Hole quality and thermal defects in drilled CFRP by nanosecond pulsed laser

doi:10.21203/rs.3.rs-3434912/v1

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Hole quality and thermal defects in drilled CFRP by nanosecond pulsed laser

https://doi.org/10.21203/rs.3.rs-3434912/v1

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published 18 Apr, 2024

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To explore the effect of nanosecond pulse laser drilling on the quality of carbon fiber reinforced polymer (CFRP). The effects of laser parameters such as laser power, scanning speed, pulse width and laser frequency on micro-hole quality and the thermal damage generated during the drilling process, were studied by using the univariate methods and the orthogonal experimental method for drilling micro-holes with a diameter of 0.4 mm using a 1064 nm fiber laser. The results indicate that with the use of pumping at the entrance, the hole diameter and the heat-affected zone (HAZ) width at the entrance and exit increased significantly with the increase of laser power, pulse width and frequency, and the decrease of scanning speed. The variation in HAZ width at the exit was correlated with whether the energy reached the carbon fiber ablation threshold. All the holes were tapered and the hole taper was closely related to the magnitude of the change in the laser parameters on the hole diameter at the entrance and exit. Holes with a taper of about 0.3 were obtained when the parameter combination was 99 W, 50 mm/s, 13 ns and 1500 kHz. Experiments indicate that laser power and pulse width are important factors affecting the quality of micro-hole processing. However, thermal damage defects such as striations, micro-cracks, delamination, voids and surface fiber ‘fish scale’ peeling can occur during nanosecond laser drilling.

Fiber laser

CFRP

Laser drilling

Thermal defects

Hole quality

With the growing demand for lightweight structural products, composite materials have attracted increasing attention. To achieve both lightweight and structure-function integration, carbon fiber reinforced polymer (CFRP) [1] has emerged as a typical representative composite material. CFRP has excellent properties such as high specific strength, low density, strong corrosion resistance, high design capability and good fatigue resistance [2], and is widely used in aerospace, transportation, medical equipment, national defense and military products and other fields [3, 4]. Currently, CFRP products cannot be molded once before it meets people's needs and still needs to be processed by cutting, drilling, and other processing. However, the anisotropy of carbon fiber composites poses great challenges to high-quality processing [5]. Traditional mechanical drilling can easily lead to the formation of defects such as high material delamination, burrs, and cracks at the edge of the hole, causing severe tool wear and increasing processing costs [6, 7].

In contrast, non-traditional machining, electrical discharge machining (EDM) [8] and ultrasonic machining [7] are inefficient and produce more hole defects [9]. The resonance frequency matching problem of ultrasonic machining also needs to be solved [7]. Abrasive water jet machining has low thermal damage, but the machining accuracy is difficult to control, resulting in defects such as high surface roughness and material delamination [10]. In addition, due to the hygroscopic properties of CFRP, the mechanical properties are significantly reduced by abrasive water jet processing [11, 12]. The laser processing technology is a new process with high machining accuracy, high efficiency, no tool wear, non-contact, low cost and easy control [13], which can effectively reduce hole defects in CFRP hole drilling. However, laser processing of CFRP achieves thermal ablation removal of materials by absorbing laser energy. Due to the large difference in thermal properties of carbon fiber and matrix material, it will inevitably cause thermal damage, especially the HAZ, which will greatly reduce the mechanical properties of the material [14, 15].

To address the above issues in the laser processing of CFRP, Herzog et al. [16] compared the effects of pulsed Nd: YAG laser, disk and CO₂ lasers on the cutting quality and static strength of CFRP at different laser powers. The results showed that pulsed lasers could reduce the contact time between the laser and the material. Compared to the other two lasers, a smaller HAZ, higher static strength (1000 MPa) and bending strength (2600 MPa) were obtained. Li et al. [17] investigated the hole quality and tensile strength of CFRP using fiber lasers in continuous and pulsed modes. It was found that the pulsed laser could achieve better processing quality. Ramanujam et al. [18] studied the hole quality of CFRP materials after continuous CO₂ laser processing and found that irregular micro-hole profiles appeared at the entrance and exit under all parameter conditions. Zhou et al. [19] analyzed the physical process of QCW fiber millisecond laser processing of CFRP. They pointed out that hole drilling involves physical processes such as thermal evaporation, melting, plasma impact, mechanical ablation, pyrolysis and carbonization. Sato et al. [20] compared the effects of N₂ and air assistance on the quality of CFRP processed by a 1064 nm pulsed fiber laser. The results showed that N₂ assistance could effectively inhibit HAZ formation and increase the cutting speed. Romoli et al. [21] showed that the epoxy resin hardly absorbs the 1064 nm laser but is absorbed by the carbon fibers. Therefore, under the laser irradiation of this wavelength, it is mainly the heat conduction of the carbon fiber that forms the HAZ. Takahashi et al. [22] analyzed the quality of Nd: YAG laser cutting of CFRP at 1064 nm and 266 nm from theoretical simulation and experiment. The results showed that the 1064 nm laser passes through the resin and heats the carbon fiber strongly. Tao et al. [23] demonstrated that the 1064 nm ultra-short pulse laser drilling of CFRP laminates produces the bowl-bottom effect of the hole taper. This is related to the Gaussian distribution characteristics of the laser beam (taper initial formation stage I), the decrease of the laser average fluence due to the expansion of the spot area and its projection area (taper accumulation stage II), and the secondary processing (bowl-bottom formation stage III ).

The laser drilling process mainly removes the molten material laterally and discharges it axially along the hole depth. In addition to the influence of laser parameter factors, the choice of drilling method also helps to reduce the recast layer and improve the geometric accuracy of laser drilling [24, 25]. Li et al. [26] explored the influence of different scanning strategies on hole drilling, and the results showed that spiral scanning could significantly reduce the width of the HAZ. Wang et al. [27] investigated the effects of nanosecond pulsed laser beam jump directions (outside-in and inside-out) and scanning modes (sequential and cyclical scanning) on the hole sidewall morphology in a spiral scanning mode. The results indicate that the outside-in laser scan path can achieve a flatter hole exit morphology at the bottom. Li et al. [28] reported that the interlaced scanning mode can effectively reduce the heat accumulation effect of adjacent trajectories compared with the default sequential scanning mode. Appropriate scanning spacing can balance the two removal mechanisms of laser ablation and mechanical denudation in laser processing [29], and improve the processing quality and efficiency.

In summary, the pulsed laser can achieve better processing quality; the spiral scanning method of hole drilling is beneficial to the reduction of thermal defects; the carbon fiber absorbs the laser energy at a wavelength of 1064 nm best, and the processing does not consider the resin matrix to absorb the laser energy at this wavelength. Meanwhile, fiber laser has the advantages of high photoelectric conversion efficiency, low cost, and high stability, etc., is widely used in cutting, hole drilling and other industrial fields, is currently a hot spot for the development of industrial applications. However, the influence of laser parameters on the quality of hole-making for nanosecond pulsed fiber laser drilling of multi-orientation fibers (MD CFRP) has not been sufficiently studied, and the analysis of the thermal damage pattern obtained by pulsed laser drilling of CFRP is still lacking.

Therefore, in this work, a 1064 nm nanosecond pulsed fiber laser was used to produce 0.4 mm diameter micro-holes in 1 mm thick CFRP to investigate the corresponding hole-drilling phenomena and patterns. A controlled variable method was used to study the effect of laser processing parameters (laser power, scan speed, pulse width, laser frequency) on the hole quality during the hole drilling process. The response indicators of micro-hole quality were the hole diameter and the heat affected zone at the entrance and exit, and the taper. The L16 orthogonal experiments were designed, and the primary factors of the influence of laser parameters on micro-hole quality were determined from the calculation results. The thermal defects were also analyzed based on the microscopic morphology of the micro-hole. This study contributes to the optimization of laser hole drilling parameters and provides a theoretical reference for the application of 1064 nm nanosecond pulse laser processing CFRP.

2.1 Experimental materials and setup

The CFRP specimen is a laminate composed of carbon fiber and epoxy resin matrix. The laminated structure is a multi-layer 0°/90° braided arrangement. The size of the length × width × thickness is 30 mm × 30 mm × 1 mm, respectively, as shown in Fig. 1a.

The experimental platform is shown in Fig. 1b and consists of a laser, beam splitter, galvanometer, 3D translation stage (movement accuracy of 1 µm), power meter, inspiratory device and computer. The laser is a pulsed nanosecond fiber laser with a wavelength of 1064 nm and an average power of > 100 W. The pulse width is adjustable from 2 ns to 500 ns, the laser frequency is adjustable from 1 kHz to 4000 kHz, and the exit beam diameter is 7 mm. Part of the laser beam is reflected through the beam splitter into the power meter for real-time monitoring of the laser power, while the other part of the laser beam enters the galvanometer and is then irradiated onto the front surface of the CFRP sample through a focusing field mirror lens with a focal length of 200 mm. The CFRP sample is placed on the sample holder of the 3D translation stage and clamped vertically to reduce the amount of carbon fiber debris adhering to the front surface of the workpiece. An inspiratory device is used to pump air onto the front surface of the specimen to filter the smoke and exhaust gases generated during the drilling process to avoid harm to the experimental operators and environmental pollution. The laser parameters used in the experiments are shown in Table 1.

Table 1

Hole-drilling parameters of the fiber laser system in the experiment
Characteristics	Symbols	Values	Units
Wavelength	λ	1064	nm
Average laser power	P_avg	> 100	W
Focal length	f	200	mm
Focused spot diameter	d₀	50	µm
Laser power	P	93,96,99,102,105	W
Pulse width	τ	6,9,13,20,30	ns
Pulse frequency	F	1000,1250,1500,1750,2000	kHz
Scanning speed	V	34,42,50,58,66	mm/s
*P_1 ~ 5 in the table are 93,96,99,102,105 W, respectively; τ_1~5 are 6,9,13,20,30 ns, respectively; F_1 ~ 5 are 1000,1250,1500,1750,2000 kHz, respectively; V_1 ~ 5 are 34,42,50,58,66 mm/s, respectively

2.2 Experimental strategy

This work focuses on the effects of laser parameters such as power, scanning speed, pulse width, and frequency on the entrance and exit hole diameters and the taper, as well as the HAZ. The spiral scanning mode and the scanning sequence from outside to inside shown in Fig. 1c were used in the experiments. The focal spot measured by the beam quality analyzer is shown in Fig. 1d with a diameter of 50 µm (1/e²). Combined with the energy distribution of the beam, the fixed pitch during scanning is 0.02 mm (i.e., a path overlap of 60%).

The influence of the laser parameters on the hole quality was analyzed using a single-factor experimental method. Based on the results of the single-factor experiment, a four-factor, four-level L16 orthogonal design experiment shown in Table 2 was designed to determine the primary and secondary relationships between the four factors of power, scanning speed, pulse width, and frequency on hole quality. The experiment was repeated at least three times for each group of parameters to avoid contingency in the results.

Table 2

Table of factor levels for orthogonal tests
Level	Factors
Level	Laser power P(W)	Scanning speed V(mm/s)	Pulse width τ(ns)	Laser frequency F(kHz)
1	93	42	6	1250
2	96	50	9	1500
3	99	58	13	1750
4	102	66	20	2000

2.3 Measurement/Characterization

Influenced by factors such as the distribution of the laser beam and the thickness of the sample, the entrance and exit diameters of the micro-holes produce a certain diameter difference, which is expressed as a tapered hole. The calculation formula is:

$$t=\frac{{D}_{Entry}-{D}_{Exit}}{T}$$

where t is the hole taper, D_Entry and D_Exit are the entry and exit hole diameters, respectively. T is the thickness of the CFRP. In the experiment, the difference in hole diameter of the micro-holes obtained for adjacent laser parameters for the same single factor variation is defined as the magnitude of the variation ($\varDelta D$), and the calculation equation is as follows:

$${\Delta }{\text{D}}_{Entry}={D}_{Entry}\left({X}_{j}\right)-{D}_{Entry}\left({X}_{i}\right)$$

$${\Delta }{\text{D}}_{Exit}={D}_{Exit}\left({X}_{j}\right)-{D}_{Exit}\left({X}_{i}\right)$$

where $\varDelta {D}_{Entry}$ and $\varDelta {D}_{Exit}$ are the magnitudes of the variation of the entry and exit hole diameters, respectively. X is a single dependent variable parameter, i.e. laser power (P), pulse width (τ), frequency (F) and scanning speed (V). i (i = 1,2,3,4,5) and j (j = 2,3,4,5) are consecutive numbers of variable parameters. Details are given in the accompanying notes to Table 1.

The thermal conductivity in the direction parallel to the carbon fiber (50 W/(m·K)) is higher than that in the vertical direction (5 W/(m·K)) [19], so the HAZ was taken as the object of inspection with the laser scanning direction perpendicular to the carbon fiber, as shown in Fig. 2a. A stereomicroscope was used to observe and measure the hole diameters at the entrance and exit, as well as the corresponding HAZ width. The measurement scheme is shown in Fig. 2. Multiple measurements were taken for each group of data, and the average value was selected for statistical analysis. Scanning electron microscopy (SEM) was used to measure the morphology of the micro-holes and to analyze the thermal damage defects.

3.1 Laser parameters versus hole diameter

Figure 3 and Fig. 4 show the entrance and exit micro-hole morphologies obtained for different laser parameters, respectively. It can be seen that D_Entry is consistently larger than D_Exit for different laser parameters. It is associated with the Gaussian distribution of the laser beam and the fact that the focal plane is maintained at the front surface during hole drilling, leading to a decrease in laser energy density with increasing depth. In addition, the high energy causes evaporation and burning of the material as the laser beam moves along the scan path. The ablative debris produced in the process carries away some of the heat. Meanwhile, the ablation material that has not been removed in time and has accumulated around the hole wall blocks some of the laser energy, creating a shielding effect that reduces the energy absorbed by the material along the depth of the hole.

Figure 3 shows the expansion of the fractured fiber ends and the convexity of the carbon fiber layers around the micro-hole, which is directly related to the significant thermal expansion of the carbon fiber and the superposition of thermal deformation of the individual fiber layers. Meanwhile, the HAZ formed during the laser irradiation and the lack of resin reinforcement of the carbon fibers became loose, which in turn led to phenomena such as carbon fiber pull-out and pick-up in Fig. 3d and h. This phenomenon becomes more pronounced as the laser power, pulse width and frequency increase, and the scanning speed decreases. In addition, the vaporized material is filtered by pumping in the experiment, and the suction of this device may also cause the carbon fibers to be picked up and then reattached to the surface. In contrast, for the rear surface shown in Fig. 4, only the broken carbon fibers were picked up, as shown in Fig. 4c, and no pulling out of the carbon fiber occurred. This may be related to the reduced energy absorbed by the material and the area of influence of the HAZ.

For any combination of laser parameters, all of the micro-holes in Fig. 3 contain the notched profile edges indicated by the arrows in Fig. 3a. This is because the start of the spiral scan path does not overlap with the adjacent spirals, and there is a distance gap between the scan spacing as shown in Fig. 1b, which leads to this defect in the drilling process. The gap is just not as obvious in some images, which is related to the amount of heat accumulation caused by different laser parameters.

Influenced by the reduced energy absorption in the thickness direction of the material, the laser dispersion outside the focal plane, the change in thermal conductivity in the fiber direction and the high processing temperature [18], the micro-holes on the rear surface shown in Fig. 4 exhibit irregular contours and elliptical holes, the circularity is much smaller than that of the front surface. Meanwhile, the energy absorbed at the rear surface of the material is not sufficient to vaporize the resin matrix directly. The temperature of heat transfer through carbon fibers is also lower than the vaporization temperature of the resin. However, it has reached its liquefaction phase transition temperature, so a residue of pyrolysis and melting and then re-curing appears in the area near the micro-holes on the rear surface as shown in Fig. 4o and q.

Figure 5 shows the quantitative relationship between the hole diameter at the entrance and exit of the micro-hole and the laser parameters shown in Fig. 3 and Fig. 4. Both D_Entry and D_Exit become larger as the laser power, pulse width and frequency increase, and as the scanning speed decreases. This is directly related to the material removal rate. As the laser power and pulse width increase, the corresponding increase in laser energy directly leads to an increase in melting and vaporization of the material, which effectively enhances the material removal capability, and in turn, leads to progressively larger hole diameters of the D_Entry and D_Exit. The laser frequency and scanning speed inevitably affect the interaction time between the laser and the CFRP material. As the laser frequency increases and the scanning speed decreases, the time taken to heat the material increases, resulting in more material being removed and an increase in the diameter of the entry and exit holes.

3.2 Laser parameters versus taper

Figure 6a-f show the relationship between the taper and the laser parameters in Fig. 3 and Fig. 4, and the corresponding variation curves of D_Entry and D_Exit. It can be seen that when the laser power, pulse width and laser frequency are changed, the variation range of the exit diameter (ΔD_Exit) is first larger than the variation range of the entry diameter (ΔD_Entry) and then smaller than ΔD_Entry. However, when the scanning speed is changed, the corresponding pattern is exactly the opposite.

According to Eq. (1), the magnitude of the taper is mainly related to the difference in diameter at the entry and exit when the material thickness T is constant. From Fig. 6a-f, it can be seen that the smallest taper is obtained when the power, pulse width and frequency are 99 W, 13 ns and 1500 kHz, respectively, which are 0.296, 0.295 and 0.304. As the power, frequency and pulse width increase, so does the taper. As can be seen from Fig. 6g and h, the taper gradually decreases as the scanning speed increases and levels off at 50 mm/s, which is related to the fact that ΔD_Exit is always greater than ΔD_Entry. After increasing the speed to 50 mm/s, the variation of the entry and exit diameters is close to each other, and finally, a micro-hole with a taper of 0.295 can be obtained. In summary, changing the laser power, pulse width and laser frequency results in the same variation trend for the taper. The opposite trend is obtained by changing the scanning speed.

It can be seen that as the laser power, pulse width and frequency increase to a certain value, the laser energy absorbed by the material on the rear surface fluctuates significantly, the energy utilization of the material removed on the rear surface increases, and ΔD_Entry is smaller than ΔD_Exit. However, as the value of the laser variable parameter continues to increase, the plasma shielding effect of the front surface increases, while the front surface also absorbs more laser energy, resulting in more intense ablation, increased material removal, and a significant increase in D_Entry. The rear surface absorbs relatively little change in energy, material removal is limited, and the trend of increasing D_Exit is relatively stable. This indicates that too much or too little laser power, pulse width and frequency will increase the hole taper. In this experiment, when the laser parameters were combined at 99 W, 50 mm/s, 13 ns, and 1500 kHz, a micro-hole with a taper of about 0.3 was obtained.

3.3 Laser parameters versus HAZ

CFRP consists of carbon fiber and epoxy resin with different components. The thermal properties of the two materials are very different, resulting in CFRP being an anisotropic material. Carbon fibers have a much higher vaporization temperature (3900 K) than epoxy resin (698 K) and have a better thermal conductivity [19]. When the CFRP is irradiated by laser, the material absorbs heat, the epoxy resin reaches the vaporization temperature earlier, the resin matrix is removed first, exposing the unremoved carbon fibers and forming a HAZ.

Figure 7a shows that as the laser power increases, the heat absorbed per unit time on the surface material increases, except for the carbon fiber material in the center of the micro-hole, which is removed, the heat accumulation around the micro-hole also increases, which does not reach the vaporization temperature, but already far exceeds the vaporization temperature of the resin matrix, making its heated vaporization area further expand, which in turn leads to a gradual expansion of the HAZ width at the entrance (HAZ_Entry). When the pulse width is increased, the heating time of the carbon fiber material becomes longer and the cooling time is shorter, which is more likely to cause heat accumulation in the irradiated area, resulting in the HAZ variation shown in Fig. 7b.

Figure 7c and d show that laser frequency and scanning speed have different effects on the HAZ. It is due to the two parameters affecting the laser spot overlap rate. According to the formula:

$$\varphi =\left(1-\frac{V}{d\times F}\right)\times 100\%$$

Where φ, V, d and F are the spot overlap rate, the scanning speed, the spot diameter and the laser frequency, respectively. According to Eq. (4), φ is proportional to the laser frequency and inversely proportional to the scanning speed when the spot diameter is constant. Increasing the frequency or decreasing the scanning speed can effectively reduce the amount of heat accumulation in the processed area of the material, and the area where the resin matrix fades is reduced, which helps to reduce the HAZ. When the pulse width is constant, increasing the laser frequency will decrease the time between adjacent pulses, decrease the cooling time in the processed area of the material, and increase the HAZ_Entry.

The HAZ_Entry versus laser parameters shown in Fig. 7 indicates that the HAZ_Entry increases as the laser power, pulse width and frequency increase, and as the scanning speed decreases. The SEM image in Fig. 8 shows that as these parameters change, it leads to more carbon fiber fracture around the micro-holes, increased fiber tip swelling, significant carbon fiber pull-out, as well as the formation of obvious cracks and striations. Meanwhile, it causes some ablation debris and matrix degradation residues to adhere to the raised carbon fibers, which inevitably affects the mechanical strength and mechanical properties of the CFRP.

Figure 9 shows the relationship between the HAZ width at the exit (HAZ_Exit) and the laser parameters. The HAZ_Exit is larger than the HAZ_Entry at 93 W power, pulse width ≤ 9 ns and scan speed ≥ 58 mm/s. Otherwise, the HAZ_Exit is smaller than the HAZ_Entry. Figure 9a shows that the HAZ_Exit first increases and then decreases with increasing laser power, with the maximum HAZ_Exit (441.163 µm) being obtained at 96 W laser power. When the laser power exceeds 102 W, the HAZ_Exit increases again, so that the minimum HAZ_Exit (377.022 µm) for this single dependent variable is obtained at 102 W. Figure 9b shows that increasing pulse width leads to a decrease and then an increase in the HAZ_Exit, with a minimum HAZ_Exit (325.942 µm) obtained at a pulse width of 13 ns. The increase slows down when the pulse width is greater than 20 ns.

Figure 9c shows that the HAZ_Exit reaches a maximum at the laser frequency of 1500 kHz. As the frequency increases, there is a significant decrease in the HAZ_Exit. When the frequency reaches 2000 kHz, the HAZ_Exit is 312.765 µm. The effect of the scanning speed is similar to that of the pulse width, and Fig. 9d shows that the smallest HAZ (268.366 µm) is obtained at a scanning speed of 50 mm/s.

The variation of HAZ_Exit is mainly related to whether the energy absorbed by the material reaches the carbon fiber ablation condition after changing the laser parameters, while the relationship between each parameter and the energy is consistent as described in section 3.1. When the energy reaches the carbon fiber ablation condition, the energy utilization increases and the energy used for heat transfer dissipation decreases, resulting in a decrease in HAZ_Exit. However, if the absorbed energy is not sufficient to remove the carbon fibers, the heat will evaporate the resin through the carbon fiber heat transfer, increasing the HAZ_Exit. In addition, if the energy density is too high, the epoxy resin matrix will be removed by both direct vaporization and indirect ablation by heat conduction, resulting in an even greater HAZ_Exit.

The range of the HAZ shown in Fig. 7 and Fig. 9 fluctuates considerably. It is because the thermal conductivity of carbon fibers along the radial direction is 10 times higher than in the axial direction [19]. The heat is mainly transmitted along the radial direction and diffuses into the interior of the material, but the total energy input remains constant, forming an elliptical heat-affected zone. The different contact locations of the laser beam with the material and the anisotropy of the carbon fibers, when the laser irradiates the carbon fibers with different weave orientations, lead to a difference in the extent of the HAZ, resulting in a large variation in the HAZ of the micro-holes obtained for the same parameters.

3.4 Micro-hole sidewall morphology and defects

Figure 10 shows the morphology of the micro-hole sidewall obtained at different laser parameters. It can be seen that striations, voids, fiber pull-out, rings, fiber fractures, tip swelling, fiber delamination, epoxy resin coverage, and fragments occur within the micro-holes. It also leads to multi-physical complexities such as the formation of rough surfaces, fiber damage, and interlayer cracks on the sidewall surface.

The striations and voids shown in Fig. 10a appear on the sidewall of the micro-holes for each combination of laser parameters. Negarestani et al. [30] attributed the striations to the unsteady motion of pyrolysis products of the epoxy, such as light gases, various hydrocarbons, and carbon. It was found that the number of striations and voids in Fig. 10f and i increased significantly when the laser frequency was less than 1000 kHz and the scanning speed was greater than 66 mm/s. This is attributed to the low overlap rate between adjacent pulse spots at these laser parameters, resulting in the presence of an incomplete removal area between the materials such that striations are visible. Voids are the thermal defects in the ablation of the resin matrix that resulting in matrix loss [26]. During laser irradiation, less energy per unit of time is used to reach the carbon fiber ablation threshold, more energy is dissipated by heat conduction rather than material removal, and the resin matrix is ablated over a wider temperature range, resulting in an increased number of surface voids. It has been found that the number of voids decreases with increasing hole depth. There are two main reasons for this phenomenon. One is that more laser energy is absorbed near the entrance of the micro-hole, the vaporization temperature of the carbon fiber and resin is inconsistent, and the heat accumulation effect between the unremoved carbon fiber is significant, resulting in more ablation of the epoxy resin matrix. Secondly, the shrinkage of the surface fiber ends increased the formation of voids. Meanwhile, varying degrees of rings in the center of the fiber end are also observed in Fig. 10a and other figures. This is related to the radial thermal melting of the carbon fiber [31]. It is just that the degree of thermal melting varies with different energy parameters and the formation of rings varies.

In addition, loosening occurs in the area near the surface of the micro-hole shown in Fig. 10b, and the outer carbon fibers are easily pulled out. This phenomenon becomes more pronounced as the laser power, pulse width, and frequency increase and the scanning speed decreases. This is related to the thermal deformation of the carbon fiber between the layers and the use of suction at the entrance. The suction pulls the heat out of the hole and carries it away to the entrance surface, thus exacerbating the heat accumulation effect within the entrance region. In addition, Fig. 10c shows that the laser scanning direction removes more material parallel to the carbon fiber direction than perpendicular to it, with more fractured and damaged carbon fibers. This is due to the higher heat conductivity along the carbon fiber direction than in the vertical direction.

As the power, pulse width and frequency increase and the scanning speed decreases, the number of delamination and cracks in Fig. 10d-h increases, and the epoxy resin covering the processing section and wrapping the carbon fiber gradually disappears. This is related to the increase of laser energy and heat accumulation. When the laser frequency reaches 2000 kHz, the processed cross-section shows a rough and uneven area shown in Fig. 10g, and the fiber ends are already slightly swollen. This may be related to thermal damage caused by shortened cooling time or may be caused by incomplete thermal fusion occurring between carbon fibers [31]. At scanning speed below 34 mm/s, significant ‘fish scale’ peel damage and micro-cracks appear on the surface of the carbon fiber as shown in Fig. 10h, where Fig. 10j shows a magnified image of the box in Fig. 10h. The thermal damage defects caused by the laser scanning of the CFRP surface over a long time are the main cause of these phenomena.

Based on the morphological analysis of the micro-hole sidewalls, it can be concluded that when the combination of 99 W, 50 mm/s, 13 ns, and 1500 kHz is used to drill the holes, the carbon fiber arrangement is not altered and leads to less thermal damage and machining defects. In addition, the t and HAZ are smaller for this combination of parameters.

3.5 Orthogonal test results and analysis

Table 3 shows the experimental results obtained from the four-factor, four-level L16 orthogonal design, and the statistical results shown in Table 4 are obtained based on the experimental data in this table. When D_Entry, D_Exit, HAZ_Entry, HAZ_Exit and t are used as indicators to evaluate the hole quality, laser power is the main factor affecting D_Entry and HAZ_Entry. Laser frequency has no significant effect on either indicator. The effect of scanning speed on D_Entry and D_Exit is more pronounced, while the pulse width only had a significant effect on D_Exit, HAZ_Exit and t. It is inconsistent with the conclusion of reference [19] that pulse width has the most significant effect on each quality factor. It may be related to the small range of pulse width in this orthogonal experiment. Meanwhile, Zhou et al. [19] used a pulse width of 0.1 ~ 0.25 ms, which is much larger than the pulse width used in this work. A larger pulse width of the laser inevitably leads to a larger thermal effect. In addition, the laser drilling of CFRP in this work uses outward pumping along the entrance of the sample, which interferes with the direction of heat conduction and affects the quality of holes at the entrance.

In summary, the effect of laser parameters on D_Entry is: laser power > scanning speed > pulse width > laser frequency. The effect of laser parameters on D_Exit is: pulse width > scanning speed > laser power > laser frequency. The effect of laser parameters on HAZ_Entry is: laser power > pulse width > laser frequency > scanning speed. The effect of laser parameters on HAZ_Exit and t is: pulse width > laser power > scanning speed > laser frequency.

Table 3

Orthogonal test results of laser drilling CFRP
No	P (W)	V (mm/s)	τ (ns)	F (kHz)	D_Entry (µm)	D_Exit (µm)	HAZ_Entry (µm)	HAZ_Exit (µm)	t
1	1	1	1	1	399.089	23.844	397.089	109.352	0.375
2	1	2	2	2	395.471	70.334	339.693	317.749	0.325
3	1	3	3	3	394.648	74.990	431.025	271.183	0.320
4	1	4	4	4	393.053	45.882	340.207	222.372	0.347
5	2	1	2	3	401.835	77.305	413.099	318.725	0.325
6	2	2	1	4	392.151	51.852	494.451	156.090	0.340
7	2	3	4	1	374.380	52.343	468.126	346.405	0.322
8	2	4	3	2	383.515	43.447	470.184	144.943	0.340
9	3	1	3	4	378.629	84.316	456.716	429.188	0.294
10	3	2	4	3	385.920	64.818	426.567	341.924	0.321
11	3	3	1	2	390.027	31.793	487.492	140.121	0.358
12	3	4	2	1	381.477	39.931	474.553	222.294	0.342
13	4	1	4	2	388.161	113.963	445.590	520.284	0.274
14	4	2	3	1	383.648	92.678	450.315	507.638	0.291
15	4	3	2	4	383.997	76.126	400.401	368.608	0.308
16	4	4	1	3	374.394	35.314	380.850	237.687	0.339

Table 4

Average response calculated based on orthogonal experimental level design
Levels					Levels
D_Entry					D_Exit
1	395.565	391.928	388.915	384.648	1	53.762	74.857	35.701	52.199
2	387.970	389.297	390.695	389.293	2	56.237	69.920	65.924	64.884
3	384.013	385.763	385.110	389.199	3	55.215	58.8130	73.858	63.107
4	382.550	383.110	385.378	386.957	4	79.520	41.144	69.251	64.544
Delta	13.015	8.819	5.585	4.645	Delta	25.758	33.713	38.157	12.685
Rank	1	2	3	4	Rank	3	2	1	4
HAZ_Entry					HAZ_Exit
1	377.003	428.124	439.970	447.521	1	230.164	344.388	160.813	296.422
2	461.465	427.756	406.936	435.740	2	241.541	330.850	306.844	280.774
3	461.332	446.761	452.060	412.885	3	283.382	281.579	338.238	292.380
4	419.289	416.448	420.122	422.944	4	408.554	206.824	357.746	294.065
Delta	84.462	30.313	45.124	34.635	Delta	178.390	137.564	196.934	15.648
Rank	1	4	2	3	Rank	2	3	1	4
t
1	0.342	0.317	0.353	0.332
2	0.332	0.319	0.327	0.324
3	0.329	0.327	0.311	0.326
4	0.303	0.342	0.316	0.322
Delta	0.039	0.025	0.042	0.010
Rank	2	3	1	4

A drilling study was carried out on the surface of carbon fiber-reinforced resin matrix composites using a 1064 nm nanosecond pulsed fiber laser based on the univariate variables and orthogonal design experiments. The effects of laser parameters such as laser power, pulse width, laser frequency and scanning speed on the processing quality were analyzed. The conclusions are as follows:

(1) The variation of micro-hole diameter and heat-affected zone width is positively proportional to the variation of laser power, pulse width and frequency, inversely proportional to the scanning speed. The hole surface without smoke accumulation at the entrance can be obtained by using the front inspiratory method, except that the taper is larger. The variation of the taper is closely related to the magnitude of the change in the diameter of the entrance and exit holes. When the laser parameters are 99 W, 50 mm/s, 13 ns and 1500 kHz, a hole taper with a minimum of about 0.3 can be obtained.

(2) The variability of heat conduction in different directions of the carbon fiber leads to the formation of an elliptical HAZ profile on the micro-hole surface. The front inspiratory direction significantly increases the width of the entrance heat-affected zone. The variation of the exit heat-affected zone was related to whether the pulse energy was used for material removal under different laser parameters.

(3) The micro-defects that occur in the laser drilling process mainly include striations, voids, delamination, micro-cracks, carbon fiber ‘fish scale’ peeling damage, fiber pull-out, fiber tip swelling, rings, rough surfaces, etc. The formation of thermal damage defects is closely related to the choice of laser parameters. The use of front pumping at the entrance will aggravate the micro-hole surface to produce a protruding layer and fiber pull-out.

(4) When the hole diameter at the entrance and exit, the width of the heat-affected zone at the entrance and exit and the taper are used as indicators to evaluate the drilling quality, the laser power is the main factor affecting the entrance diameter and the heat-affected zone compared with other parameters. The pulse width has a significant effect on the exit hole diameter and the exit heat-affected zone, as well as the taper. Smaller pulse widths have less effect on entrance hole quality. Scanning speed has some effect on the hole diameter, while laser frequency has no significant effect on any of the hole quality evaluation indicators.

Author Contributions

Xinyi Liu: Writing-original draft, Methodology, Writing-review & editing, Data curation, Formal Analysis, Conceptualization. Sen Yang: Conceptualization, Methodology, Supervision, Formal Analysis. Youdong Gao: Validation, Methodology, Data curation, Visualization, Software. Man Xu: Validation, Conceptualization, Supervision. Yu Wan: Investigation, Validation, Data curation. Meng Wu: Resources, Investigation, Supervision. Lin Li: Investigation, Validation, Data curation. Chenzhuo Wang: Data curation, Conceptualization, Validation. Mian Zhong: Writing-review & editing, Validation, Methodology, Resources. Biyi Wang: Conceptualization, Resources, Validation. Yong Jiang: Writing-review & editing, Project administration, Funding acquisition, Methodology, Supervision.

Funding

This work was supported by National Natural Science Foundation of China (11972313), Fund Project of Key Laboratory of Electro-Optical Information Control and Security Technology, China (2022JCJQLB055007).

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent for publication Not applicable.

Conflict of interest The authors declare no competing interests.

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published 18 Apr, 2024

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Editorial decision: Major Revisions Needed
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Hole quality and thermal defects in drilled CFRP by nanosecond pulsed laser

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Abstract

Figures

1 Introduction

2 Experimental Details

2.1 Experimental materials and setup

2.2 Experimental strategy

2.3 Measurement/Characterization

3 Results and Discussion

3.1 Laser parameters versus hole diameter

3.2 Laser parameters versus taper

3.3 Laser parameters versus HAZ

3.4 Micro-hole sidewall morphology and defects

3.5 Orthogonal test results and analysis

4 Conclusions

Declarations

References

Status:

Journal Publication

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