- Email: [email protected]
Improving the room temperature stretch formability of a Mg alloy thin sheet by pre-twinning
Materials Science & Engineering A 655 (2016) 1–8
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Improving the room temperature stretch formability of a Mg alloy thin sheet by pre-twinning Weijun He a,n, Qinghui Zeng a, Huihui Yu a, Yunchang Xin a, Baifeng Luan a, Qing Liu a,b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 October 2015 Accepted 21 December 2015 Available online 22 December 2015
{10 12} tensile twins were introduced by in-plane compression in a thin magnesium alloy sheet in order to tailor the texture. The room temperature stretch formability of the pre-twinned Mg alloy sheet was remarkably improved (50%). The activation of the tensile twinning in the pre-twinned region, which can effectively accommodate the through-thickness strain during stretch forming, is assumed to be the main reason of stretch formability improvement. Numerical simulation results demonstrate that multidirectional tensile twins induced by multi-steps compression are more beneficial to improve the formability of Mg alloy sheet, compared with uni-directional twins. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Thin sheet Stretch formability Twinning texture
1. Introduction It is accepted that basal slip is the most active slip system in Mg alloy, thus leading to intense basal texture in rolled Mg alloy sheet [1,13]. In a strong basal texture, the strain along the thickness direction need to be accommodated by secondary deformation modes during low temperature stretch forming of Mg sheet, in which the material often undergo in-plane tensile stress [4]. This causes the poor stretch formability of rolled Mg alloy sheet at room/low temperature. It was found that weakening and/or tilting the basal texture can effectively improve the formability of rolled Mg alloy sheet [12,21,5]. Recently, many processes were proposed to tailor the texture of rolled Mg alloy sheets without changing the chemical components, e.g., cross rolling [6,7], asymmetric rolling [19], repeated unidirectional bending [22], equal channel angular rolling [3] and high temperature rolling [10,11]. Essentially, these approaches attempt to control the texture by activating non-basal slips or altering the effect of basal slip by strain path changes or shear strain. Compared with slips, twinning can remarkably change the texture of Mg alloy within small strain. For example, the orientation of {10 12}o 1011 4 twin is rotated by about 86° and that of {10 11}o 1012 4 twin by about 56° from the matrix [2,9]. Thus, pre-inducing twining is considered as an effective method to control the texture and enhance the formability of Mg alloy sheet [16]. Xin et al. [20] found that the limited reduction ratio can be n
Corresponding author. E-mail address: [email protected] (W. He).
http://dx.doi.org/10.1016/j.msea.2015.12.070 0921-5093/& 2015 Elsevier B.V. All rights reserved.
obviously increased by performing pre-rolling along the transverse direction (TD). Park et al. [15] reported that the formability can be remarkably enhanced by pre-induced {10 12} twin. The results of Song et al. [17] indicated that pre-contraction twins combined with recrystallization annealing may weaken the basal texture of rolled Mg alloy sheet. Tailoring the texture using twinning show many advantages over other methods based on dislocation slip due to its effectiveness and easy operation. However, its applicability in thin Mg alloy sheet (thickness o 4 mm) has not been much explored yet. In this work, in-plane pre-compressions were performed on thin Mg alloy sheet in order to weaken the basal texture by activating {10 12} twinning. The stretch formability of the pre-deformed Mg alloy sheets were measured by Erichsen test, followed by microstructure characterizations and analysis of active deformation modes.
2. Material and methods The material used in this work is a commercial hot rolled AZ31( Mg–3%Al–1%Zn) Mg alloy sheet 2.2 mm thickness. For in-plane pre-compression (IPC), square sheets with a dimension of 64 mm 64 mm 2.2 mm were machined by electrical discharge wire-cutting. With the aid of a channel die, the IPC was conducted to introduce {10 12} twins, as schematized in Fig. 1. In each IPC process, three pieces of square sheet were deformed at the same time. It is worth noting that 0.4 mm thick Teflon (PTFE, polytetrafluoroethylene) films were placed between the specimens and the channel die in order to reduce friction. During
2
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 1. Schematic of in-plane compression of AZ31 Mg alloy sheet.
compression, the deformation of the Teflon films can accommodate the deformation of Mg sheet along the thickness direction so that the material will not endure plane strain condition. The channel die aims to avoid bulking. Two types of IPC samples were prepared in our work. In the first case, the IPC was along the transverse direction (TD) and samples were compressed to 2.5% strain, denoted as IPC-T samples. In the second case, the samples go through two steps of pre-compression. Firstly, the IPC was performed along TD with a strain of 2.5%. Then, the specimens were rotated 90° about the normal direction (ND) and further compressed along the rolling direction (RD) with a strain of 2.0%, resulting in as-called IPC-TR samples. The pre-compressed samples were annealed at 260 °C for 1 h to eliminate the dislocation induced hardening. Additionally, in order to remove the effect of annealing on the stretch formability, the as-received sheet was also annealed at the same condition as the IPC samples, and the annealed sheet is denoted ‘as-received sample’ in our work. Erichsen tests were carried out to determine the stretch formability of the magnesium sheets using a hemispherical punch with a diameter of 20 mm at a punch speed of 0.1 mm/s at room temperature. Tests were repeated three times for each type of sample. The microstructure before and after Erichsen test were examined using Electron backscattered diffraction (EBSD) on a FEI Nova 400 scanning electron microscope. The deformation mechanism of the magnesium sheets during Erichsen tests was analyzed by the combination of Finite element simulations and crystal plasticity calculations.
3. Results 3.1. Erichsen test results The results of Erichsen tests are displayed in Fig. 2. Erichsen value (IE) is defined as the depth of the dome at fracture initiation, which is also equivalent to the punch displacement at the maximum loading force on the load–displacement curve during Erichsen test, as shown in Fig.2b. The average Erichsen value are 3.8 mm, 5.1 mm and 5.7 mm for the as-received, IPC-T and IPC-TR samples, respectively. Compared with the as-received sheet, Erichsen values are obviously increased by 34% and 50% for the IPC-T and IPC-TR samples, respectively. That means the stretch formability of the AZ31 sheet at room temperature has been significantly improved by the IPC process. From Fig. 2b, it is interesting to note that a lower force is needed for the pre-compressed samples compared with the as-received sample at same displacement. However, higher force is supposed to be needed for the pre-compressed samples as a result of the thickening during IPC process. Lower force during Erichsen test implies that the deformation mechanisms were different in the pre-compressed
Fig. 2. Erichsen tests results: (a) typical profiles of deformed as-received sample, IPC-T sample and IPC-TR sample, and (b) typical load–displacement curves during Erichsen test.
samples. Before fracture, the load–displacement curves of the IPCT sample and the IPC-TR sample are very close to each other. 3.2. Microstructure before Erichsen test Fig. 3 shows the microstructure and the {0001} pole figures of the as-received sample, the IPC-T sample and the IPC-TR sample before Erichsen test. It is observed that the grains of the precompressed samples grew compared with the as-received sample. The as-received sample and the IPC-TR sample have a twin-free equiaxial grain structure while some twin lamellas can be found in the IPC-T sample. {0001} pole figures indicate that the as-received sample has a typical basal texture with the c-axis almost parallel to the normal direction (ND). Many c-axis of grains align with TD in the IPC-T sample, while for the IPC-TR sample some c-axis are close to TD and some c-axis are close to RD due to its two steps of pre-compression. The re-orientation of c-axis by about 90° is the evidence of {10–12} tensile twinning. No obvious twin lamellae are observed in the IPC-TR sample because of recrystallization during the annealing treatment, which is favored by its high stored strain energy (2.5% strain along TD and 2% compression along RD). The presence of very fine grain, visible in Fig. 3c, also supports this supposition. The recrystallized grains originating from the twins form some regions of grains with orientations close to the twins. Referring to their orientation, these recrystallized grains can be treated as twinned regions. On the other hand, only grain growth happened in the IPC-T sample due to its relative lower strain energy storage (2.5% strain along TD), thus some twin lamellas (twinned regions) are still visible. 3.3. Microstructure after Erichsen test Fig. 4 presents the microstructure and {0001} pole figures of the as-received sample, the IPC-T sample and the IPC-TR sample after Erichsen test. The microstructure was observed at the dome, as schematic illustrated in Fig. 4. Compared with the observation before Erichsen test, Fig. 4a shows that there is no obvious change in the microstructural characteristics for the as-received sample, including the grain size and the texture. By contrast, the IPC-TR sample displays significant change after Erichsen test, as shown in Fig. 4c. Many twin lamellas appeared. Most of the twins have been
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
3
Fig. 3. EBSD measurements showing the orientation maps, boundary information, {0001} pole figures before Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
confirmed as tensile twins according to the misorientation information. Additionally, the {0001} pole figure indicates that the texture revert to that of the as-received sample (Fig. 3a), which can be caused by the activation of tensile twinning during Erichsen test. In the IPC-T sample, some tensile twin lamellas are observed, which may remain from the sample or were newly created during the Erichsen test. From the texture change observed on the {0001} pole figure of the IPC-T sample, we conclude that some tensile twinning was activated. 3.4. Simulated results To further explore the mechanisms enhancing the stretch formability, numerical simulation was carried out to investigate the contribution of different deformation modes during the Erichsen test. Finite element method (FEM) was used to simulate
the Erichsen test. Isotropic Mises model was used in the FEM simulation. The FEM simulations are shown in Fig. 5, where the geometry and size of the sheet and the die are the same as the experimental ones. The sheet is discretized with elements of C3D8R type. For the as-received sample, the IPC-T sample and the IPC-TR sample, the FEM simulation was stop when the punch displacement reached 3.7 mm, 5.1 mm and 5.7 mm, respectively. The equivalent plastic strain (denoted as PEEQ in the figure) distribution of various samples after Erichsen test are shown in Fig. 5. It indicates that a maximum of 0.19, 0.26 and 0.29 strain is achieved for the as-received sample, the IPC-T sample and the IPCTR sample, respectively. The loading path of the element at the dome was exported to carry out meso-scale analysis. The visco-plasticity self-consistent (VPSC) model was used to analyze the deformation mechanism of AZ31 sheet during Erichsen test. The detail information on the VPSC model can be found
4
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 4. EBSD measurements showing the orientation maps, boundary information and {0001} pole figures after Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
elsewhere [14,18]. The parameter values of the VPSC model fitted for the AZ31 material in a previous study [8] are used here since we work on materials with same chemical composition and similar microstructure. Basal slip, prismatic slip, pyramidal slip and tensile twinning are assumed to be potential deformation modes. The grain size difference between the as-received material and the pre-compressed material was ignored in modeling. Only the difference on texture was considered. Simulated results are shown in Fig. 6. It is found that the basal slip is the main deformation mode at the beginning of deformation for all samples. This may be due to the fact that the basal plane is not ideally perpendicular to the ND direction of the sheet, even for the as-received sample as displayed in Fig. 3. With the increase of strain, the contribution of the pyramidal slip increases gradually. As shown in Fig. 6(b) and (c),
tensile twinning is obviously activated below 0.06 strain. For the IPC-T sample, the contribution of the tensile twinning is about 18% while it is more than 23% in the IPC-TR sample.
4. Discussions As mentioned earlier, for the IPC-T and the IPC-TR sample before Erichsen test, the material is a mixture of twinned and untwinned regions. In the twinned region, the c-axis of the twins were rotated toward the loading direction, i.e. TD or RD. As the biaxial tensile stress is the main stress state during Erichsen test, the material is subjected to tensile loading along the c-axis in the pre-twinned region, which is the favorite state for the activation of
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
5
Fig. 5. FEM model and simulation results of magnesium alloy sheet during Erichsen test. (a) FEM model and equivalent plastic strain distribution after Erichsen test of (b) asreceived sample, (c) IPC-T sample, and (d) IPC-TR sample.
the tensile twinning. The tensile twinning that happen during Erichsen test is called ‘re-twinning’ in the following discussion in order to distinguish it from the pre-twinning in IPC process. The re-twinning can accommodate the thinning of the sheet during Erichsen test and thus it helps to enhance the stretch formability
at room temperature. The present results are very different from the one reported by Park et al. [15], where the de-twinning played a key role. Compared with the IPC-T sample, much more twin lamellas appeared in the IPC-TR sample. This may be related to the
6
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 6. Contribution of the deformation modes to the deformation as a function of the equivalent strain during Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
orientation of the c-axis. In the IPC-TR sample, the c-axis of the twinned regions are close to two directions (TD and RD), which facilitate the activation of tensile twinning under biaxial tension, thus leading to a higher Erichsen value than that of IPC-T sample. Another reason, which may explain the phenomenon of more lamellas in the IPC-TR sample, is that these samples have more pre-
Fig. 7. Pole figures of different virtual samples: (a) IPC-T þ 2.0% TD (VPSC), (b) asreceivedþ 6.0% TD (VPSC), and (c) as-receivedþ3.0% TD þ 3.0% RD (VPSC).
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
7
IPC-TR sample with the same amount of pre-strain. However, we used crystal plastic modeling as a first step to explore the material behavior in the current work. To explore the mechanism that causes more twin lamellas in the Erichsen tested IPC-TR samples, three kinds of virtual experiments were carried out using the VPSC. In the first case, based on the texture of IPC-T sample, 2.0% compression along TD was simulated by the VPSC. In the second case, based on the texture of as-received sample, 6.0% compression along TD was conducted. In the third case, 3% compression strain along TD and 3% strain along RD was carried out, also on the base of the texture of as-received sample. The resulting simulated textures, shown in Fig. 7, were saved and used as virtual samples for Erichsen tests. Then, the activity of deformation modes of these virtual samples during Erichsen test were simulated by the VPSC, and displayed in Fig. 8. The IPC-T þ2.0% TD sample and the IPC-TR sample are considered to have endured same equivalent pre-strain (and so have the same amount of pre-twinned regions). The difference is within the orientation of c-axis and texture component intensities. By comparison of Figs. 6c and 8a, it is observable that the IPC-TR sample with the c-axis tilting close to two directions have more re-twinning activation than that of IPC-T þ 2.0% TD sample whose c-axis align with one direction. It is also true when the samples have greater pre-strain (6% strain), as shown in Fig. 8b and c. Although the amount of pre-strain is the same, the twinning activity during Erichsen test is different. Having some twinned regions along RD and TD (Fig. 7c) allows more twinning during the test than having all the twinned regions along TD (Fig. 7b). We can conclude that the texture containing c-axis of twinned regions aligned with two directions is more suitable for the activation of tensile twinning under biaxial stress state than that with twins along one direction.
5. Conclusion The strong basal texture of thin magnesium alloy sheet (2.2 mm) can be successfully tailored by {10 12} tensile twin introduced by in-plane compression followed by recrystallization. This kind of texture control based on the {10 12} tensile twin can be effectively used to enhance the stretch formability of magnesium alloy thin sheet at room temperature. The crystallographic orientation of the pre-twinned region is favorable for re-activation of tensile twinning in subsequent stretch forming. The re-twinning can help to accommodate the through-thickness strain, thus leading to a better stretch forming ability. Additionally, numerical simulation results indicate that multidirectional tensile twins induced by multi-steps compression can further improve the stretch formability compared with single directional tensile twin caused by single-step compression.
Acknowledgment
Fig. 8. Contribution of the deformation modes to the deformation as a function of the equivalent strain during Erichsen test: (a) IPC-T þ2.0% TD (VPSC) virtual sample, (b) as-receivedþ 6.0% TD (VPSC) virtual sample, and (c) as-receivedþ 3.0% TD þ 3.0% RD (VPSC) virtual sample.
twinned regions caused by large pre-strains. Pre-compression to a larger pre-strain was attempted: 4.5% pre-compression was performed on the thin sheets along one direction but bulking and heterogeneous deformation occurred in the current channel die. So it has not been possible to compare the IPC-T sample and the
The authors express their sincere thanks to the support by the National Basic Research Program of China (“973” Project, Grant nos. 2013CB632201 and 2013CB632204) and National Natural Science Foundation of China (Project 51421001). This work has also been funded by Chongqing Natural Science Foundation, No. cstc2014jcyjA50022 and Fundamental Research Funds for the Central University with Project no. 106112015CDJXY130002.
References [1] S. Agnew, C. Tomé, D. Brown, T. Holden, S. Vogel, Study of slip mechanisms in a magnesium alloy by neutron diffraction and modeling, Scr. Mater. 48 (2003)
8
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
1003–1008. [2] M.R. Barnett, Twinning and the ductility of magnesium alloys: part II. “Contraction” twins, Mater. Sci. Eng. A 464 (2007) 8–16. [3] Y.Q. Cheng, Z.H. Chen, W.J. Xia, T. Zhou, Improvement of drawability at room temperature in AZ31 magnesium alloy sheets processed by equal channel angular rolling, J. Mater. Eng. Perform. 17 (2008) 15–19. [4] Y. Chino, K. Kimura, M. Mabuchi, Deformation characteristics at room temperature under biaxial tensile stress in textured AZ31 Mg alloy sheets, Acta Mater. 57 (2009) 1476–1485. [5] Y. Chino, J.-S. Lee, K. Sassa, A. Kamiya, M. Mabuchi, Press formability of a rolled AZ31 Mg alloy sheet with controlled texture, Mater. Lett. 60 (2006) 173–176. [6] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Enhanced formability at elevated temperature of a cross-rolled magnesium alloy sheet, Mater. Sci. Eng. A 441 (2006) 349–356. [7] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Stretch formability at elevated temperature of a cross-rolled AZ31 Mg alloy sheet with different rolling routes, Mater. Sci. Eng. A 473 (2008) 195–200. [8] W. He, B. Luan, R. Xin, J. Xu, Q. Liu, A multi-scale model for description of strain localization in friction stir welded magnesium alloy, Comput. Mater. Sci. 104 (2015) 162–171. [9] S.-G. Hong, S.H. Park, C.S. Lee, Role of {10 12} twinning characteristics in the deformation behavior of a polycrystalline magnesium alloy, Acta Mater. 58 (2010) 5873–5885. [10] X. Huang, K. Suzuki, Y. Chino, M. Mabuchi, Improvement of stretch formability of Mg–3Al–1Zn alloy sheet by high temperature rolling at finishing pass, J. Alloy. Compd. 509 (2011) 7579–7584. [11] X. Huang, K. Suzuki, N. Saito, Textures and stretch formability of Mg–6Al–1Zn magnesium alloy sheets rolled at high temperatures up to 793 K, Scr. Mater. 60 (2009) 651–654. [12] K. Iwanaga, H. Tashiro, H. Okamoto, K. Shimizu, Improvement of formability from room temperature to warm temperature in AZ-31 magnesium alloy, J.
Mater. Process. Technol. 155 (2004) 1313–1316. [13] Q. Jin, S.-Y. Shim, S.-G. Lim, Correlation of microstructural evolution and formation of basal texture in a coarse grained Mg–Al alloy during hot rolling, Scr. Mater. 55 (2006) 843–846. [14] R. Lebensohn, C. Tomé, A self-consistent anisotropic approach for the simulation of plastic deformation and texture development of polycrystals: application to zirconium alloys, Acta Met. Mater. 41 (1993) 2611–2624. [15] S.H. Park, S.-G. Hong, C.S. Lee, Enhanced stretch formability of rolled Mg–3Al– 1Zn alloy at room temperature by initial {10 12} twins, Mater. Sci. Eng. A 578 (2013) 271–276. [16] B. Song, N. Guo, T. Liu, Q. Yang, Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: a review, Mater. Des. 62 (2014) 352–360. [17] B. Song, R. Xin, A. Liao, W. Yu, Q. Liu, Enhancing stretch formability of rolled Mg sheets by pre-inducing contraction twins and recrystallization annealing, Mater. Sci. Eng. A 627 (2015) 369–373. [18] H. Wang, B. Raeisinia, P. Wu, S. Agnew, C. Tomé, Evaluation of self-consistent polycrystal plasticity models for magnesium alloy AZ31B sheet, Int. J. Solids Struct. 47 (2010) 2905–2917. [19] H. Watanabe, T. Mukai, K. Ishikawa, Effect of temperature of differential speed rolling on room temperature mechanical properties and texture in an AZ31 magnesium alloy, J. Mater. Process. Technol. 182 (2007) 644–647. [20] Y. Xin, M. Wang, Z. Zeng, G. Huang, Q. Liu, Tailoring the texture of magnesium alloy by twinning deformation to improve the rolling capability, Scr. Mater. 64 (2011) 986–989. [21] H. Zhang, G. Huang, H.J. Roven, L. Wang, F. Pan, Influence of different rolling routes on the microstructure evolution and properties of AZ31 magnesium alloy sheets, Mater. Des. 50 (2013) 667–673. [22] L. Zhang, G. Huang, H. Zhang, B. Song, Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process, J. Mater. Process. Technol. 211 (2011) 644–649.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Improving the room temperature stretch formability of a Mg alloy thin sheet by pre-twinning Weijun He a,n, Qinghui Zeng a, Huihui Yu a, Yunchang Xin a, Baifeng Luan a, Qing Liu a,b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 October 2015 Accepted 21 December 2015 Available online 22 December 2015
{10 12} tensile twins were introduced by in-plane compression in a thin magnesium alloy sheet in order to tailor the texture. The room temperature stretch formability of the pre-twinned Mg alloy sheet was remarkably improved (50%). The activation of the tensile twinning in the pre-twinned region, which can effectively accommodate the through-thickness strain during stretch forming, is assumed to be the main reason of stretch formability improvement. Numerical simulation results demonstrate that multidirectional tensile twins induced by multi-steps compression are more beneficial to improve the formability of Mg alloy sheet, compared with uni-directional twins. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Thin sheet Stretch formability Twinning texture
1. Introduction It is accepted that basal slip is the most active slip system in Mg alloy, thus leading to intense basal texture in rolled Mg alloy sheet [1,13]. In a strong basal texture, the strain along the thickness direction need to be accommodated by secondary deformation modes during low temperature stretch forming of Mg sheet, in which the material often undergo in-plane tensile stress [4]. This causes the poor stretch formability of rolled Mg alloy sheet at room/low temperature. It was found that weakening and/or tilting the basal texture can effectively improve the formability of rolled Mg alloy sheet [12,21,5]. Recently, many processes were proposed to tailor the texture of rolled Mg alloy sheets without changing the chemical components, e.g., cross rolling [6,7], asymmetric rolling [19], repeated unidirectional bending [22], equal channel angular rolling [3] and high temperature rolling [10,11]. Essentially, these approaches attempt to control the texture by activating non-basal slips or altering the effect of basal slip by strain path changes or shear strain. Compared with slips, twinning can remarkably change the texture of Mg alloy within small strain. For example, the orientation of {10 12}o 1011 4 twin is rotated by about 86° and that of {10 11}o 1012 4 twin by about 56° from the matrix [2,9]. Thus, pre-inducing twining is considered as an effective method to control the texture and enhance the formability of Mg alloy sheet [16]. Xin et al. [20] found that the limited reduction ratio can be n
Corresponding author. E-mail address: [email protected] (W. He).
http://dx.doi.org/10.1016/j.msea.2015.12.070 0921-5093/& 2015 Elsevier B.V. All rights reserved.
obviously increased by performing pre-rolling along the transverse direction (TD). Park et al. [15] reported that the formability can be remarkably enhanced by pre-induced {10 12} twin. The results of Song et al. [17] indicated that pre-contraction twins combined with recrystallization annealing may weaken the basal texture of rolled Mg alloy sheet. Tailoring the texture using twinning show many advantages over other methods based on dislocation slip due to its effectiveness and easy operation. However, its applicability in thin Mg alloy sheet (thickness o 4 mm) has not been much explored yet. In this work, in-plane pre-compressions were performed on thin Mg alloy sheet in order to weaken the basal texture by activating {10 12} twinning. The stretch formability of the pre-deformed Mg alloy sheets were measured by Erichsen test, followed by microstructure characterizations and analysis of active deformation modes.
2. Material and methods The material used in this work is a commercial hot rolled AZ31( Mg–3%Al–1%Zn) Mg alloy sheet 2.2 mm thickness. For in-plane pre-compression (IPC), square sheets with a dimension of 64 mm 64 mm 2.2 mm were machined by electrical discharge wire-cutting. With the aid of a channel die, the IPC was conducted to introduce {10 12} twins, as schematized in Fig. 1. In each IPC process, three pieces of square sheet were deformed at the same time. It is worth noting that 0.4 mm thick Teflon (PTFE, polytetrafluoroethylene) films were placed between the specimens and the channel die in order to reduce friction. During
2
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 1. Schematic of in-plane compression of AZ31 Mg alloy sheet.
compression, the deformation of the Teflon films can accommodate the deformation of Mg sheet along the thickness direction so that the material will not endure plane strain condition. The channel die aims to avoid bulking. Two types of IPC samples were prepared in our work. In the first case, the IPC was along the transverse direction (TD) and samples were compressed to 2.5% strain, denoted as IPC-T samples. In the second case, the samples go through two steps of pre-compression. Firstly, the IPC was performed along TD with a strain of 2.5%. Then, the specimens were rotated 90° about the normal direction (ND) and further compressed along the rolling direction (RD) with a strain of 2.0%, resulting in as-called IPC-TR samples. The pre-compressed samples were annealed at 260 °C for 1 h to eliminate the dislocation induced hardening. Additionally, in order to remove the effect of annealing on the stretch formability, the as-received sheet was also annealed at the same condition as the IPC samples, and the annealed sheet is denoted ‘as-received sample’ in our work. Erichsen tests were carried out to determine the stretch formability of the magnesium sheets using a hemispherical punch with a diameter of 20 mm at a punch speed of 0.1 mm/s at room temperature. Tests were repeated three times for each type of sample. The microstructure before and after Erichsen test were examined using Electron backscattered diffraction (EBSD) on a FEI Nova 400 scanning electron microscope. The deformation mechanism of the magnesium sheets during Erichsen tests was analyzed by the combination of Finite element simulations and crystal plasticity calculations.
3. Results 3.1. Erichsen test results The results of Erichsen tests are displayed in Fig. 2. Erichsen value (IE) is defined as the depth of the dome at fracture initiation, which is also equivalent to the punch displacement at the maximum loading force on the load–displacement curve during Erichsen test, as shown in Fig.2b. The average Erichsen value are 3.8 mm, 5.1 mm and 5.7 mm for the as-received, IPC-T and IPC-TR samples, respectively. Compared with the as-received sheet, Erichsen values are obviously increased by 34% and 50% for the IPC-T and IPC-TR samples, respectively. That means the stretch formability of the AZ31 sheet at room temperature has been significantly improved by the IPC process. From Fig. 2b, it is interesting to note that a lower force is needed for the pre-compressed samples compared with the as-received sample at same displacement. However, higher force is supposed to be needed for the pre-compressed samples as a result of the thickening during IPC process. Lower force during Erichsen test implies that the deformation mechanisms were different in the pre-compressed
Fig. 2. Erichsen tests results: (a) typical profiles of deformed as-received sample, IPC-T sample and IPC-TR sample, and (b) typical load–displacement curves during Erichsen test.
samples. Before fracture, the load–displacement curves of the IPCT sample and the IPC-TR sample are very close to each other. 3.2. Microstructure before Erichsen test Fig. 3 shows the microstructure and the {0001} pole figures of the as-received sample, the IPC-T sample and the IPC-TR sample before Erichsen test. It is observed that the grains of the precompressed samples grew compared with the as-received sample. The as-received sample and the IPC-TR sample have a twin-free equiaxial grain structure while some twin lamellas can be found in the IPC-T sample. {0001} pole figures indicate that the as-received sample has a typical basal texture with the c-axis almost parallel to the normal direction (ND). Many c-axis of grains align with TD in the IPC-T sample, while for the IPC-TR sample some c-axis are close to TD and some c-axis are close to RD due to its two steps of pre-compression. The re-orientation of c-axis by about 90° is the evidence of {10–12} tensile twinning. No obvious twin lamellae are observed in the IPC-TR sample because of recrystallization during the annealing treatment, which is favored by its high stored strain energy (2.5% strain along TD and 2% compression along RD). The presence of very fine grain, visible in Fig. 3c, also supports this supposition. The recrystallized grains originating from the twins form some regions of grains with orientations close to the twins. Referring to their orientation, these recrystallized grains can be treated as twinned regions. On the other hand, only grain growth happened in the IPC-T sample due to its relative lower strain energy storage (2.5% strain along TD), thus some twin lamellas (twinned regions) are still visible. 3.3. Microstructure after Erichsen test Fig. 4 presents the microstructure and {0001} pole figures of the as-received sample, the IPC-T sample and the IPC-TR sample after Erichsen test. The microstructure was observed at the dome, as schematic illustrated in Fig. 4. Compared with the observation before Erichsen test, Fig. 4a shows that there is no obvious change in the microstructural characteristics for the as-received sample, including the grain size and the texture. By contrast, the IPC-TR sample displays significant change after Erichsen test, as shown in Fig. 4c. Many twin lamellas appeared. Most of the twins have been
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
3
Fig. 3. EBSD measurements showing the orientation maps, boundary information, {0001} pole figures before Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
confirmed as tensile twins according to the misorientation information. Additionally, the {0001} pole figure indicates that the texture revert to that of the as-received sample (Fig. 3a), which can be caused by the activation of tensile twinning during Erichsen test. In the IPC-T sample, some tensile twin lamellas are observed, which may remain from the sample or were newly created during the Erichsen test. From the texture change observed on the {0001} pole figure of the IPC-T sample, we conclude that some tensile twinning was activated. 3.4. Simulated results To further explore the mechanisms enhancing the stretch formability, numerical simulation was carried out to investigate the contribution of different deformation modes during the Erichsen test. Finite element method (FEM) was used to simulate
the Erichsen test. Isotropic Mises model was used in the FEM simulation. The FEM simulations are shown in Fig. 5, where the geometry and size of the sheet and the die are the same as the experimental ones. The sheet is discretized with elements of C3D8R type. For the as-received sample, the IPC-T sample and the IPC-TR sample, the FEM simulation was stop when the punch displacement reached 3.7 mm, 5.1 mm and 5.7 mm, respectively. The equivalent plastic strain (denoted as PEEQ in the figure) distribution of various samples after Erichsen test are shown in Fig. 5. It indicates that a maximum of 0.19, 0.26 and 0.29 strain is achieved for the as-received sample, the IPC-T sample and the IPCTR sample, respectively. The loading path of the element at the dome was exported to carry out meso-scale analysis. The visco-plasticity self-consistent (VPSC) model was used to analyze the deformation mechanism of AZ31 sheet during Erichsen test. The detail information on the VPSC model can be found
4
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 4. EBSD measurements showing the orientation maps, boundary information and {0001} pole figures after Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
elsewhere [14,18]. The parameter values of the VPSC model fitted for the AZ31 material in a previous study [8] are used here since we work on materials with same chemical composition and similar microstructure. Basal slip, prismatic slip, pyramidal slip and tensile twinning are assumed to be potential deformation modes. The grain size difference between the as-received material and the pre-compressed material was ignored in modeling. Only the difference on texture was considered. Simulated results are shown in Fig. 6. It is found that the basal slip is the main deformation mode at the beginning of deformation for all samples. This may be due to the fact that the basal plane is not ideally perpendicular to the ND direction of the sheet, even for the as-received sample as displayed in Fig. 3. With the increase of strain, the contribution of the pyramidal slip increases gradually. As shown in Fig. 6(b) and (c),
tensile twinning is obviously activated below 0.06 strain. For the IPC-T sample, the contribution of the tensile twinning is about 18% while it is more than 23% in the IPC-TR sample.
4. Discussions As mentioned earlier, for the IPC-T and the IPC-TR sample before Erichsen test, the material is a mixture of twinned and untwinned regions. In the twinned region, the c-axis of the twins were rotated toward the loading direction, i.e. TD or RD. As the biaxial tensile stress is the main stress state during Erichsen test, the material is subjected to tensile loading along the c-axis in the pre-twinned region, which is the favorite state for the activation of
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
5
Fig. 5. FEM model and simulation results of magnesium alloy sheet during Erichsen test. (a) FEM model and equivalent plastic strain distribution after Erichsen test of (b) asreceived sample, (c) IPC-T sample, and (d) IPC-TR sample.
the tensile twinning. The tensile twinning that happen during Erichsen test is called ‘re-twinning’ in the following discussion in order to distinguish it from the pre-twinning in IPC process. The re-twinning can accommodate the thinning of the sheet during Erichsen test and thus it helps to enhance the stretch formability
at room temperature. The present results are very different from the one reported by Park et al. [15], where the de-twinning played a key role. Compared with the IPC-T sample, much more twin lamellas appeared in the IPC-TR sample. This may be related to the
6
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
Fig. 6. Contribution of the deformation modes to the deformation as a function of the equivalent strain during Erichsen test: (a) as-received sample, (b) IPC-T sample, and (c) IPC-TR sample.
orientation of the c-axis. In the IPC-TR sample, the c-axis of the twinned regions are close to two directions (TD and RD), which facilitate the activation of tensile twinning under biaxial tension, thus leading to a higher Erichsen value than that of IPC-T sample. Another reason, which may explain the phenomenon of more lamellas in the IPC-TR sample, is that these samples have more pre-
Fig. 7. Pole figures of different virtual samples: (a) IPC-T þ 2.0% TD (VPSC), (b) asreceivedþ 6.0% TD (VPSC), and (c) as-receivedþ3.0% TD þ 3.0% RD (VPSC).
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
7
IPC-TR sample with the same amount of pre-strain. However, we used crystal plastic modeling as a first step to explore the material behavior in the current work. To explore the mechanism that causes more twin lamellas in the Erichsen tested IPC-TR samples, three kinds of virtual experiments were carried out using the VPSC. In the first case, based on the texture of IPC-T sample, 2.0% compression along TD was simulated by the VPSC. In the second case, based on the texture of as-received sample, 6.0% compression along TD was conducted. In the third case, 3% compression strain along TD and 3% strain along RD was carried out, also on the base of the texture of as-received sample. The resulting simulated textures, shown in Fig. 7, were saved and used as virtual samples for Erichsen tests. Then, the activity of deformation modes of these virtual samples during Erichsen test were simulated by the VPSC, and displayed in Fig. 8. The IPC-T þ2.0% TD sample and the IPC-TR sample are considered to have endured same equivalent pre-strain (and so have the same amount of pre-twinned regions). The difference is within the orientation of c-axis and texture component intensities. By comparison of Figs. 6c and 8a, it is observable that the IPC-TR sample with the c-axis tilting close to two directions have more re-twinning activation than that of IPC-T þ 2.0% TD sample whose c-axis align with one direction. It is also true when the samples have greater pre-strain (6% strain), as shown in Fig. 8b and c. Although the amount of pre-strain is the same, the twinning activity during Erichsen test is different. Having some twinned regions along RD and TD (Fig. 7c) allows more twinning during the test than having all the twinned regions along TD (Fig. 7b). We can conclude that the texture containing c-axis of twinned regions aligned with two directions is more suitable for the activation of tensile twinning under biaxial stress state than that with twins along one direction.
5. Conclusion The strong basal texture of thin magnesium alloy sheet (2.2 mm) can be successfully tailored by {10 12} tensile twin introduced by in-plane compression followed by recrystallization. This kind of texture control based on the {10 12} tensile twin can be effectively used to enhance the stretch formability of magnesium alloy thin sheet at room temperature. The crystallographic orientation of the pre-twinned region is favorable for re-activation of tensile twinning in subsequent stretch forming. The re-twinning can help to accommodate the through-thickness strain, thus leading to a better stretch forming ability. Additionally, numerical simulation results indicate that multidirectional tensile twins induced by multi-steps compression can further improve the stretch formability compared with single directional tensile twin caused by single-step compression.
Acknowledgment
Fig. 8. Contribution of the deformation modes to the deformation as a function of the equivalent strain during Erichsen test: (a) IPC-T þ2.0% TD (VPSC) virtual sample, (b) as-receivedþ 6.0% TD (VPSC) virtual sample, and (c) as-receivedþ 3.0% TD þ 3.0% RD (VPSC) virtual sample.
twinned regions caused by large pre-strains. Pre-compression to a larger pre-strain was attempted: 4.5% pre-compression was performed on the thin sheets along one direction but bulking and heterogeneous deformation occurred in the current channel die. So it has not been possible to compare the IPC-T sample and the
The authors express their sincere thanks to the support by the National Basic Research Program of China (“973” Project, Grant nos. 2013CB632201 and 2013CB632204) and National Natural Science Foundation of China (Project 51421001). This work has also been funded by Chongqing Natural Science Foundation, No. cstc2014jcyjA50022 and Fundamental Research Funds for the Central University with Project no. 106112015CDJXY130002.
References [1] S. Agnew, C. Tomé, D. Brown, T. Holden, S. Vogel, Study of slip mechanisms in a magnesium alloy by neutron diffraction and modeling, Scr. Mater. 48 (2003)
8
W. He et al. / Materials Science & Engineering A 655 (2016) 1–8
1003–1008. [2] M.R. Barnett, Twinning and the ductility of magnesium alloys: part II. “Contraction” twins, Mater. Sci. Eng. A 464 (2007) 8–16. [3] Y.Q. Cheng, Z.H. Chen, W.J. Xia, T. Zhou, Improvement of drawability at room temperature in AZ31 magnesium alloy sheets processed by equal channel angular rolling, J. Mater. Eng. Perform. 17 (2008) 15–19. [4] Y. Chino, K. Kimura, M. Mabuchi, Deformation characteristics at room temperature under biaxial tensile stress in textured AZ31 Mg alloy sheets, Acta Mater. 57 (2009) 1476–1485. [5] Y. Chino, J.-S. Lee, K. Sassa, A. Kamiya, M. Mabuchi, Press formability of a rolled AZ31 Mg alloy sheet with controlled texture, Mater. Lett. 60 (2006) 173–176. [6] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Enhanced formability at elevated temperature of a cross-rolled magnesium alloy sheet, Mater. Sci. Eng. A 441 (2006) 349–356. [7] Y. Chino, K. Sassa, A. Kamiya, M. Mabuchi, Stretch formability at elevated temperature of a cross-rolled AZ31 Mg alloy sheet with different rolling routes, Mater. Sci. Eng. A 473 (2008) 195–200. [8] W. He, B. Luan, R. Xin, J. Xu, Q. Liu, A multi-scale model for description of strain localization in friction stir welded magnesium alloy, Comput. Mater. Sci. 104 (2015) 162–171. [9] S.-G. Hong, S.H. Park, C.S. Lee, Role of {10 12} twinning characteristics in the deformation behavior of a polycrystalline magnesium alloy, Acta Mater. 58 (2010) 5873–5885. [10] X. Huang, K. Suzuki, Y. Chino, M. Mabuchi, Improvement of stretch formability of Mg–3Al–1Zn alloy sheet by high temperature rolling at finishing pass, J. Alloy. Compd. 509 (2011) 7579–7584. [11] X. Huang, K. Suzuki, N. Saito, Textures and stretch formability of Mg–6Al–1Zn magnesium alloy sheets rolled at high temperatures up to 793 K, Scr. Mater. 60 (2009) 651–654. [12] K. Iwanaga, H. Tashiro, H. Okamoto, K. Shimizu, Improvement of formability from room temperature to warm temperature in AZ-31 magnesium alloy, J.
Mater. Process. Technol. 155 (2004) 1313–1316. [13] Q. Jin, S.-Y. Shim, S.-G. Lim, Correlation of microstructural evolution and formation of basal texture in a coarse grained Mg–Al alloy during hot rolling, Scr. Mater. 55 (2006) 843–846. [14] R. Lebensohn, C. Tomé, A self-consistent anisotropic approach for the simulation of plastic deformation and texture development of polycrystals: application to zirconium alloys, Acta Met. Mater. 41 (1993) 2611–2624. [15] S.H. Park, S.-G. Hong, C.S. Lee, Enhanced stretch formability of rolled Mg–3Al– 1Zn alloy at room temperature by initial {10 12} twins, Mater. Sci. Eng. A 578 (2013) 271–276. [16] B. Song, N. Guo, T. Liu, Q. Yang, Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: a review, Mater. Des. 62 (2014) 352–360. [17] B. Song, R. Xin, A. Liao, W. Yu, Q. Liu, Enhancing stretch formability of rolled Mg sheets by pre-inducing contraction twins and recrystallization annealing, Mater. Sci. Eng. A 627 (2015) 369–373. [18] H. Wang, B. Raeisinia, P. Wu, S. Agnew, C. Tomé, Evaluation of self-consistent polycrystal plasticity models for magnesium alloy AZ31B sheet, Int. J. Solids Struct. 47 (2010) 2905–2917. [19] H. Watanabe, T. Mukai, K. Ishikawa, Effect of temperature of differential speed rolling on room temperature mechanical properties and texture in an AZ31 magnesium alloy, J. Mater. Process. Technol. 182 (2007) 644–647. [20] Y. Xin, M. Wang, Z. Zeng, G. Huang, Q. Liu, Tailoring the texture of magnesium alloy by twinning deformation to improve the rolling capability, Scr. Mater. 64 (2011) 986–989. [21] H. Zhang, G. Huang, H.J. Roven, L. Wang, F. Pan, Influence of different rolling routes on the microstructure evolution and properties of AZ31 magnesium alloy sheets, Mater. Des. 50 (2013) 667–673. [22] L. Zhang, G. Huang, H. Zhang, B. Song, Cold stamping formability of AZ31B magnesium alloy sheet undergoing repeated unidirectional bending process, J. Mater. Process. Technol. 211 (2011) 644–649.