Effect of micro-particles on cavitation erosion of Ti6Al4V alloy in sulfuric acid solution

Ultrasonics Sonochemistry 36 (2017) 270–276

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Effect of micro-particles on cavitation erosion of Ti6Al4V alloy in sulfuric acid solution D.G. Li a,⇑, Y. Long b, P. Liang a, D.R. Chen a a b

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 8 November 2016 Received in revised form 2 December 2016 Accepted 2 December 2016 Available online 3 December 2016 Keywords: Ultrasonic cavitation erosion Micro-particle Ti6Al4V alloy Open circuit potential

a b s t r a c t The influences of micro-particles on ultrasonic cavitation erosion of Ti6Al4V alloy in 0.1 M H2SO4 solution were investigated using mass loss weight, scanning electron microscopy (SEM) and white light interferometer. Mass loss results revealed that the cavitation erosion damage obviously decreased with increasing particle size and mass concentration. Open circuit potential recorded during cavitation erosion shifted to positive direction with the decreased mass loss. Meanwhile, the mass loss sharply decreased with applying a positive potential during the entire ultrasonic cavitation erosion, and the relationship between the open circuit potential and the cavitation erosion resistance was discussed. Ó 2016 Published by Elsevier B.V.

1. Introduction Cavitation erosion (CE) is a serious hydraulic problem which widely occurs in overflow components such as pumps, valves, marine propellers, turbines and pipes, and it usually makes these components failure prematurely and even to cause safety accident. The occurrence of cavitation erosion lies in the combination of shock loading and fatigue processes incurred from the stress generated by the repeated growth and collapse of cavities in a high microjet with a velocity of 130 m/s [1,2]. In such high velocity shock material surface suffers from a serious damage. Except occurrence in civil fields, CE is often found in the military field, for example, the noise caused by CE during propeller rotating can easily expose the location of submarine, and thus to threaten the safety voyage of submarine. Therefore, numerous researchers have paid attention to this issue since its invention [3–14]. Although different models are proposed to explain mechanism of cavitation erosion and no agreement is reached about the mechanism of cavitation erosion [9–14]. Additionally, it is worthy to mention that nano/microparticles can facilitate to form bubble nucleation [15]. Actually, plenty of nano-, micro and even cm-level particles exist in the rivers and sea, and these particles must have effect on the cavitation erosion of the over-flowing components, therefore, it is very necessary to investigate the particle effect on cavitation erosion. However, only several papers focused on this issue [16,17], and the effect ⇑ Corresponding author. E-mail address: [email protected] (D.G. Li). http://dx.doi.org/10.1016/j.ultsonch.2016.12.003 1350-4177/Ó 2016 Published by Elsevier B.V.

mechanism of particle on cavitation erosion is disagreement owing to different particle size, structure and chemical composition [18– 20]. Better understanding the role of particles in the cavitation erosion can be helpful for studying the cavitation erosion mechanism. Therefore, the aim of this manuscript is to investigate the influences of SiO2 particle size and concentration on the cavitation erosion of Ti6Al4V alloy in sulfuric acid solution, and based on the results the inner relationship between cavitation erosion resistance and open circuit potential is discussed. 2. Experimental section 2.1. Sample preparation The sample is cut from commercial Ti6Al4V alloy manufactured by Bao Ji Hai Ji Co., Ltd., and the sample size is showed in Fig. 1. The wafer surface of sample is abraded with 2000-grit SiC paper, polished with 0.5 um Al2O3 powder and then cleaned using doubledistilled water. The electrolyte is 0.1 M H2SO4 solution. 2.2. Ultrasonic cavitation erosion equipment Ultrasonic cavitation is produced by a JY92-ⅡDN magnetostrictive-driven apparatus (NingBo scientz Biotechnology Co., Ltd.) resonating at 20KHz with the amplitude of 60 lm. The power of this apparatus is 3KW, and the ultrasonic cavitation is continuous occurred without any suspension. The experimental instrument is sketched in Fig. 2.

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16

b

a

Fig. 1. Shape and dimension of the specimen, a) lateral view; b) topview.

4 3 2 1 5

WE

RE

CE

Fig. 2. Schematic of ultrasonic cavitation erosion equipment, 1: sound-proof enclosure, 2: breaker, 3: Ti6Al4V tip, 4: transducer, 5: EG&G M273A Potentiostat.

2.3. The SEM images of spherical SiO2 particle The morphologies of SiO2 micro-particles used in this work are showed in Fig. 3, it displays a spherical feature, and the diameters are measured to be 415 nm, 500 nm, 1560 nm and 23,000 nm, respectively. Fig. 3 also reveals that the size and profiles of all micro-particles are uniform. 2.4. The application of passive potential and the recordation of open circuit potential The applying of passive potential at sample and the recordation of the open circuit potential during the entire ultrasonic cavitation erosion are performed at an EG&G Model 273A potentiostat/gal-

vanostat. A conventional three-electrode system is used, and the counter electrode is a Pt wire. All potentials are measured against a saturated calomel electrode (SCE). 3. Results and discussions 3.1. Mass loss of Ti6Al4V alloy in H2SO4 solution containing microparticles The front SEM images and white light interferometers of Ti6Al4V alloy measured after cumulative exposure time for 30 min, 60 min and 120 min in 0.1 M H2SO4 solution containing 1 mg/L SiO2 particles with different sizes, are showed in Figs. 4– 6, respectively. In which the insert figures are the corresponded

Fig. 3. SEM images of spherical SiO2 particles added in 0.1 M H2SO4 solution, a) 300 nm, b) 500 nm, c) 1.595 lm, d) 23.34 lm.

D.G. Li et al. / Ultrasonics Sonochemistry 36 (2017) 270–276

a

b

c

d

e

1.2

Surface roughness in width / μ m

14

f

13

1.0

12

0.8

11

0.6

10 9

0.4

8 0.2

Without particle 300

500

1595

Surface roughness in height /μ m

272

23340

Particle size / nm

b

c

d

e

1.9

Surface roughness in width /μ m

a

30

f

28

1.8

26 24

1.7

22

1.6

20 18

1.5

16 14

Surface roughness in height /μ m

Fig. 4. SEM images of Ti6Al4V alloy after cavitation erosion for 30 min in 0.1 M H2SO4 solution (a) and containing 1 mg/L SiO2 particles with the diameter of 300 nm (b), 500 nm (c), 1595 nm (d), 23,340 nm (e) and roughness verse particle size plot.

1.4 Without particle

300

500

1595

23340

Particle size / nm Fig. 5. SEM images of Ti6Al4V alloy after cavitation erosion for 60 min in 0.1 M H2SO4 solution (a) and containing 1 mg/L SiO2 particles with the diameter of 300 nm (b), 500 nm (c), 1595 nm (d), 23,340 nm (e) and roughness verse particle size plot.

white light interferometers. It can be seen from Fig. 4 that sample surface suffers heavy damage after ultrasonic cavitation for 30 min, pits and micro-cracks can be observed on sample surface. However, the cavitation erosion damage significantly alleviates with adding micro-particles, and it is reverse to the findings from Refs.16 and 17. Moreover, the damage gradually decreases with increasing particle size, and some original surfaces (un-cavitation erosion areas) even appear on Fig. 4e, which indicates that micro-particles can increase the cavitation erosion resistance of

Ti6Al4V alloy. Correspondingly, the sample surface roughness no matter in width and in height evidently decreases with increasing particle size, implying the alleviated damage caused by cavitation erosion with particle. Increasing cavitation erosion time to 60 min and 120 min, material removing and cracks become apparent on sample surface, indicating the serious damage. Similarly, the insert figures of Figs. 5 and 6 display that the roughness of sample surface increases in width and in height with increasing ultrasonic cavitation erosion time, implying the aggravated damage with

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a

b

c

d

e

2.4

30 28

2.2

26

2.0

24 22

1.8

20

1.6

18 16

1.4 Without particle

300

1595

500

Surface roughness in height /μ m

Surface roughness in width /μ m

32

f

23340

Particle size / nm Fig. 6. SEM images of Ti6Al4V alloy after cavitation erosion for 120 min in 0.1 M H2SO4 solution (a) and containing 1 mg/L SiO2 particles with the diameter of 300 nm (b), 500 nm (c), 1595 nm (d) and 23,340 nm (e). In which, the insert figures are the corresponding white light interferometer, and Fig. f is the variation of surface roughness with particle size.

time. While the roughness in width and in height of sample surface clearly decreases with increasing particle size at one fixed cavitation erosion time. Meanwhile, Fig. 4f or Fig. 5f distinctly exhibits a decreased roughness in width and in height with particle size, implying the alleviated cavitation erosion damage. To quantitatively evaluate micro-particle effect on the ultrasonic cavitation erosion, the cavitation erosion loss of samples is expressed in terms of the mean depth of erosion (MDE), which is defined as:

MDEðlmÞ ¼

Dm 1000qA

ð1Þ

where Dm is the mass loss in mg, q is the density of samples, and A is the referenced area of samples. Subsequently, MDE is calculated by using Eq. (1), and Fig. 7 shows the variations of MDE and particle size in the case of cavitation erosion time. It can be observed that MDE increases with increasing cavitation erosion time, and it apparently decreases with increasing particle size at one fixed cavitation erosion time. Especially, MDE even decreases to about 60% or even less when adding 1 mg/L SiO2 with a size of 23.34 lm by com-

2.5

12

a

paring with MDE of Ti6Al4V alloy after cavitation erosion for 120 min in 0.1 M H2SO4 solution without adding particle. In order to detect particle density on cavitation erosion, Table 1 list the variations of MDE with particle density in the case of cavitation erosion for 60 min and 120 min, it can be seen that MDE apparently decreases with increasing particle concentration for every cavitation erosion time. Moreover, MDE obtained in the case of the particle size of 500 nm is always smaller than MDE obtained in the case of the particle size of 23.34 lm at the same cavitation erosion condition. According to the above results, it can be concluded that particles can increase the cavitation erosion resistance of Ti6Al4V alloy in 0.1 M H2SO4 solution, and the cavitation erosion resistance decreases with increasing particle size and concentration. The above results reveal that particle has an effective influence on cavitation erosion, and the cavitation erosion is controlled by the formation of micro-jet caused by the rapture of gas bubbles, therefore, it can be concluded that the influence of particle on cavitation erosion is related to the formation of gas bubbles. As particles can serve as nucleation sites for gas bubbles in aqueous

b

30

c

28

11

2.0

1.5

MDE / μ

MDE / μ

MDE / μ

26 10

9

24 22 20

1.0 8

18 0.5

16

7 Without particle 300nm

500nm

1595nm

Particle size / nm

23340nm

Without particle 300nm

500nm

1595nm

Particle size / nm

23340nm

Without particle 300nm

500nm

1595nm 23340nm

Particle size / nm

Fig. 7. Variations of MDE and particle size in the case of cavitation erosion for 30 min (a), 60 min (b) and 120 min (c).

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Table 1 Variations of MDE with particle size and concentration in the case of cavitation erosion for 60 min and 120 min. MDE (lm)

Particle size

23.34 lm

500 nm

Cavitation erosion time

60 min 120 min

1 mg/L

5 mg/L

10 mg/L

1 mg/L

5 mg/L

10 mg/L

9.3472 19.9609

7.3628 18.5435

6.116 17.6208

7.3524 17.2119

6.5114 16.0129

5.8475 14.2295

solution [21], the nucleation energy barrier of gas bubbles can be defined by the following equation:

DE ¼

4pr3 ð2 þ 3 cos h  cos3 hÞ 3p2

ð2Þ

where r is the gas/liquid surface tension, P is the sum of the vapor pressure, the gas pressure, and the acoustic pressure. H is the effective apparent contact angle of nucleation bubble on a particle surface in contact with a liquid. As showed in Fig. 2, the particles used in this work have the sizes of 300 nm, 500 nm, 1.595 lm and 23.34 lm, respectively. h may be close to 180° (300 nm) and be similar to that on a planar surface (1.595 lm and 23.34 lm) if the gas nucleation has a nanometer size. Based on Eq.(2), Fig. 8 sketches the variation of energy barrier of gas nucleation and h, it is clearly seen that the energy barrier of gas nucleation increases with the decreased contact angle h, and the nucleation energy barrier increases as the contact angle decreases, indicating that bubbles preferably nucleate on sub-micrometer particle surfaces. SEM and mass loss results show that adding particle can evidently alleviate the cavitation erosion damage, and the cavitation erosion resistance significantly increases with increasing particle size and concentration. The reason can be explained by Fig. 8 well, as showed in Fig. 8, the energy barrier of gas nucleation dramatically decreases with increasing h, that is numbers of gas bubbles rapidly decreases with increasing particle size. The decreased number of gas bubble means the lack of micro-jet caused by cavitation erosion, and hence to the alleviated attack of micro-jet on sample surface, it is in agreement with SEM and mass loss results. 3.2. Variations of open circuit potentials and time during cavitation erosion The above results show that the higher energy barrier between gas nucleation and particle with bigger size restricts the formation

7.00E-015

Energy barrier ΔΕ / J

6.00E-015

Gas

θ

bubble

5.00E-015

Solid

4.00E-015

particle

3.00E-015 2.00E-015 1.00E-015 0.00E+000

0

20

40

60

80

100

120

140

160

180

Contact angle / θ Fig. 8. Nucleation energy barrier of gas bubble as a function of the effective apparent contact angle h, in which the insert figure is the scheme for nucleation and growth of a cavitation bubble at a particle surface.

of gas bubbles, and thus impedes the attack of micro-jet on sample surface. Except energy barrier, open circuit potential can be used to evaluate the cavitation erosion resistance. As shown in Figs. 9b and 10, the open circuit potentials of samples during entire cavitation erosion always shift to positive direction when the mass loss decreases in the case of different conditions. In which Fig. 9a is the potentiodynamic curves of Ti6Al4V alloy during cavitation erosion in 0.1 M H2SO4 solution containing 1 mg/L particles with different sizes, it clearly shows that the sample is always in the passive state in the different solutions during entire cavitation erosion. The initial passive potential is about 0.75 VSCE, and the steady passive current density evidently decreases with increasing particle size, indicating the steady passive property with increasing particle size. Fig. 9b lists the open circuit potential verse particle size of Ti6Al4V alloy in 0.1 M H2SO4 solution for cavitation erosion for 60 min, it is seen that the open circuit potential moves to positive direction with the increased particle size. However, the corresponded MDE showed in Fig. 5b significantly reveals a sharply decreased trend with particle size. Meanwhile, Fig. 10 records the open circuit potential of Ti6Al4V alloy during cavitation erosion for 60 min in 0.1 M H2SO4 solution containing different particle concentrations, it can be seen that open circuit potential moves to positive direction with increasing particle concentration for 500 nm and 23,340 nm SiO2 particles. Similarly, comparing with MDE showed in Table 1, it can be concluded that there is some inner relationship between cavitation erosion resistance and open circuit potential, that is more positive open circuit potential higher cavitation erosion resistance, and the similar results are found by our group [22,23]. 3.3. The role of passive potential in controlling cavitation erosion It can be concluded from the mass loss and open circuit potential results that a positive open circuit potential of sample means a better cavitation erosion resistance. Therefore, if a positive potential applied at a sample, and the applied potential is more positive than the open circuit potential measured during cavitation erosion, what is the cavitation erosion result? To answer this question, Ti6Al4V sample is applied a passive potential of 0.38 VSCE during the entire cavitation erosion in 0.1 M H2SO4 solution containing SiO2 micro-particles with different size, and MDE is measured after cavitation erosion for 120 min. It is seen from Fig. 11 that MDE in the case of applying potential is obviously lower than that in the case of without applying potential, implying the increased cavitation erosion resistance with applying positive potential. MDE also decreases with increasing particle size in the case of applying positive potential, implying the positive effect of increasing particle size on cavitation erosion resistance, and it is in agreement with the above result. The SEM morphologies and white light interferometers of samples after cavitation erosion with applying a potential of 0.38 V for 120 min are showed in Fig. 12, it reveals that the cavitation erosion damage alleviates and the surface roughness in width and height clearly decreases with applying the potential, indicating the increased cavitation erosion resistance. The influence mechanism of applying potential on cavitation erosion is discussed by our group in Ref. [21], and it founds that applying

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a

Potential / VSCE

1.5 1.0

-0.15

open circuit potential, Eop / VSCE

2.0

without particle 300nm 500nm 1595nm 23340nm

0.5 0.0 -0.5 -1.0

b

-0.20 -0.25

without particle 500nm 23340nm

300nm 1595nm

-0.30 -0.35 -0.40 -0.45 -0.50 -0.55

1E-6

1E-5

1E-4

1E-3

0.01

0

500

1000 1500 2000 2500 3000 3500 4000

Current density / A⋅cm-2

Time / s

Fig. 9. Potentiodynamic curve and open circuit potential of Ti6Al4V alloy during cavitation erosion for 60 min in 0.1 M H2SO4 solution containing 1 mg/L particle with different sizes, a) potentiodynamic curve, b) open circuit potential verse time plot.

0.00

-0.35

-0.40

a 1mg/L 5mg/L 10mg/L

-0.45

open circuit potential, Eocp / VSCE

open circuit potential, E ocp / VSCE

-0.30

b 1mg/L 5mg/L 10mg/L

-0.07 -0.14 -0.21 -0.28 -0.35

-0.50 0

500

1000 1500 2000 2500 3000 3500 4000

0

500

Time / s

1000

1500

2000

2500

3000

3500

Time / s

Fig. 10. Open circuit potential of Ti6Al4V alloy during cavitation erosion for 60 min in 0.1 M H2SO4 solution containing particles with different concentrations, a) 500 nm, b) 23,340 nm.

be strongly repelled at sample surface considering the negative charge of micro-particles and bubbles.

35 30

MDE / μ

25 20

4. Conclusions

15

The influences of spherical SiO2 micro-particle on ultrasonic cavitation erosion of Ti6Al4V alloy in sulfuric acid solution are investigated in this work, from the experimental results and discussions, some conclusions can be drawn as following:

10 5 0 Without particle 300nm 500nm and without applying Particle size potential

1595nm

23340nm

/ nm

Fig. 11. Variations of MDE and particle size in the case of applying a potential of 0.38 V during the entire ultrasonic cavitation erosion.

positive potential can cause the energy band of passive film bending up, which resulting in the appearance of negative ions on sample surface, and then the micro-jet containing micro-particles can

1) Micro-particles have evident effect on ultrasonic cavitation erosion of Ti6Al4V alloy in sulfuric acid solution, in which the MDE sharply decreases with adding particles, and it also significantly decreases with increasing particle size and concentration. 2) MDE decreases with applying positive potential (which is more positive than the open circuit potential measured during entire cavitation erosion) on sample during entire cavitation erosion. 3) The open circuit potential and MDE have an inner relationship, that is higher open circuit potential meaning lower MDE.

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a

b

c

d

e

2.4

30 28

2.2

26 2.0

24

1.8

22 20

1.6

18

1.4

16

Without particle 300nm

500nm

1595nm

23340nm

Surface roughness in height /μ m

Surface roughness in width / μ m

32

f

14

Particle size / nm Fig. 12. SEM images of Ti6Al4V alloy after cavitation erosion with applying a potential of 0.38 V for 120 min in 0.1 M H2SO4 solution (a) and containing 1 mg/L SiO2 particles with the diameter of 300 nm (b), 500 nm (c), 1595 nm (d) and 23,340 nm (e). In which, the insert figures are the corresponding white light interferometer, and Fig. f is the variation of surface roughness with particle size.

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