International Journal of Minerals, Metallurgy and Materials

Accepted manuscript, https://doi.org/10.1007/s12613-019-1924-4

© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Stress-corrosion behavior and characteristics in friction stir welding of AA2198-T34 alloy

Quan-qing Zeng1,2, Songsheng Zeng1,3*, and Dongyao Wang1,2*

a. Research Institute of Light Alloy, Central South University, Changsha, Hunan, 410083, China

b. Key Laboratory of the Ministry of Education of Nonferrous Metals, Central South University,

Changsha, Hunan, 410083, China

c. Technology Center, Valin ArcelorMittal Automotive Steel Co., Ltd, Loudi, Hunan, 417009,

China

Abstract: To better understand the stress corrosion behavior of the friction stir welding (FSW), the effects of

microstructure on stress corrosion behavior of the FSW in 2198-T34 aluminum alloy were investigated. The

experimental results show that the low-angle grain boundary (LABs) of stir zone (SZ) of friction stir welding

is significantly less than that of heated affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and

parent materials (PM), but grain boundary precipitates (GBPs) T1（ Al2CuLi） are less, which has slight effect

on stress corrosion. The dislocation density in SZ is higher than that in other regions. The residual stress in SZ

is +67 MPa, which is larger than that in TMAZ. The residual stress in HAZ and PM is -8 MPa and -32 MPa,

respectively, and are both compressive stresses. The corrosion potential in SZ is obviously lower than that in

other regions. However, micro-cracks were formed in the SZ at low strain rate. This indicates that grain

boundary characters and GBPs have no significant effects on the crack initiation in stress corrosion process of

AA2198-T34. Nevertheless, residual tensile stress has significant effects on crack initiation during stress

corrosion process.

Keywords: Slow strain rate test; Residual stress; Grain boundary characteristic; AA2198-T34 alloy; Friction

stir welding

1 Introduction

FSW as a relatively novel solid-state joining and environmentally friendly technique has been widely

employed in the field of the welding of aluminum alloys [1]. The third generation of

aluminum-copper-lithium alloy such as 2050 Al-Cu-Li alloy has light weight, high specific strength, good

ductility and toughness, and is favorable in the aerospace industry [2]. However, fatal problems like severe

corrosions and residual stresses always occur in the aerospace industry, which can lead to inter-granular

corrosion, exfoliation corrosion, and stress corrosion cracking (SCC) of Al-Cu-Li alloys, especially in its

welding area, which are subjected to high stress, and more strict requirements for stress corrosion resistance

are put forward [3-5]. Therefore, it is quite critical to investigate the microstructure and stress corrosion

resistance of friction stir welding of Al-Cu-Li alloy to guide its engineering application [6].

The stress corrosion resistance of Al-Zn-Mg alloys was initially researched, and the pitting corrosion and

stress corrosion behavior of 2219 aluminum alloy were studied by many researchers, and the stress corrosion

sensitivity coefficient of 2219 aluminum alloy was acquired [7]. The narrow width of precipitation-free zone

at grain boundaries of Al-Zn-Mg alloys and the low concentration of zinc in Al matrix result in the

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enhancement of SCC resistance and electrochemical corrosion performance [8]. In FSW, high plastic

deformation and high temperature result in the different microstructure regions in the weld zone, such as SZ,

TMAZ, HAZ and PM, which lead to the different stress corrosion behavior and characteristics of friction stir

welding [9]. Blanc et al. [10] researched the IGC behavior in the SZ of FSW of AA2050. It was found that the

precipitation state was the key factor determining the preferential corrosion of SZ. Donatus et al.[11] found

that local high temperature in HAZ results in obvious coarsening of T1 phase after FSW. There is a dramatical

change on the stirring heat residual stress distribution of the seam during FSW [12]. The longest crack

initiation and propagation along the normal stress interface of grain are observed in the SZ of FSW 2050

Al-Cu-Li alloy after stress corrosion cracking test[13]. The critical value of normal stress causing SCC is

about 80% of macro stress[14].

The purpose of this paper is to study the main factors affecting the stress corrosion properties of FSW.

For the SZ, HAZ, TMAZ and PM of AA2198-T34 Al-Cu-Li alloy FSW, the microstructure, GBPs and

residual stress as well as its distribution in different regions were studied. The characteristics of the weld's

stress corrosion resistance was summed up and its impacts were assessed, and the corrosion resistance of each

region of the seam was evaluated by the polarization curve test.

2 Materials and methods

The AA2198-T34 Al-Cu-Li alloy sheet was used as the test materials. The welded specimens were cut by

wire cutting machine. The size of the specimens is 240 mm×75 mm. The chemical composition of the

specimens is exhibited in Table 1. Before the welding test, the side and upper as well as lower surfaces of the

welded parts were polished by a corner mill to remove the oxide film on the surface of the welded parts. Two

pieces of specimens were connected and then welded by FSW machine. The welding was carried out using a

tool with a shoulder diameter of 6 mm and a pin diameter of 2 mm. While the welding speed was set as 200

mm/min, the rotational speed was 900 r/min, and the inclination angle of the stirring head was 3°. Fig. 1(a) is

a schematic diagram of shape and size of the samples for welding, and Fig. 1(b) shows the setup of the FSW

process. Afterwards, these specimen，such as the cross-section specimen, parent materials specimen，welding

area specimen, residual stress specimen, electrochemical sample and the specimen for SSRT，were cut from

the welding parts by wire cutting machine. The observation surface of these sample was polished with 80

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mesh, 800 mesh, 1500 mesh and 2000 mesh abrasive paper in turn. Finally, they were polished with

diamond polishing paste.

Table 1 Chemical composition of the alloys investigated in wt.% Cu Li Mg Ag Zr Fe Zn Mn Ni Si Al 3.3 1.0 0.32 0.3 0.11 0.04 0.02 0.01 0.01 0.03 bal

Fig. 1 (a) Sampling diagram and (b) sample diagram of welded parts.

The cross-section specimen was observed by super-depth-of-field microscope （VHX-5000）, and its

corrosion depth was also measured by its 3D mode. The resulting grain sizes/sub-grain orientation as well as

grain distribution in four regions were analyzed via EBSD using a Zeiss Ultra 55SEM and the software

channel 5. The EBSD samples sectioned in the ND-RD plane were electro-polished with the solution of 70 %

methanol and 30 % nitric acid (Vol%) at -30 ℃ and a voltage of 30 V after mechanical polishing. These TEM

samples were extracted from the SZ of the PM and HAZ, and mechanically thinned to 0.1 mm. Afterwards,

these samples were punched out into round slices with a diameter of 3 mm. The mixture of nitric acid and

methanol in the ratio of 3:7 was used as the electrolyte to reduce the thickness of these round slices by double

spraying at 20 V from -20 ℃ to -30 ℃. Finally, these samples were tested by TEM（3000F,JEOL）at a voltage

of 300 kV.

Residual stress was measured by residual stress tester (Xstress3000)，and its voltage was set at 22 KV, its

target material was Ni-Ka, and the peak (311) was measured by 2θ scanning at 10 different inclination angles.

The measured diffraction peaks were fitted with a Gaussian function. The approximation of a biaxial stress

state was made for the calculation of stress-free lattice parameters and stresses. Generally, diffraction elastic

constants for the specific set of lattice planes used for the diffraction measurement was applied for calculating

stresses from the measured strains. For the sample of cross section of weld, three scanning lines were scanned

once, as shown in Fig. 2(a). The distance between continuous measuring points was 3 mm. The allowable

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deviation of residual stress was 2 MPa. The experiment was repeated when the deviation exceeded 2 MPa.

The residual stresses in coronal plane were measured for three times under each condition, and the average

value was obtained. Fig. 2(b) is physical diagram of residual stress tester.

Fig. 2 (a) Testing distribution of residual stress in cross section of Weld. (b) Physical diagram of residual stress tester

The prepared electrochemical samples were put into electrochemical workstation （CHI660E）for

electrochemical performance test with saturated calomel reference electrode （ SCE） , platinum plate

electrode and the working electrode connected with the sample. Samples were immersed in 3.5 wt.% NaCl

solution with platinum plate electrode as the working electrode. After reaching the open circuit potential, the

electrochemical measurement was conducted and the scanning rate of polarization test was set as 0.167 mV/s.

According to ASTM-G129 standard, SSRT was used to evaluate the SCC susceptibility of weld seam to

stress corrosion. The experiment was divided into two groups by using the constant-rate stretcher（RSW50）.

The first group was performed by stretching SSRT specimens directly in the air at a rate of 1×10-6 s-1. The

second group was performed by completely immersing the parallel length part of SSRT specimens in a

solution of 3.5 wt.% sodium chloride at a rate of 1×10-6 s-1. These experiments were repeated twice at least to

ensure the measurement reproducibility.

3 Results and discussion

3.1 Microstructure of friction stirring welding

3.1.1 Metallographic Structure of weld

Fig. 3 exhibits the metallographic structure of the cross section of the weld. Fig. 3(a) is the low-power

metallographic structure of the weld cross section. It shows the changes of grain morphology in each weld

region. Figs. 3(b), (c) and (d) are the enlarged maps of the PM, SZ, TMAZ and HAZ, respectively. Grain

morphologies of four zones can be observed. The parent metal is AA2198-T34 sheet with 4mm thickness. In

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the FSW process, the tool was used to intensively stir the structure of PM. The gradual structure

transformation from HAZ to SZ is shown in Fig. 3d. At the same time, a large amount of friction stir heat is

generated in the welding process and concentrated in the SZ, which lead to complete recovery and dynamic

recrystallization of grains in the SZ, resulting in uniform and fine equiaxed grains, as shown in Fig. 3c.

Fig.3 Metallogram of cross section of weld (a) cross-sectional low power metallographic maps, (b) PM magnification maps, (c) SZ magnification

maps, (d) HAZ and TMAZ magnification maps.

3.1.2 Microstructure of grain boundary of weld

Fig. 4 shows the orientation distribution map（IPF）and the grain boundary distribution map of weld cross

section. Figs.4(a), 4(c), 4(e) and 4(g) are the IPF maps of SZ, TMAZ, HAZ and PM of weld cross section,

respectively. Figs.4(b), 4(d), 4(f) and 4(h) are the grain boundary characteristic distribution maps of SZ,

TMAZ, HAZ and PM of weld cross section, respectively. In the characteristic distribution of grain boundaries,

the blue line represents the high angle grain boundaries (HABs) with an angle higher than 15°, the green line

represents the LABs between 5° and 15°, and the red line represents the LABs between 2°and 5°. In Figs.4(b),

4(d), 4(f) and 4(h), the percentage of grain boundary types in SZ, TMAZ, HAZ and PM is obtained

respectively. In Fig. 4(a), different colors of the lower left hemipolar diagram represent different grain

orientations. In SZ, most of grains are crushed by the high-speed rotation of stirring needles, while a large

amount of friction stir heat is produced, which leads to the complete recrystallization, and the anisotropy is not

obvious, but the anisotropy is relatively obvious in Figs. 4(c), 4(e) and 4(g). In Fig. 4(b), there are many

equiaxed grains in SZ, and the proportion of HABs is 73.21 %, while that of LABs is only 26.79 %. This is a

typical dynamic recrystallization process. In Fig. 4(d), severe deformation results in many sub-structures such

as dislocations and sub-crystals in TMAZ, with a large number of LABs, accounting for 65.11 %. From Fig.

4(f), it is found that the HAZ is only subjected to a small amount of stirring heat, the grain boundary distortion

energy is low, and the number of low-angle grain boundaries is slightly higher than that of TMAZ, accounting

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for 66.49 %.

3.1.3 Grain boundary precipitates in weld seam.

Fig.4. IPF and grain boundary maps of weld cross section: (a) IPF maps of SZ, (b) IPF maps of SZ, (c) IPF maps of TMAZ, (d) IPF maps of TMAZ,

(e) IPF maps of HAZ, (f) PM maps, (h) PM maps of grain boundary.

Figs. 5(a) and 5(c) are microstructures in SZ. Figs. 5(b) and 5(d) is microstructures in PM. The TEM

photographs of SZ (Figs. 5(a) and 5(c)) of the weld are observed. The number of precipitates of θ（Al2Cu） and

β（Al3Zr）is quite small. In Figs. 5(b) and 5(d), it can be seen that there are more precipitated phases, such as

θ and β, and more dislocations in the crystal. In the welding process, the SZ experiences thermal cycling and

severe plastic deformation with high temperature, resulting in the dissolution of strengthening phases.

Dynamic recrystallization occurs in the SZ, and dislocations are absorbed to form sub-grain boundaries, which

grow and rotate to form equiaxed recrystallized grains with HABs. Figs. 5(c) and 5(d) are the grain boundaries

of SZ and PM. Few GBPs are found and the width of the precipitation-free zone is narrow. There are a lot of

dislocations in the SZ, as shown in Fig. 5. Wu [15] pointed out that the S (Al2CuMg) precipitates were mainly

distributed at the LABs in Al-Cu-Li alloys, facilitating the enrichment of the elements of Cu and Mg at the

LABs. On the grain boundary（or sub-grain boundary）of AA2050, J.Stewart et al. found that the existence of

S phases did promote the formation of IGC [16]. Therefore, the existing forms of S phases precipitated at

LABs, such as GBPs and T1, were considered to be factors causing the high sensitivity and susceptibility of

Al-Cu-Li alloys to IGC [17]. During the ageing process, large number of copper atoms tend to precipitate from

the grain interior at LABs, forming chemical and electrochemical gradients, which would give the priority to

the establishment of anodic corrosion channels [18]. As what shown in Fig. 4, the smallest area of LABs is SZ,

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which possesses the smallest susceptibility to IGC, which is the area with the least IGC nucleation. However,

it is found that there are few precipitates at grain boundaries in each region of the weld with no

electrochemical gradient. Anodic corrosion channels will not be preferentially established at LABs. Therefore,

the grain boundary characteristic possesses quite slight effect on the SCC properties of the four regions of

AA2198-T34.

Fig. 5. Open-field image of 2198-T34 alloy and SAED pattern with axis (insertion) of [100] matrix region: (a) sub-grain boundary morphology

map of SZ region; (b) sub-grain boundary morphology map of PM region; (c) grain boundary morphology map of SZ region; (d) grain boundary

morphology map of PM region.

3.2 Residual stress of weld

Residual stress will intensively affect the service performance of materials and structural components,

such as fatigue life, dimensional stability, corrosion resistance and fracture toughness, especially SCC

resistance [19]. As a result, our measurements focused on the longitudinal residual stresses of this region.

Fig.6 shows the residual stress distributions as a function of distance from the depth of weld for the attach on

the weld cross section. The tensile residual stress presents in the weld zone, i.e. SZ, TMAZ and HAZ. The

largest value of residual stress (up to 67 MPa) are obtained at the SZ region, occurring at the edge of the

shoulder on the advancing side. In FSW, the stirring heat generated reduces the residual compressive stress in

the HAZ, which is only about -8 MPa. The shear stress produced in FSW transforms the residual stress in

TMAZ and SZ from compressive stress to tensile stress, and the residual stress in SZ reaches the maximum,

which is about +67 MPa. Compared to the other weld zone, the plastic extent of materials mixing in the stir

zone improves and the resistance of dislocation motion reduces, resulting in residual stress relaxation.

Therefore, the residual stress presents an “M” profile. This is due to the tool action which strains and heats the

materials. In order to better understand the residual stress, Blanc et al.[20] used finite element analysis

software to study the intergranular stress corrosion behavior in SZ of FSW of AA2050 alloy. Some literatures

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[21] show that recrystallized Cube grains in the SZ were formed by strain induced boundary migration

mechanism and Geometric dynamic recrystallization occurred in the SZ due to grain elongation and thinning

as influenced by the shear action of the tool pin. It is also found that cracks tend to nucleate at grain boundaries

with tensile stress.

Fig 6. Distribution of residual stress in the friction stir welding AA2198-T34.

3.3 Polarization curve

Fig. 7 shows the polarization curves of SZ, HAZ+TMAZ and PM measured in 3.5 wt.% NaCl electrolyte.

These three polarization curves have similar shapes, indicating that the corrosion process is the same during

initial immersion, but the potential distribution range is obviously different. At the initial stage of anodic

polarization, the current density increases with the potential, and the anodic dissolution reaction occurs in each

region. With the increase of potential, there is a slow change in the corrosion current, and the anodic

polarization curve in this range does not conform to Talfel's law. With the increase of polarization potential,

the anode current increases sharply. As the potential increases, the anion migration rate in 3.5 wt.% NaCl

solution also increases, and Cl- will easily penetrate the passive film on the surface of the samples in various

regions, resulting in the easy access to pitting potential. Corresponding electrochemical parameters such as

corrosion potential (Ecorr) and corrosion current density (icorr) can be obtained from polarization curves, as

presented in table 2. Corrosion potentials in PM, HAZ+TMAZ and SZ are -0.58 V, -0.64 V, -0.67 V,

respectively. Compared with PM, corrosion potential in SZ is significantly lower. The results show that the

PM exhibits the highest peak strength, followed by HAZ and TMAZ. In SZ, the peak strength of current

density is the smallest, and the corrosion resistance of SZ is the worst. The nucleation of Al(OH)3 is generated

in the defect and firmly adheres to the surface, hindering the entry of Cl- into the electrode substrate and the

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particle matrix. The activation reaction is effectively suppressed, the activation speed is slowed, and the

self-corrosion potential of the aluminum alloy is increased. In the region above the self-corrosion potentials,

the anodic current density increases rapidly with the enhancement of anodic polarization potentials, and the

anodic dissolution reaction occurs. As the anodic reaction develops, the rate of increase in current density

slows down and a passivation reaction occurs.

Table 2 Weld electrochemical parameters.

Region PM HAZ+TMAZ SZ

Corrosion potential, Ecorr(mV) -0.58 -0.64 -0.67

Current density, icorr(μA/cm2)

5.1×10-5 5.8×10-5 6.3×10-6

Fig. 7. Polarization curves of friction stir weld SZ, HAZ+TMAZ and PM zones.

3.4 Slow strain test of friction stir welding

3.4.1 Slow strain tensile curve of welding

Fig. 8 shows the stress-strain curves of the specimens immersed in 3.5 wt.% NaCl solution and SSRT in air,

respectively. The broken red line represents the stress-strain curve of SZ immersed in 3.5 wt.% NaCl solution,

and the broken blue line represents the stress-strain curve of SZ in air. The red line represents the stress-strain

curve of PM immersed in 3.5 wt.% NaCl solution, and the blue line represents the stress-strain curve of PM in

air. The mechanical properties of weld in 3.5 wt.% NaCl solution are similar to those of the slow strain tensile

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mechanical properties in air, and the elastic modulus is altered slightly. The tensile strength of samples

immersed in 3.5 wt.% NaCl solution is lower than that of samples immersed in air, as shown in table 3. The

hydrogen-induced delayed fracture occurs in the sample soaked in the NaCl solution, and the fracture time is

0.5 h longer than that in the sample processed in the air.

Table 3 Slow strain tensile properties of welds.

Region Tensile（MPa） Elongation（%） ISSRT

SZ-air 471 19.1

0.024

SZ-NaCl 451 17.3

PM-air 435 21.4 0.017

PM-NaCl 406 19.6

Fig.8 Stress-strain curve of slow strain tension.

3.4.2 Corrosion morphology and sensitivity analysis of slow strain specimens

Generally, the SCC sensitivity index (ISCC) is defined as the loss of area reduction after SSRT, as shown

in Formula (1).

=（ ) 100%air solSCC

air

Iψ ψ

ψ−

× （1）

Ψair and Ψsol in the formula are area reduction values in air and corrosive solution respectively, and their

corrosion sensitivity index is 0.26. The corrosion sensitivity index Iscc=ΔδSCC/δm includes three parts:

hydrogen-induced plastic loss Iscc (H) caused by hydrogen; plastic loss Iscc (AD) caused by the anodic

dissolution; and plastic loss Iscc (HAD) caused by coupling of hydrogen and anodic dissolution.

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Iscc= Iscc（H）+ Iscc（AD）+ Iscc（HAD） （2）

Fig. 9 shows the SZ, TMAZ, HAZ and PM regions of SSRT specimens immersed in 3.5 wt.% NaCl

solution by super-depth-of-field microscopy, respectively. The results show that the depth of corrosion pits in

SZ is deeper than that in other regions, and the corrosion in PM region is not obvious. The difference value

between the depth of corrosion pits in SZ and that in PM is 95.31 μm. Prominent pitting corrosion is observed

in SZ and HAZ, while the pitting corrosion of HAZ is insignificant, as shown in Figs. 9(a) and 9(c). Liu [22]

studied the effect of surface friction coefficient on pitting corrosion, and found that the smooth surface

treatment favors the reduction of the content and average size of Al-Cu-Mn-Fe intermetallic compounds, since

smooth surface treatment improves the dissolution of Al-Cu-Mn-Fe particles. Y. Ma et al. [23] assumed that

the driving anode was partially dissolved and lithium is detected in high copper-containing Al-Fe-Mn-Cu

particles with high activity. Gradual dissolution of intermetallic compounds occurs due to the tremendous

potential difference between the intermetallic compounds and the surrounding matrix, while intermetallic

compounds fall off from the matrix [24]. During FSW, more elements are migrated and segregated in the SZ,

and more T1, δ, βand θ phases are formed in the SZ [25]. In 3.5 wt.% NaCl solution, Al and Li are selectively

dissolved due to the corrosion originated from intermetallic compounds, facilitating the formation of

copper-rich residues, which are acted as effective cathodes for alloy matrix dissolution [26].

Fig. 9 Three-dimensional topography of four regions on the surface of slow strain specimens immersed in 3.5 wt.% NaCl solution: SZ, TMAZ,

HAZ and PM.

With the increase of anodic polarization potential, the hydrogen permeability is enhanced, fracture time is

decreased and the stress corrosion sensitivity coefficient is increased, so the stress corrosion sensitivity

coefficient in SZ is the highest. In the SSRT experiment, Al atoms in the matrix lose electrons, such as formula

(3). The loss of electrons of Al3+ and H2 occurs in the form of (4) ~ (6), which decreases the pH value of the

solution. As the pH value of the solution decreases, H+ accelerates the loss of electrons of Al atoms and

generates hydrogen, such as Formula (7) and (8). Therefore, the evolution of hydrogen atoms indicates the

local location of the electrochemical reaction, which can be used to locate the continuous local corrosion.

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Al= Al3++3e- （3）

Al3++H2O=Al(OH)2++H+ （4）

Al3++2H2O= Al(OH)2++2H+ （5）

Al3++3H2O=Al(OH)3+3H+ （6）

6H++2Al=2Al3++3H2↑ （7）

2H++2e-=H2↑ （8）

The hydrogen atoms in the sample are soaked in 3.5 wt.% NaCl solution and continuously enter the

sample, as what indicated by formula (5) and (6). Specimens are enriched by stress-induced diffusion during

SSRT. When the concentration of hydrogen is equal to the critical value Cth, hydrogen-induced crack

nucleation and propagation will proceed continuously until the delayed fracture occurs, as shown in Fig. 9.

In the process of SSRT test, the tensile stress is applied to the specimen, and the compressive stress in PM

region decreases the bonding stress. Once the crack nucleates, it will be compressed. Macroscopic

compressive stress will not cause stress corrosion crack growth, and will not lead to specimen fracture, thus

delaying stress corrosion in PM region. Residual tensile stress in SZ will increase the resultant force, which

will intensify the SCC in SZ. In addition, the incubation period and threshold value of compressive SCC

cracks are much higher than that of tensile stress corrosion cracks, so SCC cracks should be nucleated in SZ.

Hydrogen traps in metals can be classified into reversible and irreversible types. Reversible hydrogen

traps include dislocations, vacancies, LABs, while irreversible traps include HABs and precipitates. Due to

the blocking effect of batteries, the solution PH at the crack tip of pitting corrosion of Al-Li alloy is 3.5,

which is not associated with the surrounding solution about PH value [27]. In the solution, pitting corrosion

caused by local anodic dissolution acidizes the solution and more H atoms enter the sample. Under the

higher stress and H atom concentration, dislocation slip or creep breaks the passive film on the surface,

exposing the fresh metal, and then continues to dissolve, accelerating the stress corrosion and triggering the

formation of tunnel holes. As an effective cathode for dissolution of alloy matrix, enrichment of precipitates

in the copper mainly occurs in HAZ with more grain boundaries at LABs, which results in the more

intensified anodic dissolution in HAZ. However, the tensile stress in SZ is larger than that in HAZ. After the

SSRT stretching, the residual stress in the SZ decreases. A large number of hydrogen atoms enter the SZ,

and the hydrogen-induced cracking becomes more significant in the SZ. Interestingly, the fracture of SSRT

specimens in the SZ indicates that the stress corrosion behavior of FSW is mainly determined by the residual

stress at grain boundaries rather than precipitates at grain boundaries.

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4 Conclusion

(1) Because of the thermal effect of large amount of friction stirring, the grains in SZ are completely

recrystallized, and the uniform equiaxed grain structure with fewer LABs is obtained, while there are still

large number of as-rolled structures and low-angle grain boundaries in PM, HAZ and TMAZ. However,

less S phases and other precipitated phases are precipitated at LABs, and thus the content of LABs has

very slight effect on stress corrosion sensitivity coefficient.

(2) Residual stresses in PM, HAZ, TMAZ and SZ of FSW is transformed from compressive stress to tensile

stress in turn. The residual stress in PM region is the maximum compressive stress in the four regions,

about -32 MPa. It is difficult for cracks to nucleate and delay the stress corrosion in PM region. The

residual stress in the HAZ region is about -8 MPa. However, the residual stresses in TMAZ and SZ are

transformed from compressive stress to tensile stress, while residual stresses in the SZ are the largest,

about +67 MPa. In the SZ, cracks tend to nucleate and the corrosion sensitivity coefficient is higher.

(3) In the FSW of PM, HAZ, TMAZ and SZ, there are less precipitates such as T1 phases which could improve

stress corrosion sensitivity in PM, HAZ and TMAZ with more LABs. However, a large residual stress and

a high dislocation density are observed in the SZ. The experimental results show that the stress corrosion

sensitivity of the SZ is the highest, indicating that the residual stress possesses dominant effects on the

stress corrosion of seam of AA2198-T34.

Acknowledgements

Financial support from the Chinese Nation Science Foundation（No.51771139）. The authors also wish to acknowledge the financial support of the Hunan Natural Science Foundation（ No.2019JJ60062） .

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