Investigation of Microhole Quality of Nickel-Based Single Crystal Superalloy Processed by Ultrafast Laser

Abstract

The geometric accuracy and surface quality of thin-film cooling holes have a significant impact on the cooling efficiency and fatigue life of aeroengine turbine blades. In this paper, we conducted experimental research on the processing of nickel-based single-crystal high-temperature alloy DD6 flat plates using different femtosecond laser processes. Our focus was on analyzing the effects of various laser parameters on the geometric accuracy results of microholes and the quality of the surfaces and inner walls of these holes. The results demonstrate that femtosecond laser processing has great influence on the geometrical accuracy and surface quality results of film cooling holes. Notably, the average laser power, focus position, and feed volume exert a significant influence on the geometric accuracy results of microholes. For instance, a higher laser power can damage the microhole wall, thereby leading to the formation of tiny holes and cracks. Additionally, microholes exhibit optimal roundness and taper values when using a zero defocus volume. Moreover, increasing the feed distance results in enhanced entrance and exit roundness, whereas scanning speed has a negligible impact on microhole roundness.

1. Introduction

The performance of an aircraft engine relies heavily on the temperature at the turbine inlet, and increasing the preturbine inlet temperature is a crucial strategy for improving engine performance [1,2]. In recent years, the widespread adoption of high-temperature materials, especially nickel-based single-crystal high-temperature alloys in turbine blades, has led to a continuous rise in turbine inlet temperatures [3,4]. However, these materials have a lower temperature resistance than required for turbine inlet conditions. Consequently, thermal protection for turbine engines stands out as one of the most critical challenges in advancing aircraft engine technology. Film cooling technology is a widely adopted approach to effectively increase the operational temperature and service life of turbine engines [5,6,7,8].

Currently, the main methods used for machining microholes include electrical discharge machining (EDM), laser processing (LM), and electrochemical machining (ECM) [9,10,11,12]. Among these, EDM drilling technology is well established, thus boasting high processing efficiency and precision. However, it does generate recast layers and heat-affected zones during the process. Electrochemical drilling technology offers good processing quality, but its efficiency is limited, and it cannot process shaped holes. Laser drilling is categorized based on pulse width into two types: long-pulse lasers and ultrashort-pulse lasers. Long-pulse laser processing, such as millisecond and nanosecond lasers, eliminates material through high-temperature melting, thereby essentially making it the same as EDM. Unfortunately, this process inevitably results in heat treatment defects like recast layers and heat-affected zones (HAZ), which significantly contribute to material failure [13,14]. On the contrary, ultrashort-pulsed lasers (femtosecond lasers) possess exceptionally high peak power due to their short pulse width. The interaction of femtosecond lasers with metallic materials involves a series of complex physical processes that ultimately lead to changes in the structure of the material. Under femtosecond laser energy radiation, the free electrons in the metal will gain high energy and be in nonequilibrium after multiphoton ionization and avalanche ionization, and they then go through more and more complex processes such as the thermalization of electrons, the formation of a dense plasma, electron–phonon heat transfer, and the subsequent removal of the material. These processes are mainly related to the pulse width and pulse energy density of the femtosecond laser. This leads to significantly less damage compared to long-pulse lasers [15,16].

Many studies have delved into the theory and experimentation of ultrafast laser processing. Tsibidis et al. proposed a new model to predict the damage thresholds of metallic materials [17]. Semaltianos et al. ablated the nickel-based high-temperature alloy C263 using femtosecond laser ablation and obtained its ablation threshold as 0.26$\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$ [18]. Martins et al. investigated the effect of different femtosecond laser parameters on the heat-affected zone of a NI/AL-based RMF. The result showed that the HAZ was considerably small for all the tested parameters, thereby indicating no heat agglomeration [19]. Le et al. investigated the effects of three key parameters, the angle of incidence, FPB–FIB distance, and scanning speed, on the feed laser drilling process. The results showed that an increase in the angle of incidence and FPB–FIB distance led to a decrease in the taper angle, and, conversely, an increase in the scanning speed led to an increase in the taper angle [20]. Sun et al. explored the microhole processing of an In718 thermal barrier coating using a picosecond laser, thereby investigating the correlation between thermal accumulation and process parameters. Their findings emphasized the significance of repetition frequency in thermal accumulation [21]. Liang et al. conducted stainless steel processing through femtosecond laser technology, thus devising a novel model to predict the ablation width of the femtosecond laser. With the increase in laser energy, the laser ablation width and spot radius glow increased [22]. Yu et al. delved into the femtosecond laser processing of single-crystal high-temperature alloys with thermal barrier coatings. They segmented the femtosecond laser spot regions into distinct areas (ablation, melting, laser-induced, and radiation regions), where each region had a different mechanism of action [23]. Some studies have highlighted that alterations in the laser scanning modes and key laser parameters significantly impact the quality of the microvias during femtosecond laser processing. Wang et al. investigated the impact of different femtosecond laser parameters on the open-hole quality in a K24 alloy, thereby concluding that the laser helical scanning mode yielded the highest-quality microholes [24]. Zhang et al. explored the effects of different femtosecond laser parameters on the efficiency and quality results of ablated nickel-based single-crystal high-temperature alloys. The results showed that the ablation rate increased with the increase in laser injection and increased with the decrease in scanning speed [25]. Moreover, Du et al. scrutinized the influence of different parameter combinations on hole making through the femtosecond laser machining of a nickel-based single-crystal DD6 alloy. They optimized these parameters using a genetic algorithm [26].

Based on the preceding analysis, the geometrical accuracy and surface quality results of femtosecond laser-made holes have not been widely and deeply studied. Overcoming microhole defects like recast layers and microcracks remains a challenging endeavor. Therefore, this study primarily delves into examining how different femtosecond laser process parameters (power, defocus, feed, and scanning speed) impact the roundness, taper, and overall quality results of microhole walls. The aim is to offer theoretical optimization for femtosecond laser technology in the hole-making process, specifically for the nickel-based single-crystal high-temperature alloy DD6.

2. Materials and Experimental Methods

2.1. Materials and Experiment Setup

The material used in this study was the second-generation nickel-based superalloy DD6, and all materials were obtained from Beijing Institute of Aeronautical Materials in China. DD6 is widely employed in turbine working blades and guide vanes due to its outstanding overall performance. The elemental composition of DD6 is shown in

Table 1. Elementary composition (wt%) of single-crystal alloy DD6.

Elements	Cr	Al	Mo	Co	W	Ta	Re	Ni
wt%	3.8~4.8	5.2~6.2	1.5~2.5	8.5~9.5	7.0~9.0	6.0~8.5	1.6~2.4	Bal

.

Before laser processing, the specimens were cut into 30 mm × 10 mm × 0.8 mm pieces (Figure 1) using wire-cut electrical discharge machining followed by grinding and polishing using an ultra-precision lapping machine (LAP-1X, Lab, Shanghai, China) Subsequently, the samples were immersed in an alcohol solution for ten minutes before drilling. After drilling, the surface and pore wall morphology results of air film pores were observed using field emission scanning electron microscopy(MIRA3, TESCAN, Brno, Czech Republic).

The schematic of the ultrafast laser processing used in this experiment is depicted in Figure 2. A commercially diode-pumped solid laser (PH1-20, Light Conversion, Vilnius, Lithuania) was used in this work, thus emitting pulse laser with a wavelength of 1030 nm, a pulse width of 290 fs, a maximum power of 20 W, and a repetition frequency of 100 kHz. During the machining process, the workpiece was securely fixed on a 5-axis machining platform via a fixture. The workpiece’s status was continuously monitored through a CCD camera. When workpiece penetration was detected, it indicated the completion of machining, and the process was stopped.

2.2. Experimental Design

Laser drilling typically employs three methods: percussion drilling, ring cutting, and spiral trepanning [27]. Repeated impact drilling involves single-point impulses that abrade the center of the specimen, thereby resulting in poor machining quality and difficulties in controlling hole diameter. Concentric circular drilling can lead to difficulties in removing material from the hole as the feed depth increases. On the other hand, spiral drilling continuously processes microholes by starting from the inside and working downwards, thereby resulting in superior processing quality compared to impact drilling and concentric circle drilling. Therefore, this paper investigated the impact of laser process parameters (laser power, defocus, feed distance, and scanning velocity) on the geometrical accuracy and surface quality results of microholes using helical punching. Each test was conducted thrice, and the experimental program is provided in

Table 2. Experimental scheme for the study of laser parameters on the quality of gas film holes.

NO.	Mean Power/W	Focus Position/mm	Feed Distance/mm	Scanning Velocity/rpm
1	8	0	0.01	1500
2	10	0	0.01	1500
3	12	0	0.01	1500
4	14	0	0.01	1500
5	16	0	0.01	1500
6	12	−0.5	0.01	1500
7	12	0	0.01	1500
8	12	0.5	0.01	1500
9	12	0	0.005	1500
10	12	0	0.01	1500
11	12	0	0.015	1500
12	12	0	0.01	1000
13	12	0	0.01	1500
14	12	0	0.01	2000

.

The surface topography was examined using scanning electron microscopy (TESCAN MIRA3) equipped with EDS. Subsequently, we employed the least squares method and the minimum area method to calculate the diameter, roundness, and taper values. The least squares method involves fitting the measured circle contour of corresponding points to the circumference of a circle in such a way that the sum of the square distances from the center of the circle to these points is minimized. This method also considers the difference in radii between the measured circle contour and two concentric circles, which is indicative of the roundness error, as illustrated in Figure 3. The roundness (∆r) and taper ($\beta$) are defined as follows:

$$∆r=R_1-R_2$$

(1)

where $R_1$ represents the radius of the smallest externally tangent concentric circle, $R_2$ corresponds to the radius of the largest internally tangent concentric circle, and ∆r is the difference between these radii. A smaller roundness value indicates that the shape is closer to an ideal circle. The taper is defined as follows:

$$\beta =\frac{{\tan }^{-1}\left(\frac{D_2-D_1}{2h}\right)\times 360}{\pi }$$

(2)

where $\beta$ is the taper of the air film hole, $D_2$ is the diameter of the air film hole inlet, $D_1$ is the diameter of the air film hole outlet, and $h$ is the depth of the air film hole. In this work, h was 0.8 mm.

3. Results and Discussion

3.1. Effect of Laser Power on Hole Quality

The impact of the laser power on the geometric precision is illustrated in Figure 4. It is evident that, as the laser power increased, the diameters of the inlets and outlets of the microholes also increased. Additionally, the roundness measurements initially decreased and then started to increase, while the taper measurements of the microholes gradually decreased.

The femtosecond laser peak power and ablation radius are related to the mean laser power. The Gaussian distribution of a single-pulse laser is depicted in Figure 5. Notably, the further away from the center of the laser pulse was, the smaller the laser fluence became. The peak fluence, denoted as $F_0$, can be expressed as a function of laser power:

$$F_0=\frac{2P}{\pi {\omega }_0^{2}f}$$

(3)

where P represents the average power of the laser. It becomes apparent that a higher average power results in a higher peak power. The laser ablation diameter, denoted as D, is a function of the peak laser fluence $F_0$:

$$D^2=2{\omega }_0^2\mathrm{l}\mathrm{n}\left(\frac{F_0}{F_{th}}\right)$$

(4)

where $F_{th}$ is the ablation threshold, which is the critical laser fluence at which the material undergoes irreversible damage in the case of the DD6 alloy. In this paper, the ablation threshold of the DD6 alloy was $0.54 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$. The laser focus radius ${\omega }_0$ of the laser was 18 μm. The mean laser powers used in this experiment were 8 W, 10 W, 12 W, 14 W, and 16 W. Therefore, their peak laser fluences were 15.7, 19.6, 23.6, 27.5, and 31.4$\mathrm{J}/{\mathrm{c}\mathrm{m}}^2$, respectively, and their ablation diameters were 46.7, 48.2, 49.5, 50.5 and 51.3$\mathsf{\mu }\mathrm{m}$, respectively, according to Equations (3) and (4). Consequently, it can be inferred that as the average power increases, the peak laser fluence also increases, thereby subsequently enlarging the laser ablation diameter. This leads to an increase in the diameters of the microholes with a higher average power. Furthermore, an increase in the average laser power results in a deeper laser ablation [28], thereby effectively removing material beneath the microholes and increasing the diameters of the holes beneath the microholes. As the effective laser ablation reaches a saturation point, the changes in the microhole aperture become smaller, and the taper decreases. The taper angle is observed to decrease during this taper change.

The surface morphologies of the microholes, observed through field emission scanning electron microscopy, are presented in Figure 6. Notably, defects were evident around the peripheries of the square holes when the mean laser power was 16 W (Figure 7). The process of material removal with a femtosecond laser involves the absorption of energy by electrons, which is then followed by energy transfer to the lattice. When the lattice temperature surpasses the metal’s melting point, thermal melting occurs. Due to the extremely short pulse widths of the femtosecond lasers, the electrons absorb a significant amount of laser energy in a short timeframe. This results in substantial lattice overheating, thereby leading to microstructural alteration explosion. As the mean laser power increased, the laser fluence also increased, thereby accelerating electron thermalization processes and increasing the percentage of microstructural alteration explosions in the material removal. The high-temperature and high-pressure vapor formed by the microstructural alteration explosions of the pore walls, with energetic particles exiting through the tops of the microholes, generated a secondary process affecting the upper holes, thereby ultimately compromising the integrity of the upper pore perimeters (Figure 7a,b). Additionally, the continuous downward movement of the laser processing plane caused some of the molten metal that burst through the lower holes to adhere to the undersides of the microholes (Figure 7c,d), thereby impacting the surface integrity of each microhole.

To further analyze the qualities of the hole walls produced using femtosecond laser drilling, the specimens underwent cutting, grinding, and cleaning. Subsequently, the morphologies of the hole walls were observed using SEM, as are shown in Figure 8. It is evident that the femtosecond laser drilling process generated processing residues attached to the hole wall for each specimen (recast layer). However, this layer was considerably thinner compared to long-pulse laser methods. With an increase in laser power, the laser ablation radius and peak power increased for each microhole, and the proportion of microstructural alteration explosions in the material removal process increased, thereby creating a high-temperature and high-pressure environment in the inner walls of the micropores. Due to the blowing air pressure, the removed material cumulatively burst out below the microholes and damaged the microhole pore walls, thereby leading to the appearance of microcracks (Figure 8d), holes (Figure 8e), and defects in the walls of the microholes. With an increase in laser power, the hole wall defects became more severe.

3.2. Effect of Focus Position on Hole Quality

The influence of the focus position on the geometric precision is depicted in Figure 9. It is evident that the degree of defocusing significantly impacted the taper values of the microholes. Optimal roundness and taper values were achieved at zero defocusing. Positive defocusing resulted in larger inlet and outlet apertures, while negative defocusing yielded the poorest geometrical accuracy results.

When the focal plane was situated below the workpiece surface (negative defocus), the material above the focal plane absorbed the laser energy, thereby resulting in the creation of plasma and molten metal. These plasma and molten metal materials were then expelled from the tops of the microholes, thereby leading to enlargements in the microholes’ entrance apertures and roundness sizes. Simultaneously, the plasma and molten metal obstructed the downward laser radiation, thereby reducing the laser energy beneath the microholes and subsequently reducing the exit apertures of the microholes. On the other hand, when the focal plane was above the workpiece’s surface (positive defocus distance), the laser action time above the microholes increased, thereby leading to an increase in the diameters of the upper holes. Additionally, positive defocusing caused processing residues to be expelled from the upper holes through the microholes, thereby reducing the residues of the microholes. This reduced the refraction phenomenon of the laser and increased the energy flux acting on the undersides of the microholes, thereby ultimately enlarging the diameters of the microholes at both the inlet and outlet.

The SEM-observed morphologies of the microholes are shown in Figure 10. Notably, the position of the focal point did not significantly affect the morphologies of the microholes, and there were no apparent defects around the upper holes. Furthermore, there was always a thin layer of recast layers around the lower holes. Importantly, the position of the focal point did not appear to affect the laser energy density. It can be assumed that the recast layer is caused by the continuous downward movement of the machining plane, thereby causing molten metal and particles to adhere to the undersides of the microholes, which is a phenomenon that is seemingly unrelated to the focal point’s position.

Cross-sections of the hole walls, obtained after SEM observation, are presented in Figure 11. Compared to zero defocusing, both positive and negative defocusing reduced the area of the recast layer attached to each hole wall (Figure 11a,c). This reduction is attributed to the decreased energy density of the laser processing plane, as the laser processing plane does not remain at the same level as the focal plane. Consequently, this reduces the occurrence of phase explosions when maintaining a constant laser pulse width, thereby leading to the removal of more material in the form of plasma and mechanical fragmentation. As a result, the adhesion of molten metal to the particles is reduced.

3.3. Effect of Feed Distance on Hole Quality

The impact of the feed distance on the geometric precision is presented in Figure 12. The results reveal that as the feed distance increased, the diameters of the microholes increased, while the taper angle and roundness values decreased. The surface morphologies of the microholes, observed through SEM, are displayed in Figure 13.

Simultaneously, as the laser energy density increased, more material was removed. With a higher feed rate, the laser must remove more material on the processing plane. However, if the laser energy does not completely remove the material, the laser focus moves downward prematurely, thereby leading to incomplete laser processing. Consequently, the upper hole apertures increase, thereby improving the roundness values of the microholes. The unremoved material adheres to the microhole wall, thus forming burrs and a recast layer. Additionally, the unremoved material obstructs the downward movement of the laser energy, thereby reducing the laser energy density in the bottom hole, hindering material removal from the bottom hole, and reducing the bottom hole’s aperture.

Cross-sections of the hole walls, observed via SEM, are depicted in Figure 14. At a feed rate of 0.015, the laser focus moved downward faster than it removed the material. The incompletely removed material hindered the downward movement of the laser energy, thereby decreasing the energy density in the lower hole. Simultaneously, it impeded the material from being expelled, thereby leading to incomplete ablation of the lower hole material and machining residue sticking to the hole wall (Figure 14d). Therefore, it is important not to select an excessively high feed rate during laser machining.

3.4. Effect of Scanning Velocity on Hole Quality

The influence of the scanning velocity on the geometric precision is presented in Figure 15. The experimental results indicate that the rotational cutting speed had no significant effect on the roundness values of the microholes. However, both the diameters of the microholes and the cone angles increased with higher rotational cutting speeds. The surface morphology results of the microholes, observed through SEM, are displayed in Figure 16.

In laser drilling, the scanning velocity directly affects the spot overlap rate of the pulsed laser. As the scanning velocity increases, the laser spot overlap rate also increases, thereby resulting in a higher effective number of pulses per unit area of material removal. This enhances the efficiency of laser material removal. Zhao et al. demonstrated that the depth and width of laser-machined grooves no longer increased when the effective laser ablation number reached saturation [29]. It can be inferred that once the scanning velocity reaches a certain value, the laser’s efficiency in material removal saturates, and the aperture diameters of the microholes no longer increase. The effect of the scanning velocity on roundness is negligible. Therefore, as the scanning velocity continues to increase after reaching saturation, the roundness should decrease.

Cross-sections of the hole walls, observed through SEM, are shown in Figure 17. No significant defects were observed in the microhole walls. As the scanning speed increased, the area of the recast layer on each hole wall decreased. Material removal gradually saturated with increasing scanning speed during the machining process. The increase in scanning speed had a negligible effect on the geometric accuracy results of the microholes. However, it can reduced the area of the recast layer, thereby ultimately enhancing the machining quality results of the microholes. Hence, selecting a higher scanning velocity can improve machining quality during the process.

3.5. Microstructure

To better analyze the pore wall quality results of the microholes, the chemical compositions of the pore walls were analyzed using EDS. The cross-section of a microhole is shown in Figure 18, and the chemical compositions of region A, region B and region C are displayed in Figure 19 and

Table 3. Chemical composition (w%) of region A using EDS.

Elements	C	O	Ni	Al	Co	W	Total
wt%	8.87	2.77	66.74	5.91	6.91	5.75	96.95
at%	30.01	7.03	46.18	8.90	4.76	1.27	98.15

,

Table 4. Chemical composition (w%) of region B using EDS.

Elements	C	O	Ni	Al	Co	W	Total
wt%	7.40	5.97	62.13	5.09	8.13	5.91	94.63
at%	24.69	14.97	42.44	7.57	4.86	1.29	95.82

and

Table 5. Chemical composition (w%) of region C using EDS.

Elements	C	O	Ni	Al	Co	W	Total
wt%	4.18	3.07	73.37	5.07	7.01	4.69	97.39
at%	16.09	8.89	57.81	8.7	5.5	1.18	98.17

, respectively. In comparison to the composition of region C, regions A and B contained a significant amount of C and O elements in the EDS-layered images(Figure 20). Most of these elements were concentrated on the attachments on the inner walls of the microholes. This phenomenon may result from the instantaneous ionization of the bulk material during femtosecond laser processing, which reacts with an auxiliary gas to form oxides such as aluminum oxide, nickel oxide, chromium oxide, etc. [30,31,32]. These oxides subsequently reattach to the inner walls of the microholes, thereby forming a recast layer.

4. Conclusions

This study investigated the ultrafast laser drilling of the nickel-based single-crystal superalloy DD6, thereby focusing on the effects of the laser process parameters on the geometrical accuracy results of microholes and the morphologies of hole walls. The conclusions can be summarized as follows:

• The microholes processed using ultrafast lasers still had surface defects such as a recast layer, microcracks, and debris. However, these defects can be avoided by changing the laser process parameters.
• Among the studied laser process parameters, the laser power and focus position had the most significant influence on the quality results of the microholes. High laser power produces microstructural alteration explosion and processing defects (debris, microvoids, and microcracks). Different focus positions significantly affect the roundness values of microholes.
• Based on the experiment’s results, it is recommended to use a mean power of 12 W, a focus position of 0, a feed distance of 0.05, and a scanning velocity of 2000 rpm to achieve a higher precision and quality of microholes.