Microstructure and pitting corrosion behavior on the top surface of friction stir welded

joint in 2024 aluminum alloy

Guang LI 1, Ju KANG 1, Ruidong FU 2, Miao HE 1

(1. China FSW Center, Beijing FSW Technology Co., Ltd, Beijing 100024

2. College of Materials Science & Engineering, Yanshan University, Qinhuangdao, Hebei Prov.

066004)

Abstract: Based on the detailed analysis of the microstructure on top surface of friction stir

welded (FSW) joint of 2024 aluminum alloy by optical microscope, XRD and transmission

electron microscope. SEM was employed to obtain the in-situ corrosion development of the

joint. The results of microstructure observation show that many stringers are presented in the

shoulder active zone (SAZ) due to the extrusion action of the tool shoulder. The grains and

second phase particles are refined. The results of XRD reveal that the main precipitate in the

alloy is Al2CuMg (S phase). The results of in-situ observation of corrosion evolution indicate

that the pitting corrosion initially originates in dissolving of the S phase. The density and

degree of the pitting corrosion for the SAZ are slightly larger than those in other regions. The

corrosion resistance property of the joint is deteriorative after FSW.

Key words: 2024 aluminum alloy; friction stir welded; microstructure; immersion test;

pitting corrosion

Introduction

High strength AA2024 aluminum alloy, as a typical precipitation hardening aluminum alloy, is

difficult to join by conventional techniques, such as Gas Tungsten Arc Welding (GTAW) and

Gas Metal Arc Welding (GMAW), because of hot cracking sensitivity and significant strength

drop in the joint[1]. Friction stir welding(FSW), invented by The Welding Institute (TWI) in 1991

as a new solid state joining technique that can provide localized modification and control of

microstructures[2], has the advantage of reducing the grain size, refining the microstructure

and improving the mechanical properties compared to conventional welding. FSW is ideal for

joining aluminum alloys, especially AA2000 and AA7000 series aluminum alloys[3–5].

In the welding field, extensive research on friction stir welded joints for AA2000 or AA7000

series aluminum alloys have been carried out in the past decade focusing on micro structural

characteristics[6–8], mechanical properties[9–11], residual stress analysis[12–14],plastic flow

patterns[15,16] and numerical simulation for the temperature field[17–19]. However, there are only

a few investigations related to the corrosion properties of the FSW aluminium alloys.

For high strength age-hardening aluminum alloys it is well-known that precipitates play an

important role in corrosion performance. According to the comparison of the self-corrosion

potentials among different typical precipitates, it is known that thyself-corrosion potential of S

phase has the lowest value in AA 2024. Thus the S phase always acts as anodic polarity

taking priority of dissolving over the aluminum matrix in a NaCl solution. Since the

microstructure of the top surface is different from the cross section of the weld joint and the

in-situ observation approach can directly record the evolution of the pitting corrosion, a novel

in-situ observation method was introduced to characterize the corrosion behavior of different

regions on the top surface of a FSW joint for an AA 2024 sheet at T3. In the study, the

mechanism for the pitting corrosion on the top surface after FSW was more deeply

understood by use of the current in-situ observation technique.

1. Experimental

AA2024-T3 sheets with 1.6 mm in thickness were used in the present work. The nominal

chemical composition of this alloy is listed in Table 1. The sheets were joined using a friction

stir welding process with 8 mm shoulder made of H13 steel ate travel speed of 200 mm/min

and a spindle speed of 700 rpm in the counterclockwise direction. All the welded sheets had

been naturally aged for three months before welding.

The top surface of weld seam was polished by use of a mechanic-chemical polishing method

with 1 lm diamond paste plus 0.5 wt. % NaOH solution. Etching for metallographic observation

was carried out using Dix-Keller’s reagent (4 ml HF, 6 ml HCl, 10 ml HNO3 and 190 ml H2O).

The microstructure in the polished surface of the weld seam was observed by optical

microscopy (Ax overt 200 MAT). Scanning electron microscopy (SEM, HITACHIS-4800) with

an energy dispersive analysis system using element mapping mode (EDS, HORIBA EDAX)

was employed to obtain the chemical compositions in particles and the corrosion development

of different intermetallic compound particles in the alloy.

The corrosion immersion test was performed according to the ASTM G34-01 standard. In

order to shorten the experimental cycle, the EXCO solution was chosen as electrolyte instead

of 3.5 wt. % NaCl solution based on the fact that the evolution of the pitting and exfoliation

corrosions can be accelerated in the former solution. The EXCO test solution was prepared

as follows: 4.0 M NaCl + 0.5 M KNO3 + 0.1 M HNO3, which pH value was adjusted to 0.4 using

HNO3 (70 wt. %). And the solution temperature was maintained at 25 ℃ using a thermostat.

The current ‘‘in-situ observation” method is described as follows: first, markers were made at

the regions of interest on the shoulder active zone (SAZ), the heat-affected zone (HAZ), the

thermo-mechanical affected zone of the advancing side (TMAZ-A) and the retreating side

(TMAZ-R) of the weld seam as well as the base metal (BM), then images were taken of the

regions around the markers by SEM. Finally, the samples were removed from the SEM

chamber and immersed in the EXCO solution. After immersing for a desired time, the samples

were dried and put into the SEM chamber again. After recording the first corrosion features

around the markers, the process was repeated to provide images of the evolution of the

corrosion.

2. Results and discussion

2.1 The metallographic structure on weld surface

The microstructures on the surface layer of the FSW joint, as in the cross section of the joint,

can also be divided into five distinct regions: SAZ, HAZ, TMAZ-A, TMAZ-R and BM (as shown

in Fig. 1).For the SAZ under the shoulder of the stir tool, many regular small and short arc

shaped stringers (as marked in Fig. 1f) are observed on the as-polished surface due to the

circular extrusion action of the tool. These stringers are dissimilar to the ‘‘onion rings” feature

in the cross section of the nugget zone. The further observation of the small black stringers at

high magnification is shown in Fig. 1g. It is noted that they are high density zones of the

secondary phase particles (labeled as 1, 2 in Fig. 1g) which were corroded more severely

than the Al matrix after being etched in the Dix-Keller’s reagents that these zones show

themselves as black stringers under the optical microscope. A similar result was observed by

Suttner al. with SEM[35]. They found that there is a significant difference between high spindle

speed at low travel speed and low spindle speed at high travel speed. When spindle speed is

low, the small black stringers are long and relatively continuous. For the current welding

condition of high spindle speed and low travel speed, the small black particle stringers

become short and discontinuous. This indicates that in this region the features of the

microstructure correlated with the particle distribution of the strengthening phase, while this

distribution results from the metal plastic flow in the surface layer during FSW. As known to all,

welding parameters are the key factors affecting the metal plastic flowing the FSW process,

so this finding reveals that the distribution of the intermetallic compounds can be modified by

altering the welding parameters.

Fig.1 Microstructure on the top surface of the FSW joint. (a) overview, (b) base metal, (c) HAZ,

(d) TMAZ-R, (e) TMAZ-A, (f) SAZ, (g) intermetallics in SAZ.

In addition, the other prominent features are obvious difference of the size and shape of the

100m100m

Advanced Retreated 1mm

100m100m

10m

1

2100m

grains among the four typical regions. The grain size is distinctly larger in the HAZ (Fig. 1c)

than in BM (Fig. 1b) due to the heating effect. Comparing the microstructures of the TMAZ

(see Fig. 1d and e) and the HAZ (Fig. 1c), the grains in the areas adjacent to the tool-shoulder

are deformed and elongated as a result of heat and the tool-shoulder’s friction action, so the

TMAZ extends to the HAZ regions beyond the tool-shoulder active range. In the SAZ (Fig. 1f),

it is seen that the fine, irregular grains have a difference compared with that in the cross

section which’s attributed to the stirring action and incomplete dynamic recrystallization[8,36].

2.2. Results of the second phase particles on weld surface

The precipitate phases plays a vital role in aluminum alloy corrosion behavior：the pitting

corrosion usually originated in the precipitate phases. The precipitate phases particles

precipitated in grain boundaries makes the edge of the grain boundary precipitate free zone,

leading to intergranular corrosion as well as the incidence of exfoliation corrosion.

The Al-Cu-Mg aluminum is easy to identify two typical particles: Al2CuMg (S phase, l) and

Al-Cu-Fe-Mn phase (named as Fe-containing phase,) as well as that is of metastable phase

of θ′ or S′ phase. However, the alloy content of Cu and Cu: Mg ratio decide the main

strengthening phase in aluminum alloys. The 2024-T3 Copper content of 4.42(in wt. %) and

the Cu: Mg ration is 2.8. Combination of Al-Cu-Mg alloy phase diagram of Al-rich corner (Fig.

2), Can determine the ratio of such content and the precipitation of alloys corresponding to

S-phase-based, and there is a small amount of -phase.

Fig.2 The Al-rich corner of the Al-Cu-Mg equilibrium phase diagram

Figure 3 is the 2024 aluminum alloy base metal and weld center (NZ) of the XRD phase

analysis. Appears in the 2θ angle of 27.249°, 35.008° and 40.99° respectively, for the

emergence of crystal plane index (111), (112) and (041) of the S phase. The θ-phase only in

the 2θ angle of 42.07° occurring at the crystal plane index (220), and the diffraction peak

intensity is very weak, indicating that the alloy content in the small θ phase, which according

to Al-Cu-Mg alloy Al-rich angle phase diagram analysis results.

20 25 30 35 40 45 50 55 60

0

1000

2000

3000

4000

■■

■

■

■

◆◆

●

Inte

nsity

(cou

nts)

2 degrees(Cu-k)

BM NZ

●:Al2Cu

■:Al2CuMg

◆:Al

●

■

Fig.3 XRD spectrum of BM and NZ of FSW AA2024

Figure 4 showed the distribution of second phase particles on 2024 aluminum alloy friction stir

weld. The investigation of the effect of the Fe-containing phase on the corrosion behavior in

the HAZ and the SAZ is shown in Figs. 5and 6, respectively. Fig. 5a shows a typical

configuration of Fe-containing phase in the HAZ. An EDS result on the Fe-containing phases

shown in Fig. 4b. After being immersed for 0.5 h, slight corrosion occurred on the edge of the

phase (see the arrow position in Fig. 5c). When being immersed for 2 h, the Fe-containing

phases cathode has induced the dissolution of the matrix in their vicinity, and the corrosion

extent became more severe. There are a certain number of micro cracks in the particles

broken after rolling (see the square zone in Fig. 4a). The micro cracks with the Al matrix

constitute a micro-corrosion primary cell so that pitting corrosion preferentially occurs

between the Al matrix and its neighboring Fecontainingphase. The effect of Fe-containing

phase on the corrosion behavior in the SAZ is shown in Fig. 5. It is seen that larger particles

were broken into small pieces due to the severe mechanical stirring action of the stir tool.

Pitting corrosion occurred in the surroundings of these fine Fe-containing phases after being

immersed for 2 h. So the anodic dissolution of the matrix occurred around the fine and dense

particles of Fe-containing phases as around the coarse and thin ones in the HAZ or BM. In

other words, the pitting corrosion density is increased in the shoulder active region. Because

of this, the corrosion resistance will be reduced in this zone.

10µ

1

210µ

10µ

Fig.4 Typical phases of FSW AA2024 on the top surface at different regions

(a) base metal, (b) the EDS spectra of the particle 1, (c) HAZ, (d) SAZ.

062

130

022 000

Fig.5 TEM results of FSW AA2024 at different regions

(a) base metal, (b) HAZ, (c) TMAZ, (d) SAZ.

2.3 In-situ observation of corrosion evolution

It is well known that the best approach to investigate the corrosion evolution is the in-situ

observation technology. Here, the in-situ method has been employed, and the corrosion

features of different regions on the surface layer of FSW joints in EXCO solution during

different corrosion periods are shown in Fig. 6.The pitting corrosion occurred in every region

after being immersed in EXCO solution for 0.5 h. The initial galvanic couple of pitting

corrosion was founded in the region between the S phase particles and their adjacent

aluminium matrix. The S phase particles as anode took priority in dissolving due to the lower

self-corrosion potential compared to the adjacent aluminium matrix. With increasing

immersion time, the S phase particles became smaller and smaller, which means that the

elements Al and Mg in the S phase were dissolved continuously during the initial stage. With

decreasing content of Al and Mg element in the S phase, the change in the distribution of Cu

element from the edge to the center in the particle resulted in raising the self-corrosion

potential of the S phase. Consequently, the S phase conversely acted as the cathode and led

to the anodic dissolution of the adjacent aluminium matrix. This result supports the polarity

reverse theory for the S phase proposed by Burchett[29]. There is another likely explanation

forth dissolution of the aluminum matrix around the S phase particles. The result shown in Fig.

6 reveals that the dissolution of aluminum base is likely to be caused by a local

alkalization[39].However, this mechanism of corrosion is not fully supported by the

experimental evidence in this work, because the corresponding catholic reactions

(water/oxygen reduction which need the local PH > 9) can not occur in the EXCO solution (the

PH _ 0.4 in this solution).In addition, from the result as shown in Fig. 4, a notable interesting

feature of the corrosion (see the square marked in the SAZ) is that there exists an

Fe-containing phase surrounding an S phase in the center of the particle. The dissolution rate

of the kind of particle with the S phase plus the Fe-containing phase seems tube faster than

that of other single particle. It is because of the Fe-containing inter metallic compounds having

a self-corrosion potential of _0.35VSCE in chloride-containing solutions, 0.3VSCE positive to

aluminum matrix (_0.65VSCE) of AA2024-T3[40], so that the more active galvanic couple (S

phase plus Fe-containing phase) is founded at this position.

From the above analysis, it can be concluded that the secondary phase particles are the main

source of the pitting corrosion for FSW joint of the AA2024-T3 alloy. The welding parameters

are important factors for the redistribution of different particles, especially for the

Fe-containing phase in the SAZ. The corrosion resistance property of FSW joint could be

improved by optimizing the welding parameters.

0h 0.5h 1h 2h

BM

HA

Z

TM

AZ

-A

TM

AZ

-R

SA

Z

20µm

Fig.6 The corrosion behavior in the different regions during different corrosion periods

3. Conclusions

1) The friction stir welding to weld the surface of grains can be refined, axis active area of the

second phase particles dissolve and re-precipitation as well as mechanical fragmentation of

the role, but also refined obviously.

2) After being immersed in EXCO solution for 0.5 h, the pitting corrosion was observed for all

regions of the weld joint’s top surface. The density and degree of the pitting corrosion forth

shoulder active zone are slightly larger than those in other regions.

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