metals. A critical question in ECAP concerns the potential for scaling-up the process for industrial applications while retaining a procedure that is economically viable [7,8]. A first step in this direction was taken in a recent demonstration that it is possible to impose very high strains on a sample in a single passage through an ECAP die by constructing a multi-pass facility containing a number of consecutive shearing planes [9]. Nevertheless, there has been to date no critical assessment of the effect, on either the microstructural refinement or the subsequent mechanical properties, of an increase in the sample size in conventional processing by ECAP. Earlier results suggested a possible lower limit for satisfactory ECAP because there was evidence that frictional effects between the sample and the die walls may become critical when the sample cross-section is reduced to 5 mm [10] but there has been no corresponding evaluation of the effect of pressing samples with larger cross-sections. Accordingly, the present in-
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 3 9 - 9
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
vestigation was initiated in order to evaluate the influence of the sample size in ECAP using rod-shaped samples with diameters from 6– 40 mm. In addition, the ECAP facility used in this investigation was modified to permit a direct measure of the load imposed on each sample during pressing.
2. Experimental materials and procedures Most of the experiments conducted in this investigation used a commercial 1100 aluminum alloy received in the form of a hot-rolled billet. This alloy had a composition, in wt%., of 0.60 Fe, 0.15 Cu, 0.09 Si, 0.005 Mn, 0.002 Mg, B 0.001 Cr, 0.006 Zn with the balance as aluminum. Some additional experiments were also conducted using a commercial cold-rolled 3004 aluminum alloy with a composition, in wt%, of 0.444 Fe, 0.140 Cu, 0.206 Si, 1.015 Mn, 1.154 Mg, 0.016 Cr, 0.057 Zn, 0.017 Ti with the balance as aluminum. For the 1100 alloy, rods were machined from the billet with diameters of 6, 10 or 40 mm and with corresponding lengths of 50, 60 or 100 mm, respectively. For the 3004 alloy, rods were machined with diameters of 10 mm and lengths of 60 mm. All of these rods were annealed for 1 h at temperatures of either 500°C for the 1100 alloy or 350°C for the 3004 alloy with subsequent air cooling. Observations showed the initial grain sizes prior to ECAP were 30 mm in the 1100 alloy and 15 mm in the 3004 alloy. An earlier report gives further details of the use of ECAP to produce grain refinement in these two commercial alloys [11].
Fig. 1. Schematic illustration of the ECAP facility and the load cell for measuring the applied load during pressing.
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Full details of processing by equal-channel angular pressing were given earlier [2,3,12]. In the present investigation. all samples were pressed at room temperature ( 20°C) using a series of solid dies with the two channels forming an L-shaped configuration such that there was an internal angle of 90° between the two parts of each channel and an angle of 45° at the outer arc of curvature where the two channels intersect. It can be shown from first principles that these values for the two internal angles lead to an imposed strain of 1 for each passage of the sample through the die [13]. The dies were constructed from an SKD11 tool steel with channel diameters of either 6, 10 or 40 mm: in practice, slightly larger diameters were used for the entrance channel (by 0.3 mm) to avoid the need for lightly grinding samples between repetitive pressings [3]. Samples were pressed repetitively through the appropriate die up to a maximum of 8 passes, equivalent to a total imposed strain of 8. Prior to each pressing, samples were coated in a lubricant containing 5% MoS2. Individual specimens were pressed using three different processing routes [14]: in route A the sample is pressed repetitively without rotation. in route BC the sample is rotated by 90° in the same sense between each pass and in route C the sample is rotated by 180° between passes. Unless specified otherwise, the samples in this investigation were processed using route BC where this route was selected for the majority of the tests because it leads most expeditiously to an array of grains separated by high-angle grain boundaries [15] and it is also the optimum route for attaining high ductilities in superplasticity [16]. Measurements were taken during each pressing to determine the magnitude of the load applied during ECAP. The facility for this purpose is illustrated schematically in Fig. 1 and it consists of a load cell of 50 tons capacity attached to the upper loading platen of a hydraulic press. Using this facility, it was possible to continuously record the applied load during ECAP on a strip-chart recorder. Three orthogonal planes of sectioning are conventionally defined in ECAP and the directions associated with these planes are also illustrated in Fig. 1: the X plane is perpendicular to the direction of pressing and the Y and Z planes represent the side face and the upper face at the point of exit from the die, respectively. After ECAP, the microstructures were examined in the X plane using an Hitachi H-8100 transmission electron microscope (TEM) operating at 200 kV. Samples were prepared by sectioning the as-pressed rods perpendicular to their longitudinal axes, cutting disks with diameters of 3 mm and thicknesses of 0.15 mm from selected positions within the sample and then electropolishing for TEM using the procedure described earlier [17]. Selected area electron diffraction (SAED) patterns were taken from regions having diameters of 6.3 mm.
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Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
Measurements were taken at selected points on the X plane in the Y and Z directions using a load of 15 g applied for a total of 15 s for each separate measurement.
3. Experimental results Fig. 2. The orientations and positions of tensile specimens cut from the sample with a diameter of 40 mm.
Fig. 3. Typical appearance of the samples during ECAP for each of the three diameters used in this investigation.
Tensile specimens were machined from the samples after ECAP with gauge lengths of 5 mm and gauge sections of 2×3 mm2. No tensile tests were conducted using the samples with an initial diameter of 6 mm; for the as-pressed material with an initial diameter of 10 mm, all specimens were necessarily cut with the tensile axes lying parallel to the X or pressing direction; for the as-pressed material with an initial diameter of 40 mm, separate specimens were cut with the tensile axes lying either in the X, Y or Z directions and with the specimens positioned as illustrated schematically in Fig. 2 so that, for each separate set, specimen B was approximately at the center of the as-pressed rod and specimens A and C were located between the center and the outer edges. These specimens were tested in tension at room temperature using a testing machine operating under conditions of constant cross-head displacement and with an initial strain rate of 3.3×10 − 4 s − 1. Compression tests were also conducted at room temperature using the same initial strain rate of 3.3× 10 − 4 s − 1 and with each specimen having a height of 5 mm and a cross-section of 3×3 mm2. All of the compression specimens were taken from the center of the aspressed materials with the long axes lying parallel to the X direction. The Vickers microhardness, Hv, was measured after ECAP for the sample with a diameter of 40 mm.
3.1. Effect of sample size on microstructure The 1100 alloy used in this investigation is relatively soft and no difficulties were encountered in conducting the ECAP at room temperature. During pressing, each cylindrical sample is deformed into a rhombohedral shape as predicted by analysis [14] and as illustrated by the as-pressed samples shown in Fig. 3 for each of the three diameters. Fig. 4 shows the microstructures of the 1100 alloy after 6 passes to a strain of 6 for samples having the three different diameters used in this investigation together with the appropriate SAED patterns. These microstructures were recorded on the X plane at points close to the center of each as-pressed sample. It is apparent that the three microstructures in Fig. 4 are essentially identical and careful measurements gave average grain sizes of 0.7 mm for each material: in addition, the SAED patterns show each of these microstructures contains grains separated by boundaries having high angles of misorientation. A more detailed analysis was conducted on the largest sample with a diameter of 40 mm. On the upper left of Fig. 5 there is a schematic illustration of a section in the X plane for this largest sample with 5 separate points designated as A, B, C, D and E: A is at the center of the sample and the other 4 points are located at positions approximately 5 mm from the edge. Fig. 5 also includes the microstructures corresponding to each of these points after pressing through a total of 6 passes and the corresponding SAED patterns for points B, C, D and E are given in Fig. 6. Inspection shows the microstructures are similar at each point with a measured grain size of 0.7 mm but. whereas the grains are equiaxed at points A, B, D and E, there is evidence for the presence of some elongated grains at point C. All of the SAED patterns in Fig. 6, including for point C, show evidence for the presence of high angle grain boundaries. Hardness measurements were taken on the X plane of the sample with a 40 mm diameter after ECAP through a total of 6 passes. The result is given in Fig. 7 for measurements taken every 5 mm in the two orthogonal Y and Z directions to within 5 mm of the sample edge. These measurements show the value of Hv is essentially identical at each point and the hardness remains unchanged throughout the sample despite the lack of a fully homogeneous microstructure as revealed
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
by the presence of some elongated grains at point C in Fig. 5. It is reasonable to conclude from these observations that similar microstructures are produced by ECAP both in the centers of samples having different diameters and throughout larger samples, at least to within a distance of 5 mm of the sample edge.
3.2. Mechanical properties after ECAP Samples of the 1100 alloy were tested in tension after ECAP to 6 passes using the configurations depicted schematically in Fig. 2. Plots of true stress, |, against
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true strain, m, are shown in Fig. 8 where X(10) denotes a specimen cut in the X direction from an as-pressed sample with a diameter of 10 mm and X(40) , Y(40) and Z(40) denote the 3 separate specimens cut according to the definitions in Fig. 2 from as-pressed samples with a diameter of 40 mm. No attempt is made in Fig. 8 to distinguish between the stress-strain curves obtained for the three separate positions of A, B and C as defined in Fig. 2 because each position gave an essentially identical result. Furthermore, the stressstrain curves were essentially identical for the X(10) specimen and the three X(40) specimens in terms of the magnitude of the stress although it was observed that the ductility was slightly lower in the specimen cut from the 10 mm sample. For tensile specimens cut from the 40 mm sample, it is apparent that the flow stresses are slightly higher in the X direction, intermediate in the Y direction and lowest in the Z direction. Using these stress-strain curves, Table 1 summarizes the 0.2% proof stress, the ultimate tensile stress (UTS) and the elongation to failure for each sample: this tabulation demonstrates the reproducibility of the data for samples cut at different positions in the larger 40 mm material and, except only for the elongation to failure, the similarity between data obtained from the as-pressed samples with diameters of 10 and 40 mm. To further check on this similarity, compression tests were conducted on the 1100 alloy after ECAP to 6 passes and including samples with a diameter of 6 mm. The values of the 0.2% proof stresses measured in compression are shown in Fig. 9 for two samples cut from each of the as-pressed materials. It is concluded these results are reproducible and there is no dependence on the sample diameter at least over the range from 6–40 mm.
3.3. Magnitude of the applied load in ECAP
Fig. 4. Microstructures of the 1100 alloy after 6 passes for diameters of 6, 10 and 40 mm: the SAED patterns are also shown.
The applied load was continuously recorded throughout each pressing in this investigation and Fig. 10(a) shows the variation of the maximum load as a function of the total number of passes through the die for the 1100 alloy with a diameter of 10 mm after pressing through processing routes A, BC and C: similar data are shown in Fig. 10(b) for the 3004 alloy using samples with the same diameter. It should be noted these three processing routes are identical for a single pass through the die and therefore any difference between the experimental points after one pass is a consequence of scatter between different samples. For both alloys, it is apparent that the load is lowest in route C after 2 and more passes whereas essentially similar loads are recorded using routes A and BC. The requirement for a lower applied load when processing using route C is reasonable because this route contains a redundant strain such that the strain is fully reversed after every other pass
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Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
Fig. 5. Microstructures at selected points A, B, C, D and E in a sample of the 1100 alloy after 6 passes.
through the ECAP die [[14,18,19]]. The curves in Fig. 10 show also that the maximum load tends to increase with increasing numbers of pressings, especially when using routes A and BC and for the 3004 alloy. It is apparent that the load required for ECAP at room temperature is 1.7 to 2.5 tons for the 1100 alloy and 2.5 to 4.5 tons for the 3004 alloy. Similar measurements for the 1100 alloy with a diameter of 40 mm gave a maximum load of 32 tons after a single pass. It is instructive to compare the values of the applied loads recorded in Fig. 10 with the corresponding values of the ultimate tensile stress (UTS) for these two alloys when testing in tension. Fig. 11(a) and (b) shows, for the 1100 and 3004 alloys, the measured UTS in tensile testing as a function of the number of passes imposed on the material using the three different processing routes, where all samples were prepared from aspressed material having a diameter of 10 mm and with the tensile axes cut parallel to the pressing direction. Inspection of Fig. 11 shows the UTS increases significantly after a single pass through the die due to the initiation of grain refinement and the development of bands of subgrains [2] but thereafter the UTS increases relatively slowly with additional pressings. This increase
after the first pass is most pronounced for the 3004 alloy and, for both materials, the UTS tends to be lower when using route C and fairly similar when using routes A and BC. All of these trends match the variation of the applied load with the number of passes in ECAP as recorded in Fig. 10.
Fig. 6. The SAED patterns for points B, C, D and E in Fig. 5.
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
Fig. 7. Values of the Vickers microhardness, Hv, in the X plane of a sample of the 1100 alloy with a diameter of 40 mm after ECAP through 6 passes.
Fig. 8. Tensile stress-strain curves for specimens of the 1100 alloy tested with diameters of 10 and 40 mm and cut as illustrated in Fig. 2.
4. Discussion The results from this investigation show that grain size refinement during ECAP and the subsequent me-
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Fig. 9. Values of the 0.2% proof stress in compression for samples of the 1100 alloy after ECAP through 6 passes using diameters from 6 – 40 mm.
chanical properties after pressing are essentially independent of the sample size, at least for sample diameters in the range from 6–40 mm. At the largest diameter of 40 mm, the microstructures are identical throughout the material to a distance within at least 5 mm of the sample edge and almost identical mechanical properties are attained in the tensile testing of samples cut in three mutually orthogonal directions. All of these results suggest a potential for scaling ECAP to larger sample sizes for use in industrial applications. It is possible to make use of these results to evaluate directly the feasibility of pressing samples with larger cross-sections. However, it is first necessary to consider whether the applied load in ECAP is determined primarily by the sample strength or whether there is an additional, possibly significant, contribution from frictional effects between the pressed material and the die walls. Frictional effects have been considered in modelling simulations [20,21] but in this investigation the effect of friction was minimized by using a molybdenum disulfide lubricant.
Table 1 Values of the 0.2% proof stress, UTS and elongation to failure for samples with diameters of 40 and 10 mm: the designations A, B and C relate to the positions illustrated in Fig. 2 Properties
40 mm sample Position
0.2% proof stress (Mpa)
UTS (MPa)
Elongation to failure (%)
A B C A B C A B C
10 mm sample Tensile direction
Tensile direction
X
Y
Z
193 202 201 212 227 225 29 30 30
182 184 177 210 211 212 29 28 29
159 170 170 206 206 207 30 29 28
X
190
225
25
40
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
Considering the samples with initial diameters of 10 and 40 mm, a frictional effect will arise at the contact area between the sample and the channel wall and thus, since the periphery of a section is proportional to the radius, the increase in the frictional effect for the larger sample is by a factor of 4. Conversely, the sample strength in the absence of friction is related to the cross-sectional area of the material and, for initial diameters of 10 and 40 mm, there is an increase in strength for the larger sample by a factor of 16. As already noted, direct measurements of the maximum applied loads after a single pass for the 1100 alloy gave values of 2 and 32 MPa for the samples with diameters of 10 and 40 mm, respectively. Thus, the measured applied loads are in the ratio of l6 which demonstrates that sample strength is the dominant factor in determining the load required for ECAP. The measurements of the UTS in tensile testing provide an opportunity to make a direct estimate of the range of applied loads required for ECAP of the 1100 and 3004 alloys. For samples with a diameter of 10 mm, as used for the data in Fig. 10, the cross-sectional area is 80 mm2. Therefore, taking the UTS in the range of 170– 250 MPa for the 1100 alloy from Fig.
Fig. 10. Maximum load in ECAP as a function of the number of passes through the die when processing through routes A, BC and C for (a) 1100 alloy and (b) 3004 alloy.
Fig. 11. Values of the UTS measured in tension as a function of the number of passes in ECAP when processing through routes A, BC and C for (a) 1100 alloy and (b) 3004 alloy.
11(a), the estimated applied loads lie within the range of 1.4–2.0 tons. Similarly, the values of the UTS for the 3004 alloy lie in the range of 280–420 MPa and this requires loads of 2.2–3.4 tons. It is apparent from these calculations that the estimated loads are reasonably consistent with the measured values. To examine the feasibility of pressing large initial billets, it is first instructive to note that an ECAP sample with a size before pressing of 60×60× 150 mm3 would have the capability of providing, after pressing, a rolled sheet having a thickness of 3 mm and with dimensions larger than 400× 400 mm2. This sheet size is close to the minimum required for use in industrial superplastic forming applications. Since the dominant factor determining the maximum applied load in ECAP is the sample strength, it follows that an aluminum 1100 alloy with cross-sectional dimensions of 60× 60 mm2 will require a maximum applied load of the order of 75 tons for successful pressing at room temperature. This calculation demonstrates that, at least for relatively soft materials such as aluminum, the pressing of large-scale billets for the subsequent production of reasonably large superplastic sheets is within the capabilities of conventional hydraulic presses. For hard materials, however, successful ECAP may require the use of elevated temperatures during the pressing operation [22], slow ram speeds in the press [22] and/or the use of ECAP dies having an internal angle between the 2 parts of the channel of \ 90°: for example, an angle
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
of 110° and a pressing temperature of 1000°C has been used for the successful pressing of tungsten when using a hydraulic press with a capacity of 160 tons [23].
5. Summary and conclusions 1. Cylinders of an aluminum alloy, having diameters from 6–40 mm, were subjected to equal-channel angular pressing (ECAP) at room temperature. The results demonstrate the development of an ultrafine microstructure and the subsequent mechanical properties after pressing are independent of the initial size of the sample and, for the largest sample with a diameter of 40 mm, independent of the location within the sample at least to a distance within 5 mm of the sample edge. 2. Measurements of the applied load during ECAP show the required load is related to the sample strength and, at least for well-lubricated samples, frictional effects between the sample and the die wall make a relatively minor contribution to the measured load. 3. It is concluded that ECAP can be scaled to larger sizes for potential use in industrial applications. An example would be the production by rolling after ECAP of reasonably large sheets of metal for use in superplastic forming operations.
Acknowledgements This work was supported in part by the Super-Aluminum Project of the Japan Research and Development Center for Metals (JRCM) through the New Energy and Industrial Technology Development Organization (NEDO), in part by the Light Metals Educational Foundation of Japan and in part by the U.S. Army Research Office under Grant No. DAAD19-001-0488.
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References [1] V.M. Segal, V.I. Reznikov, A.E. Drobyshevskiy, V.I. Kopylov, Metally 1 (1981) 115. [2] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 45 (1997) 4733. [3] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317. [4] Z. Horita, M. Furukawa, M. Nemoto, A.J. Barnes, T.G. Langdon, Acta Mater. 48 (2000) 3633. [5] S. Komura, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, J. Mater. Res. 15 (2000) 2571. [6] H. Akamatsu, T. Fujinami, Z. Horita, T.G. Langdon, Scripta Mater. 44 (2001) 759. [7] A.J. Barnes, Mater. Sci. Forum 304-306 (1999) 785. [8] T.C. Lowe, Y.T. Zhu, S.L. Semiatin, D.R. Berg, in: T.C. Lowe, R.Z. Valiev (Eds.), Investigations and Applications of Severe Plastic Deformation, Kluwer, Dordrecht, The Netherlands, 2000, p. 347. [9] K. Nakashima, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A281 (2000) 82. [10] P.B. Berbon, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. 30A (1999) 1989. [11] Z. Horita, T. Fujinami, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. 31A (2000) 691. [12] T.G. Langdon, M. Furukawa, M. Nemoto, Z. Horita, JOM 52 (4) (2000) 30. [13] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 149. [14] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [15] K. Oh-ishi, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. 29A (1998) 2011. [16] S. Komura, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A297 (2001) 111. [17] J. Wang, Y. Iwahashi, Z. Horita, M. Furukawa, M. Nemoto, R.Z. Valiev, T.G. Langdon, Acta Mater. 44 (1996) 2973. [18] F.J. Humphreys, P.B. Prangnell, J.R. Bowen, A. Gholinia, C. Harris, Phil. Trans. R. Soc. Lond. A357 (1999) 1663. [19] A. Gholinia, P.B. Prangnell, M.V. Markushev, Acta Mater. 48 (2000) 1115. [20] P.B. Prangnell, C. Harris, S.M. Roberts, Scripta Mater. 37 (1997) 983. [21] S.L. Semiatin, D.P. DeLo, E.B. Shell, Acta Mater. 48 (2000) 1841. [22] S.L. Semiatin, D.P. DeLo, Mater. Design 21 (2000) 311. [23] I.V. Alexandrov, G.I. Raab, R.Z. Valiev, L.O. Shestakova, R.J. Dowding, in: M.S. Greenfield, J.J. Oakes (Eds.), Tungsten, Hardmetals and Refractory Alloys 5, The Metal Powder Industries Federation, Princeton, NJ, 2000, p. 27.
The potential for scaling ECAP: effect of sample size on grain refinement and mechanical properties Zenji Horita a, Takayoshi Fujinami a , Terence G. Langdon b,* a
b
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Uni6ersity, Fukuoka 812 -8581, Japan Departments of Aerospace and Mechanical Engineering and Materials Science, Uni6ersity of Southern California, Los Angeles, CA 90089 -1453, USA Received 3 January 2001; received in revised form 13 March 2001
Abstract The potential for scaling equal-channel angular pressing (ECAP) for use with large samples was investigated by conducting tests on an aluminum alloy using cylinders having diameters from 6 – 40 mm. The results show the refinement of the microstructure and the subsequent mechanical properties after pressing are independent of the initial size of the sample and, for the largest sample with a diameter of 40 mm, independent of the location within the sample at least to a distance of 5 mm from the sample edge. By making direct measurements of the imposed load during ECAP, it is shown that the applied load is determined by the sample strength rather than frictional effects between the sample and the die walls. The results demonstrate the feasibility of scaling ECAP to large sizes for use in industrial applications. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloys; Equal-channel angular pressing; Mechanical properties; Severe plastic deformation; Ultrafine grains sizes
1. Introduction Equal-channel angular pressing (ECAP) is a processing procedure in which a material is subjected to severe plastic deformation through simple shear [1]. This procedure differs from more conventional metal-working processes, such as rolling and extrusion. because it has the potential for introducing an exceptionally high plastic strain without any concomitant change in the crosssectional dimensions of the sample. It is now well-established that ECAP is capable of producing considerable grain refinement in metals. typically to the submicrometer level [2,3], and these very small grains provide a potential for achieving extremely high tensile ductilities. with elongations up to \ 1000%. at strain rates at and above 10 − 2 s − 1 [4,5]. Thus, there is a possibility of using ECAP as a tool for processing metals for subsequent superplastic forming at rapid strain rates. It was demonstrated recently that samples subjected to ECAP may be rolled into sheets after * Tel.: +1-213-7400491 fax: +1-213-7408071. E-mail address: [email protected] (T.G. Langdon).
pressing without any diminution in the superplastic properties [6] thereby confirming the potential of this procedure for the production of superplastic sheet metals. A critical question in ECAP concerns the potential for scaling-up the process for industrial applications while retaining a procedure that is economically viable [7,8]. A first step in this direction was taken in a recent demonstration that it is possible to impose very high strains on a sample in a single passage through an ECAP die by constructing a multi-pass facility containing a number of consecutive shearing planes [9]. Nevertheless, there has been to date no critical assessment of the effect, on either the microstructural refinement or the subsequent mechanical properties, of an increase in the sample size in conventional processing by ECAP. Earlier results suggested a possible lower limit for satisfactory ECAP because there was evidence that frictional effects between the sample and the die walls may become critical when the sample cross-section is reduced to 5 mm [10] but there has been no corresponding evaluation of the effect of pressing samples with larger cross-sections. Accordingly, the present in-
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 3 9 - 9
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
vestigation was initiated in order to evaluate the influence of the sample size in ECAP using rod-shaped samples with diameters from 6– 40 mm. In addition, the ECAP facility used in this investigation was modified to permit a direct measure of the load imposed on each sample during pressing.
2. Experimental materials and procedures Most of the experiments conducted in this investigation used a commercial 1100 aluminum alloy received in the form of a hot-rolled billet. This alloy had a composition, in wt%., of 0.60 Fe, 0.15 Cu, 0.09 Si, 0.005 Mn, 0.002 Mg, B 0.001 Cr, 0.006 Zn with the balance as aluminum. Some additional experiments were also conducted using a commercial cold-rolled 3004 aluminum alloy with a composition, in wt%, of 0.444 Fe, 0.140 Cu, 0.206 Si, 1.015 Mn, 1.154 Mg, 0.016 Cr, 0.057 Zn, 0.017 Ti with the balance as aluminum. For the 1100 alloy, rods were machined from the billet with diameters of 6, 10 or 40 mm and with corresponding lengths of 50, 60 or 100 mm, respectively. For the 3004 alloy, rods were machined with diameters of 10 mm and lengths of 60 mm. All of these rods were annealed for 1 h at temperatures of either 500°C for the 1100 alloy or 350°C for the 3004 alloy with subsequent air cooling. Observations showed the initial grain sizes prior to ECAP were 30 mm in the 1100 alloy and 15 mm in the 3004 alloy. An earlier report gives further details of the use of ECAP to produce grain refinement in these two commercial alloys [11].
Fig. 1. Schematic illustration of the ECAP facility and the load cell for measuring the applied load during pressing.
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Full details of processing by equal-channel angular pressing were given earlier [2,3,12]. In the present investigation. all samples were pressed at room temperature ( 20°C) using a series of solid dies with the two channels forming an L-shaped configuration such that there was an internal angle of 90° between the two parts of each channel and an angle of 45° at the outer arc of curvature where the two channels intersect. It can be shown from first principles that these values for the two internal angles lead to an imposed strain of 1 for each passage of the sample through the die [13]. The dies were constructed from an SKD11 tool steel with channel diameters of either 6, 10 or 40 mm: in practice, slightly larger diameters were used for the entrance channel (by 0.3 mm) to avoid the need for lightly grinding samples between repetitive pressings [3]. Samples were pressed repetitively through the appropriate die up to a maximum of 8 passes, equivalent to a total imposed strain of 8. Prior to each pressing, samples were coated in a lubricant containing 5% MoS2. Individual specimens were pressed using three different processing routes [14]: in route A the sample is pressed repetitively without rotation. in route BC the sample is rotated by 90° in the same sense between each pass and in route C the sample is rotated by 180° between passes. Unless specified otherwise, the samples in this investigation were processed using route BC where this route was selected for the majority of the tests because it leads most expeditiously to an array of grains separated by high-angle grain boundaries [15] and it is also the optimum route for attaining high ductilities in superplasticity [16]. Measurements were taken during each pressing to determine the magnitude of the load applied during ECAP. The facility for this purpose is illustrated schematically in Fig. 1 and it consists of a load cell of 50 tons capacity attached to the upper loading platen of a hydraulic press. Using this facility, it was possible to continuously record the applied load during ECAP on a strip-chart recorder. Three orthogonal planes of sectioning are conventionally defined in ECAP and the directions associated with these planes are also illustrated in Fig. 1: the X plane is perpendicular to the direction of pressing and the Y and Z planes represent the side face and the upper face at the point of exit from the die, respectively. After ECAP, the microstructures were examined in the X plane using an Hitachi H-8100 transmission electron microscope (TEM) operating at 200 kV. Samples were prepared by sectioning the as-pressed rods perpendicular to their longitudinal axes, cutting disks with diameters of 3 mm and thicknesses of 0.15 mm from selected positions within the sample and then electropolishing for TEM using the procedure described earlier [17]. Selected area electron diffraction (SAED) patterns were taken from regions having diameters of 6.3 mm.
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Measurements were taken at selected points on the X plane in the Y and Z directions using a load of 15 g applied for a total of 15 s for each separate measurement.
3. Experimental results Fig. 2. The orientations and positions of tensile specimens cut from the sample with a diameter of 40 mm.
Fig. 3. Typical appearance of the samples during ECAP for each of the three diameters used in this investigation.
Tensile specimens were machined from the samples after ECAP with gauge lengths of 5 mm and gauge sections of 2×3 mm2. No tensile tests were conducted using the samples with an initial diameter of 6 mm; for the as-pressed material with an initial diameter of 10 mm, all specimens were necessarily cut with the tensile axes lying parallel to the X or pressing direction; for the as-pressed material with an initial diameter of 40 mm, separate specimens were cut with the tensile axes lying either in the X, Y or Z directions and with the specimens positioned as illustrated schematically in Fig. 2 so that, for each separate set, specimen B was approximately at the center of the as-pressed rod and specimens A and C were located between the center and the outer edges. These specimens were tested in tension at room temperature using a testing machine operating under conditions of constant cross-head displacement and with an initial strain rate of 3.3×10 − 4 s − 1. Compression tests were also conducted at room temperature using the same initial strain rate of 3.3× 10 − 4 s − 1 and with each specimen having a height of 5 mm and a cross-section of 3×3 mm2. All of the compression specimens were taken from the center of the aspressed materials with the long axes lying parallel to the X direction. The Vickers microhardness, Hv, was measured after ECAP for the sample with a diameter of 40 mm.
3.1. Effect of sample size on microstructure The 1100 alloy used in this investigation is relatively soft and no difficulties were encountered in conducting the ECAP at room temperature. During pressing, each cylindrical sample is deformed into a rhombohedral shape as predicted by analysis [14] and as illustrated by the as-pressed samples shown in Fig. 3 for each of the three diameters. Fig. 4 shows the microstructures of the 1100 alloy after 6 passes to a strain of 6 for samples having the three different diameters used in this investigation together with the appropriate SAED patterns. These microstructures were recorded on the X plane at points close to the center of each as-pressed sample. It is apparent that the three microstructures in Fig. 4 are essentially identical and careful measurements gave average grain sizes of 0.7 mm for each material: in addition, the SAED patterns show each of these microstructures contains grains separated by boundaries having high angles of misorientation. A more detailed analysis was conducted on the largest sample with a diameter of 40 mm. On the upper left of Fig. 5 there is a schematic illustration of a section in the X plane for this largest sample with 5 separate points designated as A, B, C, D and E: A is at the center of the sample and the other 4 points are located at positions approximately 5 mm from the edge. Fig. 5 also includes the microstructures corresponding to each of these points after pressing through a total of 6 passes and the corresponding SAED patterns for points B, C, D and E are given in Fig. 6. Inspection shows the microstructures are similar at each point with a measured grain size of 0.7 mm but. whereas the grains are equiaxed at points A, B, D and E, there is evidence for the presence of some elongated grains at point C. All of the SAED patterns in Fig. 6, including for point C, show evidence for the presence of high angle grain boundaries. Hardness measurements were taken on the X plane of the sample with a 40 mm diameter after ECAP through a total of 6 passes. The result is given in Fig. 7 for measurements taken every 5 mm in the two orthogonal Y and Z directions to within 5 mm of the sample edge. These measurements show the value of Hv is essentially identical at each point and the hardness remains unchanged throughout the sample despite the lack of a fully homogeneous microstructure as revealed
Z. Horita et al. / Materials Science and Engineering A318 (2001) 34–41
by the presence of some elongated grains at point C in Fig. 5. It is reasonable to conclude from these observations that similar microstructures are produced by ECAP both in the centers of samples having different diameters and throughout larger samples, at least to within a distance of 5 mm of the sample edge.
3.2. Mechanical properties after ECAP Samples of the 1100 alloy were tested in tension after ECAP to 6 passes using the configurations depicted schematically in Fig. 2. Plots of true stress, |, against
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true strain, m, are shown in Fig. 8 where X(10) denotes a specimen cut in the X direction from an as-pressed sample with a diameter of 10 mm and X(40) , Y(40) and Z(40) denote the 3 separate specimens cut according to the definitions in Fig. 2 from as-pressed samples with a diameter of 40 mm. No attempt is made in Fig. 8 to distinguish between the stress-strain curves obtained for the three separate positions of A, B and C as defined in Fig. 2 because each position gave an essentially identical result. Furthermore, the stressstrain curves were essentially identical for the X(10) specimen and the three X(40) specimens in terms of the magnitude of the stress although it was observed that the ductility was slightly lower in the specimen cut from the 10 mm sample. For tensile specimens cut from the 40 mm sample, it is apparent that the flow stresses are slightly higher in the X direction, intermediate in the Y direction and lowest in the Z direction. Using these stress-strain curves, Table 1 summarizes the 0.2% proof stress, the ultimate tensile stress (UTS) and the elongation to failure for each sample: this tabulation demonstrates the reproducibility of the data for samples cut at different positions in the larger 40 mm material and, except only for the elongation to failure, the similarity between data obtained from the as-pressed samples with diameters of 10 and 40 mm. To further check on this similarity, compression tests were conducted on the 1100 alloy after ECAP to 6 passes and including samples with a diameter of 6 mm. The values of the 0.2% proof stresses measured in compression are shown in Fig. 9 for two samples cut from each of the as-pressed materials. It is concluded these results are reproducible and there is no dependence on the sample diameter at least over the range from 6–40 mm.
3.3. Magnitude of the applied load in ECAP
Fig. 4. Microstructures of the 1100 alloy after 6 passes for diameters of 6, 10 and 40 mm: the SAED patterns are also shown.
The applied load was continuously recorded throughout each pressing in this investigation and Fig. 10(a) shows the variation of the maximum load as a function of the total number of passes through the die for the 1100 alloy with a diameter of 10 mm after pressing through processing routes A, BC and C: similar data are shown in Fig. 10(b) for the 3004 alloy using samples with the same diameter. It should be noted these three processing routes are identical for a single pass through the die and therefore any difference between the experimental points after one pass is a consequence of scatter between different samples. For both alloys, it is apparent that the load is lowest in route C after 2 and more passes whereas essentially similar loads are recorded using routes A and BC. The requirement for a lower applied load when processing using route C is reasonable because this route contains a redundant strain such that the strain is fully reversed after every other pass
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Fig. 5. Microstructures at selected points A, B, C, D and E in a sample of the 1100 alloy after 6 passes.
through the ECAP die [[14,18,19]]. The curves in Fig. 10 show also that the maximum load tends to increase with increasing numbers of pressings, especially when using routes A and BC and for the 3004 alloy. It is apparent that the load required for ECAP at room temperature is 1.7 to 2.5 tons for the 1100 alloy and 2.5 to 4.5 tons for the 3004 alloy. Similar measurements for the 1100 alloy with a diameter of 40 mm gave a maximum load of 32 tons after a single pass. It is instructive to compare the values of the applied loads recorded in Fig. 10 with the corresponding values of the ultimate tensile stress (UTS) for these two alloys when testing in tension. Fig. 11(a) and (b) shows, for the 1100 and 3004 alloys, the measured UTS in tensile testing as a function of the number of passes imposed on the material using the three different processing routes, where all samples were prepared from aspressed material having a diameter of 10 mm and with the tensile axes cut parallel to the pressing direction. Inspection of Fig. 11 shows the UTS increases significantly after a single pass through the die due to the initiation of grain refinement and the development of bands of subgrains [2] but thereafter the UTS increases relatively slowly with additional pressings. This increase
after the first pass is most pronounced for the 3004 alloy and, for both materials, the UTS tends to be lower when using route C and fairly similar when using routes A and BC. All of these trends match the variation of the applied load with the number of passes in ECAP as recorded in Fig. 10.
Fig. 6. The SAED patterns for points B, C, D and E in Fig. 5.
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Fig. 7. Values of the Vickers microhardness, Hv, in the X plane of a sample of the 1100 alloy with a diameter of 40 mm after ECAP through 6 passes.
Fig. 8. Tensile stress-strain curves for specimens of the 1100 alloy tested with diameters of 10 and 40 mm and cut as illustrated in Fig. 2.
4. Discussion The results from this investigation show that grain size refinement during ECAP and the subsequent me-
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Fig. 9. Values of the 0.2% proof stress in compression for samples of the 1100 alloy after ECAP through 6 passes using diameters from 6 – 40 mm.
chanical properties after pressing are essentially independent of the sample size, at least for sample diameters in the range from 6–40 mm. At the largest diameter of 40 mm, the microstructures are identical throughout the material to a distance within at least 5 mm of the sample edge and almost identical mechanical properties are attained in the tensile testing of samples cut in three mutually orthogonal directions. All of these results suggest a potential for scaling ECAP to larger sample sizes for use in industrial applications. It is possible to make use of these results to evaluate directly the feasibility of pressing samples with larger cross-sections. However, it is first necessary to consider whether the applied load in ECAP is determined primarily by the sample strength or whether there is an additional, possibly significant, contribution from frictional effects between the pressed material and the die walls. Frictional effects have been considered in modelling simulations [20,21] but in this investigation the effect of friction was minimized by using a molybdenum disulfide lubricant.
Table 1 Values of the 0.2% proof stress, UTS and elongation to failure for samples with diameters of 40 and 10 mm: the designations A, B and C relate to the positions illustrated in Fig. 2 Properties
40 mm sample Position
0.2% proof stress (Mpa)
UTS (MPa)
Elongation to failure (%)
A B C A B C A B C
10 mm sample Tensile direction
Tensile direction
X
Y
Z
193 202 201 212 227 225 29 30 30
182 184 177 210 211 212 29 28 29
159 170 170 206 206 207 30 29 28
X
190
225
25
40
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Considering the samples with initial diameters of 10 and 40 mm, a frictional effect will arise at the contact area between the sample and the channel wall and thus, since the periphery of a section is proportional to the radius, the increase in the frictional effect for the larger sample is by a factor of 4. Conversely, the sample strength in the absence of friction is related to the cross-sectional area of the material and, for initial diameters of 10 and 40 mm, there is an increase in strength for the larger sample by a factor of 16. As already noted, direct measurements of the maximum applied loads after a single pass for the 1100 alloy gave values of 2 and 32 MPa for the samples with diameters of 10 and 40 mm, respectively. Thus, the measured applied loads are in the ratio of l6 which demonstrates that sample strength is the dominant factor in determining the load required for ECAP. The measurements of the UTS in tensile testing provide an opportunity to make a direct estimate of the range of applied loads required for ECAP of the 1100 and 3004 alloys. For samples with a diameter of 10 mm, as used for the data in Fig. 10, the cross-sectional area is 80 mm2. Therefore, taking the UTS in the range of 170– 250 MPa for the 1100 alloy from Fig.
Fig. 10. Maximum load in ECAP as a function of the number of passes through the die when processing through routes A, BC and C for (a) 1100 alloy and (b) 3004 alloy.
Fig. 11. Values of the UTS measured in tension as a function of the number of passes in ECAP when processing through routes A, BC and C for (a) 1100 alloy and (b) 3004 alloy.
11(a), the estimated applied loads lie within the range of 1.4–2.0 tons. Similarly, the values of the UTS for the 3004 alloy lie in the range of 280–420 MPa and this requires loads of 2.2–3.4 tons. It is apparent from these calculations that the estimated loads are reasonably consistent with the measured values. To examine the feasibility of pressing large initial billets, it is first instructive to note that an ECAP sample with a size before pressing of 60×60× 150 mm3 would have the capability of providing, after pressing, a rolled sheet having a thickness of 3 mm and with dimensions larger than 400× 400 mm2. This sheet size is close to the minimum required for use in industrial superplastic forming applications. Since the dominant factor determining the maximum applied load in ECAP is the sample strength, it follows that an aluminum 1100 alloy with cross-sectional dimensions of 60× 60 mm2 will require a maximum applied load of the order of 75 tons for successful pressing at room temperature. This calculation demonstrates that, at least for relatively soft materials such as aluminum, the pressing of large-scale billets for the subsequent production of reasonably large superplastic sheets is within the capabilities of conventional hydraulic presses. For hard materials, however, successful ECAP may require the use of elevated temperatures during the pressing operation [22], slow ram speeds in the press [22] and/or the use of ECAP dies having an internal angle between the 2 parts of the channel of \ 90°: for example, an angle
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of 110° and a pressing temperature of 1000°C has been used for the successful pressing of tungsten when using a hydraulic press with a capacity of 160 tons [23].
5. Summary and conclusions 1. Cylinders of an aluminum alloy, having diameters from 6–40 mm, were subjected to equal-channel angular pressing (ECAP) at room temperature. The results demonstrate the development of an ultrafine microstructure and the subsequent mechanical properties after pressing are independent of the initial size of the sample and, for the largest sample with a diameter of 40 mm, independent of the location within the sample at least to a distance within 5 mm of the sample edge. 2. Measurements of the applied load during ECAP show the required load is related to the sample strength and, at least for well-lubricated samples, frictional effects between the sample and the die wall make a relatively minor contribution to the measured load. 3. It is concluded that ECAP can be scaled to larger sizes for potential use in industrial applications. An example would be the production by rolling after ECAP of reasonably large sheets of metal for use in superplastic forming operations.
Acknowledgements This work was supported in part by the Super-Aluminum Project of the Japan Research and Development Center for Metals (JRCM) through the New Energy and Industrial Technology Development Organization (NEDO), in part by the Light Metals Educational Foundation of Japan and in part by the U.S. Army Research Office under Grant No. DAAD19-001-0488.
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