Enhancing corrosion resistance and mechanical properties of AZ31 magnesium alloy by friction stir processing with the same speed ratio

Journal Pre-proof Enhancing corrosion resistance and mechanical properties of AZ31 magnesium alloy by friction stir processing with the same speed ratio Fenjun Liu, Yan Ji, Zhiyong Sun, Jianbo Liu, Yanxia Bai, Zhikang Shen PII:

S0925-8388(20)30815-X

DOI:

https://doi.org/10.1016/j.jallcom.2020.154452

Reference:

JALCOM 154452

To appear in:

Journal of Alloys and Compounds

Received Date: 27 October 2019 Revised Date:

20 February 2020

Accepted Date: 21 February 2020

Please cite this article as: F. Liu, Y. Ji, Z. Sun, J. Liu, Y. Bai, Z. Shen, Enhancing corrosion resistance and mechanical properties of AZ31 magnesium alloy by friction stir processing with the same speed ratio, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154452. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Enhancing corrosion resistance and mechanical properties of AZ31 magnesium alloy by friction stir

1 2 3 4 5 6 7

processing with the same speed ratio a*

Fenjun Liu , Yan Ji , Zhiyong Suna, Jianbo Liua, Yanxia Baia, Zhikang Shenb,c a b

a

College of Energy Engineering, Yulin University, Yulin, 719000, China

Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an, 710072, China c

Centre of Advance Materials Joining, University of Waterloo, Waterloo, Canada

8 9 10 11 12 13 14 15 16 17 18 19 20 21

ABSTRACT

22 23

Keywords: AZ31 alloy; FSP; Microstructure evolution; Precipitates; Corrosion performance; Mechanical

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

1.

Friction stir processing (FSP) using the same speed ratio was introduced as an alternative method to process AZ31 magnesium alloy sheets to enhance their corrosion performance and mechanical properties. In addition to the mechanical properties, the microstructure evolution and corrosion performance of the stirred zone (SZ) were investigated using EBSD, TEM, an electronic universal testing machine and an electrochemical workstation. In addition to being significantly finer and higher than that of the AZ31 alloy, the mean grain size and the number of β-Al12Mg17 precipitates in the SZ, respectively, gradually increased as the processing speed increased. The homogeneity and dispersion of the β-Al12Mg17 precipitates in the SZ increased due to a sufficient heat input caused by increasing the processing speed. In addition to slight changes in the mechanical properties, high-speed FSP, and especially FSP with a high rotation speed, obviously improved the corrosion performance of the AZ31 alloy due to the formation of homogenized and diffused distribution of β-Al12Mg17 precipitates. The SZ obtained at 5000 rpm and 125 mm/min exhibited excellent corrosion resistance. The corrosion potential of the SZ increased from -1.563 V to -1.230 V, and the corrosion current reduced from 1.55×10-4 A to 2.87×10-5 A compared to those of the as-received AZ31 alloy.

properties

Introduction Magnesium alloys, as one of the lightest structural materials, have attracted appreciable attention

recently for using as potential functional parts in transportation vehicles and aerospace equipment, such as automobiles, rail transit and aircraft, due to their low density, good shock absorption and damping, and excellent electromagnetic shielding [1]. However, the widespread applications of magnesium alloys are restricted mainly due to their poor corrosion resistance and mechanical properties [2,3]. It is well known that the corrosion resistance and mechanical properties can be improved by grain refinement according to the Hall-Petch formula [4]. For these reasons, many researchers have tried various methods to refine the grain size for enhancing the corrosion performance and tensile properties of magnesium alloys. Large-scale plastic deformation processing techniques, such as high pressure torsion [5], accumulative roll bonding [6], equal channel angular pressing [7] and friction stir processing (FSP) [8-10], have been widely used as effective methods for grain refinement of magnesium alloys. Among these plastic deformation methods, FSP is one of the most promising candidate techniques because the ultimate strength of the stirred zone (SZ) is not reduced and it has a flexible operation. FSP, as a novel solid state severe plastic deformation processing technique derived from friction stir welding (FSW), can not only refine the grain size of the SZ but also yield homogeneous of the microstructure due to dynamic recrystallization (DRX). Zhang et al. [11] noted that the SZ prepared by single-pass FSP exhibited a homogenous and refined grain size, strong basal plane texture and less *

Corresponding author. E-mial address: [email protected] (Fenjun Liu).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

second phase, which improved the corrosion resistance of a Mg-Nd-Zn alloy. Liu et al. [12] reported that FSP could significantly modify the microstructure of the SZ by producing an obvious grain refinement and uniform distribution of fine β-Al12Mg17 precipitates, which effectively enhanced the corrosion resistance of an AZ91 alloy. Moreover, the continuous and dense distribution of β-Al12Mg17 precipitates was the main reason for improving the corrosion resistance. In the researches of FSP Mg-Y-RE alloy and Mg-9Li-1Zn alloy, Argade et al. [13] and Liu et al. [14] also demonstrated that FSP could significantly enhance the corrosion resistance by refining grains, respectively. In addition to improving the corrosion resistance of the processed zone, FSP could also improve the tensile performance by refining grains and homogenizing the microstructure. Ma et al. [15] ascertained that an enhancement in mechanical properties of the friction stir processed zone was caused by a significant refinement and homogeneous distribution of the microstructure. Zhang et al. [16] noted that FSP obviously refined the grains and altered the texture in the SZ compared to that in an as-received AZ31 alloy. It was also found that FSP improved the elongation with a significant loss in the yield strength. Azizieh et al. [17] introduced a high temperature and large plastic strain during FSP of an as-cast pure magnesium ingot and exhibited an enhancement in the average hardness due to the refinement and homogeneous intermetallic nanoparticles. It was also confirmed that the grain size increased due to an increase in the rotation speed, eventually resulting in a decrease in the average hardness values. Luo et al. [18] found that the strength and elongation of a multi-pass friction stir processed AZ61 alloy plate were significantly improved mainly due to the grain refinement and solid solution and dislocation strengthening. However, previous studies on the FSP of magnesium alloys have focused on conventional rotation speeds, which are typically less than 2000 rpm. There was still a doubt whether the effect of a high rotation speed on the microstructure evolution, corrosion resistance and mechanical performance of friction stir processed AZ31 alloys was uncertain. Whether high rotation speed FSP with the same speed ratio can further refine the grain size and significantly enhance the corrosion performance and tensile properties needs to be ascertained. In the present work, the microstructure evolution of the SZ, including the grain size, dislocation distribution and intermetallic compound distribution, was detected to ascertain the influence of high rotation speed FSP with the same speed ratio. The corrosion properties, microhardness and tensile properties were also measured to confirm the influence of the microstructure on the corrosion resistance and mechanical performance of the SZ. 2.

Material and methods A 2 mm thick hot-rolled AZ31 (Mg-3Al-1Zn, wt.%) alloy sheet was selected as the base material

(BM) for the research herein. Specimens with a size of 180×100 mm2 were produced from the as-received hot-rolled AZ31 alloy sheet using electrical discharge machining (EDM). Single-pass high-speed FSP was carried out along the rolling direction of the AZ31 alloy sheet using high-speed FSW equipment (Model: FSW-TS-F08-DZ), as shown in Fig. 1. The tool used for the FSP herein consisted of a shoulder and a pin. The cylindrical shoulder has a diameter of 10 mm, and the geometry comprises a triple helical concave surface with a concave angle of 5°. The pin has a circular truncated cone with a top diameter of 2 mm, root diameter of 3 mm and a length of 1.5 mm. The tool inclination angle and penetration depth are 0° and 1.6 mm, respectively. The selected processing parameters have the same ratio of the rotation speed to processing speed. The rotation speed of the tool is between 1000 rpm and 5000 rpm, and the corresponding processing speed is between 25 mm/min and 125 mm/min. The specific parameters used for the FSP herein are shown in Table 1. The samples were named according to their rotation speed and processing speed. For example, 5000-125 represents the sample

1 2

from the friction stir processed AZ31 alloy sheet obtained at 5000 rpm and 125 mm/min. Table 1 FSP rotation speeds and processing speeds used for sample preparation. Rotation speed

Processing

(rpm)

speed (mm/min)

1000-25

1000

25

2000-50

2000

50

3000-75

3000

75

4000-100

4000

100

5000-125

5000

125

Samples

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

28 29

The grain morphology, dislocation characteristics and precipitate distribution of the SZ were observed using EBSD (Model: TESCAN MIRA3 XMU) and TEM (Model: FEI TECANI-F30). For EBSD observations, the specimens were cut along the cross section of the SZ using EDM. The surfaces to be observed were then treated by mechanical polishing. When the surfaces were polished to mirror surfaces, electropolishing was performed to treat the mirror surfaces at 248 K for 15 s. The electrolyte contained a perchloric acid (50 ml) and alcohol (950 ml) solution. The EBSD data was collected at the center of the cross section of the SZ, as shown in Fig. 1. For TEM analysis, the specimens were cut in the SZ parallel to the processing direction. The TEM samples were then mechanically ground to a thickness of approximately 40 µm. The thin foils were then subjected to an electrolytic double-jet technique with a 5 vol.% perchloric acid (50 ml) and alcohol (950 ml) solution to produce an electron-transparent area. The observation position for the TEM was the center of the horizontal position in the SZ. A schematic diagram of the specific sampling position for microstructure observation is shown in Fig. 1. The corrosion resistance and mechanical properties of the SZ were tested using a conventional three-electrode electrochemical workstation (Model: VERSASTAT 400), a microhardness tester (Model: HMV-1), and an electronic universal material testing machine (Model: INSTRON 3382). The electrochemical test position was the surface of the SZ, and the test area was 1 cm2. During the electrochemical test, the working electrode, reference electrode and auxiliary electrode were the SZ, a saturated Ag/AgCl electrode and a platinum electrode, respectively. The microhardness distribution curve of the SZ was tested using a step size of 0.25 mm at 0.98 N for a holding time of 10 s. The microhardness testing position is shown in Fig. 1. The tensile specimens were cut parallel to the processing direction, according to the ASTM-E8 standard. The tensile properties at room temperature were tested at a rate of 1 mm/min. It must be noted that the specimens for the tensile tests also contained a BM region that was not processed at the bottom. The sampling position for the corrosion performance and mechanical properties testing is shown in Fig. 1.

Fig. 1. Schematic diagram of the (a) FSP method and (b) specific sampling positions.

1 2 3 4 5 6 7 8 9 10 11

3.

12

T = K(

13 14 15 16 17 18 19 20 21

Results and discussion

3.1 Surface macroscopic features Fig. 2 shows the surface macroscopic features of the SZ prepared by FSP with the same speed ratio. Even if the same speed ratio was adopted, the surface macro-forming becomes smooth and flat with increasing rotation speed and processing speed. Groove defects are generated in a local region of the SZ prepared by FSP using a conventional rotation speed and processing speed. This is mainly attributed to the small heat input generated during FSP with a conventional speed and the large thermal conductivity of the magnesium alloy, which together lead to poor fluidity of the thermoplastic material around the tool in the processed zone. Ultimately, the poor plastic material flow could not backfill the cavity left by the advancing tool in time, and groove defects are produced. During FSP, the peak temperature (T) of the SZ can be expressed as follows [19]: ω

2

υ ⋅ 10

α

4

)

⋅ Tm

(1)

where Tm is the melting point of the BM, υ is the processing speed, ω is the rotation speed, and K and α are constants that vary in the ranges of 0.65 ~ 0.75 and 0.04 ~ 0.06, respectively. According to Equation (1), the heat input during FSP is affected by the rotation speed being greater than the processing speed. Therefore, as the rotation speed increases, the heat input of the processed zone increases at the same speed ratio. A sufficient heat input caused by high rotation speed can significantly improve the plastic material flow around the tool in the processed zone. Zhang et al. [20] found that the balanced material flow caused by sufficient heat input can produce defect-free joints during FSW.

22 23 24 25 26 27 28 29 30 31 32 33

Fig. 2. Surface macroscopic features of the SZ for the (a) 1000-25, (b) 3000-75 and (c) 5000-125 samples.

3.2 Microstructure characteristics The microstructure evolution of the SZ is shown in Figs. 3-8. As shown in Fig. 3, the BM and SZ consist of equiaxed grains. It can also be seen from Fig. 4 that the mean grain size in the SZ is smaller than that of the BM. Furthermore, the mean grain size in the SZ gradually increases as the rotation speed and processing speed increase. Compared with that in the as-received BM, an obvious texture is produced in the SZ after FSP. Fig. 5 shows the grain boundary distributions in the SZ and BM. The BM is mainly composed of high-angle grain boundaries (HAGBs). After the BM is subjected to FSP, the proportion of HAGBs in the SZ is significantly decreased. That is, the proportion of the low-angle grain boundaries (LAGBs) in

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

the SZ is significantly increased compared to that in the BM. The ratio of the HAGBs is increased from 41.1% to 56.4% and 68.5% as the rotation speed increases from 1000 rpm to 3000 rpm and 5000 rpm, respectively. The ratio of the HAGBs gradually increases as the rotation speed increases. This is consistent with the results of the misorientation angle distributions. The misorientation angle increases with increasing rotation speed, as shown in Fig. 6. To further analyze the microstructure characteristics of the SZ, TEM was used to observe the dislocations and precipitates distributions. As shown in Fig. 7, a large number of subgrains are observed in the SZ of the 1000-25 sample, whereas the dislocation density inside the grains is small. In the SZ of the 5000-125 sample, the number of subgrains is small, whereas a large number of high-density dislocations are formed inside the grain and at the grain boundaries. It is also found that the grain size in the SZ of the 5000-125 sample is significantly larger than that of the 1000-25 sample. The distribution characteristics of the β-Al12Mg17 precipitates in the SZ and BM are shown in Fig. 8. When the AZ31 alloy is subjected to FSP, a large number of the β-Al12Mg17 precipitates are observed in the SZ. Moreover, part of the β-Al12Mg17 precipitates grows with increasing speed. The large β-Al12Mg17 precipitates are likely to form a network-like distribution. In addition, the β-Al12Mg17 precipitate distribution in the SZ is more homogeneous and dispersed than that in the BM.

18 19

Fig. 3. Inverse pole figures of the SZ for the (a) 1000-25, (b) 3000-75, (c) 5000-125 and (d) AZ31 alloy samples.

1 2

Fig. 4. Grain size distributions in the SZ for (a) 1000-25, (b) 3000-75, (c) 5000-125 and (d) AZ31 alloy samples.

3 4 5

Fig. 5. Grain boundary distributions in the SZ for the (a) 1000-25, (b) 3000-75, (c) 5000-125 and (d) AZ31 alloy samples.

1

2 3 4 5

6 7 8

Fig 6. Misorientation angle distributions in the SZ for the (a) 1000-25, (b) 3000-75, (c) 5000-125 and (d) AZ31 alloy samples.

Fig. 7. Dislocation distribution characteristics of the SZ for the (a) 1000-25 and (b) 5000-125 samples.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Fig. 8. Precipitate distribution characteristics of the SZ for the (a) 1000-25, (b) 5000-125 and (c) AZ31 alloy samples.

In the process of FSP, the SZ is subjected to intense mechanical stirring and a large heat input caused by the rotation of the tool, the frictional heat between the tool and the processed material, and the plastic deformation heat of the processed material [21-23]. As a result, dynamic recrystallization (DRX) caused by severe thermoplastic deformation occurs in the processed zone and fine recrystallized grains are formed. In addition, since the thermoplastic material rotates and squeezes in a certain direction with the tool, an obvious texture is formed in the SZ. When the processing speed, especially rotation speed, is increased, the heat input in the SZ is also increased. The large heat input causes additional severe thermoplastic deformation in the SZ, which further induces an increased DRX. As a result, in addition to the formation of additional dislocations and substructures in the SZ, the dynamically recrystallized grain size increases due to the elevated heat input, and the texture becomes random due to an increased DRX [21]. Dislocations accumulate to form high-density dislocation walls. The dislocation walls continue to absorb and block dislocations and transform into substructures and LAGBs. Part of substructures and LAGBs continue to transform into HAGBs due to the rotation and combination. Some high-density dislocations and dislocation walls remain in grains or at grain boundaries due to the inability to transform into HAGBs, forming isolated segments. Moreover, the dissolution and reprecipitation of the β-Al12Mg17 precipitates inevitably occur during FSP of the AZ31 alloy. During the heating stage, the β-Al12Mg17 precipitates are dissolved instantly into the α-Mg to form a large quantity of metastable and supersaturated solid solutions due to the increasing diffusion rate and the obvious shortening of the diffusion distance [22]. During the subsequent cooling stage, a large number of β-Al12Mg17 precipitates form and grow due to the extended high-temperature residence time caused by the high heat input. Based on the above analysis, although the same speed ratio is used, the influence of the rotation speed on the heat input is greater than the processing speed. Therefore, the SZ of the 5000-125 sample is subjected to very intense mechanical stirring and heat input, which leads to an increased DRX and thermal cycling. As a result, the ratio of the HAGBs in the SZ of the 5000-125 sample is considerably higher than that of the 1000-25 sample. Furthermore, the β-Al12Mg17 precipitate size in the SZ of the 5000-125 sample is larger than that of the 1000-25 sample. Moreover, the large heat input is favorable for the homogenization and dispersion of the precipitates. 3.3 Corrosion behavior Fig. 9 shows the room temperature corrosion performance of the BM and the SZ in a 3.5 wt.% NaCl aqueous solution. According to the extrapolated cathodic Tafel region method, the calculated corrosion potential and corrosion current are summarized in Table 2. It can be seen from the

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distribution of the potentiodynamic polarization curve that the potentiodynamic polarization curves of the SZ move to the upper left corner compared to that of the BM. This indicates that the corrosion resistance of the surface of the SZ prepared by FSP with the same speed ratio is significantly improved compared to that of the BM. In fact, in the present work, the corrosion potential is raised from -1.563 V for the as-received BM to -1.340 V ~ -1.200 V for the SZ, whereas the corrosion current is reduced from 1.55×10-4 A to 8.57×10-5 A ~ 2.77×10-5 A, respectively. The Nyquist spectra from the BM and SZ further indicate that the diameter of the capacitive loop of the SZ is significantly larger than that of the BM. This further proves that FSP enhanced the corrosion performance of the AZ31 alloy. A further comparative analysis of the corrosion current of the SZ shows that the corrosion current first increases and then decreases with increasing processing speed. The corrosion current of the 1000-25 sample is basically the same as that of the 5000-125 sample, but the corrosion potential is obviously smaller than that of the 5000-125 sample. Considering the results of the corrosion potential and corrosion current, the 5000-125 sample exhibits excellent corrosion resistance. According to previous research results, the grain size and precipitate distribution are important factors affecting the corrosion performance of Mg-Al-Zn alloys. Seifiyan et al. [24] reported that the corrosion performance of an AZ31 alloy subjected to FSP was obviously improved due to the fine grain structure. Liao et al. [25] also confirmed that the corrosion performance of an AZ31B alloy tended to improve as grain size reduced. Lunder et al. [26] found that the distribution of β-Al12Mg17 precipitates was the main factor that affected the corrosion resistance of an AZ91 alloy. The same result was also found by song et al. [27] in the study of the corrosion mechanisms of an AZ31 alloy. They also found that the continuous network-like distribution of the β-Al12Mg17 precipitates in the magnesium alloy exhibited excellent corrosion resistance. In the present study, even if the same speed ratio is employed, the mean grain size in the SZ gradually increases as the speed increases. According to previous research results, coarse grains deteriorate the corrosion performance of AZ31 alloys. However, the grains in the SZ of the 5000-125 sample are significantly larger than those in the 1000-25 sample, but the corrosion performance of the 5000-125 sample is superior to that of the 1000-25 sample. This is different from the previous research results [24,25]. In fact, the β-Al12Mg17 precipitates in the SZ of the 5000-125 sample are more homogeneous and dispersed than those in the 1000-25 sample. The homogenized and dispersed β-Al12Mg17 precipitates play a key role in enhancing the corrosion performance of the SZ. This was confirmed in our previous research [28,29]. In addition, in the present work, the number and distribution of the β-Al12Mg17 precipitates in the SZ prepared using a conventional speed or high speed are greater, more uniform and more dispersed than those in the BM, which ultimately results in the corrosion resistance of the SZ being superior to that of the BM. It can be inferred that the distribution of the β-Al12Mg17 precipitates is the main factor that enhances the corrosion performance of the AZ31 alloy SZ prepared by high-speed FSP, followed by the grain size.

1 2 3 4 5

Fig. 9. (a) Potentiodynamic polarization curves and (b) Nyquist spectra of the AZ31 alloy and SZ prepared by FSP with the same speed ratio. Table 2. Electrochemical parameters for the potentiodynamic polarization curves and the corresponding ratios calculated by cathodic Tafel extrapolation. Proportion (%) Samples

AZ31 1000-25 2000-50 3000-75 4000-100 5000-125

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Self-corrosion

Self-corrosion

potential (V)

current (A)

-1.563 -1.340 -1.200 -1.270 -1.200 -1.230

Self-corrosion

Self-corrosion

potential

current

-4

100

100

-5

85.7

17.9

-5

76.8

50.6

-5

81.3

55.3

-5

76.8

27.7

-5

78.7

18.5

1.55×10 2.77×10 7.85×10 8.57×10

4.30×10 2.87×10

3.4 Microhardness and tensile properties The microhardness distribution and room temperature tensile properties of the SZ and the BM are shown in Fig. 10. The tensile properties of the SZ and BM are summarized in Table 3. The microhardness distribution curves show that the average microhardness value of the SZ gradually decreases as the processing speed increases. Among the samples in the present study, the mean microhardness value of the 1000-25 sample is the largest due to the smallest mean grain size in the SZ, while the average microhardness value of the 5000-125 sample is the smallest due to the largest average grain size. This is consistent with previous studies [9,30,31]. The mean grain size of SZ is the main factor affecting the microhardness value. The refined average grain size can significantly improve the microhardness value of the processed zone. The average grain size in the SZ of the 5000-125 sample is basically the same as that of the BM. As a result, the microhardness value in the SZ of the 5000-125 sample is similar to that of the BM. The tensile test results for the SZ show that the tensile properties tend to increase with increasing processing speed. When the rotation speed and processing speed are increased to 3000 rpm and 75 mm/min, the tensile properties of the SZ tend to be stable. By further increasing the rotation speed and processing speed, the tensile performance of the SZ is slightly affected by the processing speed. In addition to a significant decrease in the elongation, the tensile properties of the SZ produced using high-speed FSP, especially FSP with a high rotation speed, are substantially equivalent to those of the BM. Previous studies have shown that a fine grain size can significantly improve the mechanical performance of joints [31,32]. Among the samples herein, the SZ of the 1000-25 sample has the

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

32 33 34 35

smallest average grain size, but its tensile properties are the worst. This is mainly due to the substantial microstructure difference between the processed zone and the unprocessed zone. To be as close to practical applications as possible, the size of the pin selected in the present work is small. As a result, in the thickness direction, there is a large difference in the microstructure between the processed zone and the unprocessed zone because the length of the pin is less than the thickness of the BM. The cross-section perpendicular to the rolling direction includes the BM, heat affected zone, thermal mechanical affected zone, and processed zone. There are also differences in the microstructure among the zones. Moreover, the cross-sectional dimension of the tensile specimen is significantly larger than the diameter of the pin. In this way, there are obvious microstructure differences in the thickness direction and cross-section direction of the tensile samples, which ultimately leads to a certain reduction in the tensile properties of the friction stir processed sample compared to those the BM. The microstructure differences among the adjacent areas decreases slightly with increasing heat input caused by a high rotation speed. Therefore, the mechanical properties for the samples prepared using high-speed FSP are improved compared with those for the conventional speed. In addition, groove defects are observed in the processed zone of the 1000-25 sample, which also deteriorate the tensile properties. Upon further increasing the processing speed, especially the rotation speed, the processed zone and the adjacent zone experience intense thermal cycling, which eventually reduce the differences in the microstructure among the adjacent zones and increase the tensile properties. As a result, the SZs of the 3000-75, 4000-100, and 5000-125 samples exhibit excellent tensile properties. Although the BM has coarse grains, the microstructure of the BM is significantly more uniform than that of the processed sample. As a result, the BM exhibits better tensile properties than that of the friction stir processed samples. The uniformity of the microstructure plays a vital role in improving the mechanical properties of the friction stir processed AZ31 plates. The tensile fracture surfaces of the friction stir processed AZ31 alloy plates and as-received AZ31 alloy are characterized, as shown in Fig. 11. The 1000-25 sample has a large number of cleavage surfaces and small shallow dimples, indicating a brittle failure mode, as shown in Fig. 11a. Both the 3000-75 and 5000-125 samples have a large number of large tearing ridges and small shear lips, indicating a ductile-brittle mixed fracture mode dominated by ductile fracture, as shown in Fig. 11b and c, respectively. The as-received AZ31 alloy plate has a large number of deep dimples and large tearing ridges, indicating a ductile mode, as shown in Fig. 11d. This is consistent with the tensile test results, whereby the high-speed friction stir processed samples and the BM show good elongation.

Fig. 10. (a) Microhardness distributions and (b) tensile properties of the AZ31 alloy and SZ prepared by FSP with the same speed ratio.

1

Table 3. Tensile properties and corresponding ratios for the BM and SZ. Proportion (%) Samples

UTS (MPa)

YS (MPa)

EL (%) UTS

YS

EL

AZ31

253.4

127.8

33.7

100

100

100

1000-25

234.9

124.2

10.0

92.7

97.2

29.7

2000-50

246.5

126.7

17.7

97.3

99.1

52.5

3000-75

253.3

132.6

26.3

100.0

103.8

78.0

4000-100

252.1

123.5

27.8

99.5

96.6

82.5

5000-125

255.9

123.3

27.7

101.0

96.5

82.2

2

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Fig. 11. Fracture surfaces of the (a) 1000-25, (b) 3000-75, (c) 5000-125 and (d) AZ31 alloy samples.

4.

Conclusions The AZ31 alloy plates herein were subjected to FSP with the same speed ratio. The microstructure,

room temperature corrosion performance and mechanical properties of the SZ were studied. The main conclusions obtained are as follows: (1) A sound SZ was produced using FSP with the same speed ratio, especially FSP with a high rotation speed. The average grain size and number of β-Al12Mg17 precipitates in the SZ were significantly finer and higher than those of the BM, respectively. In addition to coarsening of the mean grain size and β-Al12Mg17 precipitates, the homogeneity and dispersion of the β-Al12Mg17 precipitates increased with increasing processing speed. Moreover, the texture in the SZ became random due to DRX. (2) The corrosion resistance of the SZ prepared by high rotation speed FSP was improved significantly compared with that of the BM. The corrosion potential and corrosion current of the

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

SZ obtained at 5000 rpm and 125 mm/min reached -1.230 V and 2.87×10-5 A, for 78.7% and 18.5% of the BM, respectively. (3) The microhardness of the SZ was improved to an extent due to grain refinement. In addition to reducing the elongation, the FSP had a slight effect on the tensile performance of the defect-free SZ. Data availability statement The data used to support the findings of this study cannot be shared at this time, as the data is part of ongoing research. Author Contribution Statement Liu and Ji conceived and designed the study. Sun and Liu performed the experiments. Liu and Liu wrote the manuscript. Bai and Shen reviewed the manuscript. All authors read and approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that influenced the work reported in this paper. Acknowledgements The authors acknowledge funding from the National Natural Science Foundation of China [No. 51861034, 51975479 and 51601167], the Natural Science Foundation of Shaanxi Province [No. 2019SF-271, 2018GY-129, 2018KJXX-037 and 2016KTZDGY-04-03], and the Fundamental Research Funds for the Central Universities [No. 3102019QD0404]. References [1]

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Highlights Sound high-speed friction stir processed AZ31 stirred zone was produced with same speed ratio.

High rotation speed has a significant influence on microstructural evolution. Excellent corrosion resistance of AZ31 can be obtained using high rotation speed FSP. High rotation speed has a slight effect on tensile strength and yield strength of the stirred zone.

Dear Editors, Liu and Ji conceived and designed the study. Sun and Liu performed the experiments. Liu and Liu wrote the manuscript. Bai and Shen reviewed the manuscript. All authors read and approved the final manuscript.

Kind Regards Sincerely yours, Fenjun Liu, Yan Ji, Zhiyong Sun, Jianbo Liu, Yanxia Bai, Zhikang Shen

Dear Editors, All authors in this paper have no conflict of interest with any organization in the present research work. Moreover, the manuscript has already expressed gratitude to all organizations that have provided financial support for this research work, and there is no copyright confusion. Once this article is accepted by Journal of Alloys and Compounds, the copyright will be automatically transferred to the publisher.

Kind Regards Sincerely yours, Fenjun Liu, Yan Ji, Zhiyong Sun, Jianbo Liu, Yanxia Bai, Zhikang Shen