Investigation on mechanism of oxide removal and plasma behavior during laser cleaning on aluminum alloy

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Investigation on mechanism of oxide removal and plasma behavior during laser cleaning on aluminum alloy ⁎

Guangxing Zhanga, Xueming Huaa, , Ye Huanga, Yuelong Zhanga, Fang Lia, Chen Shena, Jian Chengb a b

Shanghai Key Laboratory of Material Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240, China School of Mechanical Engineering, Hubei University of Technology, Wuhan, Hubei 430068, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Laser cleaning Cleaning mechanism Cleaning thresholds Energy density Plasma

Through comparing the cleaning effect of aluminum alloy at different energy densities, the model of materials thermodynamics was established, illuminating the cleaning mechanism and plasma behavior. The research showed laser cleaning could remove oxide layer completely. The initial cleaning threshold was 12.7 J/cm2 and complete cleaning threshold was 25.5 J/cm2. At low energy density, phase explosion caused by laser ablation was the main cleaning mechanism. At high energy density, besides phase explosion, impact effect induced by evaporation pressure caused the splattering and removal of oxide layer. Plasma lifespan had positive correlation with energy density. Its lifespan in 25.5 J/cm2 and 51.0 J/cm2 were about 6 μs and above 10 μs, respectively. The model of materials thermodynamics showed that at low energy density, substrate evaporation pushed out molten oxide layer and formed pulse craters. At high energy density, transient energy absorption caused thermal stress coupling effect and separated substrate with oxide layer. The impact effect induced by evaporation pressure resulted in the removal of oxide layer. The vapor of Al absorbed laser energy and formed plasma. The high energy density enhanced the laser-plasma coupling. It caused high temperature and intensive electron density. The time of plasma cooling and atomization became longer.

1. Introduction

complete cleaning while the continued CO2 laser can’t. Laser-supported detonation and the thermal effect made the sputtering and removal of rust particles [12]. Laser fluence, pulse duration, pulse frequency, scanning velocity, etc. were chosen as influencing parameters to be explored. The works showed that there existed initial cleaning threshold of laser fluence for contaminant removal and the cleaning effect increased with increasing laser influence. High pulse frequency increased cleaning efficiency and the removal of contaminant can be done at low laser fluence, which compared with low pulse frequency [13,14]. Pulse duration will also affect laser ablation efficiency. Picoand femtosecond laser system allowed a higher precision while at lower ablation efficiency than that of micro- and nanosecond [16]. Li et al. found the squared diameter of ablation area was linearly related to the logarithm of pulse energy. And ablation rate decreased with increasing pulse number. They gave four possible reasons for the decrease of the ablation rate [18]. Cleaning mechanisms of different kinds of contaminants are different. For the contaminant of surface particle, Lu et al. [19,20] used laser-induced surface thermal expansion cleaning model to explain surface particle removal behavior. They showed that the cleaning force

Laser is widely applied in drilling, cutting, welding, etc. [1–3] As a new cleaning method, laser cleaning is also gradually used in manufacturing industry [4–11]. In semiconductor and disk drive industries, laser cleaning could effectively remove contaminants and particles from integrated circuit (IC) mold surface and disk surface, respectively [4]. In welding, some works showed that the welding of laser-cleaned samples showed excellent welding quality. Compared with the uncleaned sample, there were no porosities in the weld after laser cleaning [5]. In shipbuilding, laser cleaning was considered as a green manufacturing and repair method. Surface contaminants, including millscale, salt, rust, oil, could be removed effectively by properly setting the laser cleaning parameters [6]. Some works showed that laser cleaning could remove sulfide scale on compressor impeller blade and contaminants on the surface of tire mold, which improved impeller’s service life and tire quality [8,10]. In terms of the mechanism and process of laser cleaning, considerable efforts have been done and exhaustive analyses have been carried out [12–18]. Sun et al. found that pulsed YAG laser can accomplish rust ⁎

Corresponding author at: 800 Dongchuan Road, Minhang District, Shanghai 200240, China. E-mail address: [email protected] (X. Hua).

https://doi.org/10.1016/j.apsusc.2019.144666 Received 23 June 2019; Received in revised form 15 October 2019; Accepted 11 November 2019 Available online 15 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Guangxing Zhang, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144666

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of particles which induced by thermal expansion was related to laser energy and pulse duration. The larger diameter of the particle was, the easier the particle to be cleaned. For organic oil and greases, Waltersa et al. [21] used fiber laser to explore the laser cleaning mechanism. They conjectured that a thermo-mechanical effect occurred when energy was mainly absorbed by metal substrate. Then, the induced rapid expansion and some material evaporation ejected the contaminant film directly into aerosol droplets or particles which could be swept away. Grojo et al. [22] proposed that laser cleaning mechanism was surface and particle ablation, which had slight damage to substrate. However, the photochemical degradation of particle improved contaminant removal efficiency greatly. Laser cleaning mechanism is also related with laser parameters, such as energy density, laser wavelength, etc. Kumar et al. [23] used different wavelength laser to clean thin oxide on the tungsten ribbons. The results showed that removal mechanism was predominantly spallation at low energy density in 1064 nm and 532 nm, and higher energy density led to sublimation of the oxide layer. In case of 355 nm, spallation wasn’t observed at all and the removal process was sublimation over the entire energy density range. So, for different kinds of materials, contaminants and parameters, the mechanisms of laser cleaning also need to be explored in detail. Al alloy is widely used in manufacturing industry and its surface oxide layer has bad impact on welding quality [24,25]. At present, oxide removal of Al alloy mainly relies on traditional cleaning methods which have problems of pollution, low efficiency, complicated operation, etc. Laser cleaning technology could avoid those shortcomings effectively. However, due to great physical and chemical differences between aluminum and oxide layer, researches on oxide layer laser cleaning are very limited. Wang et al. [26] used tungsten inert gas to weld the laser cleaned sample and uncleaned sample. They showed that after laser cleaning, the initial oxide layer was removed completely, and no porosity existed in the fusion zone. But they didn’t illuminate the mechanism of oxide removal. Porneala et al. [27] found phase explosion will occur in aluminum when laser energy density was up to 5.2 J/ cm2. This phenomenon is likely to occur and contributes to oxide removal during laser cleaning of oxide layer. In this work, the mechanism of Al alloy laser cleaning and cleaning thresholds will be studied. High Speed Camera (HSC) was applied to observe the cleaning process of single pulse and investigate plasma behavior at low and high energy density, respectively. The model of the materials thermodynamics was proposed to explain the experimental results.

Fig. 1. Diagram of experimental device and laser spot movement.

movement. The galvanometer controlled laser spot (diameter 50 μm) movement in y direction, which enabled the maximum speeds reach 12 m/s. The diagram of experimental device and laser spot movement are shown in Fig. 1. The main working parameters of laser cleaning equipment are shown in Table 2. The pulse energy density was chosen as a variant to investigate the cleaning thresholds and the cleaning mechanism of oxide removal on Al alloy. Pulse energy density F could be calculated by Eq. (1). Experimental parameters are shown in Table 3. SEM was used to characterize the surface morphology of samples before and after cleaning. 34.0 J/ cm2 was chosen to observe the cleaning process by Veo 710 s HSC. The frames per second (FPS) was 60,000 and exposure time te was 2.9 μs.

F=

5754 Al alloy was chosen as experimental material and cut into samples whose size was 12 mm × 8 mm × 2 mm (length × width × thickness). The nominal chemical composition, as shown in Table 1. Before laser cleaning, the samples were cleaned by acetone, which aimed to clean surface oil and grease, and air drying at room temperature. Samples were kept in the constant temperature (25 °C) and humidity (32 %RH) environment for about 2 month and naturally oxidized. The Scanning Electron Microscope (SEM) was used for observing the cross-section of the sample and oxide layer. Pulse fiber laser operated at a wavelength of 1064 nm in its fundamental Gaussian mode. It delivered an average maximum power of 100 W, with maximum frequency of 500 kHz and pulse duration τ of 100 ns. Robot was used to control cleaning operation in x direction

3. Results 3.1. Oxide removal Fig. 2 shows the surface morphology of the oxide layer and crosssection morphology of the uncleaned Al alloy. The oxide layer formed in the natural environment and there were some scratches on the surface, as shown in Fig. 2(a). The cross-section of the uncleaned Al alloy was observed by SEM, as shown in Fig. 2(b). It shows an obvious oxide Table 2 Main working parameters of laser cleaning equipment. Maximum Power Wavelength Pulse Duration Maximum Frequency Scanning Speeds Temperature Request Humidity Request

Table 1 Nominal chemical composition of 5754 Al alloy (wt%). Al

Mg

Si

Cu

Ti

Zn

Mn

Cr

Fe

5754

Bal.

2.6–3.6

0.40

0.10

0.15

0.20

0.50

0.30

0.40

(1)

where P, f, d are laser power, pulse frequency and diameter of laser spot, respectively. According to the pre-experiment results, the energy density of 25.5 J/cm2 and 51.0 J/cm2 could remove the oxide layer effectively and be observed well by HSC. So, they were chosen to observe the plasma behavior of single pulse. Considering the maximum FPS limitation and in order to observe the changing behavior of single pulse plasma as much as possible, the size of photos was reduced. Photo sizes of 25.5 J/cm2 and 51.0 J/cm2 were 128 × 64 and 64 × 64 (pixel × pixel), respectively. FPS and te are 300,000, 2.9 μs and 400,000, 2.1 μs, respectively. The pulse frequency was 100 kHz.

2. Experimental

Alloy

4P f ·πd 2

2

100 W 1064 nm 100 ns 500 kHz 0–12 m/s 5–40 °C 10–95%

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Table 3 Experimental parameters of laser cleaning. Laser Power (W) Pulse Frequency (kHz) Energy density (J/cm2)

40 240 8.5

60 240 12.7

80 240 17.0

Table 4 Chemical composition measured by EDS (wt%). 100 240 21.2

60 120 25.5

80 120 34.0

100 120 42.5

Element

Oxide layer

Substrate

Center of crater (Point A)

O Al

54.7 45.3

1.3 98.7

0.5 99.5

Temperature 25 °C, Relative Humidity 65%.

by laser irradiation, but wasn’t cleaned, as shown in Fig. 6(b). When the energy density increased to 12.7 J/cm2, the traces of spot effect became clearer and the size of single pulse trace was larger. The ablative pulse craters occurred locally, and the oxide layer began to be removed, as shown in Fig. 6(c). With increasing energy density, as shown in Fig. 6(d) and (e), the number and covering areas of pulse craters increased. The Al alloy surface became cleaner while some areas still had an oxide layer. When the energy density increased to 25.5 J/cm2, the oxide layer on the surface was almost removed. Some white oxide particles were found, as shown in Fig. 6(f). The chemical composition measured by EDS showed that the mass percent concentration of aluminum and oxygen of the white particles are 84.8% and 15.2%, respectively. The concentration of oxygen in the white particles had decreased a lot. With further increasing of energy density, the sample surface was all covered by laser craters without initial surface, as shown in Fig. 6(g) and (f). Because of the mass percent concentration of oxygen in the crater was about 0.5%, we deemed that the oxide layer was removed completely. Definite Fa, Fb as initial cleaning threshold and complete cleaning threshold of the oxide layer, respectively. Results showed that cleaning effect of the oxide layer has positive correlation with energy density. When the energy density increased to Fa, cleaned traces appeared on the surface. And when the energy density increased to Fb, the oxide layer was removed completely. In this work, initial cleaning threshold was 12.7 J/cm2 and complete cleaning threshold was 25.5 J/cm2.

layer on the Al alloy surface, and its thickness is about 2 μm. The chemical composition measured by EDS, as shown in Table 4, showed that the mass percent concentration of aluminum and oxygen in the oxide layer are 45.3% and 54.7%, respectively. And the mass percent concentration of aluminum and oxygen in the substrate are 98.7% and 1.3%, respectively. At different energy density, the morphologies of Al alloy cleaning effect were shown in Fig. 3. After laser irradiation, many pulse craters formed on the surface of Al-alloy. Compared with Li [18] and Zhao’s [28] works, the depth of craters was shallow. The smooth morphology of solidified molten metal was obviously found in the crater, as shown in Fig. 3(a). With increasing energy density, the center of crater became smoother, and traces of molten metal spatter appeared, as shown in Fig. 3(d). The chemical composition measured by EDS (point A in Fig. 3(a)) showed the mass percent concentration of aluminum and oxygen in the center of crater are 99.5% and 0.5%, respectively. The concentration of crater is greatly close to that of the substrate, as shown in Table 4. Meanwhile, from the left zone of Fig. 4, the oxide layer disappeared after laser cleaning. Thus, it deemed laser cleaning could remove the oxide layer effectively and the oxidation effect during laser cleaning could be ignored. The width of the crater rim will increase first with increasing energy density then almost disappeared at 34.0 J/cm2. There were also some solidified metal droplets around the crater. Meanwhile, a relatively complete oxide layer was found at the edge of the crater, as shown in Fig. 3(c). There were also some molten oxide particles on the crater, as shown in Fig. 3(d). Laser cleaning process at 34.0 J/cm2 was observed by HSC, as shown in Fig. 5. It showed many splatters in the dotted boxes which could be the reason for the appearance of complete oxide layer and molten oxide particles in Fig. 3(c) and (d). From Fig. 4, slight surface fluctuation appeared after laser cleaning while uncleaned surface was flat with an obvious and continuous oxide layer on it. Compared with intimate contact of substrate and oxide, as shown in Fig. 2, a clear gap generated and showed the tendency of separation. Moreover, some cracks appeared in the oxide layer, which made oxide layer removal easier.

3.3. Plasma behavior When pulse frequency was 100 kHz, the energy density increased from 25.5 J/cm2 to 51.0 J/cm2, the cross-section area of laser-induced plasma increased from about 0.2 mm2 to about 0.5 mm2. The brightness of the plasma was enhanced obviously, as shown in the time of t in Fig. 7. Plasma behavior was observed by HSC in 400,000 FPS. It showed that when F = 25.5 J/cm2, the shape of laser-induced plasma liked a leaf. With time going by, after 2Δt1, the plasma gradually weakened. When at t + 3Δt1, the plasma disappeared completely, whose lifespan was about 6 μs. Thus, the plasma induced by next pulse could be deemed that had no relation with former pulse. The case was a little different when F = 51.0 J/cm2. The shape of laser-induced plasma was round and saturated. During the pulse period, the plasma become weak but didn’t disappear completely when the next pulse came. A small amount of plasma remains combined with the next pulse induced plasma, as shown in the time of t + 4Δt1 in Fig. 7. The

3.2. Cleaning thresholds Surface cleaning effect at different energy densities are shown in Fig. 6. Among them, Fig. 6(a) was an uncleaned sample on which surface was flat with some scratches. When the energy density was 8.5 J/cm2, traces of spot effect appeared in the material surface, caused

Fig. 2. SEM images of the sample: (a) surface morphology of the oxide layer; (b) cross-section morphology of the uncleaned Al alloy. 3

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Fig. 3. SEM images of Al alloy surface at different energy densities: (a) 17.0 J/cm2, (b) 21.2 J/cm2, (c) 34.0 J/cm2, (d) 42.5 J/cm2.

n=

m M

m = ρ∙V

(4) (5)

where a, b, c are the constants which are related to the properties of substance. T, m, M, ρ, V are the temperature, mass, molar mass, density and volume of substance, respectively. In order to explore the relationship between laser energy and temperature change of Al alloy, and based on measurement results of chemical composition of substrate and oxide layer, as shown in Table 4, assumptions are shown as follows: 1. The distribution of laser energy is uniform; 2. Al2O3 and Al approximately replace oxide layer and Al alloy; 3. Al2O3 and Al are considered as ideal heat insulators, which don’t transfer heat around.

Fig. 4. Cross-section morphology of laser cleaning and uncleaned (F = 17.0 J/ cm2).

The amorphous Al2O3 layer was formed in the natural environment [29]. The works showed that the shear stress and work of adhesion between Al and Al2O3 was about 20 MPa and 0.4–1 J/m2, respectively [30,31]. The thin film of Al2O3 existed a small band gap of about 2.6 eV [32]. It is transparent to laser with wavelength of 1064 nm. Thus, ignoring the energy loss of reflection and refraction of Al2O3, the laser energy transmission and absorption model is shown in Fig. 8. EL, Er, S, d1, d2 are laser incident energy, laser reflection energy, laser irradiated area and the thicknesses of Al2O3 and Al, respectively. The EAl2O3 and EAl are energy absorption of Al2O3 and Al, respectively. EAl2O3 is composed of E1 andE2 , which are energy absorption of laser incident and reflection through Al2O3, respectively. A1 and A2 are laser absorption coefficients of Al2O3 and Al, respectively. The relationship between laser energy EL with E1, EAl , and E2 is shown in Eqs. (6), (7) and (8):

plasma lifespan of 51.0 J/cm2 was more than 10 μs.

4. Discussion The experimental results showed an obvious laser ablation and splatter phenomenon. Therefore, based on the laser energy absorption of materials, the model of materials thermodynamics was established. It was used to illuminate the mechanism, cleaning thresholds and plasma behavior during laser cleaning. During substance heating, the relationship between temperature and energy Eab is showed in Eq. (2):

Eab = n·

(∫

T2

cP dT + ΔHtrans

)

E1 = A1 ·EL

(6)

where n, T1, T2, cp, ΔHtrans are the amount of substance, initial temperature, final temperature, constant-pressure molar heat-capacity and molar heats of phase transition, respectively.

EAl = A2 ·(EL − E1)

(7)

E2 = A1 ·(EL − E1 − EAl )

(8)

cp = a + bT + cT −2

Simplified Eqs. (6) (7) and (8), the absorption energy of Al2O3 and Al is shown as follows:

T1

(2)

(3) 4

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Fig. 5. Laser-induced plasma during laser cleaning, F = 34.0 J/cm2, 60,000 FPS (where τ, te, T and Δt are pulse duration, exposure time, pulse period and interval time of photos, respectively. The brightness of the picture in dotted boxes has been enhanced).

EAl2 O3 = (1 + (1 − A1 )·(1 − A2 ))·A1·EL

molar heat-capacities of Al2O3 and Al. ΔHtrans, Al2 O3 , ΔHtrans, Al are molar heats of phase transition of Al2O3 and Al. The relationship between the temperature and pressure of two phases equilibrium according to Clausius-Clapeyron Equation, as follows:

(9)

EAl = (1 − A1 )·A2 ·EL

(10)

Command α1=(1 + (1 − A1 )·(1 − A2 ))·A1, α2=(1 − A1 )·A2 , the relationship between laser energy and temperature change of Al2O3 could be deduced from Eqs. (2), (4) and (5):

ρ ·S·d1 α1·EL = 1 · M1

(∫

T2

T1

)

cp, Al2 O3 dT + ΔHtrans, Al2 O3

dP ΔHtrans = dT ΔVm·T

(11)

where ΔHtrans , ΔVm , T are the molar heats of phase transition, the volume change of phase transition and the temperature of two phases equilibrium, respectively. Due to the volume of the condensed phase is much smaller than that of the gas phase, thus, command ΔVm ≈Vgas. Taking gas phase as an ideal gas. The State Equation of Ideal Gas is shown as follows:

Then, the relationship between the energy density F1 and final temperature T2 of Al2O3 is shown as follows:

F1 =

ρ1 ·d1 · α1·M1

(∫

T2

T1

)

cp, Al2 O3 dT + ΔHtrans, Al2 O3

(12)

Similarly, the relationship between the energy density F2 and final temperature T2' of Al is shown as follows:

ρ ·d2 F2 = 2 ·⎜⎛ α2·M2 ⎝

∫T

T2'

' 1

cP, Al dT + ΔHtrans, Al⎞⎟ ⎠

(14)

P·Vgas = R·T

(15)

where P and R are pressure and gas constant, respectively. Substitute Eq. (15) into Eq. (14). The Clausius-Clapeyron Equation can be simplified as follows:

(13)

1 dP ΔHtrans · = P dT RT 2

where ρ1, ρ2 are densities of Al2O3 and Al. α1, α2 are constants that relate to laser absorption coefficients of Al2O3 and Al. M1, M2 are molar masses of Al2O3 and Al. T1, T1' are initial temperature which in this work is 298 K. T2 , T2' are final temperature. c p,Al2O3 , cP, Al are constant-pressure

(16)

Approximately consider ΔHtrans is uncorrelated to temperature. Then, double sides of the equation are integral:

Fig. 6. SEM images of Al alloy surface at different energy densities: (a) 0 J/cm2, (b) 8.5 J/cm2, (c) 12.7 J/cm2, (d) 17.0 J/cm2, (e) 21.2 J/cm2, (f) 25.5 J/cm2, (g) 34.0 J/cm2, (h) 42.5 J/cm2. 5

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Fig. 7. Plasma of single pulse and timer shaft diagram of camera and laser equipment when the energy densities are 25.5 J/cm2 and 51.0 J/cm2 (where τ, te, T and Δt1 are pulse duration, exposure time, pulse period and interval time of photos, respectively). Table 5 The constants for Al and Al2O3. Basic physical constants Substance M of substance [33] Al Al2O3

27 2.7 102 3.97

Molar heats of phase transition [34]

The constantpressure molar heat-capacity of substance [34]

lnP = −

ΔHm +C RT

where C is constant. Simplified Eq. (17) as follows:

lgP = A −

B T

where A, B are constants which are related to the properties of materials. From Eqs. (12), (13) and constants [33–36] provided in Table 5, d1, d2, A1, A2 are 0.2 μm, 0.2 μm, 0.1, 0.15, respectively. It could be calculated that when the temperature of Al2O3 increased to its melting point, the value of laser energy density was 16.0 J/cm2. When the temperature of Al increased to its melting point and boiling point, the values of laser energy densities were 3.9 J/cm2 and 16.4 J/cm2, respectively. In other word, at same energy density, the surface of Al2O3 and Al has different changes after laser irradiation due to different energy absorption. The experimental results also proofed this conclusion. The

660 2053

2519 3000

Trans.

ΔHtrans , J

Ttrans, K

Al Al Al2O3

s→l l→g s→l

10,700 25,531 107,500

933 2792 2326

b × 103 c × 10−5 Range, K

20.67 12.38 31.76 0 106.6 17.78

0 0 −28.53

298–933 (Tm) 933–2792 (Tb) 298–2326 (Tm)

Materials

300–600 nm

1.06 μm

10.6 μm

Al (smooth) Al (rough) Al2O3

– – –

0.06–0.2 0.2–0.4 0.05–0.1

0.03–0.06 0.1–0.4 0.90–0.99

Constants in evaporation pressure Equation [36]

(18)

Boiling point (°C)

Substance

Substance a

Normal, spectral absorption of materials at important Laser wavelengths [35]

(17)

Melting point (°C)

cp = a + bT + cT−2 J/mol·K

Al(s) Al(l) Al2O3

Fig. 8. Diagram of laser energy absorption model.

ρ (g/ cm3)

lgP = A − B/T Metal

A

B

Al

11.79

1.594 × 104

Fig. 9 shows laser impact comparison of oxide layer (Al2O3) and substrate (Al) with F = 8.5 J/cm2. At F = 8.5 J/cm2, there was slight laser impact trace on the oxide layer, as shown in Fig. 9(a), while obvious crater and melting trace appeared on the surface of the substrate, as shown in Fig. 9(b). The evaporation pressure P of Al in its boiling point could be calculated by Eq. (18), P = 1.2 MPa, about ten times larger than atmospheric pressure. The relationship between evaporation pressure of Al and temperature is shown in Fig. 10. The evaporation pressure of Al

6

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Fig. 9. SEM images of oxide layer and substrate with same energy density (F = 8.5 J/cm2): (a) Oxide layer (Al2O3); (b) Substrate (Al).

removal easier. When the energy density increased to 34.0 J/cm2, the temperature and ablated Al will be higher and much more. The evaporation pressure of Al will be larger. With the evaporation pressure impact process, the solid splatter will occur during laser cleaning. This phenomenon was observed by HSC, as shown in Fig. 5. The model of materials thermodynamics could explain the experimental results very well. But it needed to be noticed that the value of energy density was just a theoretical value. Considering the thermal expansion of substrate, different thicknesses of oxide layer and ablated substrate, etc. the theoretical value will less than the experimental result. Meanwhile, due to the nonuniform distribution of laser energy and different thicknesses of oxide, some local region will have craters at low energy density, as shown in Fig. 6(c). The oxide layer was not removed completely until the energy density increased to 25.5 J/cm2. From Eqs. (12) and (13), the energy density is mainly dependent on the thickness and the final temperature of oxide layer. And other parameters, which depended on the properties of materials, were taken as constant. Thus, the thickness of Al2O3 become larger, and the energy density needed to be larger to reach its melting point, while the energy density of the substrate is the same as before. In this case, a situation probably appears that at a certain energy density, the substrate reaches the boiling point and evaporates, and the oxide layer remains in the solid state. The oxide layer will be removed by the evaporation pressure and thermal stress coupling effect. The cleaning mechanism is mainly the mixed effects of phase explosion and impact effect. This model also supplied a new mind for the explanation of the mechanism of laser cleaning. It needed to consider the properties of not only the substrate but the contaminant layer. When laser irradiates on materials surface, heat electron, partly ablated materials and air ionization generate plasma. Researches showed that the temperature and the electron density were important parameters which affect plasma characteristic [39–45]. They found the temperature and electron density of plasma decreased with increasing distance of pulse irradiation zone. The shape of plasma was composed of high temperature plasma core which surrounded by low temperature halo. Those results were correspondent with experimental phenomenon, as shown in Figs. 5 and 7. Therefore, it could be deemed that the brightness of the plasma depended on temperature and electron density. The plasma of continuous cleaning process was observed by HSC, as shown in Fig. 5. After laser irradiation, the sample surface and air absorbed laser energy and then generated heat electron or ionized which formed the plasma. At same energy density of 34.0 J/cm2, the size and brightness of plasma of each pulse were similar, as shown in the time of t, t + Δt, t + 2Δt in Fig. 5. The high temperature in the plasma core generated high evaporation pressure. It made the molten oxide, substrate or oxide layer splatter combined with thermal stress coupling effect. So, in the dotted boxes, many splattering particles appeared during laser cleaning. The plasma behavior of single pulse laser is shown in Fig. 7. Without

Fig. 10. The evaporation pressure of Al varies with temperature (2792 K is Al boiling point).

increases with increasing temperature. From the model of materials thermodynamics, it is found that when the laser energy density was lower than 16.4 J/cm2, the oxide layer and substrate (Al) were melting into liquid while without force could remove the melting oxide layer. When the laser energy density increased to 16.4 J/cm2, 2 μm thickness of Al evaporated and oxide layer was still in the liquid state. The phase transition of Al in microsecond range and the evaporation pressure of Al pushed out the liquid oxide layer. Thus, craters formed on the surface, as shown in Fig. 3(a) and (b). With increasing energy density, the temperatures of Al2O3 and Al were increasing. Due to the high boiling point of Al2O3, it remained in the liquid state. However, great amount of Al rapidly evaporated or heated to high temperature after absorbing laser energy in a very short time, the pressure became larger, as shown in Fig. 10. Therefore, great pressure made the molten substrate and oxide layer splatter, which cooled and became white oxide particles on the crater, as shown in Fig. 3(c) and (d), as well as in Fig. 6(f) and (g). Due to some molten substrate splatter with molten oxide, the mass percent concentration of oxygen decreased a lot in the white particles in Fig. 6(f). During laser cleaning, the molten oxide isolated atmosphere and prevented the molten metal from oxidation. The complete oxide layer found in Fig. 3(c) is related to the separation of the oxide and the substrate. Considering the thermal expansion in the experiment, the thermal expansion coefficients of Al and Al2O3 are 2 × 10−5/K (7.85 ± 0.02) × 10−6/K, respectively [37,38]. The oxide layer will separate with substrate after energy absorption because of thermal stress coupling effect, as shown in Fig. 4. The separation of the oxide and the substrate will make the oxide layer 7

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laser irradiation, as shown in the time of t + Δt1, t + 2Δt1, and t + 3Δt1 in Fig. 7, plasma was gradually cooled and atomized. This process made the temperature and the electron density decrease. Thus, the plasma became weak and disappeared gradually. From Eqs. (12) and (13), the surface temperature was positively correlated with energy density. The evaporation of Al increased with increasing energy density and Al vapor provided more plasma. Due to the laser irradiation reflection and plasma confinement effects, laser-plasma coupling was enhanced. It drived the plasma to higher temperatures and more intensive electron densities. The high temperature and intensive electron densities needed enough time to cool and atomize [41,45]. Therefore, the plasma of 51.0 J/cm2 was larger, brighter and exists longer than that of 25.5 J/cm2.

[5]

[6]

[7]

[8]

[9]

[10]

5. Conclusion [11]

In this work, cleaning mechanism, cleaning thresholds and plasma behavior of Al alloy were investigated in detail. The model of materials thermodynamics was established and described the cleaning mechanism and phenomenon during laser cleaning. Meanwhile, the plasma behavior could be explained by the relationship between the energy density and the thermodynamics of materials. It could be concluded as follows:

[12] [13]

[14]

[15]

1. After laser irradiation, thermal stress coupling effect caused the substrate and the oxide layer to separate. At low energy density, the phase explosion caused by laser ablation was the main cleaning mechanism. When at high energy density, besides phase explosion, impact effect evaporation induced by pressure led to the splattering and removal of the oxide layer. 2. Through the experimental results, initial cleaning threshold was 12.7 J/cm2, and complete cleaning threshold was 25.5 J/cm2. The theoretical value calculated from the model was 16.4 J/cm2, which was larger than initial cleaning threshold because of the thermal expansion of materials and different thickness of the oxide layer. 3. The cross-section area, brightness and existent time of plasma were related to temperature and electron density. At higher energy density, the plasma was brighter and existed longer.

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Declaration of Competing Interest

[22]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement [25]

The authors acknowledge the assistance provided by Prof. Chen. Thanks for Chen’s help of advice writing and language checking.

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Appendix A. Supplementary material

[27]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144666.

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References

[29]

[1] R. Dewil, P. Vansteenwegen, D. Cattrysse, M. Laguna, T. Vossen, An improvement heuristic framework for the laser cutting tool path problem, Int. J. Prod. Res. 53 (2015) 1761–1776, https://doi.org/10.1080/00207543.2014.959268. [2] H. Hidai, S. Matsusaka, A. Chiba, N. Morita, Laser drilling and conducting film formation of vias in silicon, J. Electron. Mater. 44 (2015) 4928–4932, https://doi. org/10.1007/s11664-015-4097-6. [3] R. Li, F. Zhang, T. Sun, B. Liu, S. Chen, Y. Tian, Investigation of strengthening mechanism of commercially pure titanium joints fabricated by autogenously laser beam welding and laser-MIG hybrid welding processes, Int. J. Adv. Manuf. Technol. 101 (2019) 377–389, https://doi.org/10.1007/s00170-018-2922-9. [4] W.D. Song, M. Hong, L. Zhang, Y. Lu, T.C. Chong, Laser cleaning technology and its

[30]

[31]

[32]

[33]

8

application, Second International Symposium on Laser Precision Micromachining, 4426 2002, pp. 280–283, , https://doi.org/10.1117/12.456839. A. Kumar, M. Sapp, J. Vincelli, M.C. Gupta, A study on laser cleaning and pulsed gas tungsten arc welding of Ti–3Al–2.5V alloy tubes, J. Mater. Process. Technol. 210 (2010) 64–71, https://doi.org/10.1016/j.jmatprotec.2009.08.017. G.X. Chen, T.J. Kwee, K.P. Tan, Y.S. Choo, M.H. Hong, High-power fibre laser cleaning for green shipbuilding, J. Laser Micro/Nanoeng. 7 (2012) 249–253, https://doi.org/10.2961/jlmn.2012.03.0003. G. Buccolieri, V. Nassisi, A. Buccolieri, F. Vona, A. Castellano, Laser cleaning of a bronze bell, Appl. Surf. Sci. 272 (2013) 55–58, https://doi.org/10.1016/j.apsusc. 2012.03.132. Q.H. Tang, D. Zhou, Y.L. Wang, G.F. Liu, Laser cleaning of sulfide scale on compressor impeller blade, Appl. Surf. Sci. 355 (2015) 334–340, https://doi.org/10. 1016/j.apsusc.2015.07.128. C. Chunhakit, P. Kittiboonanan, W. Putchana, A. Ratanavis, The evaluation of laser cleaning of silica nanowires, J. Phys. Conf. Ser. 901 (2017) 012104, , https://doi. org/10.1088/1742-6596/901/1/012104. Y. Ye, B. Jia, J. Chen, Y. Jiang, H. Tang, H. Wang, X. Luan, W. Liao, C. Zhang, C. Yao, Laser cleaning of the contaminations on the surface of tire mould, Int. J. Mod. Phys. B 31 (2017) 1744100, https://doi.org/10.1142/s0217979217441008. S. Genna, F. Lambiase, C. Leone, Effect of laser cleaning in Laser Assisted Joining of CFRP and PC sheets, Compos. B Eng. 145 (2018) 206–214, https://doi.org/10. 1016/j.compositesb.2018.03.032. Z. Sun, J. Xu, W. Zhou, Parameters and mechanism of laser cleaning rust deposit on the steel surface, Photonics Asia 4915 (2002), https://doi.org/10.1117/12.482918. Z. Wang, X. Zeng, W. Huang, Parameters and surface performance of laser removal of rust layer on A3 steel, Surf. Coat. Technol. 166 (2003) 10–16, https://doi.org/10. 1016/s0257-8972(02)00736-3. F. Brygo, C. Dutouquet, F. Le Guern, R. Oltra, A. Semerok, J.M. Weulersse, Laser fluence, repetition rate and pulse duration effects on paint ablation, Appl. Surf. Sci. 252 (2006) 2131–2138, https://doi.org/10.1016/j.apsusc.2005.02.143. D. Grojo, M. BoyoMo-Onana, A. Cros, P. Delaporte, Influence of laser pulse shape on dry laser cleaning, Appl. Surf. Sci. 252 (2006) 4786–4791, https://doi.org/10. 1016/j.apsusc.2005.07.125. K.-H. Leitz, B. Redlingshöfer, Y. Reg, A. Otto, M. Schmidt, Metal ablation with short and ultrashort laser pulses, Phys. Procedia 12 (2011) 230–238, https://doi.org/10. 1016/j.phpro.2011.03.128. C.H. Sin, M.A.A. Bakar, D.F.H.A. Munap, G. Krishnan, N. Bidin, Optimized distance for non-damaging in laser cleaning preparation, Jurnal Teknologi 78 (2016), https://doi.org/10.11113/jt.v78.7505. F. Li, X. Chen, W. Lin, H. Pan, X. Jin, X. Hua, Nanosecond laser ablation of Al-Si coating on boron steel, Surf. Coat. Technol. 319 (2017) 129–135, https://doi.org/ 10.1016/j.surfcoat.2017.03.038. Y.F. Lu, W.D. Song, B.W. Ang, M.H. Hong, D.S.H. Chan, T.S. Low, A theoretical model for laser removal of particles from solid surfaces, Appl. Phys. A 65 (1997) 9–13, https://doi.org/10.1007/s003390050533. Y.F. Lu, W.D. Song, K.D. Ye, Y.P. Lee, D.S.H. Chan, T.S. Low, A cleaning model for removal of particles due to laser-induced thermal expansion of substrate surface, Jpn. J. Appl. Phys. 36 (1997) L1304–L1306, https://doi.org/10.1143/jjap.36. l1304. C.T. Walters, B.E. Campbell, R.J. Hull, Laser cleaning of metal surfaces, SPIE Proc. 3343 (1998) 859–865, https://doi.org/10.1117/12.321613. D. Grojo, A. Cros, P. Delaporte, M. Sentis, Experimental investigation of ablation mechanisms involved in dry laser cleaning, Appl. Surf. Sci. 253 (2007) 8309–8315, https://doi.org/10.1016/j.apsusc.2007.02.117. A. Kumar, R.B. Bhatt, P.G. Behere, M. Afzal, A. Kumar, J.P. Nilaya, D.J. Biswas, Laser-assisted surface cleaning of metallic components, Pramana 82 (2014) 237–242, https://doi.org/10.1007/s12043-013-0665-6. Y. Aoki, H. Fujii, K. Nogi, Effect of atomic oxygen exposure on bubble formation in aluminum alloy, J. Mater. Sci. 39 (2004) 1779–1783, https://doi.org/10.1023/ B:JMSC.0000016184.60145.b4. S. Shimizu, H.T. Fujii, Y.S. Sato, H. Kokawa, M.R. Sriraman, S.S. Babu, Mechanism of weld formation during very-high-power ultrasonic additive manufacturing of Al alloy 6061, Acta Mater. 74 (2014) 234–243, https://doi.org/10.1016/j.actamat. 2014.04.043. W. Qiang, G. Yingchun, C. Baoqiang, Q. Bojin, Laser cleaning of commercial Al alloy surface for tungsten inert gas welding, J. Laser Appl. 28 (2016) 022507, , https:// doi.org/10.2351/1.4943909. P. Cristian, W.D. A., Observation of nanosecond laser-induced phase explosion in aluminum, Appl. Phys. Lett. 89 (2006) 211121, doi:10.1063/1.2393158. W. Zhao, W. Wang, G. Jiang, B.Q. Li, X. Mei, Ablation and morphological evolution of micro-holes in stainless steel with picosecond laser pulses, Int. J. Adv. Manuf. Technol. 80 (2015) 1713–1720, https://doi.org/10.1007/s00170-015-7145-8. B. Goldstein, J. Dresner, Growth of MgO films with high secondary electron emission on Al-Mg alloys, Surf. Sci. 71 (1978) 15–26, https://doi.org/10.1016/00396028(78)90310-2. I. Ziv, F. Weinberg, W.J. Poole, The shear strength of alumina/aluminum alloy interfaces, Scr. Mater. 40 (1999) 1243–1248, https://doi.org/10.1016/S13596462(99)00119-0. M. Ksiazek, N. Sobczak, B. Mikulowski, W. Radziwill, I. Surowiak, Wetting and bonding strength in Al/Al2O3 system, Mater. Sci. Eng., A 324 (2002) 162–167, https://doi.org/10.1016/S0921-5093(01)01305-3. K. Shiiki, M. Igarashi, H. Kaijyu, Electronic structure of Al2O3 thin Film studied using first-principle band calculation, Jpn. J. Appl. Phys. 42 (2003) 5185–5186, https://doi.org/10.1143/jjap.42.5185. D.R. Lide, CRC Handbook of Chemistry and Physics, 84th ed., CRC Press, 2003.

Applied Surface Science xxx (xxxx) xxxx

G. Zhang, et al.

[34] David Gaskel, Introduction to the Thermodynamics of Materials, fourth ed., Taylor & Francis, 2003. [35] L.I.o. America, J.F. Ready, D.F. Farson, Lia handbook of laser materials processing, Laser Institute of America, Magnolia Pub, 2001. [36] G. Hu, X. Cai, Y. Rong, Fundamentals of Materials Science, third ed., Jiao Tong University, Shanghai, 2010. [37] P. Stoltze, K.W. Jacobsen, J.K. Norskov, Monte Carlo calculation of the thermal expansion coefficient of Al, Phys. Rev. B 36 (1987) 5035–5036, https://doi.org/10. 1103/PhysRevB.36.5035. [38] W.Q. Shao, S.O. Chen, P. Qi, D. Li, H.L. Zhu, Y.C. Zhang, Investigation on three line expansion coefficients of α-Al2O3, J. Qingdao Univ. (Nat. Sci. Edit.) 19 (2006) 35–38, https://doi.org/10.3969/j.issn.1006-1037.2006.01.009. [39] S.S. Wagal, Study of laser induced plasma at low laser intensity, IEEE Trans. Plasma Sci. 10 (1982) 211–213, https://doi.org/10.1109/TPS.1982.4316171. [40] K.J. Grant, G.L. Paul, Electron temperature and density profiles of excimer laserinduced plasmas, Appl. Spectrosc. 44 (1990) 1349–1354, https://doi.org/10.1366/ 000370290789619469. [41] K. Niemax, W. Sdorra, Optical emission spectrometry and laser-induced

[42]

[43]

[44]

[45]

9

fluorescence of laser produced sample plumes, Appl. Opt. 29 (1990) 5000–5006, https://doi.org/10.1364/AO.29.005000. T.W.L. Sanford, T.J. Nash, R.C. Mock, R.B. Spielman, J.F. Seamen, J.S. McGurn, D. Jobe, T.L. Gilliland, M. Vargas, K.G. Whitney, J.W. Thornhill, P.E. Pulsifer, J.P. Apruzese, Time-dependent electron temperature diagnostics for high-power, aluminum z-pinch plasmas, Rev. Sci. Instrum. 68 (1997) 852–857, https://doi.org/ 10.1063/1.1147707. V. Nassisi, A. Pedone, Physics of the expanding plasma ejected from a small spot illumined by an ultraviolet pulsed laser, Rev. Sci. Instrum. 74 (2003) 68–72, https://doi.org/10.1063/1.1527202. M.P. Chuchman, A.K. Shuaibov, G.E. Laslov, L.L. Shimon, Parameters of a multicomponent laser-induced plasma, Tech. Phys. 56 (2011) 151–153, https://doi.org/ 10.1134/s1063784211010075. C.D. Harris, N. Shen, A.M. Rubenchik, S.G. Demos, M.J. Matthews, Characterization of laser-induced plasmas associated with energetic laser cleaning of metal particles on fused silica surfaces, Opt. Lett. 40 (2015) 5212–5215, https://doi.org/10.1364/ OL.40.005212.