Microstructure and Corrosion Resistance of Modified Mg-5Zn Alloy via Friction Stir Processing

doi:10.11900/0412.1961.2023.00281

Acta Metall Sin

2025, Vol. 61

Issue (7): 1071-1081 DOI: 10.11900/0412.1961.2023.00281

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Microstructure and Corrosion Resistance of Modified Mg-5Zn Alloy via Friction Stir Processing

LONG Fei¹^,²^,³, LIU Qu¹^,², ZHU Yixing¹^,², ZHOU Mengran¹^,², CHEN Gaoqiang¹^,², SHI Qingyu¹^,²(

)

1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2 State Key Laboratory of Tribology in Advanced Equipment, Tsinghua University, Beijing 100084, China
3 Henan Key Laboratory of Advanced Conductor Materials, Institute of Materials, Henan Academy of Sciences, Zhengzhou 450046, China

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LONG Fei, LIU Qu, ZHU Yixing, ZHOU Mengran, CHEN Gaoqiang, SHI Qingyu. Microstructure and Corrosion Resistance of Modified Mg-5Zn Alloy via Friction Stir Processing. Acta Metall Sin, 2025, 61(7): 1071-1081.

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Abstract

Magnesium alloy is the lightest metal structure material and has one of the highest specific strength among metals. Thus, it has great potential in many applications, such as aerospace and automobile industry, to reduce the weight of components. However, magnesium alloys have very poor corrosion resistance that hinders their industrial application. Thus, several methods have been explored to improve the corrosion resistance of magnesium alloy to promote its application in industries such as automotive, aerospace, and electronics where lightweight materials are required. In this study, friction stir processing (FSP) has been applied to modify the microstructure of Mg-5Zn alloy to increase its corrosion resistance, which is a type of magnesium alloy used widely but has relatively poor corrosion resistance. Herein, three tools with different shoulder diameters of 14, 17, and 20 mm were selected to conduct FSP on the as-cast Mg-5Zn alloy. The microstructure has been observed and corrosion behavior has been investigated. The results reveal that the coarse grains of as-cast Mg-5Zn alloy are considerably refined by FSP treatment. The grain size reduces from hundreds of micrometers to a few micrometers. Furthermore, the coarse secondary phase in as-cast alloy is broken into small particles and distributed uniformly in the base material after FSP. Additionally, strong basal plane (0001) texture and low dislocation density have been observed in these FSP-treated samples, which are beneficial for increasing the corrosion resistance of magnesium alloy. Moreover, the size of the secondary phase increases with the increase of shoulder diameter, which leads to the increase in local cathode/anode area ratio, and the corrosion resistance of the three FSP-treated samples gradually reduces to 1520, 247, and 111 Ω·cm², respectively. Notably, under the FSP treatment at 800 r/min rotation speed, 40 mm/min traveling speed, and 0.3 mm plunge depth, when the tool with a shoulder diameter of 14 mm is employed, the precipitates in the Mg-5Zn alloy gets sufficiently fragmented and evenly dispersed, along with a relatively low dislocation density. The average corrosion current density of this friction stir processed sample in a 3.5%NaCl aqueous solution is reduced to 4.11 × 10^-6 A/cm² compared to that of the as-cast alloy (3.15 × 10^-5 A/cm²).

Key words: friction stir processing precipitate shoulder diameter corrosion resistance

Received: 01 July 2023

ZTFLH:

TB304

Fund: National Natural Science Foundation of China(52035005);National Natural Science Foundation of China(52175334)

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	LONG Fei
	LIU Qu
	ZHU Yixing
	ZHOU Mengran
	CHEN Gaoqiang
	SHI Qingyu

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00281 OR https://www.ams.org.cn/EN/Y2025/V61/I7/1071

Fig.1 OM image (a) and grain size distribution (b) of as-cast Mg-5Zn alloy

Fig.2 Inverse pole figures of Mg-5Zn alloys treated by friction stir processing (FSP) with shoulder diameters of 14 mm (a), 17 mm (b), and 20 mm (c) named FSP-14, FSP-17, and FSP-20, respectively (ND—normal direction, TD—transverse direction)

Fig.3 Backscattered electron (BSE) images of as-cast (a, b) and FSP-14 (c), FSP-17 (d), and FSP-20 (e) Mg-5Zn alloy samples (Fig.3b shows the locally enlarged image of square area in Fig.3a)

Fig.4 XRD spectra of as-cast and FSP-treated Mg-5Zn alloy samples

Fig.5 Hardness variations of FSP-treated Mg-5Zn alloy samples within a 6 mm region on both sides of the center of the stirred zone

Fig.6 Kernel average misorientation (KAM) maps (a, c, e) and corresponding distributions (b, d, f) in the stir zone of FSP-14 (a, b), FSP-17 (c, d), and FSP-20 (e, f) Mg-5Zn alloy samples

Fig.7 Grain boundary distribution maps of FSP-14 (a), FSP-17 (b), and FSP-20 (c) Mg-5Zn alloy samples

Fig.8 Poler figures for the stirred zone of FSP-14 (a), FSP-17 (b), and FSP-20 (c) Mg-5Zn alloy samples

Fig.9 Electrochemical impedance spectroscopy (EIS) diagrams of as-cast and FSP-treated Mg-5Zn alloy samples in 3.5%NaCl solution
(a) Nyquist plots (Z_im—imaginary part of the impedance, Z_r—real part of the impedance)
(b) Bode plots (|Z|—modulus of impedance)

Fig.10 Equivalent circuit diagram used for impedance fitting
(a) having apparent inductive loop (R_s—solution resistance; R₁ and CPE₁—resistance and capacitance of the corrosion products generated on the surface, respectively; R₂and CPE₂—charge transfer resistance and capacitance of the electrical double layer at the interface, respectively; R₃—resistance relative to the local environmental changes near the anode; L₁—an inductive component that is used to simulate the changes caused by the expansion of the active anode region)
(b) without apparent inductive loop

Table 1 Fitting results of EIS for as-cast and FSP-treated Mg-5Zn alloy samples in 3.5%NaCl solution

Fig.11 Polarization curves of as-cast and FSP-treated Mg-5Zn alloy samples in 3.5%NaCl solution (SCE—saturated calomel electrode)

Table 2 Fitting results of the potentiodynamic polarization curves of as-cast and FSP-treated Mg-5Zn alloy samples in 3.5%NaCl solution

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