Effect of post weld artificial aging and water cooling on microstructure and mechanical properties of friction stir welded 2198-T8 Al-Li joints

doi:10.1016/j.jmst.2022.01.020

Abstract

Abstract:

Friction stir welding (FSW) under both air cooling and water cooling conditions with welding parameters of 800-1200 rpm rotation rates and 50-200 mm/min welding speeds was carried out on 2198-T8 Al-Li alloys, and post weld artificial aging was performed on the air cooled joints. No welding defects other than lazy S were observed in the nugget zone (NZ) for all joints. Under air cooling condition, the lowest hardness zone (LHZ) occurred in the heat affected zone (HAZ). FSW resulted in gradual dissolution of original T₁, θ′ and δ′/β′ from the base material (BM) to the thermo-mechanically affected zone (TMAZ), and complete dissolution of all precipitates in the NZ with δ′/β′ and Guinier-Preston zones precipitating during cooling. The air cooled joints exhibited no noticeable changes in intrinsic tensile strength with a joint strength reaching 81.3% of the BM, but varied elongation with welding parameters, which was closely related to failure in the NZ and fracture along lazy S. Post weld artificial aging led to the largest hardness recovery in the TMAZ but smaller hardness recovery in the initial LHZ and the NZ. Different aging kinetics across the joint was determined by volume fraction of both original precipitate dissolution during welding and coarse particles formed during aging, and by dislocation density inherited from welding. Post weld artificial aging greatly enhanced the joint strength with the ultimate tensile strength reaching 87.3% of the BM. As compared to air cooling condition, water cooling hardly affected the NZ hardness and did not improve the joint strength, and the reason was discussed in light of precipitates, hardness changes and fracture behavior.

Key words: Friction stir welding, Aluminum-lithium alloys, Microstructure, Mechanical properties

Y. Tao, Z. Zhang, P. Xue, D.R. Ni, B.L. Xiao, Z.Y. Ma. Effect of post weld artificial aging and water cooling on microstructure and mechanical properties of friction stir welded 2198-T8 Al-Li joints[J]. J. Mater. Sci. Technol., 2022, 123: 92-112.

Figures/Tables 23

Fig. 1. (a) Schematic drawing of FSW and (b) shape of the welding tool.

Table 1. Welding parameters of FSW 2198-T8 joints.

Cooling method	Rotation rate(rpm)	Traverse speed(mm/min)	Designation
Air cooling	800	200	AC-800-200
	1200	200	AC-1200-200
	1200	50	AC-1200-50
Air cooling, followed by post weld artificial aging	800	200	AC-800-200-AA
	1200	200	AC-1200-200-AA
	1200	50	AC-1200-50-AA
Water cooling	800	200	WC-800-200
	1200	200	WC-1200-200
	1200	50	WC-1200-50

Fig. 2. Configuration and size of the tensile sample.

Fig. 3. Cross-sectional macrographs of FSW 2198-T8 joints: (a) AC-800-200, (b) AC-1200-200, (c) AC-1200-50, (d) WC-800-200, (e) WC-1200-200 and (f) WC-1200-50.

Fig. 4. Micrographs of lazy S in the areas marked by arrows in Fig. 3(b): (a) OM image and (b) magnified SEM image.

Fig. 5. Optical micrographs of various zones of FSW 2198-T8 joints.

Fig. 6. Hardness profiles of FSW 2198-T8 joints exhibiting (a) effect of welding parameter under air cooling condition; effect of post weld artificial aging and water cooling at (b) 800 rpm-200 mm/min, (c) 1200 rpm-200 mm/min and (d) 1200 rpm-50 mm/min.

Table 2. Transverse tensile properties of FSW 2198-T8 joints.

Sample	UTS (MPa)	YS (MPa)	El. (%)	Joint efficiency (%)
BM	488.5 ± 0.2	417.5 ± 21.9	16.9 ± 0.5	-
AC-800-200	397.2 ± 0.1	262.3 ± 6.5	9.1 ± 0.2	81.3
AC-800-200-AA	423.9 ± 0.5	386.1 ± 0.4	4.5 ± 0.4	86.8
WC-800-200	384.9 ± 0.6	277.1 ± 0.6	8.0 ± 0.3	78.8
AC-1200-200	388.9 ± 4.8	271.2 ± 0.0	6.0 ± 0.4	79.6
AC-1200-200-AA	426.5 ± 2.0	387.4 ± 1.2	3.2 ± 0.7	87.3
WC-1200-200	390.6 ± 3.5	286.3 ± 1.9	5.0 ± 0.4	80.0
AC-1200-50	388.7 ± 1.9	256.6 ± 2.7	9.0 ± 0.3	79.6
AC-1200-50-AA	410.7 ± 1.2	358.5 ± 1.2	4.2 ± 0.4	84.1
WC-1200-50	379.7 ± 1.9	267.9 ± 0.6	7.4 ± 1.1	77.7

Fig. 7. Fracture locations of FSW 2198-T8 joints at various parameters.

Fig. 8. Macrographic fractographs of FSW 2198-T8 joints: (a) AC-1200-200, (b) AC-1200-50, (c) AC-1200-200-AA, (d) AC-1200-50-AA, (e) WC-1200-200 and (f) WC-1200-50.

Fig. 9. Magnified fractographs of specific positions in Fig. 8: (a) and (b) magnified and further magnified images of the areas marked by arrows; (c) position A, (d) position B and (e) position C.

Fig. 10. Magnified fractographs of specific positions in Fig. 8: (a) position D, (b) position E, (c) position F and (d) position G.

Table 3. Crystal structure and precipitation characteristics for precipitates in Al-Cu-Li-Mg-Ag-Zr alloys.

Precipitate	Crystal structure	Morphology	Orientation	Refs.
T1 (Al2CuLi)	Hexagonal	Platelete (front view), Needle-like (side view)	(0001)∥(111)Al
[10-10]∥[-110]Al
[22,53]
θ′ (Al2Cu)	Tetragonal	Platelete (front view), Needle-like (side view)	[001]∥[100]Al	[22,53]
S′ (Al2CuMg)	Orthorhombic	Lath	[100]∥[100]Al	[22]
			[010]∥[02-1]Al
			[001]∥[012]Al
δ′ (Al3Li)	L12	Spherical	(111)∥(111)Al	[22,53]
β′ (Al3Zr)	L12	Spherical	(111)∥(111)Al	[22,29]

Fig. 11. TEM images of the BM: (a) low magnification bright field micrograph, (b) and (c) high magnification bright field micrographs in <110>Al and <100>Al orientations, (d), (e) and (f) SAD patterns of <100>Al, <110>Al and <112>Al zone axes.

Fig. 12. TEM bright filed micrographs of the LHZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) low magnificaiton images, (b) and (e) high magnification images in <110>Al orientation; (c) and (f) high magnification images in <100>Al orientaiton.

Fig. 13. TEM SAD patterns of the LHZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) <100>Al zone axis, (b) and (e) <110>Al zone axis, (c) and (f) <112>Al zone axis.

Fig. 14. TEM bright filed micrographs of the TMAZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) low magnificaiton images, (b) and (e) high magnification images in <110>Al orientation, (c) and (f) high magnification images in <100>Al orientaiton.

Fig. 15. TEM SAD patterns of the TMAZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) <100>Al zone axis, (b) and (e) <110>Al zone axis, (c) and (f) <112>Al zone axis.

Fig. 16. TEM bright filed micrographs of the NZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) low magnificaiton images; (b) and (e) high magnification images in <110>Al orientation; (c) and (f) high magnification images in <100>Al orientation.

Fig. 17. TEM SAD patterns of the NZ of sample (a-c) AC-800-200 and (d-f) AC-800-200-AA: (a) and (d) <100>Al zone axis, (b) and (e) <110>Al zone axis, (c) and (f) <112>Al zone axis.

Fig. 18. TEM images of the NZ of sample WC-800-200: (a) low magnification bright field micrograph; (b) high magnification bright field micrograph in <100>Al orientation; (c) SAD patterns of <100>Al zone axis.

Table 4. Summary of precipitate evolution in various zones of FSW 2198-T8 joints.

Region	Sample	Precipitate evolution
BM	—	T₁, θ′ and δ′/β′
LHZ	AC-800-200	T₁ and θ′ partially dissolved
	AC-800-200-AA	(1) pre-existing T₁ and θ′ coarsened
(2) T₁ and θ′ precipitated
TMAZ	AC-800-200	(1) T₁ further dissolved and θ′ completely dissolved
(2) δ′/β′ reprecipitated
	AC-800-200-AA	(1) The pre-existing T₁ coarsened and δ′/β′ dissolved
(2) T₁, θ′ and S′ precipitated
NZ	AC-800-200	(1) T₁, θ′ and δ′/β′ completely dissolved
(2) δ′/β′ and GP zones reprecipitated
	AC-800-200-AA	(1) pre-existing δ′/β′ dissolved
(2) T₁, θ′ and S′ precipitated
(3) θ formed
	WC-800-200	(1) T₁, θ′ and δ′/β′ completely dissolved
(2) δ′/β′ and GP zones reprecipitated

Fig. 19. Schematic diagrams of TEM bright field micrographs in <110>Al orientation showing precipitate evolution of FSW 2198-T8 joints.

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