Inhibition of Abnormal Grain Growth in Stir Zone via In-Situ Intermetallic Particle Formation During Friction Stir Welding of AA6061

doi:10.1007/s40195-022-01479-3

Abstract

Abstract:

Abnormal grain growth (AGG) has been widely observed in many friction stir welded (FSWed) joints during post-weld heat treatment (PWHT). The coarse grain structure not only reduces the strength of the joint but also limits its usage in superplastic forming. Several methods have been reported in previous studies to inhibit AGG, but all of them can only mitigate AGG. Complete inhibition of AGG was not achieved. In the current research, AGG was widely observed during the PWHT of friction stir welded AA6061. Multi-pass FSW enhanced the thermal stability of the as-welded grain structure but did not eliminate the occurrence of AGG. A new welding method was developed with the ball-milled Al-Ti powder mixture introduced into the stir zone and proved effective in inhibiting AGG in FSWed AA6061 during PWHT. The adoption of 5-pass FSW with an alternate rotation mode succeeded in producing an AGG-free sample. Microscopic characterizations conducted in the stir zone showed an evolution of Al-Ti powder mixture into different particle formations and Al₃Ti new phase. Quantitative analysis of the second phase particles (SPPs) in the stir zone confirmed the increases in both particle number and average size. The quantitative results fit well with Humphreys’ grain growth model, which theoretically explains the mechanism for AGG inhibition, i.e., the significantly enhanced particle pinning effect.

Key words: Abnormal grain growth, Friction stir welding, AA6061, Second phase particle, Particle pinning

Xin Zou, Cunli Liu, Muyang Deng, Ji Chen, Lanting Zhang, Ke Chen. Inhibition of Abnormal Grain Growth in Stir Zone via In-Situ Intermetallic Particle Formation During Friction Stir Welding of AA6061[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(4): 597-610.

Figures/Tables 12

Table 1 Chemical composition of AA6061 (wt%)

Al	Cu	Fe	Mg	Mn	Si	Cr
Bal.	0.25	0.49	0.97	0.10	0.70	0.15

Fig. 1 a Schematic demonstration of FSW with an internal tunnel (shadowed quarter circle) for holding Al-Ti powder mixture, b the welding tool used in the current study with dimensions shown

Fig. 2 a Top view of ball-milled Al-Ti powder mixture, b side view of a titanium flake

Table 2 Experimental design and sample designations

Al-Ti powder addition	State	Number of passes
Al-Ti powder addition	State	1	2	4	5
No	As-welded	NP1	NP2	NP4	-
No	After HT	NP1-HT	NP2-HT	NP4-HT	-
Yes	As-welded	P1	P2	P4	P5
Yes	After HT	P1-HT	P2-HT	P4-HT	P5-HT

Fig. 3 Cross sections of the welds without Al-Ti powder mixture: a 2-pass as-welded (NP2), b 1-pass after HT (NP1-HT), c 2-pass after HT (NP2-HT), d 4-pass after HT (NP4-HT); e-g magnified local areas indicated in a showing grain structures from top to bottom; h-j magnified local areas indicated in c showing grain structures from top to bottom

Fig. 4 Cross sections of welds produced with Al-Ti powder mixture addition after HT: a 1-pass (P1-HT), b 2-pass (P2-HT), c 4-pass (P4-HT); d-f microstructure of local areas boxed in a-c; g-i microstructure of local areas similar to d-f at higher magnification

Table 3 EDS point analyses of the locations indicated in Figs. 4 and 6

No	Al (at%)	O (at%)	Ti (at%)	Suggested phase
1	38.9	61.1	-	Oxide
2	77.2	22.8	-	Oxide
3	100	-	-	Al
4	-	-	100	Ti
5	75.8	-	24.2	Al₃Ti
6	75.9	-	24.1	Al₃Ti
7	72.1	-	27.9	Al₃Ti
8	72.9	-	27.1	Al₃Ti

Fig. 5 Magnified views via SEM in BSE mode showing the sharp difference in SPP dispersion between a AGG region of Location 1 marked in Fig. 3d in NP4-HT sample, b non-AGG region of Location 2 marked in Fig. 4c in P4-HT sample

Fig. 6 a Macroscopic view of AGG-free cross section of 5-pass weld with Al-Ti powder mixture addition after HT (P5-HT); refined grain structures in the upper part for b the left, c the middle, and d the right regions, marked I, II, and III, respectively, in a; e and f the typically transformed particles in the residual-rich area, enclosed by the white dashed line in a; g loose and dispersed cluster of Al3Ti in the center of the stir zone, marked IV in a

Fig. 7 Humphreys’ grain growth model with the stability of each characterized location predicted

Fig. 8 a and b Processed SEM images from Fig. 5a and b, showing the dispersions of SPPs with oxides colored red and the others white in NP4-HT and P4-HT, respectively; c SPP size distributions quantified using Image-Pro Plus from a and b (Note: Considering the resolution limit of SEM, particles with a radius between 0 and 20 nm were all categorized into the dp = 20 nm group.)

Table 4 Average diameters and volume fractions of SPPs, average grain radius in the pre-HT state, and computed particle pinning parameters for various locations of NP4, P4, and P5 samples

Parameter	NP4			P4				P5
	L	C	R	L	C	R	R_A	L	C	R
$\overline{{d }_{\text{p}}}$(μm)	0.103 ± 0.002	0.103 ± 0.003	0.101 ± 0.001	0.124 ± 0.003	0.137 ± 0.002	0.126 ± 0.003	0.110 ± 0.004	0.119 ± 0.002	0.126 ± 0.002	0.137 ± 0.004
F_V (%)	0.325 ± 0.019	0.333 ± 0.015	0.280 ± 0.028	1.263 ± 0.039	1.550 ± 0.094	1.266 ± 0.037	0.410 ± 0.048	1.346 ± 0.075	1.516 ± 0.039	1.938 ± 0.163
R_avg (μm)	6.71 ± 0.23	6.96 ± 0.17	6.65 ± 0.35	7.47 ± 0.29	7.22 ± 0.37	6.63 ± 0.18	6.47 ± 0.11	6.57 ± 0.36	6.96 ± 0.14	6.75 ± 0.36
Z	0.633	0.678	0.550	2.286	2.454	2.006	0.726	2.226	2.508	2.865

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