Microstructure Evolution and Mechanical Properties of Friction Stir Welded Al-Cu-Li Alloy

doi:10.1007/s40195-024-01674-4

Abstract

Abstract:

The investigation concentrates on friction stir welded (FSW) Al-Cu-Li alloy concerning its local microstructural evolution and mechanical properties. The grain features were characterized by electron back scattered diffraction (EBSD) technology, while precipitate characterization was conducted by using transmission electron microscopy (TEM) aligned along [011]_Al and [001]_Al zone axes. The mechanical properties are evaluated through micro-hardness and tensile testing. It can be found that nugget zones exhibit finely equiaxed grains evolved through complete dynamic recrystallization (DRX), primarily occurring in continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). In the thermal-mechanically affected zone (TMAZ), numerous sub-structured grains, exhibiting an elongated morphology, were created due to partial DRX, signifying the dominance of CDRX, DDRX, and geometric dynamic recrystallization (GDRX) in this region. T₁ completely dissolves in the nugget zone (NZ) leading to the formation of Guinier-Preston zones and increase of δ′, β′ and S′. Conversely, T₁ partially solubilizes in TMAZ, the lowest hardness zone (LHZ) and heat affected zone (HAZ), and the residual T₁ undergoes marked coarsening, revealing various T₁ variants. The solubilization and coarsening of T₁ are primary contributors to the degradation of hardness and strength. θ′ primarily dissolves and coarsens in NZ and TMAZ, whilst this precipitate largely coarsens in HAZ and LHZ. σ, T_B, grain boundary phases (GBPs) and precipitate-free zone (PFZ) are newly generated during FSW. σ exists in the TMAZ, LHZ and HAZ, whereas T_B nucleates in NZ. GBPs and PFZ mostly develop in LHZ and HAZ, which can cause strain localization during tensile deformation, potentially leading to LHZ joint fracture.

Key words: Al-Cu-Li alloy, Friction stir welding, Microstructure evolution, Mechanical properties

Peng Chen, Wenhao Chen, Jiaxin Chen, Zhiyu Chen, Yang Tang, Ge Liu, Bensheng Huang, Zhiqing Zhang. Microstructure Evolution and Mechanical Properties of Friction Stir Welded Al-Cu-Li Alloy[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(5): 855-871.

Figures/Tables 21

Table 1 Main precipitates in Al-Cu-Li alloy [22,24,25,26,27,28]

Phase	Crystal structure	Lattice constant (nm)			Orientation relationship	Morphology
Phase	Crystal structure	a	b	c	Orientation relationship	Morphology
T₁	Hexagonal	0.497		0.935	(0001)_T1∥(111)_Al [101]_T1∥[-110]_Al	Plate
δ'	L1₂	0.401-0.404			(111)_δ'∥(111)_Al	Spherical
β'	L1₂	0.405			(111)_β'∥(111)_Al	Spherical
θ'	Tetragonal	0.404-0.408		0.58	[001]_θ'∥[100]_Al	Plate
S'	Orthorhombic	0.404	0.925	0.718	[100]_S'∥[100]_Al [010]_S'∥[02-1]_Al [001]_S'∥[012]_Al	Lath, rod
T_B	CaF₂	0.583		0.58	[001]_TB∥[100]_Al	Rod
σ	Cubic	0.829			(100)_σ∥(100)_Al [100]_σ∥[100]_Al	Cubical

Fig. 1 T1 is marked green; θ′ is marked pink; (011)Al plane is marked blue; (001)Al plane is marked purple. Schematic diagram of a habit precipitation locations of T1 and θ′, and b morphological features of T1 and θ′ along [011]Al and [001]Al zone axes

Fig. 2 Schematic diagram of Al-Cu-Li alloy FSW process

Fig. 3 Macroscopic morphology of the cross-section of Al-Cu-Li alloy FSW joints

Fig. 4 Schematic diagram of the tensile test specimens

Fig. 5 Hardness profiles of Al-Cu-Li alloy FSW joints

Fig. 6 EBSD analysis of a BM, b NZ, c NZ-TMAZ

Fig. 7 Different grain types in a BM, b NZ, c NZ-TMAZ, and d fractions

Fig. 8 Schematic SAED patterns of the precipitates in Al-Cu-Li alloy along a [011]Al and b [001]Al zone axes

Fig. 9 STEM and SAED images of the BM along a, b [011]Al zone axis, and c, d [001]Al zone axis

Fig. 10 STEM and SAED images of the NZ along a-f [011]Al zone axis, and g, h [001]Al zone axis. (c SAED corresponding to (b); d SAED of TB; e SAED of δ′/β′; f SAED of S′; h SAED corresponding to (g))

Fig. 11 STEM and SAED images of the TMAZ along a-e [011]Al zone axis, and f-l [001]Al zone axis. (c, l SAED of σ; d, k SAED of T1(2/4#); e, i SAED of θ′; h SAED corresponding to (g); j SAED of T1(1/3#))

Fig. 12 Samples of the LHZ a prepared by combination of FIB and micro-hardness test, b observed by TEM

Fig. 13 STEM and SAED images of the LHZ along a-c [011]Al zone axis, and d-f [001]Al zone axis. (c SAED corresponding to (b); f SAED corresponding to (e))

Fig. 14 STEM and SAED images of HAZ along a-c [011]Al zone axis, and d-f [001]Al zone axis. (c SAED corresponding to (b); f SAED corresponding to (e))

Fig. 15 Engineering stress-strain curves of BM and FSW joints

Fig. 16 Fracture morphologies of a-c BM and d-e FSW joints

Fig. 17 Dynamic recrystallization in a NZ and b TMAZ

Table 2 Summary of precipitates in different regions of Al-Cu-Li alloy FSW joints

Regions	Precipitate evolution
BM	T₁, θ′, δ′/β′ and S′
NZ	T₁ completely dissolved, θ′ dissolved and coarsened, δ′ and S′ reprecipitated, β′ increased, GP zones and T_B newly formed
TMAZ	T₁ dominantly dissolved and the residual coarsened, θ′ dissolved and coarsened, δ′ reprecipitated, β′ increased, S′ dissolved, σ newly formed
LHZ	T₁ partially dissolved and the residual coarsened, θ′ coarsened, δ′ reprecipitated, β′ increased, S′ dissolved, σ, GBPs and PFZ newly formed
HAZ	T₁ partially dissolved and the residual coarsened, θ′ and S′ coarsened, δ′ reprecipitated, β′ increased, σ, GBPs and PFZ newly formed

Fig. 18 Size distribution of T1 in a BM, b LHZ and c HAZ, and d number density of T1 in different regions of NZ

Fig. 19 a Macroscopic strain distribution contour of FSW joints at different tensile test time and b precipitate observation of LHZ when deformed by 100 s

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