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Journal of Chinese Society for Corrosion and protection  2017, Vol. 37 Issue (3): 293-299    DOI: 10.11902/1005.4537.2016.030
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Intergranular Corrosion Behavior of Friction-stir Welding Joint for 20 mm Thick Plate of 7075 Al-alloy
Deqiang LIU1,2,Liming KE1,3(),Weiping XU1,Li XING1,Yuqing MAO3
1 School of Aeronautical Manufacturing and Engineering, Nanchang Hangkong University, Nanchang 330063, China
2 Jiangling Holdings Co., Ltd., Nanchang 330052, China
3 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
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Abstract  

The intergranular corrosion behavior of friction-stir welding (FSW) joint for 20 mm thick plate of 7075 Al-alloy was investigated in solution of 57 g NaCl+1000 mL H2O+10 mL H2O2 corresponding to the national standard GBT7998-2005. The microstructure of welded joint, the composition and distribution of the second phase, corrosion depth and corrosion morphology of different zones of the weld joint were characterized by optical microscope and scanning electron microscope. The results show that the corrosion severity of the center zones of the weld joint is the lightest. The corrosion severity of the thermal mechanical affected zone (TMAZ) is between that of the center zones and heat affected zone (HAZ). Downward along the thickness direction of the weld joint, the corrosion severity of the nugget zone (NZ) gradually increased, while that of the TMAZ increased first and then decreased, and that of the HAZ gradually decreased. The difference in the size and the distribution of the second phase and in the grains size of the weld joint along the thickness direction may be the main factor which caused the different corrosion severity for different zones along the thickness direction of the weld joint.

Key words:  FSW      heavy plate      7075 Al-alloy      intergranular corrosion      the second phase     
Received:  09 March 2016     
Fund: Supported by National Natural Science Foundation of China (51265043 and 51364037) and Aviation Science Foundation Projects (20140956003)

Cite this article: 

Deqiang LIU,Liming KE,Weiping XU,Li XING,Yuqing MAO. Intergranular Corrosion Behavior of Friction-stir Welding Joint for 20 mm Thick Plate of 7075 Al-alloy. Journal of Chinese Society for Corrosion and protection, 2017, 37(3): 293-299.

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https://www.jcscp.org/EN/10.11902/1005.4537.2016.030     OR     https://www.jcscp.org/EN/Y2017/V37/I3/293

Fig.1  Schematic diagram of cutting of samples for intergranular corrosion tests
Fig.2  Macroscopic morphology of cross section of FSW joint of 7075 Al alloy plate
Fig.3  Intergranular corrosion morphologies of the top surfaces of 1# (a), 2# (b), 3# (c), 4# (d) and 5# (e) samples locating at different depths of FSW joint
Fig.4  Intergranular corrosion microstructures of different zones of 3# sample: (a) NZ, (b, c) magnified images of areas A and B in Fig.4a and Fig.4c, respectively; (d) HAZ, (e, f) magnified images of areas C and D in Fig.4d and Fig.4e, respectively; (g) TMAZ, (h, i) magnified images of the areas E and F in Fig.4g and Fig.4h, respectively
Sample Area Max. depth / μm
1# NZ 23.8
HAZ 123.8
TMAZ 44.4
3# NZ 13.3
HAZ 80.0
TMAZ 68.5
5# NZ 18.8
HAZ 71.1
TMAZ 57.8
Table 1  Intergranular corrosion depths of different zones of three samples locating at different depths of FSW joint
Fig.5  Corrosion depth maps of NZ (a), TMAZ (b) and HAZ (c) of 3# sample
Fig.6  Microstructures of different zones of 1# (a, d, g), 3# (b, e, h) and 5# (c, f, i) samples along the weld thickness direction: (a) SAZ, (b) top of NZ, (c) bottom of NZ, (d) top of TMAZ, (e) middle of TMAZ, (f) bottom of TMAZ, (g) top of HAZ, (h) middle of HAZ, (i) bottom of HAZ
Fig.7  Distributions of second-phase particles in different zones of 1# sample: (a) NZ, (b) TMAZ, (c) HAZ
Element Particle1 Particle2 Particle3
Al 76.76 78.14 78.7
Fe 10.31 10.52 16.68
Cu 5.36 6.94 4.62
Zn 1.32 --- ---
Cr 2.90 2.33 ---
Si 1.91 2.07 ---
Mn 1.17 --- ---
Table 2  EDS analysis results of second-phase particles in Fig.7(atomic fraction / %)
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