An experimental investigation into the defects of laser ... - Springer Link

The International Journal of Advanced Manufacturing Technology (2018) 96:1467–1481 https://doi.org/10.1007/s00170-018-1592-y

ORIGINAL ARTICLE

An experimental investigation into the defects of laser-drilled holes in thermal barrier coated Inconel 718 superalloys Rujia Wang 1,2 & Kedian Wang 1,2 & Xia Dong 2 & Zhengjie Fan 1,2 & Wenqiang Duan 1,2 & Xuesong Mei 1,2,3 & Wenjun Wang 1,2 & Jianlei Cui 1,2 & Shuai Zhang 1,2 Received: 12 April 2017 / Accepted: 9 January 2018 / Published online: 13 February 2018 # Springer-Verlag London Ltd., part of Springer Nature 2018

Abstract Delamination and microcracks as well as spatters are known to be inherently associated with laser drilling thermal barrier coated materials. In this study, response surface method (RSM) and single-variable method were employed to correlate the laser parameters including pulse length, peak power, pulse rate, and pulse numbers as well as the interactions among them with such defects in laser percussion drilling. Detailed observation and analysis reveals that narrow pulse length, lower peak power, and low pulse rate can produce smaller cracks area on the side wall, among which the effect of pulse length is more outstanding. As for spatters around hole entrance, the interactions among these factors were identified and a combination of higher peak power, proper pulse length (in this model not exceed 1 ms), higher pulse rate, and small number of pulses are conducive to minimize the spatter volume. Furthermore, the thermal effect near the thermal barrier coating/bond coat (TBC/BC) is especially prominent in laser drilling with wide pulse length and low peak power. The removal mechanism occurred near TBC/BC also determines the delamination crack size. And higher peak power with narrow pulse length should be a good strategy in order to break through the TBC as soon as possible and shelter the TBC or TBC/BC from thermal damage. Keywords Laser percussion drilling . Thermal barrier coatings (TBCs) . Inconel 718 superalloys . Spatters and cracks . Delamination

1 Introduction With further promotion of operating efficiency in aero-engine, components are suffered from ever-increasing combustion temperatures and hostile environment [1]. Thermal barrier coatings (TBCs) and film cooling holes are raised to deal with these grim situations. TBCs are mainly plasma sprayed on the superalloys by an intermediate bond coat (termed MCrAlY, M = Ni/Co), thus forming a multilayer material system, in which the TBCs play a key role in heat insulation and bond

* Xia Dong [email protected] 1

State Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, Xi’an 710054, China

2

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China

3

Shaanxi Key Laboratory of Intelligent Robots, Xi’an Jiaotong University, Xi’an 710049, China

coat provides antioxidant capabilities. The use of TBCs has enabled modern gas-turbine engines to operate at gas temperatures exceeding the melting temperature of the Inconel superalloys by providing around 100 to 300 °C reductions in the surface temperature [2]. And laser percussion drilling has become the preferred method in application of forming film cooling holes in aircraft engine components due to the fact that it represents a significant time and cost benefits [3]. However, there came problems such as spatters, delamination, and microcracks as well as other defects when drilling this material system using laser technology. Hence, a plenty of work has been conducted to investigate the effects of laser process parameters on the processing results of both coated and uncoated materials. Duan et al. have studied parameters on burr deposition in laser drilling using Taguchi method and corresponding effects were listed and discussed carefully [4]. In the work done by Bandyopadhyay et al. [5], geometrical features (hole diameter and taper angle) and metallurgical characteristics (spatter, recast, and heat-affected zone) in laser drilling uncoated Inconel 718 sheets were carefully investigated. Corcoran et al. have adopted statistical design and signal-

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to-noise ratio method to investigate the effects of the main factors on the percussion drilling of aerospace materials with a thickness of 2 and 3.6 mm [3]. The related conclusions showed that pulse energy, pulse width, and pulse shape have close relationship with microcracking. Low et al. studied the process parameters on spatter deposition in laser percussion drilling [6, 7]. They draw the conclusion that controlling spatter geometry by means of altered processing parameters may lead to benefits in closely spaced array drilling. The work done by Ng and Li [8] mainly focused on the hole geometry repeatability in percussion drilling and effects of two factors including peak power and pulse width were identified, in which higher peak power with shorter pulse during can get a better repeatability. And a number of studies related to delamination in acute angle drilling of coated superalloys were especially concerned. Kamalu et al. [9] proposed a two-stage mechanism for the formation of the delamination through an experimental study. It is said that localized delamination was induced by thermal gradient and subsequent propagation by mechanical stress. Voisey and Clyne [10] figured out that the fatal flaw always occurred at the relatively undamaged and significantly less tough TBCs/bond coat. In the research carried out by Sezer and Li [11], melt ejection-induced stresses were identified as the key mechanisms for defects at TBCs/bond coat, while the thermal effects are responsible for defects at the bond coat/substrate interface. Girardot et al. [12] studied the difference of the laser/ceramic and the laser/substrate interaction with real time observation by using a fast movie camera during the investigation of delamination and revealed that the temperature and ejection speed are much higher for a ceramic/laser than for a metal/laser interaction. Simulation work has also been done to investigate the effects of melt-induced forces and shear stresses on the delamination in inclined holes drilling [13, 14]. However, there were few reports about the research on the cracks on the side wall of drilled holes and spatters volume around the hole entrance as well as delamination in vertical laser drilling. On the one hand, side wall cracks of drilled holes and delamination can cause great harm to the performance and service life of the coated turbine blades. On the other hand, the unknown mechanism may be hidden by the inclined drilling due to its asymmetry and complexity. Wonderful laser drilling through the coatingssubstrate material system with special attention to restrain delamination on the interface between coatings and substrates and cracks on the side wall of drilled hole as well as spatters around the hole entrance also requires further survey and research. Thus, a systematic study to recognize and get a comprehensive understanding of machining mechanism to acquire excellent processing quality in laser drilling coated materials is quite crucial. In this study, two different experiments were designed and conducted in order to grasp the effects of processing parameters and/or the interactions among them on these defects

Int J Adv Manuf Technol (2018) 96:1467–1481

mentioned above. Finally, several conclusions are drawn and may be a good advice about how to reduce such defects in laser percussion drilling thermal barrier coated superalloys.

2 Experimental procedure 2.1 Materials and equipment The material system employed in this study was comprised of plates of 2.2-mm thick Inconel 718 superalloys which was air plasma sprayed with a yttria-stabilized zirconia thermal barrier coating, approximately 270-μm thick and low temperaturehigh velocity flame sprayed a NiCoCrAlYTa bond coat with a thickness of approximately 130 μm. The drilling process was accomplished at the focal plane by a fiber delivered pulsed JK 300D Nd:YAG laser (λ = 1064 nm), emitting a spot size of 240 μm and providing a class-leading level of peak power for percussion drilling and trepanning of a range of automotive and aerospace components. Table 1 lists the laser properties and the laser drilling system is depicted in Fig. 1. The holes in experiment one and two were all drilled by percussion drilling method. Compressed air was provided through a co-axial gas nozzle assembly equipped with the lens in drilling procedure. The pressure of the assistant gas was around 0.33 MPa.

2.2 Experiment one In experiment one, the central composite design (CCD), attributed to a response surface methodology (RSM), was adopted in this study to investigate the influence of laser parameters on drilling defects including side wall cracks and spatter volumes as well as delamination phenomenon, which also served as the responses or outputs. And the laser parameters considered in the experiment consisted of pulse length, peak power, pulse rate, and pulse numbers, which acted as the numeric factors. The CCD method can predict the presence of interactions between the parameters mentioned above. The Table 1

Laser properties

Parameter

Unit

Range

Maximum average power Pulse duration Maximum peak power Repetition rate Wavelength Focal length Spot diameter Beam quality Pulse energy

W ms kW Hz nm mm μm mm mrad J

300 0.2–5 16 1–1000 1064 120 240 16 0–35


interaction effects between the factors are very crucial and can reveal how the process can be controlled in order to achieve the desirable process outputs [15]. A full scale type CCD design with 30 runs was determined according to the number of numeric factors in this experiment. Each numeric factor is varied over five levels: plus and minus alpha (axial points), plus and minus 1 (factorial points), and the center point. The numeric factors and their levels in CCD are listed in Table 2. And the arrangement of experiment one are shown in Table 3.

2.3 Experiment two In experiment two, a comparative trial was conducted by using the single-variable method. The pulse energy was set at two different levels including a high and a low level. In each level, the pulse length varied from 0.2 to 5 ms, while the single pulse energy remained a constant value. The total input energy is identical both at the high level and the low level. The details of experiment two are shown in Table 4.

1469 Table 2

Numeric factors and relative levels in CCD

Numeric factors

Actual levels

Pulse length (ms)

0.2

0.4

0.6

0.8

1

Peak power (kW)

3.6

4.8

6

7.2

8.4

Pulse rate (Hz) Pulse numbers

20 40

30 60

40 80

50 100

60 120

Coded levels

−2

−1

0

1

2

subsequent grinding and polishing and the features of holes profiles were also recorded through SEM method. In the postprocessing, the area of side wall cracks of each hole, one response of experiment one was calculated and the volume of spatters around the hole entrance, another response of experiment one was recorded by the CLSM analysis software. The same operations were also carried out for samples in experiment two.

2.4 Characterization

3 Results and discussion

The photograph of laser-drilled holes in experiments one and two are also presented in Fig. 1. Immediately after drilled, both the samples in experiments one and two were cleaned in ultrasonic cleaning machine. Then their surface morphologies were acquired by both confocal laser scanning microscope (CLSM, OLS4000), which can record the 3D information of holes and scanning electron microscope (SEM, S-3000N) equipped with energy spectrum analyzer, field emission scanning electron microscope (FESEM, SU-8010, HITACHI). The sectioned pieces were obtained by

3.1 Data analysis for experiment one

Fig. 1 Experimental setup of laser drilling system and photograph of laser drilled holes in experiment one and two

The response surface methodology fits all kinds of models including linear, two-factor interaction (2FI), quadratic, and cubic polynomials to the response. When a proper model is selected, the analysis of variance method (ANOVA) was introduced to determine the size of the controllable factors influence on the results of the study as well as confirm the adequacy of the selected model. Tables 5 and 6 present the significant factors for area of side wall cracks and volume of

1470 Table 3

Int J Adv Manuf Technol (2018) 96:1467–1481 Arrangement of numeric factors in experiment one Factor 1

Factor 2

Factor 3

Factor 4

1

A: pulse length (ms) 0.8

B: peak power (kW) 4.80

C: pulse rate (Hz) 30

D: pulse numbers 100

2 3

0.8 0.8

4.80 4.80

50 30

100 60

4 5

0.4 0.4

7.20 7.20

30 30

60 100

6 7

0.6 0.6

3.60 8.40

40 40

80 80

…

…

…

…

…

21 22

0.4 0.6

7.20 6.00

50 40

100 80

23 24

0.8 0.8

4.80 7.20

50 30

60 100

25 26 27

0.6 0.6 0.8

6.00 6.00 7.20

40 40 50

80 40 100

28 29 30

0.6 0.8 0.6

6.00 7.20 6.00

40 50 40

80 60 80

Run

spatters, respectively. The influence of different factors was evaluated as extremely significant, highly significant, significant, and not significant depending on the related p value. With further analysis by RSM method, perturbation of significant parameters to crack area and spatter volume is shown in Fig. 2 and interactions between parameters in the response of spatter volume are depicted in Fig. 3. 3.1.1 Side wall cracks Because of the main thermal interaction caused by millisecond laser, there would occur many microcracks on the side wall of drilled holes as shown in Fig. 4a. It should be clear that the side wall cracks here mainly refer to the cracks located on the substrate and especially most of them distribute on the anterior and middle portion of the substrate as shown in Fig. 5. In the Table 4

Configurations of parameters in experiment two

Parameters

High level

Low level

Pulse length (ms) Peak power density (1e10W/m2) Pulse energy (J) Pulse rate (Hz) Pulse numbers Total energy (J)

0.2, 0.5, 1, 2, 5

0.2, 0.5, 1, 2, 5

23, 9.2, 4.6, 2.3, 0.92

11.5, 4.6, 2.3, 1.15, 0.46

2.08 50 50 104

1.04 100 100 104

case of side wall cracks, there exists a reduced liner model in which the factor named pulse numbers is removed. It can be said here that this factor has no significant effect on the crack area. Table 5 clearly lists the significance of different factors according to the p value. This consistent cognition can also be acquired from Fig. 2a in which it reveals that all the main effects of factors are significant in the decreasing order of pulse length, peak power, and pulse rate. In Fig. 5a, d, the side wall cracks were quite severe when drilling under run 2 and run 22, as marked in zone 1, etc. What can be found is that they both feature a longer pulse length and residual molten materials were obviously attached to the hole walls. However, a clean and tidy hole wall (no molten materials attached to the hole wall) can efficiently restrain the crack formation, Fig. 5b, c. As for the analysis results, it can be discussed from three aspects because there were no obvious interactions among them. Firstly, pulse length behaves the utmost effect in the model because it has a strong perturbation to the crack area. It determines the time of laser irradiation on materials per pulse. The wider pulse length is, the stronger thermal effect it causes. As wrote by Chryssolouris [16], the thermal penetration depth δ, defined by the material thermal diffusivity α and the interaction time τ, reads as follow pffiffiffiffiffiffiffiffiffiffiffiffi δ ¼ ατ ð1Þ It was indicated that thick molten layer could be induced by a wider pulse length. The thick molten materials were difficult to fully get outside of the hole cavity. The imported pulse energy introduces the driving force for crack nucleation on the side wall of drilled holes. And the remaining melt attached to the side wall would experience the pulse on-and-off effect. Consequently, cracks could be easily produced there and further extended from the hole wall surface to the interior of the substrate. Secondly, the size of peak power determines whether the evaporation or melt ejection plays the dominant role in laser drilling process. As reported by Bathe and Padmanabham [17], the melt ejection fraction is greater compared to vaporization with a low power density, while material removal by vaporization is significant with a high power density. The vapor pressure caused by high peak power may throw a knock-on effect on the side walls. It indicates that the side wall will suffer from erosion caused by metal vapor flow during the drilling process. Yellow marked zones in Fig. 5 show the SEM micrograph of cracks distribution on the side wall of drilled holes from different parameter sets. It can also be found that there are tiny or no cracks near the exit of drilled holes. As mentioned before, most of cracks were located on the anterior and middle portion of the substrate where lots of solidified materials attached there. The reason derived from the result may be that the peak power density drops along with the increasing of the hole depth. As a result, the terminal portion of substrate was free from erosion by vapor pressure. Besides, the laser beam can easily break

Int J Adv Manuf Technol (2018) 96:1467–1481 Table 5

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ANOVA table for area of side wall cracks based on a reduced linear model

Source

Sum of squares

df

Mean square

F value

p value prob. > F

Influence

Model A-pulse length

9.490E + 007

3

3.163E + 007

19.69

< 0.0001

Extremely significant

7.218E + 007

1

7.218E + 007

44.92

< 0.0001


B-peak power C-pulse rate

1.710E + 007 5.626E + 006

1 1

1.710E + 007 5.626E + 006

10.64 3.50

0.0031 0.0726

Highly significant Significant

Residual Lack of fit

4.178E + 007 3.039E + 007

26 21

1.607E + 006 1.447E + 006

0.64

0.7890

Not significant

Pure error

1.138E + 007

5

2.277E + 006

Cor total

1.367E + 008

29

Extremely significant, p < 0.0001; highly significant, p < 0.05; significant, p < 0.1; not significant, p > 0.1

through the materials producing a through hole at the final drilling stage and almost all the remaining materials were expelled through the hole exit, alleviating the damage to the side wall of drilled holes. Thirdly, although pulse rate has a relatively less perturbation than other factors, it also deserves to be noticed. It is the characterization of the time interval between two adjacent pulses. A large time interval is good to thermal stress release. A high-cycle thermal fatigue can be induced by a high pulse rate, which may be the reason for the crack propagation and increase of cracks area. In this section, it indicates that a narrow pulse length, a lower peak power, and a low pulse rate can produce a smaller cracks area on the side wall. 3.1.2 Spatters around hole entrance In the case of spatters around hole entrance as shown in Fig. 4b, a reduced 2FI model was selected for this response, in which the interactions between parameters are especially significant as can be seen in Table 6. Figure 2b shows the perturbation of significant parameters to spatter volume. It highlights the effect of peak power on the spatter volume while others are not significant individually. It can be seen that a higher peak power can produce small spatter volume. Table 6

In Fig. 3, the interactions between different factors in the response of spatter volume are clearly presented in the form of 3D surfaces. Thereinto, the interaction between pulse length and peak power is extremely significant followed by pulse length and pulse numbers, peak power and pulse numbers, and peak power and pulse rate in an order of decreasing importance. In the case of higher peak power, at wide pulse length (Fig. 3a), small number of pulses (Fig. 3c), and higher pulse rate (Fig. 3d), the spatter volume can be reduced. It should be noted here that wide pulse length may be not good to reduce spatter volume. However, the effect of pulse length should not be discussed individually since interactions are significant in the model. Higher peak power density can produce a thin melt front and a wide pulse length (in this model not exceed 1 ms) can provide sufficient time for producing continuous vapor pressure to expel melt, while at narrow pulse length, decreasing pulse numbers comes out small spatter volume as shown in Fig. 3b. It can be summarized that higher peak power, proper pulse length, higher pulse rate, and small number of pulses are conducive to minimize the spatter volume. Furthermore, higher peak power is the most preferred choice when considering the processing parameters in millisecond laser percussion drilling.

ANOVA table for volume of spatters based on a reduced 2FI model

Source

Sum of squares

df

Mean square

F value

p value prob. > F

Influence

Model

1.448E + 015

5

2.895E + 014

11.38

< 0.0001


B-peak power AB AD BC BD Residual Lack of Fit Pure Error Cor Total

1.763E + 014 7.425E + 014 2.952E + 014 1.068E + 014 1.268E + 014 6.105E + 014 3.433E + 014 2.672E + 014 2.058E + 015

1 1 1 1 1 24 19 5 29

1.763E + 014 7.425E + 014 2.952E + 014 1.068E + 014 1.268E + 014 2.544E + 013 1.807E + 013 5.344E + 013

6.93 29.19 11.61 4.20 4.98

0.0146 < 0.0001 0.0023 0.0515 0.0352

Highly significant Extremely significant Highly significant Significant Highly significant

0.34

0.9614

A-pulse length, B-peak power, C-pulse rate, D-pulse numbers

Not significant

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Fig. 2 Perturbation of significant parameters to crack area (a) and spatter volume (b)

3.1.3 Effect of pulse energy input on the responses

3.2 Defects analysis in experiment two

Figure 6 shows the relationships between the pulse energy input with the responses, respectively. Figure 6a shows that the crack area is positively proportional to the pulse energy, whose relation coefficient is R = 0.84 (R2 = 0.7111), indicating that the crack area increases with increasing pulse energy. When pulse energy is around 5 J, the crack phenomenon is very serious. These processing parameters are all featuring a longer pulse length with high peak power. This is consistent with the discussion in Section 3.1.1. In Fig. 6b, there is no clear relationship between pulse energy and spatter volume. However, a general trend is that too low (< 1.5 J) or too high (> 4.5 J) pulse energy contribute to smaller spatter volume. This judgment is based on the indicated value by the dotted line in Fig. 6b. As previously mentioned, high pulse energy features a high peak power density in the experiment, and material removed by vaporization is prominent when providing a high peak power density, leading to small spatter around hole entrance. A low pulse energy cannot provide efficient material removal, consequently small spatter volume produced.

3.2.1 Spatters and delamination

3.1.4 Delamination phenomenon The delamination phenomenon was also investigated in experiment one. It was observed that severe delamination occurred at both the coating/bond coat interface (termed TBC/BC) and the BC/substrate interface (termed BC/substrate) at higher pulse energy, wide pulse length. The thermal effect has great relationship with this defect and further discussion will be stated in the next section.

It should be noted here that the spatter volume was measured by the CLSM analysis software and the cavity volume was calculated by boundary fitting and 3D reconstruction, and actually, it is only an approximation of the real value of drilled holes. Figure 7 describes the schematic diagram of the whole operating process for obtaining the approximate solution of volume of drilled holes. The detail operation is presented here carefully. First of all, it was assumed that the hole cavity is an axisymmetric structure because of the distribution characteristics of the laser. The cavity structures and corresponding volumes were acquired in a 3D software according to the SEM micrograph of cross section of drilled holes. Then the final values were attained by scale conversion. And the conversion coefficient and the actual volume are deduced from the equations below: b

V 0 ¼ V x ¼ π∫a f 2 ðxÞdx

ð2Þ

0 0 b0 V x ¼ V x0 ¼ k coe3 *π∫a0 f 2 x dx

ð3Þ

V ¼ V 0 =K

ð4Þ

COE ;

K

COE

¼ k coe3

where k_coe denotes a known image scale and K_COE the conversion coefficient. As shown in Fig. 8, it depicts the variation curve of spatter-cavity ratio at various pulse length. The spatter volume as well as the spatter-cavity ratio shows an increasing trend with respect to pulse length, while this increasing trend behaves dramatically under the condition of 2.08 J, as can be seen in Fig. 8a. As listed in Table 4, the


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b Spatter volume (μm^3)

Spatter volume (μm^3)

a

0.80

0.40

0.70

0.50

0.60 A: Pulse length (ms) 0.50 Actual factors: C: pulse rate=40Hz D: pulse numbers=80

0.40 7.20

5.40 6.60

4.80

6.00

B: Peak power (kW)

100.00 0.60 92.00 84.00 A: Pulse length (ms) 0.70 76.00 Actual factors: 68.00 B: Peak power=6kW 0.80 60.00 D: Pulse numbers C: Pulse rate=40Hz

d Spatter volume (μm^3)

Spatter volume (μm^3)

c

4.80 7.20

5.40

6.00 B: Peak power (kW) 6.60

6.60

92.00

84.00 Actual factors: 76.00 A: Pulse length=0.6ms 68.00 7.20 60.00 C: Pulse rate=40Hz D: Pulse numbers

100.00

6.00 B: Peak power (kW) 5.40 Actual factors: A: Pulse length=0.6ms D: Pulse numbers=80

Fig. 3 a–d Interactions between parameters in the response of spatter volume

Fig. 4 SEM micrograph of defects in laser drilled holes: side wall cracks (a) and spatters (b)

4.80 50.00

30.00 35.00

40.00 45.00

C: Pulse rate (Hz)

1474


Fig. 5 SEM micrograph of cracks distribution on the side wall from different parameter sets: a run 2, b run 5, c run 21, and d run 22

calculated peak power density can be 2.3e11W/m2 with 0.2 ms at 2.08 J while only get to 9.2e9 W/m2 with 5 ms. It can be derived from Fig. 8a, b that higher peak power density gains a significant vaporization. And the distribution of spatters was becoming more and more concentrated and thick along with increasing pulse length.

Especially when pulse length exceeded 2 ms, the spatters showed a directional distribution. This may be determined by whether the vapor pressure or assist gas pressure plays the dominant role in melt ejection. The vapor pressure can be calculated from the Clausius–Clapeyron equation [18, 19]. Thus, the recoil vapor pressure, always taken as 0.54

Fig. 6 The relationships between pulse energy input and response. a Crack area and b spatter volume


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Fig. 7 Schematic diagram of operating process for obtaining the approximate solution of volume of drilled holes

of the vapor pressure in equilibrium on the melting surface, can be expressed as follows: Lv 1 1 pvap ≅0:54 p0 exp ð5Þ − R T vap T s where p0 denotes the atmospheric pressure, Lv the latent heat of vaporization, R the universal gas constant, Tvap the vaporization temperature, and Ts the surface temperature of molten pool. The curve which relates the surface temperature to vapor pressure is shown in Fig. 9. As the surface temperature rises, the recoil vapor pressure goes upward dramatically. When recoil pressure gains a great advantage over the assist gas pressure (here the value equals to 0.33 MPa), it may dominate in melt expulsion during laser drilling. In addition, forming a large recoil pressure also requires higher peak power density. And the molten materials were mainly expelled by assist gas pressure at lower peak power density due to that it would not induce sufficient vaporization, let alone large recoil pressure. As a result, the spatters showed a directional distribution with wide pulse length (2 ms, 5 ms), which is quite different from those with such pulse length as 0.2, 0.5, and 1 ms. As the conclusion demonstrated by Voisey et al., the average ejection velocity of molten droplets increases with increasing pulse intensity [20]. It indicates that the distribution of spatters can be controlled by regulating the size of peak powers.

Figure 10 shows the hole depth and diameter as well as their ratio at various pulse length with 2.08 J (Fig. 10a) and 1.04 J (Fig. 10b). It indicates that the depth of holes shows a decreasing trend while the diameter of holes a reverse trend with respect to pulse length. And the depth-diameter ratio can be attained at around 8 at higher peak power while it can only be around 2 at lower peak power. Besides, the hole depth at 0.2 ms can be twice as deep as that at 0.5 ms with 1.04 J as shown in Fig. 10b. A higher peak power can be obtained when narrow pulse length is selected at constant pulse energy. It can be said that a higher peak power can promote the drilling efficiency due to the fact that the total input energy remained the same for each hole. What can be found is that narrow pulse length with higher peak power and higher pulse energy can generate small spatter volume and spatter-cavity ratio as well as diameter, but deep holes in depth and great depth-diameter ratio. Thus, various parameter sets can result in different processing orientation. At higher peak power with narrow pulse length (HPNP), the feature of holes exhibits a preferred orientation in depth direction. Conversely, the feature of holes shows a preferred development in diameter direction at lower peak power with wide pulse length (LPWP). The statement above can be obviously observed in Fig. 11, in which the profiles of holes drilled at various pulse length with different pulse energy are demonstrated, respectively. In the case of delamination, what can be observed is that the interfacial crack shows preferred location at TBC/BC in

1476 Fig. 8 Histogram of cavity and spatter volume and their ratio at various pulse length with 2.08 J (a) and 1.04 J (b)

Fig. 9 Variation of recoil vapor pressure for increasing surface temperatures



1477

Fig. 10 Histogram of depth and diameter and their ratio at various pulse length with 2.08 J (a) and 1.04 J (b)

millisecond laser drilling. And the interfacial crack became severer and severer in size of both length and width along with the increasing pulse length. Thus, it can be indicated that the HPNP can produce slight interfacial cracks, while the LPWP may result in serious interfacial cracks from the processing results. The difference in crack size mainly lies in the removal mechanism that occurred near TBC/BC. Large crack size was induced by prominent melting of materials. And more vaporization that happened near the interface may contribute to tiny crack size because it did not change the structure dramatically. 3.2.2 Further discussion In experiment two, the total energy imported to each hole as well as the time consumption was the same. As described above, the HPNP can give rise to small spatter-cavity ratio and great depth-diameter ratio as well as slight interfacial crack while the LPWP exactly the opposite. Narrow pulse

length can restrain from the heat propagation and accumulation in the diameter direction, while higher peak power is conducive to emission of molten materials boosting the tip of bore advancing in the depth direction. In order to understand these two different drilling orientations, it is important to analyze the effect of laser peak power density on the drilling process. Before the phase transition occurs, it will undergo a heating stage. The duration of heating stage th can be calculated as [16, 17] πkρc ðT m −T 0 Þ 2 4 th ¼ ð6Þ αI 0 where k, ρ, c, Tm, T0, α, and I0 are the thermal conductivity, density, heat capacity, melting temperature, initial temperature, absorptivity, and peak power density, respectively. Figure 12a shows that duration of heating stage can be reduced with increasing of the peak power density. In Fig. 12b, a high peak power density (G2) features a larger high-power zone

1478


Fig. 11 SEM micrograph of holes drilled at high level and low level pulse energy. a SEM micrograph of hole profiles at various pulse length with 2.08 J. b SEM micrograph of hole profiles at various pulse length with 1.04 J

(effective drilling threshold range) than lower peak power density (G1), d2 > d1. It indicates that HPNP can facilitate the materials quickly from heating stage to melting stage even to vaporization stage. Firstly, higher peak power can generate a thin melt front. And narrow pulse length contributes to a thin thermal penetration depth, restraining heat loss caused by heat conduction, consequently promoting the drilling efficiency. Secondly, it also can result in a high recoil vapor pressure in the cavity, facilitating the expulsion of the melt timely and preventing materials from re-solidifying, consequently avoiding laser interacting with the re-solidified materials again. Furthermore, under the condition of HPNP, evaporation may dominate in the material removal, lowering the damage to the bonding interfaces from material melting. In addition, as marked in Fig. 12a, the difference in th between TBC and

substrate was carefully calculated. The difference can be significantly reduced from 411 to 26 ns with the increasing of peak power density and this can be further reduced by increasing the peak power density. The shortening of time difference is conducive to removing the materials near the interface simultaneously. A temporal hysteresis in phase transition between TBC and substrate is not good for delamination prevention. As seen in Fig. 12a, duration of heating stage for TBC is longer than substrate especially when peak power density is very low. This time-lag may induce that matrix material is removed prior to the ceramic layer near the interface and crack initiation starts from here, which further develops into delamination (see Fig. 11) caused by melt ejection or recoil vapor pressure in hole cavity. Thus, this HPNP configuration is quiet beneficial for drilling coated materials. However, the result is


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Fig. 12 a The relationship between duration of heating stage and peak power density. b Peak power density distribution

not satisfying when drilled at LPWP in which the delamination is apparently so severe, as shown in Fig. 11. For one thing, the thermal effect is so prominent due to wide pulse length, and another, the drilling efficiency is so poor at lower peak power (more time is consumed in heating stage and smaller effective drilling threshold range) that lingers for much more time on/ above the TBC/BC, which means that lots of heat stays near the TBC/BC. The variation of distance from hole tip to TBC/ BC at different pulse length is shown in Fig. 13 and the crack size at TBC/BC is also included. In spite of the same input energy, the crack size at 0.2 ms is about 3 μm while around 12 μm at 5 ms. Such increase in crack size may have a close relationship with the distance. It was indicated that the distance from hole tip to TBC/BC acted as a prominent role in heat accumulation near TBC/BC due to the fact that a large crack size was induced by the close range and smaller peak power. As is known, the thermal conductivity and coefficient of

Fig. 13 Distance from hole tip to TBC/BC and crack size at various pulse length

thermal expansion of TBC and BC are quite distinguishing. The performance of thermal response can be very distinct in temperature gradient and thermal-induced deformation. What’s more, the thermal effect is fairly predominant in laser drilling at wide pulse length as can be seen in Fig. 14. There are lots of cluster-like microstructures growing on the rim of hole entrance (only when the pulse length is above 2 ms). It can be indicated that the material forming the microstructures originated from the substrate according to the EDS pattern as shown in Fig. 14. And the heat induced by laser may provide the energy source for the growth of these microstructures. Thus, it reveals that the TBC/BC or TBC may suffer from the thermal damage problem under the condition of LPWP. As a result, it will be a wise move choosing higher peak power with narrow pulse length in drilling thermal barrier coated superalloys, accelerating the laser beam breaking through the TBC, and consequently reducing the thermal damage to TBC or TBC/BC.

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Fig. 14 Elemental distribution of microstructure on the rim of hole entrance measured by EDS (SEM, 1.04 J/5 ms)

4 Conclusion The defects including cracks on the side wall and spatters around the hole entrance as well as delamination of laserdrilled holes in thermal barrier coated Inconel 718 superalloys were investigated through the two experiments. The main findings of the work achieved based both on the RSM analysis and single-variable method are presented here. It should be noted that the laws for response values in the RSM analysis are only valid with respect to employed variable parameter ranges. The conclusions drawn are as follows: &

&

In the case of side wall cracks, among the factors investigated, pulse length should be the key issue. And the other two factors including peak power and pulse rate should also be of concerned. Thick melt front could be induced by a wider pulse length and the residual melt attached to the side wall creates a perfect breeding ground for the cracks. The peak power determines the size of vapor pressure, while the pulse rate, the combination of heat and cold, introduces the effects of mechanical erosion and thermal stress fatigue, respectively. As for spatters around the hole entrance, higher peak power is the most preferred choice when considering to reduce spatters. Through 3D reconstruction and calculation of spatter-cavity ratio, high peak power density (> 9.2e10 W/m2) can contribute to vaporization removal

&

&

being prominent in drilling process. HPNP contributes to a thin thermal penetration depth, restraining heat loss caused by heat conduction and consequently promoting the drilling efficiency and small spatters produced. The delamination is mainly discussed in experiment two. The thermal effect near the TBC/BC is especially prominent in laser drilling under the condition of LPWP. Temporal hysteresis in phase transition between TBC and substrate should also be noticed. In order to break through the TBC as soon as possible and shelter the TBC or TBC/BC from thermal damage, HPNP should be a good strategy in drilling thermal barrier coated superalloys. The removal mechanism that occurred near TBC/ BC also determines the delamination crack size. Large crack size was induced by prominent melting of materials. And more vaporization happened near the interface may contribute to tiny crack size because it did not change the structure dramatically. Finally, conflicts appear where one would like to minimize all the defects mentioned above. Thus, multi-objective optimization problem arises and further investigation is quite in demand.

Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Grant No. 51575432), National Natural Science Foundation of China (Grant No. 51375374), and the Program for ChangJiang Scholars and Innovative Research Team in University (Grant No. IRT_15R54) for supporting this work.


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