Effects of hot extrusion and annealing treatment on microstructures, properties and texture of AZ31 Mg alloy

ZHANG Hui(张 慧), YAN Yun-qi(闫蕴琪), WENG Wen-ping(翁文凭),

ZHONG Hao(钟 皓), CHEN Qi(陈 琦)

Suzhou Institute for Non-ferrous Metals Processing Technology, Suzhou 215021, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Effects of extrusion deformation and heat treatment on microstructures, mechanical properties and texture of AZ31 Mg alloy were investigated．The results show that the microstructures of as-cast AZ31 alloy are markedly refined after hot extruding, the average grain size is about 25 μm and strong fiber texture  exists in the extruded AZ31 alloy. The mechanical properties are improved obviously. The grain size is somewhat inhomogeneous and strip structure emerges along the extrusion direction due to incomplete dynamic recrystallization during the extrusion process. With increasing annealing temperature, the small grain grows up and turns into equiaxed grain, and the texture is weakened with the visible growing up of grains.

Key words：AZ31 alloy; Mg alloy; texture; mechanical properties; hot extrusion

1 Introduction

Recently, magnesium alloys are attractive for engineering applications for their extremely low density, high specific strength and stiffness[1-3]. However, magnesium alloys often show poor formability and limited ductility at room temperature ascribed to their hexagonal close-packed (HCP) crystal structure with less independent slip systems[4]. Thus its industrial applications are limited. Grain refinement is an effective way to improve both strength and ductility of magnesium alloys[3]. During hot working, the grain refinement and low deformation resistance result from dynamic recrystallization[5]. Extrusion is in tri-directional compression stress condition, which makes it available to obtain perfect form and compact structure[6]. In this study, the effects of extrusion deformation and heat treatment on microstructures, mechanical properties and texture of AZ31 Mg alloy were investigated.

2 Experimental

The nominal composition (mass fraction, %) of the studied alloy is Mg-3Al-1Zn. Ingots were homogenized at 400 ℃ for 10 h and then machined into bars of d110 mm×400 mm, followed by extrusion at 350-400 ℃ with an extrusion ratio of 39?1, and then annealed at 150-500 ℃ for 0.5-2 h, respectively. The microstruc- tures were observed using Nikon Epiphot 200 optical microscope, phase and texture analysis was performed by JSM6480 equipped with TSL OIM 2000 system, and mechanical properties tests at room temperature were carried out using CSS—44100 testing machine and hardness testing machine.

3 Results and discussion

3.1 Microstructures observation

As-cast microstructures of AZ31 alloy are shown in Fig.1. It can be known that the main phases in AZ31 alloy are α-Mg and the others that depict like particles or short rods are secondary phases, Mg₁₇Al₁₂ and Al-Mn phases distribute in the Mg solid solution. For the secondary phases have significant effect on reducing the formability of magnesium alloys, cast ingots are often carried out homogeneous heat-treatment before extruding process.

As shown in Fig.2(a), after hot extruding, the as-extruded microstructures of alloys are markedly refined, the grain boundaries of the extruded alloys are well defined, and the average grain size for the AZ31 alloy after extrusion is about 10 μm. However, the grain size is somewhat inhomogeneous and the strip structure emerges along the extrusion direction due to incomplete dynamic recrystallization during the extrusion process. Figs.2(b)-(f) show the microstructures of alloys annealed from 150 ℃ to 500 ℃ for 0.5-2 h,  respectively. It can be noted that the microstructure does not change significantly compared with the extruded structure (Fig.2(a)) at 200 ℃ for 1 h (Fig.2(b)). The microstructure still shows inhomo- geneous and strip structure, which occurs during the extrusion process. After annealing heat-treatment at 300 ℃ for 1 h(Fig.2(c)), areas of fine grains can be observed, whereby those non-recrystallized areas still present. After annealing heat- treatment at 350 ℃ for 0.5 h, a relatively fine-grained structure can be obtained (Fig.2(d)). The grain growth can be observed clearly, while the grains grow from 10 to 200 μm after heat-treatment at 400 ℃ for 1 h. A further increase of annealing time and temperature leads to a coarser structure, and the microstructure of AZ31 alloy sheet is not uniform. The grains partially grow very large, whereas the secondary dynamic recrystallization occurs during the annealing process.

Fig.1 Microstructures of as-cast AZ31 magnesium alloy: (a) OM image; (b) SEM image

Fig.2 OM images of AZ31 alloys annealed under different conditions: (a) As-extruded; (b) Annealed at 200 ℃ for 1 h; (c) Annealed at 300 ℃ for 1 h; (d) Annealed at 350 ℃ for 0.5 h; (e) Annealed at 400 ℃ for 1 h; (f) Annealed at 500 ℃ for 1 h

3.2 Textures

The texture extruded AZ31 alloy sheet was analyzed by using electron back-scattered diffractometry (EBSD). Figs.3 and 4 present the results of the micro- orientations of the extruded and annealed AZ31 sample. It can be noted that the misorientation angle of less than 60? was observed at the extruded alloys and a very strong fiber texture  formed in the extruded- processed samples. After annealing, the misorientation angle distribution of the boundaries is shown in Fig.4(c), which shows that the misorientation angle increases to 90?, and most are low angle grain boundaries, i.e. misorientation angle, θ＜10?. With increasing annealing temperature and time, the texture intensity decreases in the extruded sample.

3.3 Mechanical properties

The mechanical properties of the as-cast, as-extruded and as-annealed specimens are compared in Table 1. It can be seen that hot extruding processing can improve the tensile strength about 120%, and hardness about 46%. Moreover, the elongation increases after extrusion. It is known that the Hall-Petch coefficient of magnesium is K_y=280 MPa?m^1/2, which is 4.1 times greater than that of aluminum[7]. This means that the grain refining strengthening effect of Mg alloys is much more significant than that of Al. As a result of dynamic recrystallization, fine grains are obtained in AZ31 alloy after extruding, which results in the improvement of mechanical properties of extruded alloys. After annealing process, both tensile strength and yield strength of the alloy decrease, while the elongation of alloy increases contrarily.

Table 1 Comparisons of mechanical properties of AZ31 alloy after various processes

Fig.3 Analytic results of micro-orientations for extruded AZ31 samples: (a) Orientation mapping; (b) Grain size distribution; (c) Misorientation angle distribution; (d) ODF

Fig.4 Analytic results of micro-orientations determined by SEM-EBSD for extruded AZ31 sample after annealing: (a) Orientation mapping; (b) Grain size distribution;      (c) Misorientation angle distribution; (d) Macro-texture

4 Conclusions

1) Microstructures of as-cast AZ31 alloy consist of α-Mg, Mg₁₇Al₁₂ and Al-Mn phases.

2) During the hot extrusion process, dynamic recrystallization had occurred，the microstructures of extruded AZ31 alloys were greatly refined. Microstructure shows inhomogeneous and strip structure, while the mechanical properties of AZ31 alloy sheet increase after hot extrusion.

3) The annealing heat treatment at 350 ℃ for 0.5 h proves to be favorable to enhance the mechanical properties for extruded AZ31 alloys.

4) After annealing treatment strong fiber texture  exists in the extruded AZ31 alloy is weakened.

Acknowledgements

The authors would like to thank for the financial support of CHALCO S&T Foundation (2005KJA13), and the cooperation of Ms ZHANG Ying-nan on EBSD data and Mr. ZHANG Yue-jie on extruding process was appreciated.

References

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(Edited by Long Huai-zhong)

Foundation item: Project(2005KJA13) supported by the Aluminium Corporation of China Limited S&T Foundation

Corresponding author: ZHANG Hui; Tel: +86-512-62585621; Fax: +86-512-62585618; E-mail:maydaynini@126.com