Scholarly article on topic 'Enhanced mechanical properties of AZ31 magnesium alloy sheets by continuous bending process after V-bending'

Enhanced mechanical properties of AZ31 magnesium alloy sheets by continuous bending process after V-bending Academic research paper on "Materials engineering"

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{"AZ31 magnesium alloy" / "Continuous bending" / V-bending / "Mechanical properties" / Formability}

Abstract of research paper on Materials engineering, author of scientific article — Tingzhuang Han, Guangsheng Huang, Yougen Wang, Guangang Wang, Yanchun Zhao, et al.

Abstract The effects of V-bending process, continuous bending process and combination process on the microstructure and mechanical properties and formability of an AZ31 magnesium alloy sheet were investigated. The experimental results showed that no twins were found in the microstructure of all samples after processes due to the fine grain. The V-bending and continuous bending processes were proved to be an effective approach to modify the mechanical properties and formability. While the samples after the combination process exhibited better mechanical properties and formability than the single processed sample. The yield strength significantly decreased with the value of 100MPa and the fracture elongation enhanced to 18.3% at room temperature. The Erichsen value was 5.0mm which was significantly increased by 117% compared with as-received sample. The superior formability of combination processed samples was mainly attributed to the smaller r-value and n-value.

Academic research paper on topic "Enhanced mechanical properties of AZ31 magnesium alloy sheets by continuous bending process after V-bending"

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SdenCeDireCt Progress _ in Natural

Science Materials International

ELSEVIER Progress in Natural Science: Materials International ■ (■■■■) III III -

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Original Research

Enhanced mechanical properties of AZ31 magnesium alloy sheets by continuous bending process after V-bending

Tingzhuang Hana,b, Guangsheng Huanga,b,c,*,:L, Yougen Wanga,b, Guangang Wanga,b,

Yanchun Zhaoa,b, Fusheng Pana'b'c'*'2

aState Key Laboratory of Mechanical Transmission, College of Materials Science and Engineering, Chongqing University, Chongqing 400044 China bNational Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044 China cChongqing Research Center for Advanced Materials, Chongqing Academy of Science & Technology, Chongqing 401123 China

Received 20 July 2015; accepted 2 November 2015

Abstract

The effects of V-bending process, continuous bending process and combination process on the microstructure and mechanical properties and formability of an AZ31 magnesium alloy sheet were investigated. The experimental results showed that no twins were found in the microstructure of all samples after processes due to the fine grain. The V-bending and continuous bending processes were proved to be an effective approach to modify the mechanical properties and formability. While the samples after the combination process exhibited better mechanical properties and formability than the single processed sample. The yield strength significantly decreased with the value of 100 MPa and the fracture elongation enhanced to 18.3% at room temperature. The Erichsen value was 5.0 mm which was significantly increased by 117% compared with as-received sample. The superior formability of combination processed samples was mainly attributed to the smaller r-value and n-value. © 2016 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: AZ31 magnesium alloy; Continuous bending; V-bending; Mechanical properties; Formability

1. Introduction

As the lightest metal structural material, magnesium alloys have recently attracted a lot of research interest because of their potential application for lightweight structural components particularly in electron, automobile industries [1-3]. However, the magnesium alloys always exhibit a poor ductility and formability at room temperature due to the hexagonal close-packed (HCP) crystal structure. For magnesium alloys, the critical resolved shear stresses (CRSS) of non-basal slip systems on prismatic and pyramidal planes are much higher than that of a basal slip system at low temperature (from room

nCorresponding authors.

E-mail addresses: gshuang@cqu.edu.cn (G. Huang), fspan@cqu.edu.cn (F. Pan).

1Tel.: +86 23 6511 2239.

2Tel.:+86 23 6511 2635.

Peer review under responsibility of Chinese Materials Research Society.

temperature to 473 K). Thus, the basal slip is expected to be the dominant system at room temperature. But the basal slip system provides only two independent slip systems, far fewer than the requirement of five independent systems for homogeneous deformation [4-6]. On the other hand, normal rolled and extruded magnesium alloy sheets generally exhibit a strong basal texture, i.e. c-axes are mostly perpendicular to rolling plane. This induces a high normal anisotropy in sheet and increases the difficulty in deformation accompanied with thickness reduction, and consequently leads to a very limited formability at ambient temperatures [7,8].

It has been known that the stretch formability can be improved by changing the (0002) basal texture. Thus, a great effort has been devoted to reducing the texture intensity or inclining the basal poles with the purpose of enhancing the formability. The equal channel angular pressing (ECAP) processes can produce texture with a strong basal component located at 45° between extrusion and normal direction. Kim et al. [9] reported that the

http://dx.doi.org/10.1016/j.pnsc.2016.01.005

1002-0071/© 2016 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

2 T. Han et al. / Progress in Natural Science: Materials International I (IIII) III III

ductility of AZ61 Mg alloy bars were improved due to the grain refinement and texture weakening induced by ECAP. AZ31 magnesium alloy sheets processed by ECAP were investigated by Suh et al. [10]. They indicated that the drawing depth of the ECAPed sheet at 225 °C was improved by more than 50% as compared with the as-rolled condition. Tang et al. [11] analyzed the texture evolution of AZ31 alloy sheets processed by cross rolling. They suggested that (0002) basal texture can be effectively weakened and the ductility and drawability were significantly improved by cross rolling. The differential speed rolling (DSR) process was carried out on an AZ31B magnesium alloy sheets. The Erichsen values of the DSR processed sheet with an inclination of basal pole in the rolling direction significantly increased by about 1.5 times at room temperature [7]. Thus, texture control can effectively improve the formability of magnesium alloy sheets at room temperature. Furthermore, the addition of RE elements such as yttrium (Y) [12], neodymium (Nd) [12], Gadolinium (Gd) [13] and Cerium (Ce) [14,15] can develop a more random texture enhancing ductility in magnesium alloys.

Wang and Huang et al. [6,16] studied the evolution of neutral layer of AZ31 magnesium alloy during V-bending. They indicated that twinning dominated the compression region while the prismatic slip dominated the tension region. For the previous studies, the authors payed more attention to the evolution of microstructures, while the effects of V-bending on formability of magnesium alloy sheets were out of consideration. For this reason, it is necessary to investigate systematically the influence of V-bending on formability of magnesium alloys. In addition, due to the tension-compression asymmetry between the tension and compression region, the present work applies a novel continuous bending device to investigate the influence of continuous bending process on the microstructure and properties of AZ31 magnesium alloy sheets. On the other hand, the influence of microstructure evolution on the continuous bending properties was rarely studied. Thus, the authors pay attention to the effect of the combination of V-bending and continuous bending on the ductility and formability of AZ31 magnesium alloy sheets.

2. Experimental

The as-received AZ31 magnesium alloy sheets (Mg-3 wt% Al-1 wt% Zn) with a thickness of 0.6 mm, cut into rectangular samples of 1000 mm x 100 mm (length x width), were used in this investigation. Table 1 shows the chemical compositions of the as-received AZ31 magnesium alloy sheets. Fig. 1a shows an abridged general view of V-bending process (VB), where the

Table 1

Chemical composition of AZ31 magnesium alloy sheets (wt%).

Al Zn Mn Si Ni Fe Mg

3.01 0.9 0.5 0.04 0.005 0.005 Balance

magnesium alloy sheet was bent on a cylindrical support under a constant force F and a constant speed v. Due to the different deformation mode between inner and outer region during V-bend-ing, the sheets was bent for two passes and the sheet was turned over and the bending orientation was also changed in the second pass. Fig. 1b shows the abridged general view of continuous bending process (CB). The combination bending process is that the sheet is underwent firstly by V-bending, and then underwent for continuous bending, named for VB + CB process. After bending processes, the samples were annealed at 533 K for 60 min. The different bent samples followed by annealing were marked as VBA, CBA and VB + CBA samples, respectively.

Tensile samples with a gauge length of 40 mm, a width of 10 mm and a thickness of 0.6 mm were machined along the RD, TD and 45° by wire-cutting. The uniaxial tensile tests were carried out on a CMT6305-300kN electronic universal testing machine with an initial strain rate of1 x 10_3s_1 at room temperature. The tensile test was repeated 3 times to get representative results. In addition, Lankford values (r-value) were measured using the samples deformed to a true tensile-direction strain of 10%. The strain-hardening exponents (n-value) were obtained from the uniform plastic deformation region of the tensile stress-strain curves.

The Erichsen tests were carried out to investigate the press formability of AZ31 Mg alloy sheets at room temperature. The schematic diagram and geometry dimension of mold are shown in Fig. 2. The Erichsen tests were conducted on the rectangular samples with a gauge size 50 mmx 50 mm using a hemispherical punch with a diameter of 20 mm, and the Erichsen values were the punch stroke at fracture initiation. The punch speed and the blank holder force were 3 mm/min and 10 kN, respectively. Graphite grease was used as a lubricant.

Support

Mg sheet

T / \ T

'////////////////////

Fig. 1. (a) Schematic diagram of V-bending and (b) schematic diagram of continuous bending devices.

T. Han et al. / Progress in Natural Science: Materials International I (IIII) III III 3

3. Result and discussion

Fig. 3 shows the optical microstructures of different magnesium alloy sheets. The grain sizes were measured by a mean line-intercept method to analyze its change. For as-received sample, the equiaxed grains distributed in the microstructure and the average grain size was 6.5 ^m, as shown in Fig. 3a. It is interesting to note that no twins have been found in the continuous bending, V-bending and the combination

Fig. 2. Schematic diagram of mold (rp = 10 mm; rd=0.75 mm; Lp = 16.5 mm; Ld = 13.5 mm).

Fig. 3. Optical microstructures of (a) as-received, (b)

samples, with the average grain size of 6.4 |m, 6.5 |m and 6.8 |m, respectively. The free-twin microstructure can be due to the small grain size. Huang et al. [17] suggested that the smaller grain size restricted tensile twinning activity. Li et al. [18] and also indicated that the compressive deformation behavior at room temperature depended on grain size. Cepeda-Jimenez et al. [19] reported that for magnesium alloys, a transition from twinning-dominated to slip-dominated flow is often observed at grain sizes of 1-10 |m. The disappearance of twins could suggest that the slip is the dominate deformation mode during bending process. Fig. 4 shows the microstructures of bent samples after annealing. The average grain size are 7.7 |m, 7.8 |m and 8.9 |m for CBA, VBA and VB + CBA samples, respectively. It can be seen that the grain sizes slightly increased for the samples underwent different bending processes after annealing.

Fig. 5 shows the true stress-strain curves of different samples obtained in annealed condition. The yield strength (YS), ultimate tensile strength (UTS), fracture elongation (FE), r-value and n-value are summarized in Table 2. The average value of YS, UTS and FE are shown in Fig. 6. Comparing with the as-received sample, the yield strengths of continuous bending, V-bending and the combination samples decreased obviously from 177 MPa to 145, 124, 100 MPa, respectively (Fig. 6a). While the ultimate tensile strength changed slightly, as shown in Fig. 6b. The decrease in the YS can be attributed

'¿s*;;: ù ''VSHSHCw ^«■¿•^flSSSf

yÇ 50|im

>, (c) VB and (d) VB + CB samples without annealing.

0 -'-1-'-1-'-L"-1 I I i

0.00 0.05 0.10 0.15 0.20

strain

Fig. 5. The true strain-stress curves for various samples.

Table 2

Mechanical properties of various samples.

Samples YS (MPa) UTS (MPa) FE (%) r-value n-value

As-received 177 292 13.6 2.65 0.231

CBA 145 288 14.3 1.85 0.247

VBA 124 332 15.9 1.30 0.277

VB + CBA 100 291 18.3 0.97 0.284

to the weakened basal texture. A weak basal texture can result in a large Schmid fact or of the basal slip and can thus lead to a decrease in YS. The low yield strength of magnesium alloy sheets can help reduce the spring-back after shape forming. Fig. 6c shows the fracture elongation of different samples. It can be seen that compared with the as-received sample, the fracture elongations of bent samples increased from 13.6% to 14.3%, 15.9%, 18.3%, respectively. The enhancement of the FE can be attributed to the improvement in the n-value (Fig. 7b). The larger n-value leads to a lower sensitivity to strain localization in the form of necking, which helps the material achieve high fracture elongation without fracture [20].

The r-values and n-values of the samples are shown in Fig. 7. Compared with the as-received sample, the r-values decreased significantly after different bending process (Fig. 7a). High r-values are observed to promote good deep drawability in many steels. However, for magnesium alloy with the HCP structure, a low r-value meant a tendency to decrease in the thickness, which could coordinate the deformation and favored the improvement of stretch formability [21]. Yi et al. [22] also indicated that the high r-values of AZ31 alloy sheets could be interpreted as a large mechanical anisotropy and general difficulty of deformation, which resulted in the lower formability and early fracture. The nvalues of the samples are shown in Fig. 7b. The n-value is one of the important factors which control metal sheets' resistance

T. Han et al. / Progress in Natural Science: Materials International I (IIII) III—III

as-received CBA VBA VB+CBA

samples

Fig. 6. Yield strength (a), ultimate tensile strength (b) and fracture elongation (c) of various samples.

Fig. 7. (a) r-value and n-value of different samples.

to plastic instability. Compared with the as-received sample, the n-values increased significantly after different bending process. The sheet after VB + CB process with annealing condition exhibited the n-value of 0.284. The high n-value improves the ability of sheets to resist localized necking, and is responsible for the increase in the uniform elongation [23]. Generally speaking, a large n-value meant a strong ability to

resist fracture during tension and shape forming. Previous studies [20,21,24] also indicated that the AZ31 sheets underwent by different processes exhibited high n-values and the improvement in n-values enhanced their formability. Therefore, the AZ31 magnesium alloy sheets undergoing different bending process exhibited lower r-value and larger n-values, which favored the improvement of ductility and formability.

6 T. Han et al. / Progress in Natural Science: Materials International I (IIII) III III

] _I_._I_I_I_._I_

as-received CBA VBA VB+CBA

samples

Fig. 8. Erichsen values (IE) of various samples.

Fig. 8 shows the Erichsen values (IE) of AZ31 magnesium alloy sheets underwent different bending after annealing. The Erichsen values were 2.3, 3.3, 4.2 and 5.0 mm for samples of as-received, CB, VB and VB + CB, respectively. It can be seen that the formability was greatly enhanced for the sheet that underwent different bending processes followed by annealing. Erichsen value of VB + CBA sample was remarkably enhanced to 5.00 mm (enhanced by 117% compared to the as-received sample), which can be attributed to the lowest rvalue and highest n-value. The formability was assessed on the basis of plastic strain ratio (r) and strain hardening coefficient (n) from extensive tensile testing. The width strain can contribute to the total strain for uniaxial tensile extension, while the thickness strain is most necessary for stretch forming under a biaxial tension stress state, and it is believed that a low r-value and a high n-value can enhance the deformation capability of sheet forming [25]. In addition, the larger grains may produce positive effect on formability at room temperature. Chino et al. [26] and Kang et al. [27] reported that increasing grain size may enhance cold stretch formability. Thus, the AZ31 magnesium alloy sheets with larger grains exhibited a lower r-value and higher n-value, which lead to a superior stretch formability at room temperature.

4. Conclusions

The effects of V-bending, continuous bending and the combination of V-bending and continuous bending process on the microstructure, mechanical properties and stretch formability of AZ31 magnesium alloy sheets have been investigated. After the bending process, the microstructure exhibited well-equiaxed grains without twins, and the disappearance of twins can be related to the small grain size. Compared with the as-received sample, the bent samples showed lower yield strengths and higher fracture elongations. The VB + CBA sample exhibited lowest yield strength of 100 MPa and the largest fracture elongation (18.3%). For the Erichsen test, the bent samples showed better formability. The Erichsen value 5.0 mm significantly increased by 117%

compared with as-received sample. The superior formability of combination processed samples is mainly attributed to the smaller r-value and n-value.

Acknowledgment

This work is supported by Fundamental Research Funds for the Central Universities (No. CDJZR13130081) and Demonstrative Project of Chongqing Science and Technology Commission (No. CSCT2014FAZKTJCSF50004).

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