Microstructure Evolution and Mechanical Properties of 6061 Aluminum Alloy Fabricated by Friction Stir Additive Manufacturing

doi:10.11900/0412.1961.2023.00220

Acta Metall Sin

2025, Vol. 61

Issue (8): 1129-1140 DOI: 10.11900/0412.1961.2023.00220

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Microstructure Evolution and Mechanical Properties of 6061 Aluminum Alloy Fabricated by Friction Stir Additive Manufacturing

YANG Fan, PEI Shichao, LUO Xinrui, CHEN Yuxiang, LI Ningyu, CHANG Yongqin(

)

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Cite this article:

YANG Fan, PEI Shichao, LUO Xinrui, CHEN Yuxiang, LI Ningyu, CHANG Yongqin. Microstructure Evolution and Mechanical Properties of 6061 Aluminum Alloy Fabricated by Friction Stir Additive Manufacturing. Acta Metall Sin, 2025, 61(8): 1129-1140.

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Abstract

Friction stir additive manufacturing (FSAM) is an advanced solid-phase forming technology based on the principle of friction stir. As no heat source is required, the FSAM process can avoid metallurgical defects during melting and solidification of the material. The FSAM also conserves energy and protects the environment. To replace the traditional forming technology with a heat source, current research aims to optimize the forming process parameters of aluminum and magnesium alloys. Similarly to the friction stir welding process, increasing the temperature in the FSAM process will dissolve part of the strengthening phase, coarsening the particles and softening the welding core area. In addition, during layer-by-layer stacking in FSAM, the stir tool will re-stir the previously formed area, introducing a new thermal cycle during the stirring process. Because the changes in the temperature and stress field are more complex in the FSAM process than in the friction stir welding process, the influence of microstructure evolution on the mechanical properties of materials in the FSAM process is worthy of investigation. In this study, a multilayer defect-free FSAM material was fabricated from 2-mm-thick 6061 aluminum alloy sheets. The microstructural evolution along the building direction was observed during the FSAM process to investigate its effect on the microhardness and tensile properties. Dynamic crystallization formed fine equiaxed grains in the stir zone, which were further refined after re-stirring in the overlapping interface regions. The tensile strength and elongation of the FSAM material were 47.7%-55.2% and 144.6%-148.8% those of the base material, respectively. Multiple thermal cycling weakens the performance of the overlapping interface regions. The spherical α-Al(MnCrFe)Si phase plays a strengthening role in the matrix. Extensive dissolution of the strengthening phase during the FSAM process is mainly responsible for the performance deterioration of the FSAM material. After heat treatment at 520 ^oC for 1 h and at 165 ^oC for 18 h of aging, the properties of the FSAM material were largely improved: the hardness slightly increased from that of the base material and the tensile strength reached 87.2%-91.9% that of the base material.

Key words: friction stir additive manufacturing Al-Si-Mg-Cu alloy post-weld heat treatment hooking defect abnormal grain growth

Received: 12 May 2023

ZTFLH:

TG453.9

Fund: National Natural Science Foundation of China(11775017);National Natural Science Foundation of China(51971021)

Corresponding Authors: CHANG Yongqin, professor, Tel: 13522569036, E-mail: chang@ustb.edu.cn

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	YANG Fan
	PEI Shichao
	LUO Xinrui
	CHEN Yuxiang
	LI Ningyu
	CHANG Yongqin

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00220 OR https://www.ams.org.cn/EN/Y2025/V61/I8/1129

Fig.1 Photograph of the stir tool (unit: mm) (a) and schematic of friction stir additive manufacturing (FSAM) process (b)

Fig.2 Tensile specimen dimension of formed part (unit: mm) (a) and schematic of sampling location of the tensile specimen (b)

Fig.3 Cross-sectional OM images of the 6061 aluminum alloy fabricated by FSAM
(a) the overall morphology
(b) advancing side (AS) (c) retreating side (RS)
(d) hooking defects in AS (e) hooking defects in RS

Fig.4 Sampling positions for the EBSD observation

Fig.5 Inverse pole figures (IPFs) (a-d) and grain boundary maps (e-h) of 6061 aluminum alloy base metal (BM) and the FSAM samples at different locations shown in Fig.4 (The red lines and blue lines denote low angle grain boundaries (LAGBs) with misorientation angle (θ) 2°-15° and the high angle grain boundaries (HAGBs) with θ ≥ 15°, respectively)
(a, e) BM (b, f) FSAM-1 (c, g) FSAM-2 (d, h) FSAM-3

Table 1 Average grain sizes and number fractions of LAGBs and HAGBs in 6061 aluminum alloy BM and the different locations shown in Fig.3a

Fig.6 Orientation distribution function (ODF) maps and texture distributions (Black dots in Figs.6a1-a4 and b1-b4 corres-pond to the positions of the textures in Figs.6c and d, respectively)
(a1-a4) ODF maps for BM (a1), FSAM-1 (a2), FSAM-2 (a3), and FSAM-3 (a4) at Euler angle φ₂ = 0° (b1-b4) ODF maps for BM (b1), FSAM-1 (b2), FSAM-2 (b3), and FSAM-3 (b4) at φ₂ = 45° (c) texture at φ₂ = 0° (Φ, φ₁—Euler angles) (d) texture at φ₂ = 45°

Fig.7 Hardness distributions of the 6061 aluminum alloy fabricated by FSAM (Insets show the position and direction of hardness test sample, Z—distance between the test position and the upper surface of the sample)
(a) building direction (b) vertical building direction

Fig.8 Hardness distributions of the 6061 aluminum alloy fabricated by FSAM and followed by post-weld heat treatment (PWHT) along building direction (a) and PWHT sample along vertical building direction (b) (Inset in Fig.8b shows the position and direction of hardness test sample)

Fig.9 Engineering stress-strain curves of 6061 aluminum alloy samples in different states
(a) FSAM (b) PWHT

Table 2 Tensile properties of 6061 aluminum alloy samples in different states

Fig.10 OM images of the 6061 aluminum alloy BM (a) and 6061 aluminum alloy fabricated by FSAM followed by PWHT (b)

Fig.11 TEM bright field images of the 6061 aluminum alloy BM (a), 6061 aluminum alloy fabricated by FSAM (b), and then followed by PWHT (c)

Fig.12 Particle size distributions (a) and average diameters and number densities of precipitates (b) for 6061 aluinum alloy BM, 6061 aluminum alloy fabricated by FSAM, and then followed by PWHT

Fig.13 High-angle annular dark-field (HAADF) image (a) and corresponding EDS element distribution maps of Fe (b), Cr (c), Mn (d), Al (e), Si (f), Mg (g), and Cu (h) for 6061 aluminum alloy fabricated by FSAM followed by PWHT

Fig.14 TEM image (a) and EDS analysis results of precipitated phases P1 (b) and P2 (c) in Fig.14a for 6061 aluminum alloy fabricated by FSAM followed by PWHT

Fig.15 TEM bright field images and corresponding selected area electron diffraction (SAED) patterns (insets) of rod-like (a) and spherical (b) precipitates for 6061 aluminum alloy fabricated by FSAM followed by PWHT

Fig.16 SEM images of macroscopic morphology (insets) and microstructure of tensile fracture for 6061 aluminum alloy BM (a), 6061 aluminum alloy fabricated by FSAM (b), and then followed by PWHT (c)

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