Microstructure and Mechanical Properties in Friction Stir Welded Thick Al-Zn-Mg-Cu Alloy Plate

doi:10.1007/s40195-022-01373-y

Abstract

Abstract:

In this work, 20-mm-thick aluminum-alloy plates were joined via friction stir welding. The temperature gradient was reduced by reducing the surface welding heat input to achieve uniformity of the mechanical properties across the thick plate joints. The welding temperature was measured using thermocouples. The microstructures were observed via electron backscatter diffraction and transmission electron microscopy. The tensile properties of the samples sliced along the thickness direction of the joint were evaluated. The results show that the highest welding peak temperature is 430 °C on the advancing side on the top surface of the joint. The grain size gradually decreased along the thickness direction, and grain refinement was due to the combination of continuous, discontinuous, and geometric dynamic recrystallization. The tensile properties of the sliced samples were found to be uniform, and the ultimate tensile strength reached 62% of that of the base metal. The main strengthening mechanism of the Al-Zn-Mg-Cu alloy joints consists of precipitation strengthening. In addition, the η` → η phase transition and grain coarsening in the heat-affected zone were found to be responsible for the fracture of the joints.

Key words: Friction stir welding, Aluminum alloy thick plate, Dynamic recrystallization, Mechanical properties, Precipitated phase

Fei Qiang, Wen Wang, Ke Qiao, Pai Peng, Ting Zhang, Xiao-Hu Guan, Jun Cai, Qiang Meng, Hua-Xia Zhao, Kuai-She Wang. Microstructure and Mechanical Properties in Friction Stir Welded Thick Al-Zn-Mg-Cu Alloy Plate[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(8): 1329-1342.

Figures/Tables 17

Table 1 Chemical composition of the Al-Zn-Mg-Cu alloy (wt%)

Zn	Mg	Cu	Si	Fe	Zr	Ti	Al
4.57	2.01	1.92	0.01	0.03	0.05	0.01	Bal.

Table 2 Mechanical properties of the Al-Zn-Mg-Cu alloy

Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation (%)	Hardness (HV)
530 ± 5	580 ± 4	17.5 ± 2.6	203 ± 8

Fig. 1 a Illustration of FSW and sampling positions for microstructure observation and property testing, b schematic drawing of the FSW tool. WD, RD, TD, ND represent welding, rolling, transversal and normal directions, respectively. AS and RS are the advancing and retreating sides, respectively

Fig. 2 Schematic diagram of temperature measurement: a distribution of temperature measurement points along the thickness direction, b distribution of temperature measurement points along the transverse direction

Fig. 3 a Dimension of tensile sample, photography of b the FSW joint, c the sliced samples

Fig. 4 Temperature profiles along different thicknesses of the plate: a changing curves of AS, b changing curves of RS, c peak temperature in different thicknesses

Fig. 5 Distribution of transverse temperature profiles: a changing curves of AS, b changing curves of RS, c peak temperature in different thicknesses

Fig. 6 Microstructures of the BM: a EBSD morphology, b grain boundary distribution map, c TEM bright field phase, d [001]Al electron diffraction pattern, e [001]Al standard diffraction pattern

Fig. 7 Macrostructures of the FSW joint: a surface, b cross section

Fig. 8 Microstructures of different zones of the FSW joint: a1 SZ-Layer 1, a2 SZ-Layer 2, a3 SZ-Layer 3, a4 SZ-Layer 4, b TMAZ, c1 HAZ-Layer 1, c2 HAZ-Layer 2, c4 HAZ-Layer 4

Fig. 9 Grain boundary distribution in different zones of the FSW joint: a1 SZ-Layer 1, a2 SZ-Layer 2, a3 SZ-Layer 3, a4 SZ-Layer 4, b TMAZ, c1 HAZ-Layer 1, c2 HAZ-Layer 2, c4 HAZ-Layer 4 (Blue arrows represent fine recrystallized grains; Black arrows represent HAGBs bulging; Red arrows represent the grains divided by LAGBs)

Fig. 10 Precipitated phase morphology and selected area diffraction pattern (SADP): a1 SZ-Layer 1, a4 SZ-Layer 4, b TMAZ, c1 HAZ-Layer 1, c4 HAZ-Layer 4, d, e SADP of the precipitated phase in a1 c1, respectively. (Green arrows represent η`; red arrows represent Al3Zr; blue arrows represent η)

Fig. 11 a Cross-sectional hardness distribution of the FSW joint, b the lowest hardness coordinates linear fitting results, (AS-LHC: the lowest hardness coordinates in the advancing side; RS-LHC: the lowest hardness coordinates in the retreating side), c shape coordinates of the stir tool

Fig. 12 Engineering stress-strain curves of the BM and the FSW joint

Table 3 Tensile test results of the joint

Samples	Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation (%)	Efficiency of UTS (%)
FSW Joint	257 ± 4	369 ± 2	4.0 ± 0.6	64
Layer 1	257 ± 2	360 ± 7	2.4 ± 0.5	62
Layer 2	246 ± 5	360 ± 3	4.6 ± 0.6	62
Layer 3	242 ± 4	361 ± 6	5.3 ± 0.7	62
Layer 4	247 ± 3	363 ± 3	6.0 ± 0.4	63

Fig. 13 Fracture position of a the FSW joint, b the sliced samples

Fig. 14 SEM morphologies of fracture surface of the joint and the sliced samples: a BM, b FSW joint, c Layer 1, d Layer 2, e Layer 3, f Layer 4

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