Electrochemical Corrosion Behavior of Friction Stir Welding Weld for 6061 Aluminum Alloy

Friction and Wear Research Vol. 1 Iss. 3, October 2013

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Electrochemical Corrosion Behavior of Friction Stir Welding Weld for 6061 Aluminum Alloy Yadong Zhao*1, Qiang He2 School of Mechanical Engineering, Anyang Institute of Technology China zhaoyadong1983@163.com; 2aystar@163.com

*1

Abstract Microstructure of the Friction Stir welded was characterized by its much finer grains as contrast with the grains of parent material. At the ambient temperature 0.2 M NaHSO3 and 0.6 M NaCl solution, the electrochemical corrosion behavior of friction stir welded 6061 aluminum alloy and 6061 parent material were comparatively investigated by static weight loss experiment (gravimetric test), potentiodynamic polarization curve and scanning electron microscopy (SEM) observation. The corrosion rate of the weld with the tool rotation rate of 800 r/min, the traverse speed of 160 mm/min, was less than that of the parent material; and the weld showed more positive corrosion potential, less current density, larger Rp (polarization resistance) than the parent material. SEM observation showed that a few shallow pits were presented on the surface of the weld. However, a large number of deeper pits emerged on the surface of the parent material.

Keywords Friction Stir Welding (FSW); Electrochemical Corrosion; Weld; 6061 Aluminum Alloy

Introduction Friction stir welding (FSW) was invented at The Welding Institute (TWI) of UK in 1991 as a solid-state joining technique that unites thermal and mechanical aspects to produce a high quality joint, and it was initially applied to aluminum alloys (Thoms et al. 1991; Colligan et al. 1998; Murr et al. 1998). Because it is a solid state joining process that no melt metal emerges during welding, and it can avoid many defects in fusion welding techniques, FSW has been used on many alloys that are typically difficult to be welded (Liu et al. 2007), and many advantages of friction stir welding make FSW extremely attractive for the joining of aerospace aluminum alloys and magnesium alloy.

In a relatively short duration after invention, quite a few successful applications of FSW have been demonstrated.

Meanwhile, many researchers have devoted to

systematically investigating the microstructural evolutions (Mahoneyet al. 1998), the residual stress and hardness distributions (Peelet al.2003), the mechanical properties (strength and ductility) (Satoet al.2001), fatigue and fracture toughness (Lomolinoaet al.2005) of FSW joints, however, the electrochemical corrosion behaviours have been rarely studied.

To authors’ knowledge, for a wide range of engineering applications of FSW, the corrosion susceptibility, especially the ambient electrochemical corrosion behavior of the high-strength aluminum alloy FSW weld is a concern. In the present communication, firstly, the microstructure of FSW weld and 6061 parent material were characterized.

Then at the ambient temperature 0.2M NaHSO3 and 0.6MNaCl solution, by static weight loss experiment (gravimetric test), potentiodynamic polarization (PDP) curve, electrochemical impedance spectra (EIS) and scanning electron microscopy (SEM) observation, the electrochemical corrosion behavior of FSW weld and parent material were comparatively investigated. Some interesting and surprising results of electrochemical corrosion behavior of 6061 FSW weld were obtained.

Experiment Apparatus Static weight loss experiments were carried out in a rectangular glass tanker with a capacity of 3000 ml.

After the immersion tests for a given interval of 24h, the samples were treated by the procedure adopted by K. S. Ferrer et al (Ferrerand Kelly.2001), dried, then weighed by a type-FA1104A electrical balance (accuracy: 0.1 mg), finally observed by SEM. Potentiodynamic polarization tests were carried out using CHI600B electrochemical station at ambient temperature, using a pyrex glass, single compartment, three-electrode cell with a capacity of 100ml. Electrode

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Friction and Wear Research Vol. 1 Iss. 3, October 2013

potentials were measured with respect to an aqueous KCl saturated calomel electrode (SCE). A Pt sheet was used as the counter electrode. The potential was scanned from -1.5 V to 1.5 V versus the open current potential (Ecorr). Scan speed was set at 0.01V/Sec. EIS measurement was carried out with the EG&G PARC Parstat 2273 controlled by POWERSINE at Eocp and an AC sine wave amplitude of 10 mV was utilized.

The applied frequency ranged from 105 to 10-2 Hz. The samples for this experiment were also treated the same as in PDP test. The impedance data, Rs was the solution resistance, Rp was the polarization resistance, Ro was the oxide film resistance, and C was the interfacial capacitance, which were analyzed by Zsimpwin software.

The morphologies of corrosion surfaces for weld and parent material after static weight loss experiment (gravimetric test) were analyzed by JSM-6360LV scanning electrode microscope (SEM).

Chemicals and Materials The tests were carried on with 6061 aluminum alloy plate with thickness of 4mm. The chemical composition of 6061 aluminum alloy was listed in Table 1. The FSW samples were prepared by a Suspensory apparatus of FSW manufactured in China FSW Center. The process parameters were chosen as the tool rotation rate of 800 r/min, the traverse speed of 160 mm/ min. All chemicals used were of the purity of reagent grade. Each wire-cut sample was encapsulated with epoxy resin and supported by a PVC holder, then ground with silicon carbide (SiC) papers, finally polished to a diamond paste. The exposure area of the sample was 10mmĂ—10mm.

TABLE 1 CHEMICAL COMPOSITION OF 6061 ALUMINUM ALLOY

Element

Si

Fe

Cu

Mg

Mn

Cr

Wt%

0.4

0.70

0.15

0.8

0.15

0.04

Zn

Ti

0.25 0.15

Al bal

grains, as shown in Fig. 1(a).

Fig. 1(b) showed FSW weld zone center grain structure in contrast to the parent material grain structure.

While the slightly elongated and larger grain structure in the parent material was significantly recrystallized in contrast to parent material. Correspondingly, the equiaxed, dynamically recrystallized grain structure in the weld zone center was changed in terms of actual, avrange grain size, from the parent material (Corral et al. 2000).

The contribution of intense plastic deformation and high-temperature exposure within the stirred zone during FSW resulted in recrystallization. Intense plastic deformation and frictional heating during FSW resulted in generation of a recrystallized fine-grained microstructure within stirred zone. In the interior of the recrystallized grains, usually there was low dislocation density (Mishra and Ma. 2005). The microstructure of 6061 parent material was characterized by its coarse 30

200 Âľm

60 Âľm

FIG. 1 MICROSTRUCTURESOF (a) 6061 PARENTMATERIAL AND (b) FSWWELDZONECENTER

Static Weight Loss Experiments Herein, 0.2M NaHSO3 and 0.6MNaCl solution was picked up as the corrosive media in order to simulate the polluted acidic industrial atmosphere. The electrolyte solution’s temperature was ambient temperature. For each kind of material (FSW weld and 6061 parent material), there were three parellel samples. The average corrosion rate was calculated by eq 1, and all data for immersion tests were listed in Table 2.

đ?‘Ł=

đ?‘š1 −đ?‘š2

(1)

đ?‘ Ă—đ?‘Ą

TABLE 2 RESULTS OF IMMERSION TEST FOR FSW WELD AND PARENT MATERIAL

Sample F

Results and Discussion Microstructure

b

a

P

sĂ—10-6 (m2) 1160

m1 (g) 6.4494

m2 (g) 6.4428

t (h) 24

vĂ—10-1 (g/(m2h)) 2.37

vďż˝ Ă—10-1 (g/(m2h)) 2.53

1160

6.4478

6.4411

24

2.41

1160

6.5848

6.5770

24

2.8

1160

6.4463

6.4346

24

4.2

1160

6.4525

6.4418

24

3.84

1188

6.6798

6.6702

24

3.37

3.8

Sample FSW weld was coded as F, 6061 parent material as P, m1 and m2 were the sample mass before and after corrosion, respectively, s was the area exposed in solution, t was the time of corrosion, v was corrosion rate of each measurement, and đ?‘ŁĚ… was average corrosion rate of three parallel samples. According to eq 1, đ?‘ŁĚ… of FSW weld and parent metal were 0.253 g/ (m2h) and 0.38 g/(m2h), respectively. This result

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Poteniodynamic Polarization Curves Fig. 2 was the potentiodynamic polarization curves of 6061 FSW weld and parent material in 0.2M NaHSO3 and 0.6M NaCl solution at ambient temperature. The electrochemical parameters of FSW weld and parent material were listed in Table 3. Ecorr of FSW weld was more positive than that of parent material, while Icorr was less than that of parent material. The whole potentiodynamic polarization curve of FSW weld was on the top left-hand side of the potentiodynamic polarization curve of the parent metal over the entire potential range. Fig.2 and Table 3 also indicated that the corrosion resistance of FSW weld in 0.2 M NaHSO3 and 0.6 M NaCl solution was improved, in comparison with parent material. These results were in accordance with those obtained from static weight loss experiments.

TABLE 3 ELECTROCHEMICAL PARAMETERS OF FSW WELD AND PARENT

electrode. There is no acknowledged explanation about the inductive arc at low frequency. CAO et al (Cao et al. 1989) suggested that inductive resistance could be produced in inducing period of pit corrosion of oxide film. Meanwhile, from Fig. 3, the radius of the capacitance loop of weld was remarkably larger than parent metal. The results indicated that the polarization resistance of the parent metal was smaller than that of weld, and the electrochemical reactions took place easier and more quickly.

3000

F P

2000

Zim/ohm.cm2

demonstrated that the corrosion resistance of FSW weld was improved, in comparison with that of parent material.

1000

0

-1000

-2000 -1000

0

1000

2000

3000

4000

5000

6000

7000

Zre/ohm.cm2

FIG. 3 NYQUIST PLOTSOF FSW WELDAND PARENT MATERIAL

MATERIAL

Icorr

C μF·cm2

Sample

Ecorr mVSCE

F

-559.85

0.26

8.597

3496

2343

6.3

p

-685.11

1.45

7.852

911.9

1422

12.6

-0.2

µ

RsΩ·cm2 RpΩ·cm2 R0Ω·cm2

Acm-2

F P

FIG. 4 EQUIVALENT CIRCUIT OF EIS FOR FSW WELD AND PARENT MATERIAL

-0.4

E/VSCE

-0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -7

-6

-5

-4

-3

-2

-1

0

logI/A.cm2

FIG. 2 POTENTIODYNAMIC POLARIZATION CURVES OF FSW WELD AND PARENT MATERIAL

EIS Results Fig. 3 was the Nyquist plots of FSW weld and the parent material, from which all the Nyquist complex planes were composed of two semi-arcs. For the high frequency region, a capacitance loop appeared, which should be attributed to a polarization reaction processed on the surface of the electrode, with the attenuation of frequency, a inductive loop emerged which was induced by the continuous absorption and desorption of the reactants on the surface of the

Fig.4 was the equivalent circuit of EIS for FSW weld and the parent metal. The exact values of all the equivalent circuit parameters can be fitted by Zsimpwin software and were listed in table 3. Among them, Rp was an important parameter coupled with the corrosion rate, and the more the polarization resistance was, the slower the corresponding corrosion rate was. The EIS results indicated that the corrosion rate of FSW weld was slower than that of the parent metal. It was consistent with static weight loss experiments and potentiodynamic polarization curves test.

SEM Results Fig.5 (a) and (b) showed the SEM morphologies for FSW weld and 6061 parent material. By SEM observation, the morphology of FSW weld after ambient temperature static immersion test was relatively even, revealing the classical formation of the elliptical ‘onion’ structure in the centre weld which 31

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Friction and Wear Research Vol. 1 Iss. 3, October 2013

was characteristic of the FSW weld, and the pits were nearly uniform, no visible superficial porosity or macroscopic defects existed, which indicated that localized attack for FSW weld in the given media can be neglected.

For 6061 parent material, the morphologies were relatively rough, and some deep pits can be obviously observed. The localized attacks of 6061 parent material were supposed to result from the microcells formed due to the chemical nonhomogenization. a

material. The corrosion rate of FSW weld of the tool rotation rate of 800 r/min, the traverse speed of 160 mm/min, was less than that of 6061 parent material.

The corrosion potential of the weld was more positive than that of 6061 parent material, while the corrosion current density was less than that of 6061 parent material.

The polarization resistance of FSW weld of the tool rotation rate of 800 r/min, the traverse speed of 160 mm/min, was larger than that of 6061 parent material.

SEM observation showed that a few shallow pits were presented on the surface of FSW weld, however, a large number of deeper pits emerged on the surface of 6061 parent material.

REFERENCES

100 µm b

Cao, Chunan, Wang, Jia, Lin, Haichao. “Effect of Cl− on the impedance of passive-film-covered electrodes.” 9(1989): 261−270.

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Corral, J, Trillo, E A, Ying, L et al. “Corrosion of friction-stir welded aluminum alloys 2024 and 2195.” 19(2000):2117 –

100 µm FIG. 5 SEM MORPHOLOGIES (a) FSW WELD AND (b) PARENTMATERIAL

Discussion During FSW process, the material undergoes intense plastic deformation at elevated temperature, resulting in generation of fine and equiaxed recrystallized grains. The dynamic recrystallization grains caused to the density of dislocation of the sample decreased. At the same time, after severe plastic deformation (SPD), the change of actual microstructure of FSWed sample made the weld chemically homogenized. Based on the principle of electrochemistry, the homogenization of the chemical constituent made less prone to forming the localized microcells; meanwhile, coupled with the decrease of the density of the dislocation, the crystal linear defect, or the number of the weld reduced, and the decreased active sites (herein, the high-density dislocations) for the anodic dissolution of weld metal enhanced the ability of the corrosion resistance.

2122.

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Conclusions

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The microstructure of the FSW weld consisted of very fine grains in comparison of the grains of parent

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Friction and Wear Research Vol. 1 Iss. 3, October 2013

governing hardness in friction-stir welds of solidsolution-hardened Al Alloys.” 32(2001):3033-3042.

Thoms, W M, Nicholas, E D, Needdham, J C et al. “Friction

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Yadong Zhao, M. E., Lecturer, School of Mechanical Engineering, Anyang Institute of Technology, Anyang 455000, China, Tel: 0086-372-2909844. E-mail: zhaoyadong 1983@163.com

Stir Butt Welding” GB Patent 9125978 (1991).

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