­­­ Comparison of microstructure and properties of AZ31 Mg alloy sheet produced through different routes

LUO Jin-ru(罗晋如)¹, CHEN Xing-pin(陈兴品)², XIN Ren-long(辛仁龙)²,

HUANG Guang-jie(黄光杰)², LIU Qing(刘 庆)^1,2

1. Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China;

2. College of Material Science and Engineering, Chongqing University, Chongqing 400044, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Slabs fabricated by means of three different ingot breakdown modes: hot-rolling, extrusion-rolling and twin-roll strip continuous casting-cum-rolling, were rolled into sheets and then annealed. Both the rolled and annealed sheets were investigated by SEM-EBSP, BSE, X-ray diffraction and tensile test, and compared in terms of microstructure, texture, and mechanical properties. Effects of different processing methods on the microstructure, texture and the related mechanical properties were discussed based on the experimental results.

Key words: AZ31 Mg alloy; texture; mechanical property; anisotropy

1 Introduction

Magnesium alloys are now attractive candidates for structural applications due to their low density and good electromagnetic shielding[1]. However, because of the lower efficiency of the sheet processing and yield, being easy to form the basal texture and difficult to deform, the application of magnesium alloy sheet is limited. Many studies have been focused on improving the texture of the sheets through different processing routes in order to enhance the performance, efficiency and yield of the sheet. For instance, primary processing such as hot-rolling and hot-extrusion had been studied early in 1980s[2-3]; and equal channel angular pressing performed on an AZ31 magnesium alloy had successfully been used to fabricate ductile alloy[4]. VALLE et al[5] compared magnesium alloy AZ31 produced by large-hot rolling and AM60 produced by equal channel angular pressing; KIM et al[6] and HUANG et al[7] obtained Mg-Al-Zn alloy with a tilted basal texture by different speed rolling; WATANABE et al[8] had superplastically deformed the AZ31 magnesium alloy aiming for ductile alloy. In this work, three processing routes (hot-rolling, extrusion-rolling and twin-roll strip continuous casting-cum-rolling) were chosen to prepare the plates for the final cold (or warm) rolling. Both the rolled and annealed sheets were investigated by SEM-EBSP, BSE, X-ray diffraction and tensile test, and compared in terms of microstructure, texture and mechanical properties. The effects of different processing methods on the microstructure, texture and the related mechanical properties were discussed based on the experimental results.

2 Experimental

The alloys used in present study were commercial Mg-Al-Zn alloys (AZ31, nominally 3%Al, 1%Zn and balance Mg). Hot-rolling slab, extrusion-rolling slab and twin-roll strip continuous casting-cum-rolling slab were received as the raw materials. The hot-rolling slab was obtained by hot rolling the AZ31 alloy ingot above 400 ℃, and the extrusion-rolling slab was gained by extruding a d 155 mm×500 mm AZ31 ingot into a 180 mm×3 mm slab at about 390 ℃ and then hot-rolling above 400 ℃. The twin-roll strip continuous-cum- casting slab was fabricated by feeding the molten metal into the space between a pair of counter-rotating, internally cooled rolls and then hot-rolling it above 400 ℃. All the three slabs were subsequently annealed at the temperature of 300 ℃ for 50 min and cooled down in atmospheric air to room temperature, and then rolled into sheets until their thickness was about 0.8 mm. Finally, all three types of rolled sheets were annealed in a Lenton tube furnaces at 220 ℃ for 120 min and cooled down to room temperature in atmospheric air. The flowchart is illustrated in Fig.1.

The BSE microstructure observation and SEM- EBSP were carried out by a FEI Nova NanoSEM. Samples for EBSP indexing were prepared using standard metallographic polishing method and then electro-polishing in commercial magnesium polishing solution ACⅡ for 60-70 s. As for the samples for BSE photographing, they were subsequently etched in an acetic picral solution (5 g picric acid, 10 mL acetic acid, 70 mL ethanol, and 10 mL water) for 5-10 s after electro-polishing. All the observed surfaces were on the cross-section of the sheets.

Texture was analyzed by X-ray diffraction using the Schulz reflection method. The (0001) incomplete pole figures were obtained at α-angles ranging from 20? to 90? using a Rigaku D/max-rB Rota flex X-ray diffracto- meter with Cu K_α radiation at 40 kV and 100 mA. The rolled sheet samples were ground to mid-section because of the low indexing rate of EBSP. And the pole figures of annealed sheets were calculated from the EBSP data throughout the sheet cross-section.

Sub-sized tensile samples were machined by wire-EDM in accordance with ASTM standard E8 for monoaxial tensile tests. The dog-bone shaped samples with gage length of 25 mm, width of 6 mm, and the same thickness as that of the sheet were machined, parallel to the rolling direction (RD) and transverse direction (TD) respectively. Then the samples were tensioned at room temperature at a constant crosshead speed of 0.5 mm/min. A clip-on extensometer was used to measure strain in loading direction. Additionally, all samples were machined from the center part of the cold-rolled sheets.

3 Results and discussion

3.1 Microstructures

Fig.2 shows the microstructures on the cross-section of specimens. There are massive twins in all the three samples, and the twin boundaries of hot-rolling sheet were identified in EBSP map shown in Fig.2(d)). It is clearly seen that grains are well refined for sheets produced through extrusion-rolling and twin-roll strip continuous casting-cum-rolling routes. The extrusion- rolling sheet has well uniformly refined grains, while the twin-roll strip continuous casting-cum-rolling sheet shows a microstructure composed of fine elongated grain layer and coarse equiaxed grain layer on the cross- section, and the hot-rolling sheet has the coarsest grains among the three samples. However, those differences in the microstructures are reduced after annealing the sheets. The EBSP maps (Fig.3) show the microstructures of the annealed sheets. Twins disappear and the grains are fine and equiaxial with several small grains located at the triple grain junctions.

3.2 Texture

Fig.4 shows the (0001) incomplete pole figures of the three rolled sheets. The intensity (CPS: counts per seconds) in the basal pole figure presented as a function of tilting from ND towards RD and TD, and 45? (45? between RD and TD on the ND plane) is illustrated in Fig.5. And the textures of annealed sheets are illustrated in Fig.6.

It can be observed from Fig. 4 and Fig.5 that:

1) After processed, all the alloys develop a strong basal texture with angular spread in the pole density being greater towards the RD than towards the TD.

Fig.1 Process flowchart of tested rolled sheets

Fig.2 Cross-sectional microstructures of three different rolled sheets: (a) BSE of hot-rolling sheet; (b) BSE of extrusion-rolling sheet; (c) BSE of twin-roll strip continuous casting-cum-rolling sheet; (d) EBSP of hot-rolling sheet with special grain boundaries identified

Fig.3 Cross-sectional microstructures of three annealed sheets: (a) EBSP of annealed hot-rolling sheet; (b) EBSP of annealed extrusion-rolling sheet; (c) EBSP of annealed twin-roll strip continuous casting-cum-rolling sheet

Fig.4 (0001) pole figures of rolled sheets: (a) Hot-rolling sheet; (b) Extrusion-rolling sheet; (c) Twin-roll strip continuous casting- cum-rolling sheet

Fig.5 Texture comparison with C-axis distribution intensity in basal pole figures plotted as function of tilting from normal direction: (a) Toward RD; (b) Toward TD; (c): Toward 45? (between RD and TD and on ND plane); (d) Comparison of asymmetry

Fig.6 (0001) pole figures of annealed sheets: (a) Annealed hot-rolling sheet, Max. M.U.D=9; (b) Annealed extrusion-rolling sheet, Max. M.U.D=12; (c) Annealed twin-roll strip continuous casting-cum-rolling sheet, Max. M.U.D=19

2) The intensity of hot-rolling sheet poles is much higher than that of the other two, and the shape of plotting curve for hot-rolling sheet is sharper as shown in Fig.5. Therefore, the hot-rolling sheet develops a stronger and more asymmetric basal texture, indicating more grains in this sheet with their C-axis tilting towards RD.

Usually, magnesium sheet exhibits a typical strong basal texture, where the majority of grains are oriented as their basal (0001) planes close to the rolling plane of the sheet. Such a texture places most grains in an orientation difficult to deform, which results in stress localization and premature failure[9-10]. This strong basal texture is typical for rolled HCP materials[11-13].

3.3 Mechanical properties

Sheets were tensioned at room temperature along the RD and the TD until fracture. The true stress—true strain curves for rolled sheets and annealed sheets are illustrated in Fig.7 and Fig.8, respectively.

It can be concluded from Fig.7 and Fig.8 that:

1) Hot-rolling sheet tensions weaker in the directions of both RD and TD, and is most anisotropic in terms of yield strength in either rolling or annealing state.

Fig.7 True stress—true strain curves of RD and TD uniaxial tensile of rolled sheets: (a) Hot-rolling sheet; (b) Extrusion- rolling sheet; (c) Twin-roll strip continuous casting-cum-rolling sheet

Fig.8 True stress—true strain curves of RD and TD uniaxial tensile of annealed sheets: (a) Annealed hot-rolling sheet;    (b) Annealed extrusion-rolling sheet; (c) Annealed twin-roll strip continuous casting-cum-rolling sheet

2) The TD samples are almost stronger than the RD samples for the rolled sheets, and the mechanical anisotropy could be reduced by annealing, especially for the extrusion-rolling and twin-roll strip continuous casting-cum-rolling sheet.

3) The ductility of the AZ31 sheets is greatly enhanced after annealing compared with the fracture true strains, while the yield strength decreases with the appearance of distinct yield point.

It is found that the extrusion-rolling sheet behaves the best among the three sheets for its homogeneous fine grained microstructure as shown in Fig.2(b) and Fig.3(b). For all the three sheets, the ductility can be enhanced by means of annealing.

Researchers reported that the greater spreading of the basal poles towards the TD than towards the RD would result in the lower flow stresses along the TD tension than along the RD tension[14-15]. Similarly, these experimental results showed that the TD samples were stronger than the RD samples on account that all sheets have similar greater RD spreading basal texture. The most mechanical anisotropic sheet is the hot-rolling one either in rolling or annealing state because it has the most anisotropic RD spreading basal texture.

4 Conclusions

1) All the typical Mg alloy sheets fabricated through hot-rolling, extrusion-rolling and twin-roll strip continuous casting-cum-rolling manufacturing processes exhibit similar basal texture with greater RD spreading after the same final rolling procedure. Meanwhile, massive twins are observed in all the three sheets. Annealing has no distinct effect on the sheet textures.

2) There is significant relationship between the texture and the mechanical properties anisotropy of both the rolled and annealed sheets. It is found that the extrusion-rolling sheet behaves the strongest among the three due to the finest grain. For all the three sheets, the ductility can be enhanced by means of annealing.

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(Edited by YUAN Sai-qian)