Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability by a Combination of Rolling and High-Temperature Annealing

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1 Materials Transactions, Vol. 51, No. 4 (2010) pp. 749 to 755 #2010 The Japan Institute of Light Metals Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability by a Combination of Rolling and High-Temperature Annealing Masahide Kohzu, Kenji Kii* 1, Yuki Nagata* 2, Hiroyuki Nishio* 3, Kenji Higashi and Hirofumi Inoue Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai , Japan To improve the cold formability of AZ31 magnesium alloy sheets, we investigate texture control by and annealing. The texture ideal for forming close to random orientation was obtained by annealing at K before and after isothermal at 298 K. For the randomizing process, such a high temperature in pre-annealing was essential, whereas a slightly lower temperature was acceptable for final annealing, assuming a sufficiently long annealing time. The randomized sheet could be obtained in a wide range of temperatures and reductions. It could also be produced easily with a standard mill without roll heating. The microstructure of the randomized sheet consisted of relatively homogeneous grains 25 mm large on average. In a 90 degree V-bending test, a well randomized sheet could be bent without cracking with a minimum bending radius per thickness R=t ¼ 1:4, which was about half of that in commercial AZ31D-O sheets, in spite of manganese content of over 0.6%. [doi: /matertrans.l-m ] (Received July 3, 9; Accepted December 14, 9; Published March 3, 2010) Keywords: AZ31 magnesium alloy, texture randomization, isothermal, high-temperature annealing, cold formability, V-bending 1. Introduction Magnesium alloys with high specific strength are already being used extensively as structural materials for portable electronics. Potential application to transportation devices, especially to automotive parts, is now attracting attention with the goal of reducing CO 2 production. 1) Most magnesium alloy structures are produced by die-casting or thixomolding even for shell structures at present, although sheet forming via had been expected to improve productivity and toughness early on. To replace magnesium casting or aluminum pressing with magnesium pressing, it is indispensable to lower the forming temperature preferably to room temperature. Cold formability depends above all on texture. 2,3) In a rolled sheet, a basal texture, in which {0001} planes are oriented parallel to the rolled surface, often forms and consequently seriously impairs cold formability. Rolling with additional shearing strain to tilt the basal planes has been tried in order to control the texture. However, it did not lead to a significant improvement of cold formability. 4 6) Some authors have reported that commercial AZ31 rolled sheets with a double-peak texture had excellent low-temperature formability, 2) although the cold forming was hard. However, such a sheet was confined to a subset of narrow coiled strips. Thus, the process of texture evolution during deformation was systematically investigated by isothermal eccentric roll drawing as simulation. 7) Double peak textures were obtained at K with reductions of 22% and above. About the cold formability, there has been a report on AZ31 sheets annealed at high temperature ( K) being drawn with a drawing ratio of ,9) In this study, texture control for improving the cold formability of AZ31D magnesium alloy sheets has been investigated, via a combination of isothermal and high-temperature annealing. We have also studied the change in the microstructure in each procedure. In addition, cold formability has been evaluated by a V-bending test. This study is based on the idea that annealing is indispensable for sheet forming and final textures are not determined by plastic processing only. 2. Experimental Procedure Figure 1 shows the scheme of the eccentric-roll drawing 7) (hereinafter called isothermal ), in which reduction changes continuously, and the rolls can be heated uniformly in a furnace together with the specimen. A commercial AZ31D-O rolled sheet 1.6 mm thick was used as a specimen after being cut into 40 mm 2 rectangles along the direction. Table 1 shows the chemical composition of the sheet. The specimens were processed first by pre-annealing, by isothermal and then by final annealing under the conditions shown in Table 2. During the annealing, specimens were covered with aluminum foil to prevent oxidation. Annealing above K Test piece after drawing Fracture by tension * 1 Graduate Student, Osaka Prefecture University, Present Address: SRI Sports Limited, Kobe , Japan * 2 Graduate Student, Osaka Prefecture University * 3 Undergraduate Student, Osaka Prefecture University, Present Address: Graduate Student, Osaka University, Suita , Japan Fig. 1 The scheme of eccentric-rolls drawing.

2 750 M. Kohzu et al. Table 1 The chemical composition of the specimen. (mass%) Al Zn Mn Si Cu Fe Ni Mg Bal. Table 2 The positive combinations of conditions in the texture cont process. (a) Rolling temperatures for focused pre-annealing conditions. Pre-annealing K K Final-annealing min 1 h 473 K 5 min 473 K 298, 393, 423, 473 K K min 298, 473 K 298, 473 K 693 K 24 h 473 K 723 K min, 1 h 473 K 10 sec, 20 sec 473 K sec, 1 min 298, 473 K K 2 min, 4 min 473 K 20 min 298, 393, 473 K 298, 393, 423, 473 K 1 h 473 K 473 K (b) Rolling temperatures for other pre-annealing conditions. Pre-ann. 693 K 723 K K Final ann. 24 h 1 h 2 h min 473 K 5 min 473 k 473 k 473 k K 20 min 298, 393, 473 K 298, 393, 473 K 473 k 473 k Standard reductions for each temperature: 298 K-4, 7%; 393 K-4, 11, 22%; >423 K-4, 11, 22, 32%. Especially, 298 K-4, 7, 9, 11%; 423 K-4, 7, 11, 16, 22, 27, 32%. was not tried to avoid ignition and self-weight deformation. Isothermal was performed at 100 mm s 1 using the eccentric-roll drawing apparatus. Additionally, we carried out with a standard mill without heating the rolls at the same rate for specimens preheated at 523,, 623 and 673 K. The microstructures and {0001} pole figures of the rolled surfaces were examined at each stage of the process. The textures were analyzed by Schulz reflection method using a copper X-ray tube operating at 40 kv and 100 ma. The normalized scan tilt angle range was 075 degree around the normal of the rolled surface. The diffraction intensity was normalized to unity for magnesium powder. Specimens for microstructures observation were etched with acetic picral after buffing, and the average grain size was determined by the linear intercept method. The samples after were taken from locations on the tapered specimen mainly corresponding to 4, 11, 22 and 32% reduction. The details of the experimental methods omitted here are identical to those in our previous paper. 7) Test pieces for 90 degree cold V-bending were made up into 20 (RD) mm 3 plates by cutting and grinding from the thickness tapered samples following texture measurements. Punches with tip radii R shown in Table 3 were prepared, and a bending limit R=t was evaluated by a strict criterion as shown in Fig. 2. The test was conducted at 0.17 mm s 1 and was stopped when the bending load reached about five times its plateau level. Table 3 The tip-radii of punches and their ratios to sheet thickness, t=r,in V-bending (t ¼ 0:9 mm). R /mm t/r R /mm t/r Results and Discussion 3.1 Discovery of texture randomizing phenomenon The texture randomization, in AZ31 was first discovered by annealing at K for 1 h before and after isothermal at 473 K. Later, it was found that the final annealing for 20 min was sufficient and preferable for texture randomization. Requirements for texture randomization were investigated around this processing condition hereinafter referred to as the standard randomizing process. A texture of as-received sheet is shown as a {0001} pole figure in Fig. 3 together with its RD ( direction) section and optical microstructure. The pole figure exhibits a typical basal texture, consisting of concentric rings strongly oriented toward its center (0 degree). The microstructure consists of equiaxial crystal grains about 12 mm large on average. 3.2 Conditions for texture randomization Figure 4 shows changes in {0001} pole figures, their RD sections and optical microstructure by pre-annealing, isothermal and final annealing. The left and the right columns correspond to pre-annealing at K for min and at K for 1 h, respectively. In either column, was performed at 473 K and final annealing at K for 20 min. The right column corresponds to the above-mentioned t R R/t = Fig. 2 An example of the success/failure criterion in V-bending ( : rough surface stopping short of cracking. In this case, the bending limit is evaluated as R=t ¼ 4:2). Intensity Fig. 3 The {0001} pole figure, its RD-section and microstructure of the received AZ31D rolled sheet.

3 Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability Pre-annealing at K for 1 h After pre-annealing Pre-annealing at K for min (A) before (a) before (0 %) Intensity 8.9 After at 473 K (f) 4 % Intensity 7.9 After final annealing at K for 20 min Intensity 7.3 Intensity 7.3 Intensity 6.4 Intensity 2.7 (H) 22 % (i) 32 % Intensity 12.1 (G) 11 % (h) 22 % Intensity 11.0 Intensity 6.6 (F) 4 % (g) 11 % Intensity 10.6 (E) 32 % (e) 32 % Intensity 7.5 Intensity 9.5 (D) 22 % (d) 22 % Intensity 6.4 (C) 11 % (c) 11 % Intensity 8.2 Intensity 12.9 (B) 4 % (b) 4 % Intensity Intensity 3.6 (I) 32 % Intensity 4.7 Fig. 4 The {0001} pole figure, its RD-section and microstructure after pre-annealing, and final annealing (effect of hightemperature annealing).

4 752 M. Kohzu et al. standard randomizing process. Low-temperature pre-annealing at K causes little change of texture and microstructure, as shown in Fig. 4(a). However, high-temperature pre-annealing at K produces a stronger basal texture and a coarse grain microstructure containing abnormally grown grains, as shown in Fig. 4(A). By under high reduction, the texture formed by the high-temperature preannealing changed to a more pronounced double-peak texture, as shown in Fig. 4(D) (E), than that formed by the low-temperature pre-annealing, as shown in Fig. 4(d) (e). For the low-temperature pre-annealed sheet, during dynamic recrystallization progresses from the sites of deformation twins formed under low reduction. The twins disappear under high reduction, as shown in Fig. 4(b) (e). For the high-temperature pre-annealed sheet, dynamic recrystallization does not occur during, and deformation twins are ever-increasing with an increase in reduction, as shown in Fig. 4(B) (E). By the final high-temperature annealing, only samples rolled with medium reductions of 11 22% after high-temperature pre-annealing are randomized with a significant reduction of peak intensity. For either pre-annealing condition, the final annealing produces a homogeneous grain structure of 25 mm, as shown in Fig. 4(f) (i) and (F) (I) without mixing of abnormally grown grains as in Fig. 4(A). Based on the standard randomizing process shown in the right column of Fig. 4, the pre-annealing, isothermal and final annealing conditions are changed one by one. A degree of randomization after final annealing is shown in Fig. 5 as peak intensities of the pole figures, where the number in parentheses is the average grain size after preannealing (except abnormally grown grains). As shown in Fig. 5(a), pre-annealing up to 723 K does not lead to final texture randomization even if it is prolonged. Pre-annealing for min is insufficient for final randomization even at K. In addition, pre-annealing at 693 K for 24 h is adequate to dissolve tiny amounts of -phase (Mg 17 Al 12 ) precipitated at grain boundaries. Grain size after this preannealing was comparable to that at K for 1 h, and was larger than that at K for min. However, the final peak intensity was higher than that in the case of pre-annealing at K for either pre-annealing time. From this it follows that formation of a coarse grain structure does not always lead to a final texture randomization. As shown in Fig. 5(b), temperature has little effect on the final peak intensity, which suggests that precise temperature control is not required during. As shown in Fig. 5(c), final annealing as low as 693 or 723 K can reduce peak intensity to almost the level reached by annealing at K for 20 min, but a lot more time is required at lower temperature. At K, annealing times greater than 20 min do not improve the randomization. Figure 5(a) (c) shows that final peak intensities are low at medium reductions of 11 22%. It does not matter for randomization whether the texture becomes double peak or not, since the threshold is between 11 and 22%. The wide range of conditions for final randomization suggests the potential applicability of standard to this process. Table 4 shows the effect of pre-heating temperature on final peak intensity in standard, without roll heating. The rate is 100 mm s 1 which is Intensity, I Intensity, I Intensity, I (a) (b) (c) 693K-24h(26) 723K-1h(21) 723K-2h(25) K-min(21) : 298K : 393K : 423K : 473K 723K-min 693K-24h 723K-1h Effect of pre-annealing ( ) : grain size, d/µm K-1h(27) Effect of Effect of final annealing K-20min Rolling reduction, r (%) Fig. 5 The effect of experimental conditions in each procedure on the final peak intensity (based on pre-annealing at K for 1 h, at 473 K and final annealing at K for 20 min). Table 4 The effect of pre-heating temperature and reduction on final peak intensity in standard without roll heating (pre-annealing at K for 1 h and final annealing at K for 20 min). Temperature Reduction 523 K K 623 K 673 K 17.5% % the same as in the isothermal. Rolling at pre-heating temperature between 523 and K leads to low final peak intensities comparable to those of the standard randomizing process using isothermal. However, excess preheating at 673 K creates a strong basal texture after final annealing, although dynamic recrystallization does not occur during. A high-temperature annealed sheet has better low-temperature rollability than a normally annealed sheet. 3.3 Discussion of texture and microstructure Figure 6 shows changes of texture and microstructure in the process for which pre-annealing in the standard randomizing process is replaced with 24 h at 693 K. This preannealing serves as a solution treatment as described above. In the pre-annealed state of Fig. 6(a), the texture, existence of

5 Pre-annealing for 24 h at 693 K Texture Randomization of AZ31 Magnesium Alloy Sheets for Improving the Cold Formability (a) Rolling with 22 % (a) before Intensity 12.6 Rolling at 473 K Intensity 5.5 (c) 22 % Fig. 7 The {0001} pole figure, its RD-section and microstructure after preannealing at 693 K for 24 h, at 398 K and final annealing at K for 20 min. Intensity 7.6 (d) 11 % Final annealing at K for 20 min Intensity 7.6 (b) After Final annealing (b) 11 % Intensity Intensity 6.3 (e) 22 % Intensity 5.5 Fig. 6 The {0001} pole figure, its RD-section and microstructure after preannealing at 693 K for 24 h, at 473 K and final annealing at K for 20 min. abnormally grown grains and average grain size of normally grown grains differ little from those in the state of Fig. 4(A) pre-annealed at K for 1 h (which is sufficient for randomization). However, in Fig. 6(a), subsequent causes dynamic recrystallization, shown in Fig. 6(b) (c), and the final texture is a concentric ring (basal texture), as shown in Fig. 6(d) (e). This transition is similar to pre-annealing at K for min shown in the left column of Fig. 4. but with lower peak intensities. Generally, in AZ31 magnesium alloys with coarse grains, twins play a major role in a plastic deformation. For a sheet pre-annealed at or below 723 K, deformation twins disappear by being replaced with fine dynamically recrystallized grains. In general, a larger strain is required for dynamic recrystallization at lower temperatures,10) so only temperature was lowered to 398 K in the process shown in Fig. 6. The result is demonstrated in Fig. 7. In the microstructure after with 22%, many deformation twins are produced without dynamic recrystallization (see Fig. 7(a)). Although this microstructure is similar to that in the standard randomizing process (right column of Fig. 4), the final texture becomes a concentric ring (Fig. 7(b)) like those of Fig. 6(d) (e). The microstructures of Figs. 4(D) and 7(a) which contain many deformation twins are magnified and compared in Fig. 8. In both cases, thin and partially-crossed twins were mainly observed. The yet-unidentified type of the twins, may be mainly the twin of f101 2g h101 1i, since these play a major role in plastic deformation with a limited number of slip systems.11,12) Figure 9 shows the variation of the RD section of {0001} pole figure as a function of final annealing condition, where pre-annealing and conditions are as in the standard randomizing process. Even for final annealing at 723 or 693 K, the texture may be randomized. Nevertheless, K is highly favorable since there randomization progresses much faster than up to 723 K. We note that annealing longer than the 20 min in the standard randomizing process has no effect. The above-mentioned requirements for final texture randomization can be summarized as follows. (1) Hightemperature pre-annealing at K (1 h) is absolutely necessary. (2) High-temperature pre-annealing makes the grain-structure coarse and leads to a microstructure containing many deformation twins without dynamic recrystallization in the subsequent process. (3) Coarse grain structure and twinning which do not evolve into dynamic recrystallization are not sufficient. (4) The range of successful conditions (temperature and reduction) is wide, hence standard mill without heating the rolls should also be successful. (5) Although high-temperature final annealing at K is not always necessary, it leads to a randomization much faster than up to 723 K. 3.4 Correlation between peak intensity and bendability Although the excellent formability of a randomized sample was exhibited by 90 degree V-bending test, a statistical correlation between the V-bending limit and the peak intensity of a pole figure had not still been confirmed down to such a low intensity level, since such AZ31 sheets do not exist. By using many samples obtained in search of satisfactory randomizing conditions, we can plot the correlation diagram as in Fig. 10. The test pieces were ground down to a thickness of 0.9 mm. The V-bending limit, R=t, shows a good correlation as a whole with peak intensity,

6 754 M. Kohzu et al. (a) Magnified microstructure of Fig. 4 (D) 10 µm 10 µm (b) Magnified microstructure of Fig. 7 (a) Fig. 8 The comparison between magnified microstructures of Fig. 4(D) and Fig. 7(a). (a): Rolling at 473 K with 22% after pre-annealing at K for 1 h. (b): Rolling at 393 K with 22% after pre-annealing at 693 K for 24 h. As rolled K min 693 K 24 h 723 K min 723 K 1 h K s K 1 min 11 % 22 % Fig. 9 The variation in the RD-section of {0001} pole figure due to the final annealing ( at 473 K after pre-annealing at K for 1 h). I. The excellent bendability of R=t < 2:0 is limited to the randomized samples of I < 4:5. But in samples with higher intensity, the R=t distribution is wide because it is affected not only by peak intensity of the pole-figure but also by its pattern. 4. Conclusion Texture randomization of magnesium alloy sheets was achieved with a standard commercial wrought alloy, AZ31, within the standard process without the use of special apparatus or technique. In this process, high-temperature annealing at K before and after the final pass significantly weakens the basal orientation of sheet. This results in considerably improved cold-formability. The allowed range of conditions is wide, therefore industrial mass production is possible given demand. However, the mechanism of this transition is still poorly understood and many questions remain to be answered. In the future, multiple verification of formability in view of the final practical application is required, in parallel with the exhaustive analysis of the mechanism. Acknowledgments We express our deepest appreciation to Professor Mototsugu Katsuta of Nihon University and Mitsubishi Aluminum Co., Ltd. for their research cooperation.

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