Equal channel angular drawing of aluminium sheet

Materials Science and Engineering A 427 (2006) 123–129

Equal channel angular drawing of aluminium sheet A.A. Zisman a,∗ , V.V. Rybin a , S. Van Boxel b , M. Seefeldt b , B. Verlinden b a

b

CRISM “Prometey”, 49 Shpalernaja, 191015 St.-Petersburg, Russia Department MTM, Katholieke Universiteit Leuven, B-3001 Leuven (Heverlee), Belgium

Received 19 August 2005; received in revised form 12 April 2006; accepted 14 April 2006

Abstract A method of equal channel angular drawing of sheet metals (ECADS) is presented and results of its application to a thin sheet of commercially pure aluminium are reported. The obtained mechanical properties, textures and microstructures are compared to those resulting from closely related processing techniques: (a) ECAD of rods, and (b) continuous shearing of sheets (CSS). © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminium; Equal channel angular drawing; Deformation structure; EBSD; Texture

1. Introduction Grain fragmentation at large plastic strains has attracted a lot of research interest as a mechanism to produce bulk metals with sub-micrometer sized crystals separated by high-angle boundaries [1]. Progress in this field has particularly been advanced by the concept of equal channel angular pressing (ECAP), invented by Segal et al. [2], where enormous degrees of plastic strain may be accumulated because the main dimensions of a processed billet are kept invariant. In this technique the billet is repeatedly pushed through intersecting channels of the same cross-section, preserving its transversal dimensions. ECAP and other related techniques, mostly based on simple shearing and advisedly used to refine polycrystalline structures [3], acquired the generic term “severe plastic deformation” (SPD). Although many alternative SPD methods have been proposed (see [4] for example), ECAP still remains the most popular among materials scientists. Even this efficient technique, however, has important drawbacks preventing its wider industrial application. First of all, it is not suitable for continuous processing of long billets, as the inlet channel length is limited by the buckling instability inherent to compressive loads. For the same reason, in particular, ECAP is unsuitable for treating sheet metals, which are much in demand. Further-

∗

Corresponding author. Tel.: +7 812 521 3981; fax: +7 812 7103756. E-mail address: crism [email protected] (A.A. Zisman).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.007

more, since channel dimensions are fixed, only a certain billet size can be processed; any change of the billet cross-section would necessitate a replacement of the device. In what follows we will touch on some efforts to obviate these ECAP limitations. The idea of Suriadi and Thomson to pull the billet through intersecting channels, thus eliminating problems related to the compressive loads of ECAP, resulted in the equal channel angular drawing (ECAD) method [5–7] which is potentially more suitable for continuous processing. A main drawback of ECAD is some plastic elongation along the drawing direction, inherent to tensile loads, and the related cross-section reduction. This effect intensifies when the channel intersection angle Φ decreases: a too sharp intersection may even lead to billet rupture during processing. Therefore, the die angle should be sufficiently blunt. This limitation is also important because at sharper die angles simple shear, uniform through the billet thickness, as the dominating deformation mode will be substituted by bending. The problem aggravates in the multi-pass ECAD process. Once the billet cross-section is reduced in the first pass, it no longer matches the die cross-section, and such a misfit, increasing pass by pass, results in billet bending [9]. On the other hand, excessively high magnitudes of Φ, close to 180◦ , would be senseless as it would give low plastic strain per pass and hence diminish the efficiency with respect to grain refinement [8]. Apparently for such reasons, the inventors have proposed alternative ECAD applications as hardening of metal rods or improving their initial texture [7]. Similar opportunities, however, were not applied to sheet metals.

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A way to avoid the buckling instability, while keeping the compressive load, has been proposed by Saito et al. [10,11] for the continuous shearing of sheet metals (CSS). In their method the sheet is pushed through an equal channel die by a specifically designed rolling system. Comparable processing schemes, called the continuous confined strip shearing (CCSS) and the dissimilar channel angular pressing (DCAP), were somewhat later reported by Lee et al. [12–14]. For brevity we will use “CSS” as a common abbreviation for both techniques. Like ECAD, they can use only a sufficiently blunt angle Φ which limits the shear deformation per pass. Accordingly, apart from the grain refinement, CSS is considered as a means to harden metals and/or to improve their textures. Although similar techniques of plastic treatment are less intensive than ECAP, they remain very promising from an industrial viewpoint as they are applicable to sheet metals. An obstacle on this way, however, is that each particular CSS device (both the die and the feeding system) is only suitable to process sheets of one thickness. Another kind of plastic treatment applied to sheet metals is accumulative roll-bonding (ARB) [15]. It provides very high strain degrees with no principal limitation on the sheet thickness. It is complicated, however, due to the necessity to cut a preprocessed sheet, to carefully treat the surfaces of its halves and to stack them together before each next pass. The present paper briefly describes a new simple method to process sheet metals, which combines to some extent the advantages of ECAD and of CSS. It is the equal channel angular drawing of sheet metals (ECADS) with adjustable die opening. In order to accumulate higher strains, corresponding to the SPD concept, ECADS can repeatedly be applied to the same sheet many times. The goal of this work, however, is to present the processing technique. The deformation applied up to now remains restricted to a moderate true strain of 1.25. Trial ECADS of commercially pure (CP) Al and mechanical tests were performed at CRISM “Prometey” (St.-Petersburg, Russia), where the device was designed and fabricated. The microstructures and textures of as-received and processed samples were analyzed in the Department of Metallurgy and Materials Engineering of K.U. Leuven (Leuven, Belgium).

2. Equal channel angular drawing of sheet (ECADS): device and processing In ECADS, schematically represented in Fig. 1(a), the equal channel die is formed by a base and a plunger, Fig. 1(b), where the plunger position may be adjusted to a desired sheet thickness by special screws, situated on both sides of the processing zone and directed parallel to the die bisector. Though the size of the plunger tip (working part) is limited in order to reduce friction, the total plunger cross-section is left large enough to obtain an appropriate bending stiffness that keeps the deformation homogeneous over the sheet width. The best sheet-to-die fit is maintained by narrow spacers, cut from the sheet before each next pass and fixed between the base and the plunger by the screws, Fig. 1(b). Such a tool does not only allow for a minor thickness reduction taking place each pass under the pulling force, but also enables the processed sheets to vary in initial thickness H < L, where L is the working arm of the plunger, Fig. 1(a). Besides, the screw tools facilitate a careful bending of the sheet, necessary to insert it into the die. For the pulling force, either standard testing machines or appropriate rolling drives may be employed. One more advantage of the device, not yet used in the present work, is the accessibility of the sheet just in front of the shearing zone. This should ease sheet heating and temperature control, when necessary. The base and the plunger were made of the Russian steel XB, commonly used in stamping equipment and, in this case, heat treated to hardness HRC = 60 sufficient to process mild steels and commercially pure Al and Ti. Before each processing a MoS2 -based lubricant is applied to the die surfaces and the sheet. As in case of ECAD or CSS, the selection of the ECADS channel intersection angle is the most delicate point. Preliminary estimates and, to greater extent, experiments proved Φ ≤ 135◦ to be too sharp. This also corresponds to data [9] on the closely related ECAD kinematics recorded on grid specimens of aluminium. According to these data (Fig. 4 of [9]), the principal deformation mode at Φ = 90◦ and at Φ = 120◦ is bending where cracks readily appear on the stretched surface. The deformed grid pattern at Φ = 150◦ shows the simple shear mode and is almost uniform through the specimen

Fig. 1. Principal scheme of sheet processing by ECADS (a) and general appearance of the plunger and the base of ECADS device (b).

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thickness. Therefore, Φ = 150◦ has been applied in the present work, and the simple shear per pass has been estimated as γ = 2 tan (90◦ − Φ/2) = 0.536. The true plastic strain ε per pass is expressed as: √
 2 3 2 1/2 2 2 2 ε= (εx − εy ) + (εx ) + (εy ) + γ 3 2     2 1/2  2  γ εy γ εy = √ 1+4 , (1) ≈ √ 1+2 γ γ 3 3 where the strain components εx and εy = −εx are also taken into account, corresponding to the elongation and the related thickness reduction per pass, respectively. In our experiments |εy | ≈ 0.07 was observed, so √ that (εy /γ)2 ≈ 0.02 in expression (1) is relatively small and ε ≈ γ/ 3 = 0.31 is a satisfactory approximation. The true strain rate D is roughly evaluated as D≈

εv , H

(2)

where v is the drawing velocity. Trial experiments have shown that D < 10−1 s−1 , as found with expression (2), is safe for CP Al sheet of 0.7–2.5 mm thickness. Aimed at a similar thickness range, our pilot ECADS device has L = 4 mm. It is worth noting that successive shear amounts γ are not simply added up in the multi-pass ECADS. Indeed, both the shear plane and direction, introduced in a previous pass of the equal channel angular treatment, are tilted with respect to the sheet during each next pass [18]. The planes of successive shears thus intersect each other. Consequently, only a relatively slow development or even some randomization of the crystallographic texture can be expected after a number of ECADS passes, and the cumulative shear effect on the grain shape should also be moderate. Unlike γ, the equivalent strain ε defined by Eq. (1) is additive, as well as its effect on the final work hardening. After each ECADS pass, except for the last one, processed strips were turned by 180◦ about the transversal direction in

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order to compensate a possible weak bend in the next pass. This rotation maintaining both the plane and the direction of previous shearing with respect to the material, did not deflect processing from route A, as classified in conventional ECAP terms [16]. When treating sheet metals, the only alternative in these terms would be route C, i.e. the strip rotation through ϕ = 180◦ about its longitudinal axis, whereas ϕ = 90◦ (route B) is impossible in ECADS. According to ECAP data [16,17] obtained with the die angle Φ = 90◦ , route C is somewhat more efficient than route A in terms of grain refinement. However, taking into account the different angle (Φ = 150◦ ) and the bending effect, the ECADS route selection should be particularly careful. In order to have a formal framework to compare different SPD routes, let us employ (following [18]) the strain-path change parameter s which is mostly applied to sheet forming processes [19–21]. This parameter, related to the abruptness of microstructure transformations, depends on the strain rate tensors just before (D1 ) and after (D2 ) the strain-path change, respectively: s = (D1 : D2 )(D1 : D1 )−1/2 (D2 : D2 )−1/2 ,

(3)

where the colon indicates the double contracted product. The most intensive is the “orthogonal” change with s = 0, whereas both the strain reversal with s = −1 and the monotonic strainpath with s = 1 are essentially weaker in this sense. According to Ref. [18], the microstructural responses to equal channel angular treatments, particularly at Φ = 90◦ , satisfactorily follow s rather than the conventional ABC classification. Omitting formal details [18], let us consider the s(ϕ) dependencies, Fig. 2, calculated at Φ = 90◦ , typical for ECAP, at Φ = 150◦ , used in our ECADS tests, and at Φ = 115◦ optimal in CSS [11]. Except for Φ = 90◦ , the graphs clearly indicate the predominance of route A over C. Moreover, route A in ECADS (Φ = 150◦ ) gives |s| = 0.5, i.e. the same intensity of strain-path change as route B at Φ = 90◦ , considered most efficient in ECAP. This change and, presumably, the related microstructure transformation could be even more abrupt in ECADS route A with Φ closer to 135◦ where

Fig. 2. Dependencies of strain-path change parameter [19] on the billet rotation about its longitudinal axis between successive passes of equal channel angular processing. Conventional routes A–C correspond to ϕ = 0◦ , 90◦ and 180◦ , respectively.

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s approaches 0 (orthogonal case). However, in order to realize this opportunity while avoiding rupture, particular precautions should be taken, such as pre-heating the processed sheet, appropriate rounding of the inner die angle, reducing the drawing velocity, etc. 3. Material and experimental procedures Trial ECADS (Φ = 150◦ ) treatments were performed at room temperature on a strip of 40 mm width and 1 mm thickness, cut in the rolling direction from the cold rolled and annealed sheet of CP Al. Four drawing passes by route A at constant D = 3 × 10−2 s−1 were carried out on a tensile machine (5 kN maximum load), the ECADS device being fixed. As expected, sheet bending after each pass was relatively weak: H/R < 0.005 where H is the sheet thickness and R is the bending radius. The total thickness reduction for four passes reached 30%. Mechanical testing, texture measurements and microstructure analysis by electron backscattering diffraction (EBSD) were performed on the as-received material and after two and four ECADS passes. Equivalent strains of 0.62 and 1.25 were accumulated during two and four ECADS passes, respectively, roughly corresponding to one and two passes in CSS with Φ = 115◦ reported in Ref. [11]. The latter treatment applied to CP Al sheet of 2 mm in thickness has provided us with appropriate reference data. Flat specimens with working part length and width of 40 and 10 mm, respectively, were cut from the strip in the drawing (former rolling) direction. They were subjected to standard tensile tests at room temperature with a strain rate of 10−3 s−1 , carried out on the tensile machine also employed to draw strips during ECADS. The crystallographic textures were determined by X-ray diffraction on square 10 mm × 10 mm samples, using a Siemens D500 goniometer with Cu K␣ radiation. The drawing direction, coincident with the previous rolling direction RD is denoted by DD and the common transverse direction is referred to as TD. Four incomplete pole figures {1 1 1}, {2 0 0}, {2 2 0} and {1 1 3} were measured on each sample, and the orientation distribution functions (ODF) were then reconstructed, using the MTM-FHM software [22]. The microstructures were analyzed on longitudinal sections of the strip prepared by standard metallographic procedures, using electrochemical polishing in 15% perchloric acid–ethanol solution at −5 ◦ C as the final step. Orientation mapping with EBSD was carried out in a hexagonal grid, following [23], on a Philips XL30 SEM with a tungsten filament at an accelerating voltage of 20 kV. EDAX-TSL software was used for pattern indexing. The step size and angular accuracy were 0.3 ␮m and 1◦ , respectively. 4. Mechanical properties The stress–strain diagrams of tensile tests prior to ECADS and after two and four passes are shown in Fig. 3, and the corresponding mechanical properties are represented in Table 1. Note that the indicated stress and deformation are the engineering stress and strain. They are scaled to the initial cross-section and

Fig. 3. Stress–strain diagrams of flat CP Al specimens at room temperature prior to ECADS and after two and four ECADS passes. Table 1 Mechanical properties of CP Al sheet in the drawing (rolling) direction Property

As received

Two ECADS passes

Four ECADS passes

Yield stress (MPa) Tensile strength (MPa) Ultimate elongation (%)

48.9 78.5 24.9

80.0 89.0 6.8

96.4 101.6 4.0

length of the specimen work part, respectively, thus neglecting the necking effect which is rather strong for the ECADS specimens. According to these data, after four ECADS passes the yield stress exceeds the initial value of 49 MPa by a factor of 2 and the tensile strengths the initial one by a factor of 1.3. The ultimate elongation is decreased sharply by the first two passes and much more slowly during the last two. In all, this behavior is essentially similar to that reported for the CSS processed CP Al sheet [11], where one and two passes are correlated to our two and four, respectively. At the same time, the ultimate logarithmic strains, in particular 19% after two CSS passes, are significantly larger than our ductility parameters after four ECADS passes. The apparent difference is mostly due to the strain localization that could not be properly reflected by the nominal elongation of flat specimens. According to [11], the pronounced strain localization takes place on CSS processed CP Al after relatively small (3–5%) uniform deformations. Other data, also relevant in this context, were obtained on CP Al rods of a square cross-section 15 mm × 15 mm processed at Φ = 135◦ by ECAD [7]. When correlating one and two ECAD passes (equivalent strains of 0.5 and 1.0) to two and four ECADS passes, respectively, the mechanical responses to these two SPD treatments satisfactorily correspond to each other as well. 5. Texture evolution The {1 1 1} pole figures on RD-TD projection planes are shown in Fig. 4(a–c) for the as received (cold rolled and annealed) Al sheet and after two and four ECADS passes,

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Fig. 4. {1 1 1} Pole figures prior to ECADS processing (a) and after two (b) and four (c) passes; successive intensity levels are 0.8, 1.0 1.3, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0 and 6.4. The drawing direction DD coincides with the former rolling direction RD.

respectively. The corresponding intensities (relative to a random distribution) of the main texture components, determined within 5◦ from ideal orientations, are listed in Table 2. The initial state, Fig. 1(a), displays a typical recrystallization texture of CP aluminium with a sharp cube component {1 0 0} 0 0 1, though weaker residuals of the cold rolling texture (S, Cu, Goss and brass) are still perceptible. Similar to one and two CSS passes [11], ECADS first disintegrates the initial recrystallization texture, and then (during the third and the fourth pass) results in a weak texture evolution. Thus, the most intensive randomization takes place after two passes, Fig. 4(b), when the thickness reduction of about 16% is still relatively small. As for the following evolution, Fig. 4(c), some restoration of the initial texture should be noted, except for the Goss component. This is particularly pronounced for the rotated cube {1 0 0} 0 1 1, as in case of CSS [11], although the Cu, S and brass components also follow the trend. Considering the progressive sheet thickness reduction together with the respective elongation as a kind of cold rolling continuation (the total thickness reduction of 30% after four passes), such a development does not seem surprising. Indeed, during the multi-pass ECADS the axes of reduction and elongation were kept invariant with respect to the sheet unlike the shear plane and direction varying from pass to pass, as noted in Section 2.

6. Microstructure The intragranular misorientations determined with EBSD on longitudinal sections near the mid-plane of the as-received CP Al sheet and after two and four ECADS passes are represented in Fig. 5(a–c), respectively. The shading of a data point is proportional to the misorientation angle δ between the lattice orientation of the data point and the average orientation of the grain containing this data point; white and black accordingly correspond to δ = 0◦ and δ ≥ 10◦ . Grains are defined as areas enclosed by boundaries, which have misorientation angles θ a ≥ 6◦ between adjacent data points. Therefore, interfaces with θ a ≥ 6◦ , which do not end at triple junctions, will not be identified as grain boundaries. The grain boundaries, found with the above convention, are indicated by closed black lines in Fig. 5(a) and by white lines in Figs. 5(b and c). Fragment boundaries are conventionally determined inside grains with a criterion of θ f ≥ 2◦ . Such boundaries, absent in Fig. 5(a), appear in Fig. 5(b and c) as short black segments. Isolated black spots in Fig. 5(a) indicate data points with wrongly indexed diffraction patterns, which have not been taken into account in the calculations. In the sections near the sheet mid-plane there are no clear indications of shearing in the grain shapes after two and four ECADS passes (Fig. 5(b and c), respectively). This might be due to the fact that the shear amount of γ = 0.54 imposed per

Table 2 Relative (to random) intensities of main texture components in CP Al sheet Texture component

As received

Two ECADS passes (ε = 0.62)

Four ECADS passes (ε = 1.25)

Cube Goss Rotated cube Cu S Brass

21.59 3.38 1.26 3.01 6.51 0.56

3.93 1.07 0.27 1.60 3.22 1.09

5.58 0.86 0.78 2.29 4.07 1.68

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Fig. 5. In-grain misorientation maps by EBSD in the longitudinal sections of CP Al sheet in the as-received state (a) and after two (b) and four (c) ECADS passes. The shading corresponds to the misorientation angle from the average orientation of the respective grain. Grain boundaries are indicated in black (a) or white (b and c) and fragment boundaries (θ f ≥ 2◦ ) in black (b and c). The drawing direction is along the long edges of the maps. Table 3 Characteristics of intragranular disorientations (◦ ) averaged over all measured grains in longitudinal sections of CP Al sheet Reference orientations

Disorientation term

Average over whole grains Adjacent data points Adjacent data points with

θ ≥ 2◦

 δ¯   θ¯ a   θ¯ f

pass is only a moderate one. Note for comparison that even after one CSS pass (Φ = 115◦ ) with a notably higher shear of γ = 1.27 distinctly sheared grains were detected only in a surface layer of the processed Al sheet where friction effects took place [11]. For each grain three different magnitudes are calculated to quantify the degree of its subdivision. δ¯ represents the average deviation angle of local orientations from the orientation averaged over the grain, θ¯ a is the average misorientation angle between adjacent data points inside the same grain, and θ¯ f is calculated like θ¯ a with only misorientations larger than 2◦ involved ¯ θ¯ a and θ¯ f , now aver(fragment boundaries). The magnitudes δ, aged over all measured grains prior to ECADS and after its two and four passes, are represented in Table 3. As is evident from Fig. 5 and Table 3, the degree of subdivision   increases   with the number of passes. Indeed, the growth of δ¯ and θ¯ a indicates that a wider spread of local lattice orientations inside the grains is built up with an increasing number of processing steps. Fragmented substructures, absent before the processing, also evolve inside the grains: the misorientations between adjacent fragments grow pass by pass of ECADS, approaching 4◦ after four passes. Furthermore the fractions of adjacent data point pairs with θ a ≥ 2◦ have been calculated inside all grains and averaged over the same data sets. After two and four ECADS passes, respec-

Number of ECADS passes 0

2

4

0.52

3.17

4.92

0.35

0.86

1.35

–

3.61

3.87

tively, they are equal to 7.04% and 15.71%, indicating that the density of fragment boundaries is increasing with the number of ECADS passes. The low-angle character of the considered deformation microstructures is in no way disappointing. Even larger true strains of 1.5 and 3.0 applied at room temperature to Al sheets by CSS [11] and to Al rods by ECAD [7], respectively, resulted only in low-angle boundaries. Similarly, in ECAP with Φ = 90◦ at room temperature [16], deformation-induced high-angle boundaries have not been found in CP Al until four passes, that is ε > 4.0. An important reason for the observed slow rate of fragmentation is that room temperature corresponds in Al to a rather high homologous temperature T/Tm > 0.3, where Tm is the melting point. Generally, grain fragmentation by high-angle boundaries notably accelerates at T/Tm < 0.2, where it becomes the main structure–relaxation mechanism under conditions of progressing plastic deformation [1]. This requirement, automatically satisfied by most metals at 20 ◦ C, would suggest Al to be treated at about −100 ◦ C or lower. 7. Conclusion A device for equal channel angular drawing of sheet metals (ECADS) has been designed and tested at room temperature

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on thin strips of CP Al. The principal deformation mode was simple shear supplemented by some elongation along the drawing direction, inherent to corresponding tensile loads, and the related thickness reduction. Owing to a sufficiently blunt die angle (Φ = 150◦ ), bending was present in insignificant amounts. Trial treatments by ECADS have resulted in microstructures and properties similar to those obtained at comparable strains by CSS [10,12], the most advanced SPD techniques applied to sheet metals as yet. Despite some thickness reduction, ECADS proved to be an efficient method to strengthen Al sheets and to control their textures. A strong recrystallization texture was randomized and the initial yield stress increased by a factor of 1.6 after two ECADS passes with a relatively low thickness reduction of 16%, whereas the accumulated equivalent strain of about 0.64 was mostly due to the simple shear. The initial yield stress was doubled after four ECADS passes with the total equivalent strain of 1.25 at the thickness reduction of 30%. In contrast to ECADS, the same equivalent strain in rolling corresponds to a thickness reduction of more than 80%, although similar microstructures and strengths during rolling are obtained at smaller equivalent strains. The two processes are also essentially different in texture. Since the sheet thickness reduction limits both the equivalent strain per pass and the total strain that can be accumulated in the multi-pass ECADS processing, the latter can hardly provide the rate of grain refinement usually observed in ECAP [15,16] of metal rods. Although these limitations are somewhat stronger than those of CSS [10,12], the very simple design of ECADS and allowance for varying sheet thickness offer other important advantages from the research and, potentially, industrial viewpoints. The absence of high-angle boundaries in the deformation microstructures, obtained in this work after two and four ECADS passes, may be partly attributed to a relatively high homologous temperature of Al at room temperature. In order to estimate the ECADS potential in producing high-angle microstructures with sub-micron crystallites (fragments), further investigations should be undertaken. References [1] V.V. Rybin, Large Plastic Strains and Fracture of Metals (in Russian), Metallurgy, Moscow, 1986.

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