Crystallographic Orientation Distribution Control by Means of Continuous Cyclic Bending in a Pure Aluminum Sheet

Transcription

1 Materials Transactions, Vol. 48, No. 8 (27) pp to 1997 pecial Issue on Crystallographic Orientation Distribution and Related Properties in Advanced Materials #27 The Japan Institute of Light Metals Crystallographic Orientation Distribution Control by Means of Continuous Cyclic Bending in a Pure Aluminum heet Yoshimasa Takayama 1; * 1, Yuji Uchiyama 1; * 2, Tsuyoshi Arakawa 1; * 2, Masakazu Kobayashi 2 and Hajime Kato 1 1 Department of Mechanical ystems Engineering, Utsunomiya University, Utsunomiya , Japan 2 Department of Production ystems Engineering, Toyohashi University of Technology, Toyohashi , Japan Continuous cyclic bending (CCB) has been proposed as a useful straining technique to produce the high strain on the surface layers and the lower strain in the central layer of the sheet. A pure aluminum sheet is worked by the CCB using the Roll traveling device and Roll driven machine. It is found that a sharp orientation is formed by the CCB and subsequent annealing in the pure aluminum sheet. Influences of the CCB/annealing process parameters such as partial or final annealing temperature, strain inside grains and repeating effect, on the sharpening are investigated. The sharper orientation is obtained for the higher temperature of the final annealing after the roll traveling device CCB. While a considerably strong texture appears after one routine of the roll driven machine CCB/773 K-3.6 ks, the texture is weakened after two routines repeated to change into a random texture. The 1 Pass and 2 Pass CCBs with a comparatively low degree of working before partial annealing lead to a high fraction more than 6%. [doi:1.232/matertrans.l-mra2787] (Received February 2, 27; Accepted April 26, 27; Published July 19, 27) Keywords: aluminum, texture, EM/EBP technique, crystallographic orientation, continuous cyclic bending 1. Introduction Recently, necessity of improving resource productivity has been emphasized from an aspect of sustainable development, and the importance of recycling has been considered. Improvement of recyclability in materials requires the simplification of chemical composition, the unification of alloy kinds and non addition of rare or poisonous elements. Therefore, importance to control of production process rather than chemical composition is attached to the development of material. In other words, the microstructural control only by thermomechanical treatment (TMT) without small amount of minor elements should be noted for development of innovative materials or improvement of common commercial materials. One of strategies to control the microstructure by TMT is to make the proper crystallographic orientation distribution. Controlling the crystallographic orientation should be an attractive way for the improvement of properties. In order to improve the capacitance of high voltage capacitors, it is required to increase the surface area of high purity aluminum foils. texture ({1} h1i) is effective to increase the surface area through an etching process, because the etch pits form along crystallographic h1i-direction into the foils. Accordingly, the aluminum foils for the electrolytic capacitors have a strong component of texture. Many studies concerned with a formation of texture were performed widely in the production process from the hot rolled sheets to the final annealed foils up to date. 1 11) It was considered in the previous studies that -oriented nuclei formed during the hot rolling or cold rolling, they became stable grains in partial annealing process, and in final annealing process, the grains were able to grow preferentially with strain-induced grain boundary migration (IBM) driven by difference in stored energy * 1 Corresponding author, takayama@mech.utsunomiya-u.ac.jp * 2 Graduate tudent, Utsunomiya University introduced by additional rolling process. It was mentioned that grains were less affected than other oriented grains by deformation of additional rolling 12 14) or were able to recover earlier than other oriented ones. 12,15) The aim of the present study is to investigate the possibility of crystallographic orientation distribution control by means of continuous cyclic bending (CCB), which has been proposed as a straining technique that can produce the high strain in the surface layer and the low strain in the central part of metal sheet. 16) Further, after CCB and subsequent annealing, a pure aluminum sheet and foil are analyzed by orientation image microscope (OIMÔ) using scanning electron microscopy/electron backscattered diffraction pattern (EM/EBP) technique. 17,18) 2. Experimental Procedure 2.1 amples ample was a 99.99% pure aluminum sheet, which contains following elements: 28 Fe, 33 i, 17, 2 Mn, 1 Cr, 6 Zn and.5 Ti in ppm. The as-received sheet of 1.5 mm thick had been manufactured by the process of casting, hot rolling, cold rolling and annealing. From the sheet, workpiece was machined parallel to rolling direction (RD) with a 2 mm length, 2 mm width and a 1.5 mm thickness. The workpiece was subjected to the continuous cyclic bending (CCB), partial annealing (PA), additional CCB and final annealing (FA). PAed and FAed refer to partially annealed and finally annealed, respectively, below. CCB was performed using the Roll traveling device or Roll driven machine as illustrated in Figs. 1 and 2, respectively. PA was carried out in vacuum of 2.3 kpa with a heating rate of.28 to.35 K/s at a temperature ranging from 463 K to 53 K for 1.8 ks. FA was in salt-bath after CCB by the roll traveling device or in vacuum after CCB by the roll driven machine at a temperature of 673 K to 773 K for 3.6 ks. The preparation process of sample is shown in Fig. 3.

2 Crystallographic Orientation Distribution Control by Means of Continuous Cyclic Bending in a Pure Aluminum heet 1993 Roll 1 φ 22 t= % Aluminum heet Continuous Cyclic Bent (1 5P) Roll 2 18 sheet Partially annealed at K for 1.8ks CCBent (1P) Roll CCBent sheet PAed PA+CCBent (5P) sheet sheet Finally Annealed in 673K, 773K for 3.6ks Fig. 3 The preparation process of sheet sample. Table 1 Analysis area and scanning step Roll 4 amples Analysis Area tep ize Fig. 1 Roll traveling CCB device. Before FA 8 mm 8 mm 5 mm After FA 3 mm 3 mm 1525 mm φ Roll 1 R2=8.5 Roll 2.5 Roll 3 Roll 4 Roll Fig. 2 R1= Crystallographic orientation analysis by EM/ EBP method pecimens were polished to mechanically and electrochemically (HClO 4 : ethanol ¼ 1:8, 7 V, 283 K, 48 to 6 s). Crystallographic orientation analysis was performed using EM/EBP technique by HITACHI -35H equipped with TL Orientation Image Microscopy (OIM) system. Analyzed area and scanning step were selected for the grain size of each sample shown in Table 1. Unclear data points were included partially since results of EBP analysis were affected by cleanness and a roughness on the surface of samples. Therefore, EBP maps were modified by clean-up treatment of the OIM software. Grains are colored by the color-code that is assigned to the texture components of recrystallization after rolling in aluminum, i.e. {112}h111i, {123}h634i, {11}h112i, and {1}h1i orientations. The orientation components were recognized with misorientation within 1 degrees so as to avoid overlap of the components. Grain or area with lower misorientation from a ideal orientation is given a deeper color. Fine and bold lines show low-angle (LAGB) and highangle grain boundaries (HAGB). LAGB and HAGB are φ Advancing direction Roll driven CCB machine. t=1.5 Change in dimensions (%) Length Roll traveling Roll driven Width Thickness mulative true strain, εcum commonly defined here misorientations between two grains as 5 to 15 degrees and more than 15 degrees, respectively. Consequently, we adopt the threshold angle of 5 degrees for grain boundaries. Red lines represent coincidence site lattice (CL) boundaries of 3 to 19b. 3. Results and Discussion 4N Al Fig. 4 Changes in dimensions after continuous cyclic bending by the roll driven machine and the roll traveling device. 3.1 Change in Dimensions by Continuous Cyclic Bending Figure 4 shows change in dimensions after continuous

3 1994 Y. Takayama, Y. Uchiyama, T. Arakawa, M. Kobayashi and H. Kato a b c LAGB HAGB Σ3 19b 5P/673K-3.6ks d RD TD 5P/773K-3.6ks Fig. 5 Experimental results of the FAed sheet after the CCB by roll traveling device: (a) Orientation maps for as-received and annealed samples for 3.6 ks at (b) 673 K, (c) 773 K, and (d) fractions of distribution. 5P refers to 5 Passes of CCB. a b.4 5Pass RD.2 CCB direction Fraction.4 5Pass TD Misorienation degrees Fig. 6 Experimental results of the 5 Pass CCBent sheet: (a) Orientation map (colors show the same orientations as in Fig. 5) and (b) misorientation distribution. cyclic bending (CCB) using both types of the CCB in the pure aluminum workpiece. mulative true strain is taken as abscissa in the figure since true strains per a single pass are not the same for both the CCBs. In the roll traveling CCB, the dimensional change of the workpiece is considerably large as well as in titanium sheet, 19) in particular, increase in length reaches 23.4% at a cumulative strain of Because the increase in length for a high purity (HP) titanium was only 6.9% at the same strain, 19) such dimensional change probably depends on the tensile strength. On the other hand, change in

4 Crystallographic Orientation Distribution Control by Means of Continuous Cyclic Bending in a Pure Aluminum heet P/773K-3.6ks (5P/773K-3.6ks) 2 (5P/773K-3.6ks) 3 (5P/773K-3.6ks) 4 Pole Figure Max Max 16.5 Max Max Fraction of Boundaries 5P/773Kñ3.6ks (5P/773K-3.6ks) 2 (5P/773K-3.6ks) 3 (5P/773K-3.6ks) Fig. 7 Repeating effect of CCB/annealing by the roll driven device. 5P refers to 5 Passes of CCB. (a) (b) 1P /FA 5P/PA+1P/FA 463K 2P/FA 5P/PA+2P/FA 483K 5P/FA 53K 5P/PA+5P/FA Fig. 8 Changes in fractions of primary components for effect of (a) partial annealing (PA) temperature and (b) comparison between with the PA and without. 1P, 2P and 5P and refer to 1, 2 and 5 Passes of CCB, respectively.

5 1996 Y. Takayama, Y. Uchiyama, T. Arakawa, M. Kobayashi and H. Kato length due to the roll driven CCB reaches 8.9% at a strain of even for the pure aluminum. Decrease in width is less than.8% to be regarded as almost unchanged for both of CCBs. Change in thickness seemed to correspond to that in length. The roll traveling CCB lowered thickness by about 2% in a strain of Therefore, CCB by the roll driven machine were closer to an ideal one accompanied with no dimensional change, and plane strain state, which remains in rolling, was almost held in both CCBs. 3.2 Crystallographic orientation distribution control by roll traveling CCB device Effect of FA temperature on microstructure Figure 5 represents the result of crystal orientation analysis as an orientation map on the surface of the sheet specimen FAed at 673 K and 773 K after the roll traveling CCB. The average grain sizes were evaluated for as-received, 673 K- FAed and 773 K-FAed specimens as 51.7 mm, 212 mm and 39 mm, respectively, from the orientation maps. The FAed specimens have about four and eight times larger grain size than the as-received one, due to marked grain growth. There is no difference in grain shape in the microstructures of the three specimens. However, colored grains in the maps indicate different distributions of crystallographic orientation. The typical texture components of recrystallization after rolling in aluminum are displayed as an area fraction in Fig. 5(d). In the as-received sheet, the rolling recrystallization texture appears as expected. While the fraction of oriented grains is a high percentage over 25%, oriented grains have a fraction as low as 2.9%. After CCB and annealing, the fraction of orientation increases up to 17.4%, and that of orientation decreases to 1.8%, in the 673 K-FAed. For the higher temperature FAed at 773 K, the orientation reaches very high fraction of 65.8%, and orientation shows only 1.9%. The grains probably consumed the other orientations, such as orientation by a driving force of the stored strain energy given by CCB. As a result, texture was formed quickly Intragranular Misorientation due to CCB Here, the intragranular misorientation was examined in a 5-pass CCBent sheet specimen. Figure 6 shows the orientation map and the intragranular misorientation distributions in rolling and transverse directions for the CCBent specimen. The orientation map shown in Fig. 6(a) is colored by only primary orientations. The gradation of the color in the map shows large amount of strain inside grains by CCB passes. Local concentration of strain, that is sharp gradation, is found not only near grain boundaries but inside grains. As well as gradation, the thin lines representing the low angle boundary with misorientation of 5 to 15 degrees are observed inside the grains uniformly. However, the thin lines have anisotropy to be parallel to TD, in other words, vertical to CCB direction, which makes clear the strain stored by CCB. The histograms (Fig. 6(b)) show the intragranular misorientation distributions for the and oriented grains in both of RD and TD. The intragranular misorientations were measured along transverse lines in each grain for points with intervals of.5 micrometers, totaled to 1 grains for both orientations. These histograms reveal that the misorientation of grains is concentrated on the classes of small values while those of grains were distributed in the wider range over 6. degrees, especially, in RD. As mentioned above, grains have the small misorientation produced by CCB compared with grains. Because magnitude of the intragranular misorientation corresponds to amount of geometrically necessary dislocations, 2) grains have smaller amounts of stored strain energy. Moreover, a part of the present authors reported that grains were easier to recover than the other grains in rolled samples. 21) Therefore, the reason why grains grow preferentially is probably their recovery in an early stage resulting from the small amounts of the stored energy and ease of recover. We are interested in the fact that the intragranular misorientations of grains are clearly lower than that of the other oriented ones. According to Ridha and Hutchinson 12) the grains have had lower stored energy than the other oriented grains after deformation as explained by Taylor factor. 22) Moreover, It was understood that orientations were able to recover faster than the others were. When the grains are difficult to introduce or easy to release the stored energy, the driving force for grainboundary migration arise from grains to the neighbor other-oriented grains with higher stored energy. 5) Then the grains grow preferentially. This mechanism, which is well known as strain induced grain boundary migrations (IBM), 23) evolve the texture during early annealing stages in the additionally rolled foils. 3.3 Crystallographic orientation distribution control by roll driven CCB machine In this section, control of crystallographic orientation distribution using roll driven CCB machine was investigated. The roll driven CCB is close to an ideal bending process as described above Repeating effect of CCB/Annealing process To give the CCB and the annealing once more to the annealed sample after the CCB is very interesting for the texture evolution in the present samples. Figure 7 represents {111} pole figures and a bar graph showing the fraction of each orientation after repeating routine of CCB/Annealing up to 4 times. After one routine of 5 Pass/773 K-3.6 ks, the fractions of and orientations are 29.7% and 5.7%, respectively. The considerably strong texture has appeared after the first routine. However, as shown in the pole figures, primary orientations including orientation decrease after two routines repeated, and the random texture is formed. The third and the forth routines lead to a little evolution of the primary components, especially orientation. These variations with repeating the routines can be explained as follows. Grain coarsening occurs after the first routine of CCB/annealing, which is accompanied by the formation of texture. In the second routine, the CCB introduce severe strain into coarse grains even with orientation to form new recrystallization nuclei having the orientations other than. Thus, the subsequent annealing produces comparatively fine recrystallized grains with random texture. After the third and forth routines, the

6 Crystallographic Orientation Distribution Control by Means of Continuous Cyclic Bending in a Pure Aluminum heet 1997 primary components evolve in the same mechanism as the first routine. It should be noted that there is a critical grain size of grain to recover and grow up after deformation Effect of partial annealing (PA) It is known that in the industrial manufacturing process of electrical capacitor a texture is developed by cold rolling, partial annealing (PA), additional rolling and final annealing (FA). In this study, the additional rolling was replaced by an additional CCB. Then, how the PA temperature and subsequent CCB/annealing affect microstructure was investigated. Figure 8 indicates the changes in fractions of primary components on the surface of the specimens subjected to 5- pass CCB, PA, additional 1-pass CCB and FA in a pure aluminum sheet. As shown in Fig. 8(a), the fraction of orientation is over 5% for PA at 463 K and 483 K. The fractions of an orientation are 26.3% and 4.8% for both specimens. On the other hand, the fraction of a orientation was a low percentage of 36% at a PA temperature of 53 K. This means that orientations are able to recover faster or at lower temperature than the others. train of oriented grains is released at a low temperature where the marked difference appears in the stored energy between and oriented grains. oriented grains can grow up preferentially compared with oriented grains during FA owing to the difference of the stored energy. Comparing the specimens with and without PA of 463 K- 1.8 ks FA is set as 773 K-3.6 ks. It is found that the number of additional CCB passes after PA affects final fractions of crystallographic orientations. The 1 Pass and 2 Pass CCB with the comparatively low degree of working lead to a high fraction more than 6%. For the 5 Pass CCB with the high degree of working, the orientation occupies a fairly small percentage of 36.8%. This is due to considerably large amounts of strain stored in the oriented grains by the additional 5 Pass CCB. In the 483 K-PAed sheets, a marked difference in the initial grain size between the and the other oriented grains is favorable for the formation of clusters through the foil thickness. The partial annealing at the relatively low temperatures of 463 K and 483 K is helpful to the formation of the large grains and small other oriented grains, because the grains can recover and grow well in this temperatures range. Larger grains can grow preferentially. 4. Conclusions In this study, in order to investigate the possibility of crystallographic orientation distribution control by means of continuous cyclic bending (CCB) and annealing in a pure aluminum sheet and foil have been analyzed by OIM using electron backscattered diffraction pattern (EM/EBP) technique. The conclusions are summarized as follows. 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