Effect of Ultrasonic Vibration on Microstructure Evolution at the Mg/Al Dissimilar Alloy Friction Stir Welded Lap Joint Interface

doi:10.11900/0412.1961.2025.00198

Acta Metall Sin

2026, Vol. 62

Issue (1): 133-147 DOI: 10.11900/0412.1961.2025.00198

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Effect of Ultrasonic Vibration on Microstructure Evolution at the Mg/Al Dissimilar Alloy Friction Stir Welded Lap Joint Interface

YIN Jialin, SHI Lei, WU Chuansong(

)

Key Laboratory for Liquid-Solid Structure Evolution and Materials Processing, Ministry of Education, Shandong University, Jinan 250061, China

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YIN Jialin, SHI Lei, WU Chuansong. Effect of Ultrasonic Vibration on Microstructure Evolution at the Mg/Al Dissimilar Alloy Friction Stir Welded Lap Joint Interface. Acta Metall Sin, 2026, 62(1): 133-147.

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Abstract

With growing emphasis on energy saving and emission reduction, lightweight structures have become a key development focus in vehicle manufacturing. Mg and Al alloys, as lightweight materials with excellent properties, have broad applications in aerospace, automobile manufacturing, 3C products, and other fields. Mg/Al composite components can fully leverage the advantages of both alloys. Therefore, achieving high-quality and high-efficiency joints of Mg/Al dissimilar alloys has become a critical challenge in the manufacturing industry. Among the important structural types of Mg/Al thin-plate dissimilar welded joints, lap joints have attracted considerable attention. Friction stir welding (FSW), a solid-state joining process, offers distinct advantages for producing high-quality, defect-free Mg/Al joints. Ultrasonic-assisted FSW can further broaden the process window and enhance joint strength. However, the mechanism by which ultrasound influences joint formation during welding remains unclear. In this study, ultrasonic vibration enhanced friction stir lap welding experiments were conducted on Mg alloy and Al alloy sheets. The optimal process parameters were determined to be a rotation speed of 800 r/min and a welding speed of 90 mm/min. During welding, the keyhole region of the lap joint was subjected to a sudden stop + freezing treatment. Material flow behavior was characterized on vertical cross-sections at various angles around the keyhole and on horizontal cross-sections. Microstructures of the characteristic regions on the horizontal cross-section around the keyhole and along the weld centerline passing through keyhole center near the Mg alloy side were characterized. The influence of ultrasonic vibration on the mechanical properties of the lap joints was also evaluated. The results show that introducing ultrasonic vibration during friction stir lap welding enhanced both the tensile shear strength and the effective sheet thickness of the Mg/Al lap joints. Furthermore, ultrasonic vibration increased the volume of material driven by the tool, promoting enhanced material flow and mixing. During joint formation, the grain size distribution around the keyhole on the Mg alloy side became more uniform, and the grains behind the tool underwent a significantly higher degree of recrystallization.

Key words: Mg alloy Al alloy lap joint friction stir lap welding ultrasonic vibration microstructure evolution

Received: 08 July 2025

ZTFLH:

TG456.9

Fund: National Natural Science Foundation of China(52035005)

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	YIN Jialin
	SHI Lei
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https://www.ams.org.cn/EN/10.11900/0412.1961.2025.00198 OR https://www.ams.org.cn/EN/Y2026/V62/I1/133

Table 1 Chemical compositions of base materials

Fig.1 Schematic of ultrasonic vibration enhanced friction stir lap welding (UVeFSLW) system (FSW—friction stir welding)

Fig.2 Schematic of lap configuration and relative pose between sonotrode and tool (AS—advancing side, RS—retreating side. unit: mm)

Fig.3 Schematics of sampling position (a) and sample dimensions (b) at the lap joint (unit: mm)

Fig.4 Schematics of keyhole sample extraction from the lap joint (WD—welding direction, LS—leading side, TS—trailing side)
(a) overview of sample extraction
(b) sample extraction around the keyhole center at 0°, 90°, 180°, and 270°
(c) sample extraction along the original lap surface

Fig.5 Selections of characterization regions near keyhole on horizontal cross section of Mg alloy side (800 r/min, 90 mm/min) (R—radius of shoulder, r—radius of characterization regions around keyhole, FSLW—friction stir lap welding)
(a) FSLW (b) UVeFSLW

Fig.6 OM images of vertical cross sections of lap joints produced by FSLW (a1-e1) and UVeFSLW (a2-e2) processes under different rotation speeds and welding speeds (d_EST—effective sheet thickness)
(a1, a2) 600 r/min, 90 min/mm (b1, b2) 800 r/min, 90 min/mm
(c1, c2) 1000 r/min, 90 min/mm (d1, d2) 800 r/min, 60 min/mm
(e1, e2) 800 r/min, 120 min/mm

Fig.7 d_EST of lap joints under various parameters and processes
(a) welding speed 90 min/mm (b) rotation speed 800 r/min

Fig. 8 Tensile shear strengths of lap joints within the process window

Fig.9 OM images of vertical cross sections at various angles around the keyhole center of FSLW (a1-d1) and UVeFSLW (a2-d2) samples (800 r/min, 90 mm/min, the same below)
(a1, a2) 0° (b1, b2) 90° (c1, c2) 180° (d1, d2) 270°

Fig.10 Horizontal cross-sectional OM images showing material flows around keyhole on Mg alloy (a1, a2) and Al alloy (b1, b2) matrix sides produced by FSLW (a1, b1) and UVeFSLW (a2, b2) processes (White dotted circles represent ideal boundary of material mixing area, yellow dotted lines represent actual boundary of material mixing area)

Fig.11 Horizontal cross-sectional inverse pole figures (IPFs) of the characteristic regions A-H in Fig.5a on the Mg alloy side of the FSLW joint (RD_o—rotation direction)

Fig.12 Cross-sectional IPFs of the characteristic regions A-H in Fig.5b on the Mg alloy side of the UVeFSLW joint

Fig.13 Statistics of grain size (a), LAGB fraction (b), and recrystallization degrees (c) of A-H regions in Fig.5 (LAGB—low angle grain boundary)

Fig.14 Horizontal cross-sectional IPFs of characteristic regions in Fig.5a along the welding path through the keyhole center on the Mg alloy side of the FSLW joint
(a) C2 (b) C1 (c) C (d) G (e) G1 (f) G2

Fig.15 Horizontal cross-sectional IPFs of characteristic regions in Fig.5b along the welding path through the keyhole center on the Mg alloy side of the UVeFSLW joint
(a) C2 (b) C1 (c) C (d) G (e) G1 (f) G2

Fig.16 Statistics of grain size (a), LAGB fraction (b), and recrystallization degree (c) of different characterization regions in Fig.5

Fig.17 Effect of ultrasonic energy on Al alloy flow stress^[27] (UV—ultrasonic vibration)

Fig.18 Calculated materials flow stress results of FLSW (a) and UVeFLSW (b) samples (800 r/min, 30 mm/min)^[29]

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