Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two - stage rolling

Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two - stage rolling

Author’s Accepted Manuscript Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two - stage rolling Ru Ma, Yiqu...

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Author’s Accepted Manuscript Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two - stage rolling Ru Ma, Yiquan Zhao, Yinong Wang

PII: DOI: Reference:

S0921-5093(17)30278-2 MSA34783

To appear in: Materials Science & Engineering A Received date: 6 January 2017 Revised date: 27 February 2017 Accepted date: 28 February 2017 Cite this article as: Ru Ma, Yiquan Zhao and Yinong Wang, Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two stage rolling, Materials Science & Engineering A, This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Grain refinement and mechanical properties improvement of AZ31 Mg alloy sheet obtained by two - stage rolling Ru Ma, Yiquan Zhao, Yinong Wang* School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China *

Corresponding author. Tel.: +86 0411 [email protected] (Y.N. Wang)

Abstract: A simple process named two - stage rolling was proposed and investigated to obtain a fine-grained microstructure of AZ31 Mg alloy sheet. The first stage rolling is deformed at 773K with a large reduction by single pass, while the second stage process is a multi-pass warm rolling deformed at 573K. Microstructure and texture were measured on sheets subjected to rolling experiments of different reductions, and the tensile test was also carried out. After the two-stage rolling and subsequent annealing, reductions as large as 89% were achieved without surface cracking; the AZ31 alloy sheet exhibits a homogenous microstructure with fine grains and weak basal texture, where the final average grain size is stabilized around 3 μm; thus, it shows the high strengths and good ductility. Keywords: magnesium alloys; large strain hot rolling; warm rolling; grain refinement; mechanical properties 1. Introduction Magnesium (Mg) and its alloys have been widely applied in the industrial area, particularly in some automotive applications, since they could replace heavier materials such as steel or aluminum alloys due to their low density (1.74 g cm3) and good specific properties [1]. However, the strength and ductility of Mg alloys are relatively low at room temperature due to their hcp lattice. To overcome these difficulties, important efforts are being devoted to microstructural design of Mg alloys. Several researchers found that the room temperature ductility and strength of the Mg alloy can be enhanced by controlling texture and grain refinement [2, 3]. It is well known that the yield strength of Mg alloys at room temperature may be significantly increased by grain refinement per the Hall-Petch law, and grain refinement in Mg alloys can be achieved in different ways. Several methods based on severe plastic deformation (SPD), such as equal channel angular processing (ECAP) [4, 5]; differential speed rolling (DSR) [6, 7]; single roller drive rolling (SRDR) [8]; and accumulative roll bonding (ARB) [9] have allowed engineering of fine grained microstructures with even submicron or nanocrystalline grains. The drawbacks of these processing routes are, however, remarkable, the forming limitations, process complexity and expensive cost. The significant grain refinement can also be achieved using more conventional thermomechanical processing methods, such as large strain rolling (LSR) [10, 11], extrusion of the as-cast material [12], when the values of

Zener–Hollomon parameter (Z = ε exp (Q/RT)) is high (high strain rates or low temperatures). The idea behind LSR is to produce the largest amount of deformation through the smallest number of rolling passes without causing material failure. Its simplicity and the possibility of forming large parts allow LSR to be scaled-up for industrial use. However, the effect of parameters such as deformation temperature, thickness reduction, rolling pass, initial texture and others on the microstructural development during LRS still need to be investigated. To explore the feasibility of grain refinement in AZ31 Mg alloy by LSR, a simple thermomechanical processing (TMP) consisted of two stages of rolling were proposed and thus named as two - stage rolling. The first stage (I-stage) is the hot rolling that carried out at high temperature by a single pass with a large reduction, while the second stage (II-stage) is the warm rolling that carried out at a lower temperature with multiple passes. The microscopic deformation mechanisms that allow significant grain refinement were investigated by means of macro- and micro-texture analysis and optical microscopy. Additionally, the mechanical properties of the annealed AZ31 Mg alloy sheets were also investigated by tensile test at room temperature. 2. Experimental procedures As-cast AZ31 Mg alloy (Mg, 3 mass % Al and 1 mass % Zn) plate ingot with 9 mm in thickness was prepared as the starting material for processing. Before rolling, the ingot was annealed at 673K for 5h to homogenize the microstructure. The two - stage rolling consisting of hot and warm rolling was utilized to obtain fine-grained microstructure. In the first stage, the plate ingots were hot rolled at 773K with different reductions of 30%, 50%, 65% and 80%, respectively, by a single pass. And the II-stage rolling was a multi-pass warm rolling that done upon the I-stage rolled sheet with a reduction ratio of about 20% per pass at a lower temperature of 573K. The sheets were reheated to the target temperature and holding for 5 min to stabilize the rolling temperature between passes. All specimens were air cooled after each rolling experiment and the surfaces were examined to check whether the side-cracking occurred. Finally, the rolled sheets were annealed at 573K for 5min prior to tensile tests. The rollers with a diameter of 200mm were used for the rolling experiments and without heating during deformation to improve the simplicity of the process. The rolling speed is 440 mm/s (4.2 rpm) and graphite was used as a lubricant. Microstructure of the transverse direction (TD) plane closed to the middle position of the rolled and annealed sheets was observed by optical microscope. The grain size was measured by the line intercept method [13]. Macro-texture of the rolled sheets was measured at the mid-thickness in normal direction (ND) by X-ray diffraction (XRD) using an EMPYREAN equipment in the reflection mode with Cu Ka radiation. The microstructure and micro-texture of the rolling plane of specimens were also 2

measured by the electron backscattered diffraction (EBSD), using an automated high-resolution EBSD system (JEOL7001F) with the Oxford HKL Channel 5 software in a Zeiss Supra 55 FEG-SEM. The working distance for the EBSD measurement was 16mm with a tilt angle of 70º. Tensile tests were carried out using samples with a gauge length of 10 mm and a width of 3 mm along the rolling direction (RD). The triplicate universal tensile tests were conducted to ensure the reproducibility of tensile results using a standard universal testing machine (Instron 4206) at room temperature with a normal strain rate of about 1×10-3 s-1. 3. Results and discussion 3.1 Microstructure and texture of the sheets rolled by I-stage rolling The optical microstructure (OM) of the initial homogenized AZ31 Mg alloy sheet is illustrated in Fig. 1. It can be found the microstructure of the cast sample was sufficiently homogenized and consisted of large grains (d > 100μm) without precipitations.

Fig. 1. OM of the initial homogenized AZ31 Mg alloy sample

Fig. 2 shows the OM of the AZ31 alloy sheets rolled at 773K with different reductions. After the I-stage rolling, dynamic recrystallization (DRX) were taken place in all rolled sheets resulting in the grain refinement despite the microstructures were not homogeneous. When the Mg alloys processed at a high temperature, the activation energy of plastic flow increased to a value close to the activation energy for magnesium self-diffusion; i.e., the controlling process is dislocation climb [14]. Microscopic strain localization at slip lines causes formation of bulges of grain boundaries which leads to nucleation of DRX grains [15], as observed in Fig 2a to 2c. And the subsequent grain growth is processed by strain-induced boundary migration. This phenomenon indicates that the discontinuous dynamic recrystallization (DDRX) was taken place in AZ31 alloy during I-stage rolling, and dislocation climb controlled by self-diffusion is the controlling process for plastic deformation. It’s worth noting that the recrystallization fraction increased with increasing the 3

rolling reduction, while the size of new recrystallized grains decreased gradually. However, large grains are always present in the microstructure, especially in the rolled sheet with 80% reduction, a sharply bimodal microstructure developed which consisted of large grains and very fine recrystallized grains. To get the details of deformation behavior of the rolled sheets, the microstructures were also measured by EBSD, as exhibited in Fig. 3. It can be found that the large grains (as indicated by A) may be the original grains with their c-axis parallel to ND and insensitive to basal slip and dynamic recrystallization, while the finer recrystallized grains can be attributed to the larger strain that led to more recrystallizations. By comparing the distribution characteristics of grain size of all the rolled sheets, it can be concluded that the sheet with a reduction of 65% exhibits a relatively more homogeneous microstructure.

Fig. 2. OM of the AZ31 alloy sheets rolled at 773K with the reductions of (a) 30%; (b) 50%; (c) 65%; (d) 80%. The RD is horizontal.

Fig. 3. Inverse pole figure (IPF) maps of the AZ31 alloy sheets rolled at 773K with the reductions of (a) 50%; (b) 65%; (c) 80%. The RD is horizontal 4

The (0002) pole figures of I-stage rolled sheets are shown in Fig. 4. From the distribution characteristics of pole figure, it can be inferred the basal texture is progressively strengthened with rolling reduction increased. As indicated by A in Fig. 3, the large basal orientated grains were retained after deformation. During rolling, the resolved shear stress of the large grains with their c-axis parallel to ND is nearly zero. Thus, the large grains are not favorable for basal slip and less hardened compared with other orientated grains. Consequently, both nucleation and growth of new grains are retarded in the large grain, i.e., the large basal orientated grain remains relatively stable during deformation [16]. For the same reason, if a new recrystallized grain with basal orientation is nucleated, as indicated by B in Fig 3, the grain is also considered to be relatively stable and insensitive to DRX. While other orientated grains may easily be consumed by repeated DRX. As the recrystallization fraction increased with increasing the rolling reduction, more and more grains with their c-axis parallel to ND would survive and the basal texture is strengthened. Nevertheless, there are still many original grains with a basal orientation occurred DRX due to the activation of non-basal slips at high temperature and weakened the intensity of basal texture of the rolled sheets. Thus, the maximums of the texture intensity were 8.0, 7.6, 8.2 and 7.9 for the rolled sheet with 30, 50, 65 and 80 % thickness reductions, respectively, which can be said the intensity of basal texture is not strong.

Fig. 4. The (0002) pole figures of AZ31 alloy sheets rolled at 773K with the reductions of (a) 30%; (b) 50%; (c) 65%; (d) 80%

In addition, it is noteworthy that the AZ31 Mg alloy can accommodate very large reductions by a single pass at high temperature. However, for rolling reduction large to 80% some edge cracking was observed despite the crack length was small. Therefore, considering the microstructural characteristics and deformability of the rolled sheet, we selected the hot rolled sheet with a 65% reduction (final thickness ≈ 3.1mm) as the starting material for the II-stage warm rolling. 3.2 Microstructure of the sheets processed by II-stage rolling The details of the II-stage rolling carried out are listed in Table. 1. Fig. 5 presents 5

the microstructures of the rolled sheets with different rolling reductions and after subsequent annealing. For the rolled sheet processed by II-1P, significant microstructural changes occurred: grain boundaries became wavy, extensive twinning was clearly apparent, and some shear bands (as indicated by the black lines) together with little recrystallized grains were also observed (Fig. 5a). As the rolling reduction increased to 82% (II-3P), both the large grains and twinning decreased, on the contrary, the amount of shear bands increased with more recrystallized grains (Fig. 5c). With the largest reduction of 89%, the II-5P rolled sheet exhibits a lot of shear bands and most recrystallizations for its severe plastic deformation and stored energy (Fig. 5e). In addition, all the two - stage rolled sheets had a good surface quality although some edge cracking in small length was observed in the II-5P rolled sheet. Table.1. Routes of two - stage rolling used for grain refinement in the AZ31 alloy Routes


Total reduction

Final thickness/mm













Generally, the warm rolling changed the shape of grains and greatly increased the total grain boundary area. The new grain boundary was created with the incorporation of dislocations which were continuously generated during the deformation process [2]. Extensive twinning occurred at the initial stage of warm rolling, especially in the large grains with basal orientation, which suggested that most of the strain of large grains was accommodated by the twinning. Besides, the twins are considered to be {10-12} twinning which is most easily activated in Mg [17]. Additionally, the grain refinement caused by DRX mainly occurred in the shear bands because the main portion of the plastic deformation was thought to take place in them [18]. Therefore, with the rolling strain increased, there appeared more shear bands corresponding to more recrystallizations occurred and thus a larger number of grain refinement was achieved. After annealing, the deformation microstructure of all the rolled sheets changed to a well-equiaxed structure together with fine well-delineated grain boundaries without twinning and large grains. This result can be attributed to the static recrystallization (SRX) that occurred during the subsequent heat treatment. Twins generally play an important role and may serve as nucleation sites [19], and the new grains were usually formed within compression twins during SRX. In addition, the nucleation and grain growth during SRX occurred along the shear bands resulting in fine grains after the intermediate annealing. As the thickness reduction increased, the microstructure of annealed sheets tended to be more homogenous with decreasing grain sizes. The 6

average grain sizes of the as-annealed sheets rolled by II-1P, II-3P, II-5P are 7, 4.3 and 3 μm, respectively. Compared with the I-stage rolled sheet with a 65% reduction, it can be found that grain refinement takes place mainly during the first pass, and after several passes, the grain size stabilizes around 3 μm. And this result is consistent with that in the AZ31 sheet rolled via accumulative roll bonding as reported by M.T. Perez-Prado et al [9], which suggested that, once a critical minimum grain size is achieved, subsequent passes do not have any noticeable refining effect.

Fig. 5. OM taken in the longitudinal section of the sheets processed by two - stage rolling and subsequent annealing: (a, b) II-1P; (c, d) II-3P; (e, f) II-5P. The RD is horizontal.

3.3 Texture evolutions of the sheets processed by II-stage rolling The (0002) pole figures of II-stage rolled sheets with different rolling passes were 7

obtained from XRD analysis and shown in Fig. 6. After one pass rolling, a weak typical basal texture was already formed (Fig. 6a), and consistent with the texture of the hot rolled sheet (Fig. 4c). At the initial stage of warm rolling, the {10-12} twinning is the dominant deformation mechanism and resulting in a basal texture. This basal texture is further strengthened by the subsequent rollings, which may attributed to the repeated DRX during deformation [20]. It is worthy to note that a double peak in the texture becomes apparent with increasing the rolling passes. In the II-3P and II-5P rolled sheets, the intensity maximum of the (0002) pole figure is displaced from the ideal basal orientation by a rotation of 15o around TD (Fig. 7b, c). Since (0002) plane of most grains are parallel to the rolling direction after the first pass rolling, basal slip Schmid factor of grains with those orientation is nearly zero, and it is not easy for basal slip activation in the following roll process. However, pyramidal slip might be activated due to the decreased grain size [16], which caused by the continuously recrystallizations with the deformation. Correspondingly, addition of pyramidal slip will lead to a characterized double-peak distribution of (0002) pole figure [21].

Fig. 6. The (0002) pole figures of AZ31 alloy sheets processed by two - stage rolling: (a) II-1P; (b) II-3P; (c) II-5P.

In order to get the detail of texture evolution, the microtexture of as-annealed sheets was investigated by EBSD. Fig. 7 illustrates the inverse pole figure maps and (0001) pole figures of the as-annealed sheets. The microstructures of the as-annealed sheets obtained from EBSD were consistent with their OM and exhibited a homogenous structure full of equiaxed grains, indicating the SRX process was completed. From the (0001) pole figures, it can be observed that all the as-annealed sheets exhibit a typical basal texture, and the maximum intensities of the micro texture were 9.7, 9.4 and 11, respectively, larger than those of the as-rolled sheets (Fig. 6). The recrystallization texture of sheets was always influenced by the initial texture and new recrystallized grains, as well as their growth during the heat treatment.


Fig. 7. IPF maps and (0001) pole figures of the as-annealed sheets rolled by (a) II-1P; (b) II-3P; (c) II-5P. The RD of the IPF maps is vertical and the scale bar at the bottom is 50 μm.

During SRX process, twins occurred during warm rolling generally serve as nucleation sites and the new grains usually leads to the persistence of a strong basal texture [22]. The difference in grain growth behavior among the SRXed grains is also likely to be responsible for the change in texture during annealing. It is interesting to note that for the annealed sheet rolled by II-3P and 5P (Fig. 7b, c), the split basal pole was disappeared and replaced by a single basal pole without inclination. This phenomenon is quite like that reported by Mackenzie et al [23], in which the double peaks in the RD were replaced by a single peak after annealing, a phenomenon that was suggested to be due to grain growth. 3.4 Mechanical properties The curve of the engineering stress - strain obtained at room temperature of the as-annealed AZ31 alloy sheets processed by two - stage rolling is illustrated in Fig. 8, 9

and the hot rolled sheet with a 65% reduction was also tested for a comparison. Obviously, after the II-stage rolling and annealing, the tensile strength of all the sheets was increased significantly, especially the yield strength (YS) which was higher than 60MPa compared with the hot rolled sheet. Moreover, these high strength values are attained without much loss of ductility, as there is only a small difference between the failure elongations (FEs) of all the sheets. With increasing the rolling passes during II-stage deformation, the YS and FE of sheet changed slightly, while the ultimate tensile strength (UTS) increased progressively with a larger value.

Fig. 8. The stress - strain curves of the hot rolled and as-annealed sheets.

Magnesium and its alloys show a large Taylor factor due to a lack of slip systems, and hence they exhibit a grain size strongly dependence on stress, allowing high strength to be attained by grain refinement [24]. For this reason, grain refinement during the two - stage rolling is an important factor for the significantly enhancement of mechanical properties (YS and UTS) of the sheets. On the other hand, the basal slip is preferentially activated during tensile test. However, as all the sheets present a basal-type texture, i.e., these basal planes are oriented parallel to the tensile axis, the critical resolved shear stress (CRSS) increases dramatically, leading to a remarkable strength increase. The grain sizes and tensile properties, including the values of YS, UTS and FE attained in the two - stage rolled sheets are compared in Table. 2 with the values reported in the literatures (Refs) [5-8] for the AZ31 Mg alloy. Compared with other processing routes, the two - stage rolling can effectively refine the grain sizes of AZ31 alloy, and the strength and ductility values of the two - stage rolled sheets are relatively higher. Similar tensile property values have been previously achieved mainly only by DSR with more complex processes [6]. It can also be inferred from Table 2 that the YS values of AZ31 alloy obtained via ECAP are significantly lower 10

than those obtained in this study. This phenomenon may be attributed to the inclination of basal plane during EACP as the grains were orientated more favorably for basal slip, and leading to the reduction of CRSS [5]. Therefore, the difference in the tensile values with respect to the two - stage rolled sheets can be attributed to a combination of texture and grain size effects. Table. 2. Comparison of the room temperature properties of AZ31 alloy using different processing Processing method


Grain size























































C 73
























4. Conclusions With the aim of obtaining a fine-grained microstructure of AZ31 Mg alloy sheet, a simple thermomechanical processing method, consisting of two stages of rolling, was proposed and investigated. The I-stage rolling was processed at 773K with a large reduction by a single pass, while the II-stage rolling was a multi-pass rolling deformed at 573K. Some important conclusions are summarized as follows: 1) After the I-stage rolling, the sheet with a reduction of 65% exhibits a relatively more homogeneous microstructure and a weak basal texture. 2) The grain size could be significantly refined by the two - stage rolling, and the largest reduction of rolled sheets can be achieved to 89% while the final grain size stabilizes around 3 μm. 3) Two - stage rolling is a simple and effective method for fabricating high strength AZ31 Mg alloy sheets. The tensile strengths of the sheet increased significantly after the II-stage rolling, especially the yield strength which was about 220MPa that much higher than the hot rolled sheet. Acknowledgements Authors are grateful for the support of the National Natural Science Foundation of China (NO: 51271046). 11

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