Effect of Electropulse on Dynamic Precipitation and Microstructure of AZ91 Magnesium Alloy During Warm Extrusion

doi:10.11900/0412.1961.2024.00256

Acta Metall Sin

2025, Vol. 61

Issue (1): 129-142 DOI: 10.11900/0412.1961.2024.00256

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Effect of Electropulse on Dynamic Precipitation and Microstructure of AZ91 Magnesium Alloy During Warm Extrusion

WANG Binshan¹^,², XU Guang¹^,², REN Rui¹^,², ZHANG Qiang¹^,²^,³, SHAN Zhaohui⁴, FAN Jianfeng¹^,²^,³(

)

1 Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
2 College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
3 Shanxi Key Laboratory of Advanced Magnesium Based Materials, Taiyuan University of Technology, Taiyuan 030024, China
4 College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China

Cite this article:

WANG Binshan, XU Guang, REN Rui, ZHANG Qiang, SHAN Zhaohui, FAN Jianfeng. Effect of Electropulse on Dynamic Precipitation and Microstructure of AZ91 Magnesium Alloy During Warm Extrusion. Acta Metall Sin, 2025, 61(1): 129-142.

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Abstract

The electropulse-assisted forming process has been widely used in various plastic deformation applications owing to its advantages in improving formability and refining microstructure. However, the influence of electropulse on the dynamic extrusion deformation process remains unclear. In this study, the effects of electropulse on dynamic precipitation and microstructure evolution of AZ91 magnesium alloy during extrusion were investigated using electropulse-assisted extrusion (EPAE) technology. The results demonstrate that under critical deformation conditions for complete dynamic recrystallization, the EPAE process reduces the volume fraction of the β-Mg₁₇Al₁₂ phase, promotes its spheroidization, and enhances both the average grain size and the maximum basal texture intensity. These effects become more pronounced with increasing peak current density. Specifically, with a peak current density of 6.4 × 10⁷ A/m² during the EPAE process, the volume fraction of the β-Mg₁₇Al₁₂ phase decreased from 76.9% to 16.5%, the average grain size increased from 1.07 μm to 3.54 μm, and the maximum basal texture intensity increased from 3.39 to 5.92, compared to conventional hot extrusion. The bimodal structure observed in the EPAE-processed AZ91 alloy was attributed to the pinning effect caused by the inhomogeneous distribution of the β-Mg₁₇Al₁₂ phase. Experimental and theoretical analyses indicated that the increase of Gibbs free energy variation and atomic diffusion flux during extrusion of AZ91 alloy caused by the combined thermal and athermal effects of the pulsed current was the main reason for the experimental phenomena, which promoting the solution of β-Mg₁₇Al₁₂ phase and uniform distribution of Al solute atoms nearby while also increasing the grain boundary migration rate. Moreover, the electropulse strengthened the basal texture in β-Mg₁₇Al₁₂ particle-depleted regions by accelerating basal <a> slip.

Key words: AZ91 magnesium alloy electropulse assisted extrusion β-Mg₁₇Al₁₂ phase dynamic precipitation grain growth

Received: 14 August 2024

ZTFLH:

TG146.2

Fund: Natural Science Foundation of Shanxi Province(20210302123134);Natural Science Foundation of Shanxi Province(202203021221071);Natural Science Foundation of Shanxi Province(202203021211157);Shanxi Scholarship Council of China(2022-045);Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering(2021SX-FR005)

Corresponding Authors: FAN Jianfeng, professor, Tel: 13935107463, E-mail: fanjianfeng@tyut.edu.cn

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	WANG Binshan
	XU Guang
	REN Rui
	ZHANG Qiang
	SHAN Zhaohui
	FAN Jianfeng

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00256 OR https://www.ams.org.cn/EN/Y2025/V61/I1/129

Fig.1 Schematic of the EPAE process and square wave generated by electric pulsing (EPAE—electropulse-assisted extrusion, ED—extrusion direction, J_m—peak pulse current density, τ_p—pulse width, τ_s—pulse spacing)

Table 1 The extrude process parameters of AZ91 magnesium alloy

Fig.2 OM (a, c) and SEM-BSE (b, d) images of the as-cast (a, b) and as-soluted (c, d) AZ91 magnesium alloys (Inset in Fig.2d shows the EDS results, x_M —atomic fraction of element M)

Fig.3 XRD spectra (a) and evolution of dislocation density (b) of as-soluted and extruded AZ91 magnesium alloys

Fig.4 Low (a-d) and high (e-h) magnification OM images and SEM-BSE images (i-l) of the extrude AZ91 magnesium alloy samples (The black dashed areas in Figs.4b-d are coarse-grained areas, and the black arrows in Figs.4f-h are long-strip second phases)
(a, e, i) 250EX (b, f, j) EPAE-1 (c, g, k) EPAE-2 (d, h, l) EPAE-3

Table 2 Microstructural parameters and post-temperature of extruded AZ91 magnesium alloys

Fig.5 Evolutions of average size and proportion of different types of grains (a) and β-Mg₁₇Al₁₂ phase (b) in extruded AZ91 magnesium alloy

Fig.6 SEM-SE images (a, b, d-l) and EDS results (c) of the 250EX (a, b), EPAE-1 (d-f), EPAE-2 (g-i), and EPAE-3 (j-l) AZ91 magnesium alloy samples
(a, d, g, j) SEM-SE images of fine grain region (b, e, h, k) SEM-SE images of the whole region (f, i, l) SEM-SE images of coarse grain region

Fig.7 SEM-SE image of the transition region from fine grain to coarse grain (a) and TEM image of fine grain region (b) of the EPAE-3 sample (The black dotted line in Fig.7b is the grain boundary)

Fig.8 Typical EBSD results of 250EX alloy (a-c) and EPAE-3 alloy (d-f) (TD—transverse direction, f_LAGB—fraction of low angle grain boundary)
(a, d) invers pole figures (IPFs) (b, e) (0001) pole figures (PFs) (c, f) grain boundary misorientation angle distributions

Fig.9 True stress-strain curves of as-soluted and extruded samples at room temperature

Table 3 Mechanical properties of as-soluted and extruded AZ91 alloys

Fig.10 Schematic of Gibbs free energy changes during solution of β-Mg₁₇Al₁₂ phase under different extrusion conditions (ΔG_d—thedifference in free energy between state 0 and state 1 in Curve 1, ΔG—thedifference in free energy between state 0 and state 1 in Curve 2, ΔG_e—additional energy change caused by pulse current)

Fig.11 Typical EBSD results of EPAE-3 sample in coarse grain region (a, b) and fine grain region (c, d)
(a, c) IPFs (b, d) (0001) PFs

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