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Microstructure and mechanical properties of Mg-5Li-1Al sheets processed by cross accumulative roll bonding
Journal of Manufacturing Processes 46 (2019) 139–146
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Microstructure and mechanical properties of Mg-5Li-1Al sheets processed by cross accumulative roll bonding
T
⁎
Huajie Wua, Tianzi Wangc, Ruizhi Wua,b, , Legan Houa, Jinghuai Zhanga, Xinlin Lia, Milin Zhanga a
Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, PR China College of Science, Heihe University, Heihe University, Heihe, 164300, PR China c Suzhou Changfeng Avionics Co., LTD., Suzhou, 215151, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mg-Li alloy Cross accumulate roll bonding Dynamic recrystallization
Mg-5Li-1Al alloy was processed by cross accumulative roll bonding (CARB). The processed Mg-5Li-1Al sheet possesses ultra-fine grains, and its strength increases both in the rolling direction (RD) and transverse direction (TD) with the elongation remaining 16˜18%. The proper CARB processing is helpful to improve the isotropy of mechanical properties of Mg-5Li-1Al sheet. At the same time, the CARB process has a beneficial effect on improving the plasticity of Mg-5Li-1Al sheet. With the change of the direction between the CARB passes, the basal texture of Mg-5Li-1Al sheet is weakened, and the non-basal texture is strengthened. Both prismatic slip and pyramidal slip are initiated. The dominant deformation mechanism changes from twin deformation to slip deformation, and finally shear deformation. The grain refinement mechanism changes from twin dynamic recrystallization to continuous dynamic recrystallization, and finally rotational dynamic recrystallization.
1. Introduction With the advantages of low density, high specific strength and good electromagnetic shielding properties, magnesium alloys attract much attention from researchers, especially Mg-Li alloy is famous for its ultralight. Wu et al. [1] demonstrated the preparation, processing technologies and surface processing technologies of Mg-Li alloys, which possess lower density (1.35–1.65 g/cm3). Moreover, Zhang et al. [2] suggested the addition of lithium also improves the plasticity of the Mg alloy. Compared with Al-Li alloy [3], the absolute strength of Mg-Li alloy is insufficient. Zhang et al. [4] held the view that rare earth element microalloying is an effective strengthening method, but leading to an increase of the density. Then we expect to refine the grain through severe plastic deformation (SPD) to improve its strength. Tsuji et al. [5] first put forward accumulative roll bonding (ARB) as a SPD technology, which can realize continuous production of ultra-fine grain (UFG) materials on a large-scale. At the same time, Tsuji pointed out that the uniform elongation of materials processed by ARB is limited due to the plastic instability. In order to research the microstructure evolution rule in ARB process, Kamikawai et al. [6] investigated the effect of residual shear strain on the microstructure and texture of IF steel, which generated the microstructure inhomogeneity throughout the thickness at low pass, and the uniform UFG structures requiring more passes. Wu
et al. [7] applied ARB process to improve the strength of Mg-Li alloy, and improved its plasticity by proper annealing treatment. Hou et al. [8] have prepared Mg-5Li-1Al sheets by conventional ARB technology, which have good mechanical properties and interfacial bonding properties. However, there still exist some structural inhomogeneity and anisotropy in mechanical properties along the direction of RD and TD. Fatemeh et al. [9] considered that cross rolling (CR) can promote dynamic recrystallization, and obtain isotropic AA2024 sheet compared with other strain paths. In order to make the reinforcements evenly distributed in the matrix, Alizadeh et al. [10] put forward the cross accumulative roll bonding (CARB) process, during which the rolling direction between two cycles changes by 90° around the normal direction (ND) so that the uniformity of the microstructure and the mechanical property of the alloy is improved. Ardakani et al. [11] investigated that microstructure and mechanical properties of the Al/Al2O3 composites with uniform distribution of Al2O3 particles by CARB. The average grain size of the composites was 71 nm, and the tensile strength of the composites was up to 344 MPa. Verstraete et al. [12] found that CARB not only weakens the texture and anisotropy, but also makes the strength and plasticity of the sheet higher than that of ARB. Zhang et al. [13] prepared bulk Cu/Ta layered composites with high strength (UTS = 950 MPa) and thermal stability by CARB and intermediate annealing, because this method can inhibit edge crack and
⁎ Corresponding author at: Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, PR China. E-mail address: [email protected] (R. Wu).
https://doi.org/10.1016/j.jmapro.2019.09.004 Received 3 June 2019; Received in revised form 30 August 2019; Accepted 1 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 46 (2019) 139–146
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shows that the grain refinement of the alloy is mainly based on twinning-induced dynamic recrystallization supposed by Peng et al. [18]. Due to the change of rolling direction between passes, the distribution of fiber structure along the direction of RD decreases and there is a tendency of dispersion along the direction of TD. With the increase of CARB pass, the distribution uniformity of microstructure increases, especially after 4 passes (Fig. 1(d)), the distribution of fibrous structure almost shows no specific orientation. The recrystallized grains continuously increase and are refined. Sheets processed by CARB6 have the most uniform microstructure and fine recrystallization grain. Compared with the previous ARB rolled sheets [19], the reduction of the twins and fibrous structures is faster, the microstructure uniformity is more obvious, but the grain is slightly coarser. Fig. 2 shows the RD-ND microstructure of the CARB-processed Mg5Li-1Al sheet. There appears a small amount of macroscopic shear bands along 45° with the RD direction in the vicinity of surface of the CARB1 plate (Fig. 2(a)). With the increase of CARB passes, the macroscopic shear bands are refined and the number of them increases. After 4 CARB passes, the macroscopic shear bands are finer and more uniform, and a small number of fine dynamic recrystallization grains appear between the shear bands (Fig. 2(c)). There still remain some fine macroscopic shear bands in the microstructure of CARB6 sheets (Fig. 2(d)). TEM and SAED patterns of Mg-5Li-1Al alloys processed by CARB are shown in Fig. 3. The average grain size of CARB1 sheet is 500–700 nm, as shown in Fig. 3(a), and the grain orientation difference is small. The twins at high magnification are shown in the bottom left corner of Fig. 3(a), which indicates that there are many twins in the alloy after CARB1. After CARB2, grains are obviously refined, about 250–300 nm (Fig. 3(b)). The number of twins decreases rapidly, and there are a few short dislocation lines in the grains, and some parallel dislocation walls distribute at the grain boundaries, but the overall distribution is more dispersed. In the SAED, the diffraction spots are isolated and reticulated, and the degree of isolation is greater than that of CARB1, which indicates that the inter-grain orientation difference of sheets decreases because of the changing of rolling direction. For CARB4 sheets, the fine grain distribution is more uniform, as shown in Fig. 3(e). There are many dislocation tangles at the grain boundary and sub-grains. The diffraction spots are almost connected to a ring, indicating the orientation difference between the grains increases. The average grain size of CARB6 alloy is 150–200 nm (Fig. 3(d)), and the number of dislocations is slightly higher compared with that of CARB4 alloy. The SAED pattern in CARB6 shows isolated spots indicating decrease in orientation difference. This is mainly because with the increase of cumulative strain in CARB6 sheet, fine grains begin to be produced in the macroscopic shear bands, indicating that the rotating dynamic recrystallizing mechanism is started. With the change of rolling direction between passes, the grain size of Mg-5Li-1Al alloy decreases rapidly after the second pass, and the microstructure uniformity of each pass sheet increases. The pole figures of CARB-processed Mg-5Li-1Al sheets are shown in Fig. 4. The CARB1 sheet exhibits typical magnesium alloy rolling texture, and (10–11) and (10–12) pyramidal slip systems are also slightly initiated. With the increase of CARB pass, the maximum pole density of (0002) decreases from 10.756 m.r.d (Fig. 4(a)) to 8.216 m.r.d (Fig. 4(d)), and that of CARB 6 is almost the same as CARB4 sheet. During the CARB process, the contour shape of the basal plane and the c-axis orientation of the grains also change obviously. The pole contour of CARB1 is flattened along TD, and the deflection degree of the c-axis along RD is large, dR/dT =1.79, as shown in Fig. 4(a). After CARB2 process, the pole contour produces 90° rotation as a whole, the deflection direction of c-axis translates into the TD, dR/dT =0.64 (Fig. 4(b)), indicating that the change of rolling direction also changes the grain orientation of the sheets. With the increase of CARB pass and the change of rolling direction, the pole contour of basal plane gradually changes from ellipse to circle, and the dR/dT value gradually tends
inter-laminar plastic instability. Duan et al. [14] studied the texture distribution of pure nickel proceed by ARB and CARB and concluded that the texture of the latter was significantly weakened and CARB process could reduce the plane anisotropy. They all agreed that the composite microstructure prepared by the CARB was more uniform, and the mechanical properties were improved, and the anisotropy was reduced, compared with the conventional ARB process. As the microstructure of Mg alloy after deformation was inhomogeneous and the anisotropy was obvious due to hexagonal closepacked (HCP) structure, as described in Davis et al. [15], CARB may be an effective processing method. Although the addition of lithium can improve the lattice symmetry, the microstructure of Mg-5Li-1Al alloy is still α-Mg matrix based on HCP structure, which can be seen from the research of Wang et al. [16]. In this work, Mg-5Li-1Al alloy was carried out 6-pass CARB process. The microstructure, mechanical properties and texture evolution of each pass were investigated. The plastic deformation mechanism and grain refinement mechanism in the process of CARB were discussed. 2. Experiment Mg-5Li-1Al alloy was prepared by induction melting with argon protection, and the raw materials consist of commercial pure magnesium ingot (99.9 wt.%), commercial pure lithium ingot (99.9 wt.%) and commercial pure aluminum ingot (99.9 wt.%). Because of the high activity of pure lithium, the lithium ingot is soaked in kerosene. Before usage, lithium ingot was taken out from kerosene and defatted cotton was used to remove kerosene from the surface of ingot. Then it was weighed and put into crucible with other raw materials, and finally melted under the protection of argon atmosphere. The melt was poured into a permanent mould to obtained as-cast specimen. Then the as-cast specimen was homogenized at 300 °C for 12 h. The received ingot was cut into original sheets with the dimensions of 50 mm × 30 mm × 2 mm for the subsequent CARB process. In the CARB process, the sheet was heated at 350 °C for 10 min before the first pass and for 5 min at the interval between two passes. The reduction of each pass was 55%. In each pass, the rolling direction was rotated 90° around normal direction of sheet. RD and TD in the first pass during CARB were defined as the directions of RD and TD in the paper. The detail procedures about ARB process can also be found in our previous literature [17]. The microstructure of RD-TD surface and RD-ND surface of Mg-5Li1Al sheet was observed by LEICA DM IRM metallographic microscope. Specimens were etched with 4% nitric acid alcohol for 10–15 s. The grain size and dislocation of Mg-Li alloy sheet during CARB were observed by JEM-2100 transmission electron microscope (TEM), and the crystal structure of the alloy was determined by selective area electron diffraction (SAED). The TEM images are taken in RD-ND plane, and 20 TEM images were used to estimate the grain size. The texture test was carried out by X' Pert Pro X-ray diffractometer, during which the Cu Kα (λ =0.1542 nm) was selected. The tensile test at room temperature was carried out on the WDW3050 electronic universal tester with a strain rate of 10−3 s-1. JEOL SM-6360LV scanning electron microscope (SEM) was used to analyze the fracture morphology. 3. Results 3.1. Microstructure The RD-TD microstructure of Mg-5Li-1Al alloy is shown in Fig. 1. Fig. 1(a) shows the pre-CARB structure in which many twins appear. In Fig. 1(b), a number of twins and fibrous structures distribute along the rolling direction, and a few dynamic recrystallization grains appear around the twin structure, as shown in Fig. 1(f). After the second pass, the number of twinning is rapidly reduced, and the number of recrystallized grains is obviously increased, as shown in Fig. 1(c), which 140
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Fig. 1. Microstructure of the Mg-5Li-1Al sheets (RD-TD plane) (a) as-rolled, (b)CARB1, (c)CARB2, (d) CARB4, (e) CARB6, (f)CARB1 at high magnification.
The inverse pole figures of CARB-processed Mg-5Li-1Al sheets are shown in Fig. 5. The dense poles are distributed between < 0002 > and < 11−20 > , and tend to be about 20° in the direction of < 11−20 > . With the increase of CARB passes, the main slip direction does not change obviously, but the maximum pole density decrease first and then increase. The maximum pole density decreases from 19.066 m.r.d of CARB1 to 16.862 m.r.d of CARB4, and the number of contours between < 11−20 > and < 10−10 > increases, indicating that the basal plane texture intensity decreases and the non-basal plane texture intensity increases, which is beneficial to the decrease the anisotropy. However, the CARB6 maximum pole density increases abruptly to 26.182 m.r.d, and the contours decrease in other directions, which reflects the increase of the basal plane texture and the difficulty of continuing sliping.
to 1, which suggests the deflection of c-axis of grain in sheets is getting smaller and smaller. In CARB6, dR/dT = 1.02 (Fig. 4(d)), showing that the grain tends to be consistent along the direction of RD and TD, which is beneficial for the weakening of the anisotropy in Mg-5Li-1Al sheets. In addition, in the (0002) pole figure of CARB6 sheets, the area of the maximum pole density is distributed in the position where the c axis is the center, the TD direction is symmetrical axial, deflecting ± 15° along the RD direction, forming a bimodal shape along the RD direction, as shown in Fig. 4 (d). S. R. Agnew et al. [20] suggested this shift is caused by the start of < a+c > slip mode. According to the subsequent analysis, it can be concluded that the double peak characteristic of the basal texture of CARB6 sheets is mainly caused by the basal plane slip due to the complex deformation. In addition, in the CARB process, the pole distribution in the nonbasal pole figure also rotates with the change of rolling direction. As shown in Fig. 4(b), the pole distributions of (10-10), (10–11) and (10–12) pole figures in CARB2 sheets are changed by 90°. In the nonbasal plane pole figure of CARB6 sheets, the poles are evenly distributed on the circle with a certain angle centered on the c-axis (Fig. 4(d)). By comparing the non-basal plane pole intensity, with the increase of CARB passes, the prismatic texture intensity increases, while the pyramidal texture intensity decreases a little, but the distributions of them are all around the center, indicating that both prismatic slip and pyramidal slip are initiated in CARB process.
3.2. Mechanical properties The stress-strain curves of Mg-5Li-1Al sheets along TD and RD are shown in Fig. 6. With the increase of the CARB pass, the yield strength and tensile strength in RD and TD both increase, and the tensile strength of CARB6 sheets reaches 306.29 MPa and 304.49 MPa along RD and TD, respectively. However, the elongation of Mg-5Li-1Al sheets increases first, then decreases slowly before the CARB5 pass, remaining 16–18%. But after the CARB6 pass, the elongation along TD decreases 141
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Fig. 2. Microstructure of the CARB-processed Mg-5Li-1Al sheets (RD-ND plane). (a)CARB1, (b)CARB2, (c) CARB4, (d) CARB6.
held the view that indicator of plane anistropy (IPA) is always used to characterize the anisotropy of metallic plate, which needs three values from different directions. In this work, for simplicity, a new parameter, m, is defined as tensile strength anisotropy, m=|UTSRD/UTSTD-1|, where UTS is the ultimate tensile strength. As can be seen from Fig. 7, after 2 ARB passes, the anisotropy of tensile strength decreases obviously. While as for CARB process, after 1 pass, the anisotropy decreases obviously and it is always lower than that of ARB. It can be concluded that appropriate CARB process is helpful to decrease the anisotropy of Mg-5Li-1Al sheet.
obviously. When the plastic deformation is carried out to a higher pass (CARB6), the continuous increase of rolling pass will only enhance the rotating dynamic recrystallization. At the same time, the pole strength of < 110 > slip direction in the reverse pole figure increases sharply in CARB6, indicating that the basal slip is difficult and easy to cause the stress concentration and local shear fracture of the alloy, so the plasticity of CARB6 sheet along the RD direction and the TD direction is obviously different. The anisotropy of mechanical properties of Mg-5Li-1Al sheets processed by ARB [8] and CARB is shown in Fig. 7. Medjahed et al. [21]
Fig. 3. TEM images of the CARB-processed Mg-5Li-1Al sheets (a) CARB1, (b) CARB2, (c) CARB4, (d) CARB6, (e) CARB4 20000×, (f) CARB6 20000×. 142
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Fig. 4. Pole figures of the CARB-processed Mg-5Li-1Al sheets. (a) CARB1, (b) CARB2, (c) CARB4, (d) CARB6.
Fig. 5. Inverse pole figures of the CARB-processed Mg-5Li-1Al sheets with respect to ND (a)CARB1, (b)CARB2, (c)CARB4, (d)CARB6.
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Fig. 6. The stress-strain curves of the CARB-processed Mg-5Li-1Al sheets. (a) Along RD, (b) Along TD.
The tensile fracture morphologies of the CARB-processed Mg-5Li1Al sheets are shown in Fig. 8. There are many equiaxed dimples in the fracture microstructure of CARB1, CARB2 and CARB4 sheets. With the increase of CARB pass, the number of dimples increases, but the dimples become more shallow and smaller, indicating a stable range of the elongation. The fracture mode is mainly ductile fracture. For the CARB6 sheet (Fig. 8(d)), the number of dimples decreases obviously, and a small number of cleavage planes appear, indicating a relatively poor plasticity. The fracture mode of the sheet is a mixture of ductile fracture and cleavage fracture. As a result, the elongation of the CARB6 sheets decreased significantly. In addition, the interface layers do not crack during the tensile process, showing a good bonding during CARB. 4. Discussions 4.1. Force analysis in CARB process
Fig. 7. Tensile strength anisotropy of Mg-5Li-1Al sheets after the ARB (Hou L.G., 2018) and CARB process.
The stress state and flow of the metal in the rolling process are
Fig. 8. Fracture morphology of the CARB-processed Mg-5Li-1Al sheets. (a) CARB1, (b)CARB2, (c) CARB4, (d) CARB6. 144
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Fig. 9. The stress diagram of the alloy during rolling process. (a) Entering into the rolling mill, (b) Stable rolling stage.
crucial to the microstructure and mecha(Q − F ) + 2Tx 2Px nical properties of the sheet. At the moment of entering into the rolling mill, the sheet is subjected to positive pressure P, friction force T, external thrust Q and inertia force F, as shown in Fig. 9(a). Under the combined action of the four forces, the sheet is bitten into the roll. The force condition satisfies the Equation 4-1.
(Q − F ) + 2Tx > 2Px
(4-1)
Where Px is the horizontal component of P, Tx is the horizontal component of T. After the front end of the sheet reaches the center of the two rolling rolls, it comes to the stable rolling stage (Fig. 9(b)). The action of Q and F is no longer required, that is, the rolling process can be successfully completed under the following critical equilibrium relation, Equation 4-2.
2Tx − 2Px = 0
(4-2)
After entering the stable rolling stage, the friction component Tx increases and the rolling resistance Px decreases gradually. So the residual friction (Ts) becomes more and more abundant, Ts=Tx-Px. According to Sakai et al. [22], the residual friction force acts on the surface of the sheet, resulting in inhomogeneous deformation in the direction of thickness. Fig. 10 is a schematic diagram of the stress during rolling. The rolling deformation zone is roughly divided into four parts, ADB, CGE, ADGC and BDGE. According to the law of minimum resistance, the friction force can be divided into longitudinal friction σ3 and transverse friction σ2. The metal in the regions of ADB and CGE flows along the transverse direction, while the metal in the regions of ADGC and BDGE flows along the longitudinal direction. Because the metal in the center of the longitudinal region is elongated without broadening, the longitudinal elongation at the center is larger than that at the edges, and there must be a stress σAB at the center, which is opposite to σ3. The σAB is the shear stress at the edges during CARB process.
Fig. 10. The transverse flow in the deformation zone of the alloy during rolling process.
reduction. Accordingly, the metallurgical bonding is realized at the interface between the two metal sheets. For the CARB2 sheets, the residual shear friction generated on the surface during CARB1 is brought into the interior of sheet, while the shear friction on the two surfaces continues to increase, causing the formation of a large number of macroscopic shear bands on the RD-ND surface, as shown in Fig. 2. Generally speaking, the longitudinal extension of the alloy is larger than that of the transverse extension (σ2 < σ3) during rolling, the stress along RD is larger and the dynamic recrystallization is more likely to occur. Therefore, the strength and elongation of the CARB1 sheets along RD are higher than TD. Due to the change of the rolling direction in the CARB2 sheet, the friction of σ2 and σ3 also rotates 90° along the width direction, the direction of σAB changes accordingly. The shear stress along TD increases. Therefore, with the increase of CARB pass, the anisotropy of Mg-5Li-1Al alloy decreases gradually. The CARB process also plays a beneficial role in the texture of
4.2. Effect of rolling route on the anisotropy During the CARB1 process, a large amount of residual shear friction is produced on the surface of the sheet due to the large amount of 145
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Declaration of Competing Interest
sheets. Ruppert [23] demonstrated that the grain orientation of each pass changes due to the change of the direction of shear stress in the CARB process. Many potential slip systems can be activated. As shown in Fig. 4, with the increase of CARB pass, the basal plane texture of Mg5Li-1Al alloy weakens, the dR/dT value gradually tends to 1, and the non-basal plane slip system is activated. Agnew et al. [24] showed that in the alloy with strong texture distribution, the shear stress along the basal plane is almost zero and the slip is difficult to carry out. Accordingly, it was easy to cause stress concentration and local shear fracture of the alloy. When the base texture of (0002) is weakened, the plasticity of the alloy is enhanced. Therefore, compared with ARB process, CARB is beneficial for the improvement of the plasticity of Mg5Li-1Al sheets.
None. Acknowledgments This paper was supported by National Natural Science Foundation of China (51671063, 51771060, 51871068), Heilongjiang Province Natural Science Foundation (ZD2017010), the Fundamental Research Funds for the Central Universities (HEUCFG201834), Harbin City Application Technology Research and Development Project (2017RAQXJ032). References
4.3. Deformation mechanism in CARB process [1] Wu RZ, Yan YD, Wang GX. Recent progress in magnesium-lithium alloys. Int Mater Rev 2015;60(2):65–100. [2] Zhang TL, Tokunaga Ohno M, Zhang ML. Fabrication of Al-coated Mg-Li alloy sheet and investigation of its properties. Acta Metall Sin 2019;32(2):169–77. (English Letters). [3] Wang Y, Zhang Z, Wu RZ, Sun JF, Jiao YL, Hou LG, et al. Ambient-temperature mechanical properties of isochronally aged 1420-Sc-Zr aluminum. Mater Sci Eng A 2019;745:411–9. [4] Zhang JH, Liu SJ, Wu RZ, Hou LG, Zhang ML. Recent developments in high-strength Mg-RE-based alloys: Focusing on Mg-Gd and Mg-Y systems. J Magnes Alloy 2018;6:277–91. [5] Tsuji N, Ito Y, Saito Y, et al. Strength and ductility of ultrafine grained aluminum and iron produced by ARB and annealing. Scr Mater 2002;47(12):893–9. [6] Kamikawa N, Sakai T, Tsuji N. Effect of redundant shear strain on microstructure and texture evolution during accumulative roll-bonding in ultralow carbon if steel. Acta Mater 2007;55(17):5873–88. [7] Wu HJ, Wang TZ, Wu RZ, Hou LG, Zhang JH, Li XL, et al. Effects of annealing process on the interface of alternate α/β Mg-Li composite sheets prepared by accumulative roll bonding. J Mater Process Technol 2018;254:265–76. [8] Hou LG, Wang TZ, Wu RZ. Microstructure and mechanical properties of Mg-5Li-1Al sheets prepared by accumulative roll bonding. J Mater Sci Technol 2018;34(2):317–23. [9] Fatemeh G, Roohollah J. Effect of strain path during cold rolling on the microstructure, texture, and mechanical properties of AA2024 aluminum alloy. Mater Res Express 2019:6. [10] Alizadeh M. Processing of Al/B4C composites by cross-roll accumulative roll bonding. Mater Lett 2010;64:2641–3. [11] Ardakani MRK, Amirkhanlou S, Khorsand S. Cross accumulative roll bonding- A novel mechanical technique for significant improvement of stir-cast Al/Al2O3 nanocomposite properties. Mater Sci Eng A 2014;591:144–9. [12] Verstraete K, Helbert AL, et al. Microstructure, mechanical properties and texture of an aa6061/aa5754 composite fabricated by cross accumulative roll bonding. Mater Sci Eng A 2015;640:235–42. [13] Zhang T, Zeng LF, Gao R, Fang QF, et al. High strength and thermal stability of bulk Cu/Ta nanolamellar multilayers fabricated by cross accumulative roll bonding. Acta Mater 2016;110:341–51. [14] Duan JQ, Quadir MZ, Ferry M. Engineering low intensity planar textures in commercial purity nickel sheets by cross roll bonding. Mater Lett 2016;188:138–41. [15] Davis AE, Robson JD, Turski M. Reducing yield asymmetry and anisotropy in wrought magnesium alloys-a comparative study. Mater Sci Eng A 2019;744:525–37. [16] Wang TZ, Zhu TL, Sun JF, Wu RZ, Zhang ML. Influence of rolling directions on microstructure, mechanical properties and anisotropy of Mg-5Li-1Al-0.5Y alloy. J Magnes Alloys 2015;3:345–51. [17] Wang TZ, Zheng HP, Wu RZ. Preparation of fine-grained and high-strength Mg–8Li–3Al–1Zn alloy by accumulative roll bonding. Adv Eng Mater 2016;18:304–11. [18] Peng JH, Zhang Z, Li YZ, Zhou W, Wu YC. Twinning-induced dynamic recrystallization and micro-plastic mechanism during hot-rolling process of a magnesium alloy. Mater Sci Eng A 2017;699:99–105. [19] Zhong F, Wang T, Wu RZ, et al. Microstructure, texture, and mechanical properties of alternate Mg-Li composite sheets prepared by accumulative roll bonding: microstructure, texture and mechanical properties of alternate α/β composite sheets. Adv Eng Mater 2017;19(5):16–7. [20] Agnew SR, Horton JA, Yoo MH. Investigation of c+a dislocation structures in Mg and Mg-Li α-solid solution alloys. Metall Mater Trans A 2002;23:813–22. [21] Medjahed A, Moula H, Zegaoui A, Derradji M, Henniche A, Wu RZ, et al. Influence of the rolling direction on the microstructure, mechanical, anisotropy and gamma rays shielding properties of an Al-Cu-Li-Mg-X alloy. Mater Sci Eng A 2018;732:129–37. [22] Sakai T, Saito Y, Matsuo M, et al. Inhomogeneous texture formation in high speed hot rolling of ferritic strainless steel. ISIJ Int 1991;31:86–94. [23] Ruppert M, Hoppel HW, Goken M. Influence of cross-rolling on the mechanical properties of an accumulative roll boned aluminum alloy AA6014. Mater Sci Eng A 2014;597:122–7. [24] Agnew SR, Duygulu O. Plastic anistropy and the role of non-basal slip in magnesium alloy AZ31B. Int J Plast 2005;21:1161–93.
During the CARB process, the plastic deformation and grain refinement mechanism of Mg-5Li-1Al alloy also change with the change of rolling direction. According to the results of TEM (Fig. 3), there are a lot of twins in the microstructure of CARB1 sheet, and there exist fine dynamic recrystallized grains around them. The number of twins in the CARB2 sheet decreases rapidly, and the number of dynamic recrystallization grains increases. It can be concluded that the dominant deformation mechanism of Mg-5Li-1Al alloy in the first two passes of CARB is twinning deformation, and the grain refinement mechanism is twinning induced dynamic recrystallization. In addition, a small number of dislocation lines and dislocation walls also appear in the CARB2 sheet. Dislocation entanglement occurs at the grain boundaries and sub-crystals appear in the local region of the grains. Moreover, every CARB-processed sheet shows typical rolling texture because of large plastic deformation, and the prismatic slip and pyramidal slip system are initiated after changing the rolling direction. According to the results, it can be concluded that the dominant deformation mechanism of Mg-5Li-1Al alloy is slip deformation, which includes the basal plane slip, the prismatic plane slip and the pyramidal plane slip, and the grain refinement mechanism accords with the characteristics of continuous dynamic recrystallization. As can be observed from Fig. 2, due to the continuous increase and accumulation of rolling reduction, fine grains are formed in some macro shear deformation bands. At the same time, the rotational dynamic recrystallization is enhanced. The pole intensity of < 11−20 > in the reverse pole figure increases sharply in the CARB6 sheet, and the slip deformation becomes gradually difficult, indicating that the plastic deformation of the sheet is mainly shear deformation. 5. Conclusions 1 With the increase of CARB pass, the surface residual shear friction produced by the previous pass is gradually brought into the sheet interior, and the whole sheet is finally subjected to complex shear stress. 2 The average grain size of CARB6 sheet is 150–200 nm Compared with the traditional ARB process, the dislocations disperses slightly and the grain refinement rate slows down, but the microstructure uniformity increases due to the change of the rolling direction. 3 With the increase of CARB pass, the basal plane texture of Mg-5Li1Al alloy weakens, the dR/dT value gradually tends to 1. The nonbasal slip is activated. 4 With the increase of the CARB pass, the yield strength and tensile strength of the sheet show an increasing trend. The tensile strength of the CARB6 sheet is up to 306.29 MPa and 304.49 MPa in RD and TD, respectively. CARB is helpful to decrease the anisotropy of Mg5Li-1Al sheet.
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Microstructure and mechanical properties of Mg-5Li-1Al sheets processed by cross accumulative roll bonding
T
⁎
Huajie Wua, Tianzi Wangc, Ruizhi Wua,b, , Legan Houa, Jinghuai Zhanga, Xinlin Lia, Milin Zhanga a
Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, PR China College of Science, Heihe University, Heihe University, Heihe, 164300, PR China c Suzhou Changfeng Avionics Co., LTD., Suzhou, 215151, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mg-Li alloy Cross accumulate roll bonding Dynamic recrystallization
Mg-5Li-1Al alloy was processed by cross accumulative roll bonding (CARB). The processed Mg-5Li-1Al sheet possesses ultra-fine grains, and its strength increases both in the rolling direction (RD) and transverse direction (TD) with the elongation remaining 16˜18%. The proper CARB processing is helpful to improve the isotropy of mechanical properties of Mg-5Li-1Al sheet. At the same time, the CARB process has a beneficial effect on improving the plasticity of Mg-5Li-1Al sheet. With the change of the direction between the CARB passes, the basal texture of Mg-5Li-1Al sheet is weakened, and the non-basal texture is strengthened. Both prismatic slip and pyramidal slip are initiated. The dominant deformation mechanism changes from twin deformation to slip deformation, and finally shear deformation. The grain refinement mechanism changes from twin dynamic recrystallization to continuous dynamic recrystallization, and finally rotational dynamic recrystallization.
1. Introduction With the advantages of low density, high specific strength and good electromagnetic shielding properties, magnesium alloys attract much attention from researchers, especially Mg-Li alloy is famous for its ultralight. Wu et al. [1] demonstrated the preparation, processing technologies and surface processing technologies of Mg-Li alloys, which possess lower density (1.35–1.65 g/cm3). Moreover, Zhang et al. [2] suggested the addition of lithium also improves the plasticity of the Mg alloy. Compared with Al-Li alloy [3], the absolute strength of Mg-Li alloy is insufficient. Zhang et al. [4] held the view that rare earth element microalloying is an effective strengthening method, but leading to an increase of the density. Then we expect to refine the grain through severe plastic deformation (SPD) to improve its strength. Tsuji et al. [5] first put forward accumulative roll bonding (ARB) as a SPD technology, which can realize continuous production of ultra-fine grain (UFG) materials on a large-scale. At the same time, Tsuji pointed out that the uniform elongation of materials processed by ARB is limited due to the plastic instability. In order to research the microstructure evolution rule in ARB process, Kamikawai et al. [6] investigated the effect of residual shear strain on the microstructure and texture of IF steel, which generated the microstructure inhomogeneity throughout the thickness at low pass, and the uniform UFG structures requiring more passes. Wu
et al. [7] applied ARB process to improve the strength of Mg-Li alloy, and improved its plasticity by proper annealing treatment. Hou et al. [8] have prepared Mg-5Li-1Al sheets by conventional ARB technology, which have good mechanical properties and interfacial bonding properties. However, there still exist some structural inhomogeneity and anisotropy in mechanical properties along the direction of RD and TD. Fatemeh et al. [9] considered that cross rolling (CR) can promote dynamic recrystallization, and obtain isotropic AA2024 sheet compared with other strain paths. In order to make the reinforcements evenly distributed in the matrix, Alizadeh et al. [10] put forward the cross accumulative roll bonding (CARB) process, during which the rolling direction between two cycles changes by 90° around the normal direction (ND) so that the uniformity of the microstructure and the mechanical property of the alloy is improved. Ardakani et al. [11] investigated that microstructure and mechanical properties of the Al/Al2O3 composites with uniform distribution of Al2O3 particles by CARB. The average grain size of the composites was 71 nm, and the tensile strength of the composites was up to 344 MPa. Verstraete et al. [12] found that CARB not only weakens the texture and anisotropy, but also makes the strength and plasticity of the sheet higher than that of ARB. Zhang et al. [13] prepared bulk Cu/Ta layered composites with high strength (UTS = 950 MPa) and thermal stability by CARB and intermediate annealing, because this method can inhibit edge crack and
⁎ Corresponding author at: Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, 150001, PR China. E-mail address: [email protected] (R. Wu).
https://doi.org/10.1016/j.jmapro.2019.09.004 Received 3 June 2019; Received in revised form 30 August 2019; Accepted 1 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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shows that the grain refinement of the alloy is mainly based on twinning-induced dynamic recrystallization supposed by Peng et al. [18]. Due to the change of rolling direction between passes, the distribution of fiber structure along the direction of RD decreases and there is a tendency of dispersion along the direction of TD. With the increase of CARB pass, the distribution uniformity of microstructure increases, especially after 4 passes (Fig. 1(d)), the distribution of fibrous structure almost shows no specific orientation. The recrystallized grains continuously increase and are refined. Sheets processed by CARB6 have the most uniform microstructure and fine recrystallization grain. Compared with the previous ARB rolled sheets [19], the reduction of the twins and fibrous structures is faster, the microstructure uniformity is more obvious, but the grain is slightly coarser. Fig. 2 shows the RD-ND microstructure of the CARB-processed Mg5Li-1Al sheet. There appears a small amount of macroscopic shear bands along 45° with the RD direction in the vicinity of surface of the CARB1 plate (Fig. 2(a)). With the increase of CARB passes, the macroscopic shear bands are refined and the number of them increases. After 4 CARB passes, the macroscopic shear bands are finer and more uniform, and a small number of fine dynamic recrystallization grains appear between the shear bands (Fig. 2(c)). There still remain some fine macroscopic shear bands in the microstructure of CARB6 sheets (Fig. 2(d)). TEM and SAED patterns of Mg-5Li-1Al alloys processed by CARB are shown in Fig. 3. The average grain size of CARB1 sheet is 500–700 nm, as shown in Fig. 3(a), and the grain orientation difference is small. The twins at high magnification are shown in the bottom left corner of Fig. 3(a), which indicates that there are many twins in the alloy after CARB1. After CARB2, grains are obviously refined, about 250–300 nm (Fig. 3(b)). The number of twins decreases rapidly, and there are a few short dislocation lines in the grains, and some parallel dislocation walls distribute at the grain boundaries, but the overall distribution is more dispersed. In the SAED, the diffraction spots are isolated and reticulated, and the degree of isolation is greater than that of CARB1, which indicates that the inter-grain orientation difference of sheets decreases because of the changing of rolling direction. For CARB4 sheets, the fine grain distribution is more uniform, as shown in Fig. 3(e). There are many dislocation tangles at the grain boundary and sub-grains. The diffraction spots are almost connected to a ring, indicating the orientation difference between the grains increases. The average grain size of CARB6 alloy is 150–200 nm (Fig. 3(d)), and the number of dislocations is slightly higher compared with that of CARB4 alloy. The SAED pattern in CARB6 shows isolated spots indicating decrease in orientation difference. This is mainly because with the increase of cumulative strain in CARB6 sheet, fine grains begin to be produced in the macroscopic shear bands, indicating that the rotating dynamic recrystallizing mechanism is started. With the change of rolling direction between passes, the grain size of Mg-5Li-1Al alloy decreases rapidly after the second pass, and the microstructure uniformity of each pass sheet increases. The pole figures of CARB-processed Mg-5Li-1Al sheets are shown in Fig. 4. The CARB1 sheet exhibits typical magnesium alloy rolling texture, and (10–11) and (10–12) pyramidal slip systems are also slightly initiated. With the increase of CARB pass, the maximum pole density of (0002) decreases from 10.756 m.r.d (Fig. 4(a)) to 8.216 m.r.d (Fig. 4(d)), and that of CARB 6 is almost the same as CARB4 sheet. During the CARB process, the contour shape of the basal plane and the c-axis orientation of the grains also change obviously. The pole contour of CARB1 is flattened along TD, and the deflection degree of the c-axis along RD is large, dR/dT =1.79, as shown in Fig. 4(a). After CARB2 process, the pole contour produces 90° rotation as a whole, the deflection direction of c-axis translates into the TD, dR/dT =0.64 (Fig. 4(b)), indicating that the change of rolling direction also changes the grain orientation of the sheets. With the increase of CARB pass and the change of rolling direction, the pole contour of basal plane gradually changes from ellipse to circle, and the dR/dT value gradually tends
inter-laminar plastic instability. Duan et al. [14] studied the texture distribution of pure nickel proceed by ARB and CARB and concluded that the texture of the latter was significantly weakened and CARB process could reduce the plane anisotropy. They all agreed that the composite microstructure prepared by the CARB was more uniform, and the mechanical properties were improved, and the anisotropy was reduced, compared with the conventional ARB process. As the microstructure of Mg alloy after deformation was inhomogeneous and the anisotropy was obvious due to hexagonal closepacked (HCP) structure, as described in Davis et al. [15], CARB may be an effective processing method. Although the addition of lithium can improve the lattice symmetry, the microstructure of Mg-5Li-1Al alloy is still α-Mg matrix based on HCP structure, which can be seen from the research of Wang et al. [16]. In this work, Mg-5Li-1Al alloy was carried out 6-pass CARB process. The microstructure, mechanical properties and texture evolution of each pass were investigated. The plastic deformation mechanism and grain refinement mechanism in the process of CARB were discussed. 2. Experiment Mg-5Li-1Al alloy was prepared by induction melting with argon protection, and the raw materials consist of commercial pure magnesium ingot (99.9 wt.%), commercial pure lithium ingot (99.9 wt.%) and commercial pure aluminum ingot (99.9 wt.%). Because of the high activity of pure lithium, the lithium ingot is soaked in kerosene. Before usage, lithium ingot was taken out from kerosene and defatted cotton was used to remove kerosene from the surface of ingot. Then it was weighed and put into crucible with other raw materials, and finally melted under the protection of argon atmosphere. The melt was poured into a permanent mould to obtained as-cast specimen. Then the as-cast specimen was homogenized at 300 °C for 12 h. The received ingot was cut into original sheets with the dimensions of 50 mm × 30 mm × 2 mm for the subsequent CARB process. In the CARB process, the sheet was heated at 350 °C for 10 min before the first pass and for 5 min at the interval between two passes. The reduction of each pass was 55%. In each pass, the rolling direction was rotated 90° around normal direction of sheet. RD and TD in the first pass during CARB were defined as the directions of RD and TD in the paper. The detail procedures about ARB process can also be found in our previous literature [17]. The microstructure of RD-TD surface and RD-ND surface of Mg-5Li1Al sheet was observed by LEICA DM IRM metallographic microscope. Specimens were etched with 4% nitric acid alcohol for 10–15 s. The grain size and dislocation of Mg-Li alloy sheet during CARB were observed by JEM-2100 transmission electron microscope (TEM), and the crystal structure of the alloy was determined by selective area electron diffraction (SAED). The TEM images are taken in RD-ND plane, and 20 TEM images were used to estimate the grain size. The texture test was carried out by X' Pert Pro X-ray diffractometer, during which the Cu Kα (λ =0.1542 nm) was selected. The tensile test at room temperature was carried out on the WDW3050 electronic universal tester with a strain rate of 10−3 s-1. JEOL SM-6360LV scanning electron microscope (SEM) was used to analyze the fracture morphology. 3. Results 3.1. Microstructure The RD-TD microstructure of Mg-5Li-1Al alloy is shown in Fig. 1. Fig. 1(a) shows the pre-CARB structure in which many twins appear. In Fig. 1(b), a number of twins and fibrous structures distribute along the rolling direction, and a few dynamic recrystallization grains appear around the twin structure, as shown in Fig. 1(f). After the second pass, the number of twinning is rapidly reduced, and the number of recrystallized grains is obviously increased, as shown in Fig. 1(c), which 140
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Fig. 1. Microstructure of the Mg-5Li-1Al sheets (RD-TD plane) (a) as-rolled, (b)CARB1, (c)CARB2, (d) CARB4, (e) CARB6, (f)CARB1 at high magnification.
The inverse pole figures of CARB-processed Mg-5Li-1Al sheets are shown in Fig. 5. The dense poles are distributed between < 0002 > and < 11−20 > , and tend to be about 20° in the direction of < 11−20 > . With the increase of CARB passes, the main slip direction does not change obviously, but the maximum pole density decrease first and then increase. The maximum pole density decreases from 19.066 m.r.d of CARB1 to 16.862 m.r.d of CARB4, and the number of contours between < 11−20 > and < 10−10 > increases, indicating that the basal plane texture intensity decreases and the non-basal plane texture intensity increases, which is beneficial to the decrease the anisotropy. However, the CARB6 maximum pole density increases abruptly to 26.182 m.r.d, and the contours decrease in other directions, which reflects the increase of the basal plane texture and the difficulty of continuing sliping.
to 1, which suggests the deflection of c-axis of grain in sheets is getting smaller and smaller. In CARB6, dR/dT = 1.02 (Fig. 4(d)), showing that the grain tends to be consistent along the direction of RD and TD, which is beneficial for the weakening of the anisotropy in Mg-5Li-1Al sheets. In addition, in the (0002) pole figure of CARB6 sheets, the area of the maximum pole density is distributed in the position where the c axis is the center, the TD direction is symmetrical axial, deflecting ± 15° along the RD direction, forming a bimodal shape along the RD direction, as shown in Fig. 4 (d). S. R. Agnew et al. [20] suggested this shift is caused by the start of < a+c > slip mode. According to the subsequent analysis, it can be concluded that the double peak characteristic of the basal texture of CARB6 sheets is mainly caused by the basal plane slip due to the complex deformation. In addition, in the CARB process, the pole distribution in the nonbasal pole figure also rotates with the change of rolling direction. As shown in Fig. 4(b), the pole distributions of (10-10), (10–11) and (10–12) pole figures in CARB2 sheets are changed by 90°. In the nonbasal plane pole figure of CARB6 sheets, the poles are evenly distributed on the circle with a certain angle centered on the c-axis (Fig. 4(d)). By comparing the non-basal plane pole intensity, with the increase of CARB passes, the prismatic texture intensity increases, while the pyramidal texture intensity decreases a little, but the distributions of them are all around the center, indicating that both prismatic slip and pyramidal slip are initiated in CARB process.
3.2. Mechanical properties The stress-strain curves of Mg-5Li-1Al sheets along TD and RD are shown in Fig. 6. With the increase of the CARB pass, the yield strength and tensile strength in RD and TD both increase, and the tensile strength of CARB6 sheets reaches 306.29 MPa and 304.49 MPa along RD and TD, respectively. However, the elongation of Mg-5Li-1Al sheets increases first, then decreases slowly before the CARB5 pass, remaining 16–18%. But after the CARB6 pass, the elongation along TD decreases 141
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Fig. 2. Microstructure of the CARB-processed Mg-5Li-1Al sheets (RD-ND plane). (a)CARB1, (b)CARB2, (c) CARB4, (d) CARB6.
held the view that indicator of plane anistropy (IPA) is always used to characterize the anisotropy of metallic plate, which needs three values from different directions. In this work, for simplicity, a new parameter, m, is defined as tensile strength anisotropy, m=|UTSRD/UTSTD-1|, where UTS is the ultimate tensile strength. As can be seen from Fig. 7, after 2 ARB passes, the anisotropy of tensile strength decreases obviously. While as for CARB process, after 1 pass, the anisotropy decreases obviously and it is always lower than that of ARB. It can be concluded that appropriate CARB process is helpful to decrease the anisotropy of Mg-5Li-1Al sheet.
obviously. When the plastic deformation is carried out to a higher pass (CARB6), the continuous increase of rolling pass will only enhance the rotating dynamic recrystallization. At the same time, the pole strength of < 110 > slip direction in the reverse pole figure increases sharply in CARB6, indicating that the basal slip is difficult and easy to cause the stress concentration and local shear fracture of the alloy, so the plasticity of CARB6 sheet along the RD direction and the TD direction is obviously different. The anisotropy of mechanical properties of Mg-5Li-1Al sheets processed by ARB [8] and CARB is shown in Fig. 7. Medjahed et al. [21]
Fig. 3. TEM images of the CARB-processed Mg-5Li-1Al sheets (a) CARB1, (b) CARB2, (c) CARB4, (d) CARB6, (e) CARB4 20000×, (f) CARB6 20000×. 142
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Fig. 4. Pole figures of the CARB-processed Mg-5Li-1Al sheets. (a) CARB1, (b) CARB2, (c) CARB4, (d) CARB6.
Fig. 5. Inverse pole figures of the CARB-processed Mg-5Li-1Al sheets with respect to ND (a)CARB1, (b)CARB2, (c)CARB4, (d)CARB6.
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Fig. 6. The stress-strain curves of the CARB-processed Mg-5Li-1Al sheets. (a) Along RD, (b) Along TD.
The tensile fracture morphologies of the CARB-processed Mg-5Li1Al sheets are shown in Fig. 8. There are many equiaxed dimples in the fracture microstructure of CARB1, CARB2 and CARB4 sheets. With the increase of CARB pass, the number of dimples increases, but the dimples become more shallow and smaller, indicating a stable range of the elongation. The fracture mode is mainly ductile fracture. For the CARB6 sheet (Fig. 8(d)), the number of dimples decreases obviously, and a small number of cleavage planes appear, indicating a relatively poor plasticity. The fracture mode of the sheet is a mixture of ductile fracture and cleavage fracture. As a result, the elongation of the CARB6 sheets decreased significantly. In addition, the interface layers do not crack during the tensile process, showing a good bonding during CARB. 4. Discussions 4.1. Force analysis in CARB process
Fig. 7. Tensile strength anisotropy of Mg-5Li-1Al sheets after the ARB (Hou L.G., 2018) and CARB process.
The stress state and flow of the metal in the rolling process are
Fig. 8. Fracture morphology of the CARB-processed Mg-5Li-1Al sheets. (a) CARB1, (b)CARB2, (c) CARB4, (d) CARB6. 144
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Fig. 9. The stress diagram of the alloy during rolling process. (a) Entering into the rolling mill, (b) Stable rolling stage.
crucial to the microstructure and mecha(Q − F ) + 2Tx 2Px nical properties of the sheet. At the moment of entering into the rolling mill, the sheet is subjected to positive pressure P, friction force T, external thrust Q and inertia force F, as shown in Fig. 9(a). Under the combined action of the four forces, the sheet is bitten into the roll. The force condition satisfies the Equation 4-1.
(Q − F ) + 2Tx > 2Px
(4-1)
Where Px is the horizontal component of P, Tx is the horizontal component of T. After the front end of the sheet reaches the center of the two rolling rolls, it comes to the stable rolling stage (Fig. 9(b)). The action of Q and F is no longer required, that is, the rolling process can be successfully completed under the following critical equilibrium relation, Equation 4-2.
2Tx − 2Px = 0
(4-2)
After entering the stable rolling stage, the friction component Tx increases and the rolling resistance Px decreases gradually. So the residual friction (Ts) becomes more and more abundant, Ts=Tx-Px. According to Sakai et al. [22], the residual friction force acts on the surface of the sheet, resulting in inhomogeneous deformation in the direction of thickness. Fig. 10 is a schematic diagram of the stress during rolling. The rolling deformation zone is roughly divided into four parts, ADB, CGE, ADGC and BDGE. According to the law of minimum resistance, the friction force can be divided into longitudinal friction σ3 and transverse friction σ2. The metal in the regions of ADB and CGE flows along the transverse direction, while the metal in the regions of ADGC and BDGE flows along the longitudinal direction. Because the metal in the center of the longitudinal region is elongated without broadening, the longitudinal elongation at the center is larger than that at the edges, and there must be a stress σAB at the center, which is opposite to σ3. The σAB is the shear stress at the edges during CARB process.
Fig. 10. The transverse flow in the deformation zone of the alloy during rolling process.
reduction. Accordingly, the metallurgical bonding is realized at the interface between the two metal sheets. For the CARB2 sheets, the residual shear friction generated on the surface during CARB1 is brought into the interior of sheet, while the shear friction on the two surfaces continues to increase, causing the formation of a large number of macroscopic shear bands on the RD-ND surface, as shown in Fig. 2. Generally speaking, the longitudinal extension of the alloy is larger than that of the transverse extension (σ2 < σ3) during rolling, the stress along RD is larger and the dynamic recrystallization is more likely to occur. Therefore, the strength and elongation of the CARB1 sheets along RD are higher than TD. Due to the change of the rolling direction in the CARB2 sheet, the friction of σ2 and σ3 also rotates 90° along the width direction, the direction of σAB changes accordingly. The shear stress along TD increases. Therefore, with the increase of CARB pass, the anisotropy of Mg-5Li-1Al alloy decreases gradually. The CARB process also plays a beneficial role in the texture of
4.2. Effect of rolling route on the anisotropy During the CARB1 process, a large amount of residual shear friction is produced on the surface of the sheet due to the large amount of 145
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Declaration of Competing Interest
sheets. Ruppert [23] demonstrated that the grain orientation of each pass changes due to the change of the direction of shear stress in the CARB process. Many potential slip systems can be activated. As shown in Fig. 4, with the increase of CARB pass, the basal plane texture of Mg5Li-1Al alloy weakens, the dR/dT value gradually tends to 1, and the non-basal plane slip system is activated. Agnew et al. [24] showed that in the alloy with strong texture distribution, the shear stress along the basal plane is almost zero and the slip is difficult to carry out. Accordingly, it was easy to cause stress concentration and local shear fracture of the alloy. When the base texture of (0002) is weakened, the plasticity of the alloy is enhanced. Therefore, compared with ARB process, CARB is beneficial for the improvement of the plasticity of Mg5Li-1Al sheets.
None. Acknowledgments This paper was supported by National Natural Science Foundation of China (51671063, 51771060, 51871068), Heilongjiang Province Natural Science Foundation (ZD2017010), the Fundamental Research Funds for the Central Universities (HEUCFG201834), Harbin City Application Technology Research and Development Project (2017RAQXJ032). References
4.3. Deformation mechanism in CARB process [1] Wu RZ, Yan YD, Wang GX. Recent progress in magnesium-lithium alloys. Int Mater Rev 2015;60(2):65–100. [2] Zhang TL, Tokunaga Ohno M, Zhang ML. Fabrication of Al-coated Mg-Li alloy sheet and investigation of its properties. Acta Metall Sin 2019;32(2):169–77. (English Letters). [3] Wang Y, Zhang Z, Wu RZ, Sun JF, Jiao YL, Hou LG, et al. Ambient-temperature mechanical properties of isochronally aged 1420-Sc-Zr aluminum. Mater Sci Eng A 2019;745:411–9. [4] Zhang JH, Liu SJ, Wu RZ, Hou LG, Zhang ML. Recent developments in high-strength Mg-RE-based alloys: Focusing on Mg-Gd and Mg-Y systems. J Magnes Alloy 2018;6:277–91. [5] Tsuji N, Ito Y, Saito Y, et al. Strength and ductility of ultrafine grained aluminum and iron produced by ARB and annealing. Scr Mater 2002;47(12):893–9. [6] Kamikawa N, Sakai T, Tsuji N. Effect of redundant shear strain on microstructure and texture evolution during accumulative roll-bonding in ultralow carbon if steel. Acta Mater 2007;55(17):5873–88. [7] Wu HJ, Wang TZ, Wu RZ, Hou LG, Zhang JH, Li XL, et al. Effects of annealing process on the interface of alternate α/β Mg-Li composite sheets prepared by accumulative roll bonding. J Mater Process Technol 2018;254:265–76. [8] Hou LG, Wang TZ, Wu RZ. Microstructure and mechanical properties of Mg-5Li-1Al sheets prepared by accumulative roll bonding. J Mater Sci Technol 2018;34(2):317–23. [9] Fatemeh G, Roohollah J. Effect of strain path during cold rolling on the microstructure, texture, and mechanical properties of AA2024 aluminum alloy. Mater Res Express 2019:6. [10] Alizadeh M. Processing of Al/B4C composites by cross-roll accumulative roll bonding. Mater Lett 2010;64:2641–3. [11] Ardakani MRK, Amirkhanlou S, Khorsand S. Cross accumulative roll bonding- A novel mechanical technique for significant improvement of stir-cast Al/Al2O3 nanocomposite properties. Mater Sci Eng A 2014;591:144–9. [12] Verstraete K, Helbert AL, et al. Microstructure, mechanical properties and texture of an aa6061/aa5754 composite fabricated by cross accumulative roll bonding. Mater Sci Eng A 2015;640:235–42. [13] Zhang T, Zeng LF, Gao R, Fang QF, et al. High strength and thermal stability of bulk Cu/Ta nanolamellar multilayers fabricated by cross accumulative roll bonding. Acta Mater 2016;110:341–51. [14] Duan JQ, Quadir MZ, Ferry M. Engineering low intensity planar textures in commercial purity nickel sheets by cross roll bonding. Mater Lett 2016;188:138–41. [15] Davis AE, Robson JD, Turski M. Reducing yield asymmetry and anisotropy in wrought magnesium alloys-a comparative study. Mater Sci Eng A 2019;744:525–37. [16] Wang TZ, Zhu TL, Sun JF, Wu RZ, Zhang ML. Influence of rolling directions on microstructure, mechanical properties and anisotropy of Mg-5Li-1Al-0.5Y alloy. J Magnes Alloys 2015;3:345–51. [17] Wang TZ, Zheng HP, Wu RZ. Preparation of fine-grained and high-strength Mg–8Li–3Al–1Zn alloy by accumulative roll bonding. Adv Eng Mater 2016;18:304–11. [18] Peng JH, Zhang Z, Li YZ, Zhou W, Wu YC. Twinning-induced dynamic recrystallization and micro-plastic mechanism during hot-rolling process of a magnesium alloy. Mater Sci Eng A 2017;699:99–105. [19] Zhong F, Wang T, Wu RZ, et al. Microstructure, texture, and mechanical properties of alternate Mg-Li composite sheets prepared by accumulative roll bonding: microstructure, texture and mechanical properties of alternate α/β composite sheets. Adv Eng Mater 2017;19(5):16–7. [20] Agnew SR, Horton JA, Yoo MH. Investigation of c+a dislocation structures in Mg and Mg-Li α-solid solution alloys. Metall Mater Trans A 2002;23:813–22. [21] Medjahed A, Moula H, Zegaoui A, Derradji M, Henniche A, Wu RZ, et al. Influence of the rolling direction on the microstructure, mechanical, anisotropy and gamma rays shielding properties of an Al-Cu-Li-Mg-X alloy. Mater Sci Eng A 2018;732:129–37. [22] Sakai T, Saito Y, Matsuo M, et al. Inhomogeneous texture formation in high speed hot rolling of ferritic strainless steel. ISIJ Int 1991;31:86–94. [23] Ruppert M, Hoppel HW, Goken M. Influence of cross-rolling on the mechanical properties of an accumulative roll boned aluminum alloy AA6014. Mater Sci Eng A 2014;597:122–7. [24] Agnew SR, Duygulu O. Plastic anistropy and the role of non-basal slip in magnesium alloy AZ31B. Int J Plast 2005;21:1161–93.
During the CARB process, the plastic deformation and grain refinement mechanism of Mg-5Li-1Al alloy also change with the change of rolling direction. According to the results of TEM (Fig. 3), there are a lot of twins in the microstructure of CARB1 sheet, and there exist fine dynamic recrystallized grains around them. The number of twins in the CARB2 sheet decreases rapidly, and the number of dynamic recrystallization grains increases. It can be concluded that the dominant deformation mechanism of Mg-5Li-1Al alloy in the first two passes of CARB is twinning deformation, and the grain refinement mechanism is twinning induced dynamic recrystallization. In addition, a small number of dislocation lines and dislocation walls also appear in the CARB2 sheet. Dislocation entanglement occurs at the grain boundaries and sub-crystals appear in the local region of the grains. Moreover, every CARB-processed sheet shows typical rolling texture because of large plastic deformation, and the prismatic slip and pyramidal slip system are initiated after changing the rolling direction. According to the results, it can be concluded that the dominant deformation mechanism of Mg-5Li-1Al alloy is slip deformation, which includes the basal plane slip, the prismatic plane slip and the pyramidal plane slip, and the grain refinement mechanism accords with the characteristics of continuous dynamic recrystallization. As can be observed from Fig. 2, due to the continuous increase and accumulation of rolling reduction, fine grains are formed in some macro shear deformation bands. At the same time, the rotational dynamic recrystallization is enhanced. The pole intensity of < 11−20 > in the reverse pole figure increases sharply in the CARB6 sheet, and the slip deformation becomes gradually difficult, indicating that the plastic deformation of the sheet is mainly shear deformation. 5. Conclusions 1 With the increase of CARB pass, the surface residual shear friction produced by the previous pass is gradually brought into the sheet interior, and the whole sheet is finally subjected to complex shear stress. 2 The average grain size of CARB6 sheet is 150–200 nm Compared with the traditional ARB process, the dislocations disperses slightly and the grain refinement rate slows down, but the microstructure uniformity increases due to the change of the rolling direction. 3 With the increase of CARB pass, the basal plane texture of Mg-5Li1Al alloy weakens, the dR/dT value gradually tends to 1. The nonbasal slip is activated. 4 With the increase of the CARB pass, the yield strength and tensile strength of the sheet show an increasing trend. The tensile strength of the CARB6 sheet is up to 306.29 MPa and 304.49 MPa in RD and TD, respectively. CARB is helpful to decrease the anisotropy of Mg5Li-1Al sheet.
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