Investigation on cutting of CFRP composite by nanosecond short-pulsed laser with rotary drilling method

2.1 CFRP composite

The CFRP composite used in the experiment was composed of carbon fibers and epoxy resin, with a volume ratio of 62% for carbon fibers and 38% for epoxy resin. The epoxy resin is Glycidyl end-capped poly(bisphenol A-co-epichlorohydrin), while the carbon fibers are continuous and have a diameter of approximately 7 µm, as shown in Figure 1. The CFRP specimens have dimensions of 500 mm × 250 mm × 2 mm, comprising ten layers in total, with each layer being 0.2 mm thick. The CFRP plate is woven in a two-dimensional style. Table 1 presents the key physical parameters of the CFRP material utilized in the study.

Figure 1

SEM observations of CFRP composite: (a) Morphology of carbon fibers array and (b) detailed feature of carbon interface.

2.2 Laser processing

In the present experiment, the nanosecond short-pulsed laser processing system was applied to drill the CFRP composite laminate. Taking full use of the instantaneously high energy, the short-pulsed laser could vaporize the carbon fiber and epoxy resin periodically, but the short pulse also restrained the overheat of local region. Such a design would utilize its advantages of high-density energy and micro-region ablation. As shown in Figure 2(a), the laser processing system mainly comprised the nanosecond laser (Cypress-532-35B), four-axis motion system, scanning galvanometer, controlling platform, laser beam delivery unit, gas blowing, and dust remover. The nanosecond laser had maximum power of 35 W, pulse duration of 150 ns, repetition frequency ranged between 50 and 500 kHz, and beam circularity exceeding 90%.

Figure 2

Laser processing system and route: (a) Nanosecond short-pulsed laser processing system and (b) laser rotary drilling route for hole fabrication.

Considering the heating effect from sequential laser ablation, the scanning path and speed were adjustable through a scanning galvanometer system. The laser beam was regulated by the scanning galvanometer to achieve rapid rotating during its scanning forward, as shown in Figure 2(b). With the help of scanning galvanometer, the laser beam rotated and cut along the outer contour of shape. During this process, the laser beam rotated rapidly with the radius of r = 0.5 mm, step size of d = 0.5 mm, and cutting aperture of r = 2.5 mm. The laser beam rotated around the circumference of the hole, which was similar to the mechanical drilling tool. Such a laser rotary drilling could realize the cutting with less heat concentration in specific region. Moreover, the rotation enhanced the slit width and increased processing efficiency. The shapes for laser processing were square, triangle, circle, and regular hexagon. For the circle, its diameter ranged from 0.2 to 5 mm.

To explore the impact of various scanning speeds and laser powers on the quality of CFRP laser drilling, the orthogonal experiment was designed. The laser repetition frequency and pulse width were set as 60 kHz and 150 ns, respectively. While the laser power (P) was changed as 21, 24.5, 28, and 31.5 W. The scanning speed (V) was set as 600, 800, 1,000, and 1,200 mm/s. Then, 16 experimental groups could be established, as shown in Table 2. During the laser drilling, nitrogen gas, with purity of around 99%, was employed as a protective gas at an approximate flow rate of 8 L/min. The nitrogen gas helped the cooling of laser processed CFRP composite and minimized the HAZ. it also facilitated debris removal and enhanced hole quality.

2.3 Characterization

The laser processed CFRP composite samples were ultrasonically cleaned in ethyl alcohol to remove residual debris. The surface morphology was analyzed by using a Keyence VK-X200 laser scanning confocal microscope (LSCM). The size and diameter of the hole after drilling were measured using an MV5000 metallographic microscope. Based on the experience, HAZ was observed and analyzed. The morphology and microstructure of the processed hole wall was characterized by Phenom Pro scanning electron microscopy (SEM). The tensile test was performed on a UTM4304 universal testing machine in accordance with the ASTM D3039/D3039M, as shown in Figure 3(a). The test temperature is 27°C and tensile rate is 0.1 mm/s. The dimensions of the tensile specimens were 200 mm in length, 36 mm in width, and 2 mm in thickness. The specimens were divided into two groups. One group has a processed hole with a diameter of 6 mm in center, while the other group has no change, as shown in Figure 3(b).

Figure 3

Tensile properties test for CFRP with and without laser processing: (a) Tensile testing machine and (b) CFRP tensile specimen with processed hole in the center.

During laser drilling, the continuous irradiation on CFRP composite resulted in high temperatures, which induces the thermal expansion. Comparatively, the bottom would absorb more heat and experience higher temperature, which induced the gradually enhanced thermal expansion. As a consequence, it affected the dimensions of the laser processed hole from surface to bottom, resulting in a tapered shape, as illustrated in Figure 4. Actually, the tapered shape would influence the subsequent application and the taper angle was set as the main factor. The calculation for taper angle could be expressed by the following formula:

Figure 4

Schematic representation of taper angle in a hole drilled in CFRP.

D 1 represented the entrance diameter of the hole, D 2 represented the exit diameter of the hole, and h represented the thickness of the hole in CFRP.

3.1 Processing accuracy and hole taper of CFRP laser drilling

Based on the nanosecond short-pulsed laser and rotary drilling technique, the holes with different sizes were processed in the CFRP composite plate. The parameters exhibited in Table 2 are utilized to process the hole in CFRP composite. Generally, it takes approximately 90 s to finish a single hole and the holes display circular shape without evident burrs, as shown in Figure 5(a). Further observation on the hole wall reveals its smooth and flat morphology. To study the laser parameter effect, the diameters of hole entrance and exit are tested. The typical diameter of hole entrance is 5,035 μm, while that of hole exit is 4,984 μm, as shown in Figure 5(b) and (c). This difference leads to a machining precision of approximately 35 μm, but it already well solves the tapered phenomenon in laser drilling. With the change in the laser scanning path, the rotary drilling technique also could realize the fabrication of shape such as square, regular hexagon, and triangle, as shown in Figure 5(d) and (e). In addition, the holes processed by this method range from 0.1 to 50 mm.

Figure 5

Macroscopic feature of the laser processed CFRP plate: (a) Morphology of laser processed holes; (b) tested diameter of typical hole entrance; (c) tested diameter of typical hole exit; (d) different shapes processed by laser in CFRP; and (e) the laser processed shapes in CFRP.

Due to the thermal expansion, the laser drilled hole always has the taper angle, which influences the precision. To evaluate the influence of laser parameters on hole dimension, the diameters of hole entrance and exit are measured. As shown in Figure 6(a), the diameters of laser drilled holes exhibit a fluctuated variation with the increase in laser scanning speed and laser power. When the laser scanning speed is set, the diameter of most holes’ entrance increases first and then decreases with increased laser power. The hole drilled at laser scanning speed of 1,200 mm/s exhibits the different variation that decreases first and then increases again. The diameters of hole entrances all have a higher diameter than target value of 5 mm, and the hole drilled at laser scanning speed of 1,000 mm/s and laser power of 28 W has the highest diameter of hole entrance (5.025), while the hole drilled at laser scanning speed of 1,200 mm/s and laser power of 28 W has the lowest diameter of hole entrance (5.008). The deviation of hole entrance diameter from target value is less than 2.5%. Such a diameter variation should be mainly attributed to different absorbed energy by CFRP. Generally, the increasing of laser energy would benefit the ablation of CFRP, which increases the hole diameter. When the laser energy increases further, it would increase the depth but exerts little effect on radial direction. The increasing of laser scanning speed could contribute to the homogeneous energy absorption, which increases laser ablation along radial direction and benefits diameter. When laser scanning speed exceeds a certain value, it would decrease the laser ablation rate and diameter. Therefore, the diameter of hole entrance almost increases with the increased laser scanning speed, except the hole drilled at laser power of 31.5 W and laser scanning speed of 1,200 mm/s.

Figure 6

The variation in laser processed hole diameter with scanning speed and laser power: (a) The diameter of top hole and (b) The diameter of bottom hole.

Differently, the diameter of hole exit demonstrates a diversified tendency and they almost have no relationship with laser parameter, as shown in Figure 6(b). When the laser scanning speed is 600 mm/s, the diameter of hole exit increases from 4.994 mm at laser power of 21 W to 5.007 mm at laser power of 28 W and then decreases to 5.001 mm at laser power of 31.5 W. When the laser scanning speed is 800 mm/s, the diameter of hole exit fluctuates from 5.006 to 5.009 mm. When the laser scanning speed is 1,000 mm/s, the diameter of hole exit decreases from 4.998 to 4.992 mm. When the laser scanning speed is 1,200 mm/s, the diameter of hole exit obtains its highest value of 5.006 mm at laser power of 21 W and lowest value of 4.994 mm at laser power of 24.5 W. Exactly, the hole exit diameter ranges from 4.992 to 5.009, and the deviation from target value is less than 0.9%.

Based on the measured diameter of hole entrance and exit, the hole taper angle could be calculated and corresponding results are shown in Figure 7. It can be seen that the hole taper angle exhibits a diversified changing with the laser scanning speed and power. When the laser scanning speed is set as 600 mm/s, the hole taper angle exhibits decreasing tendency with increased power, i.e., the value decreases from 0.3° to 0.18°. If the laser scanning speed increases to 800 mm/s, the increased power increases hole taper angle from about 0.2° to 0.27° and then drops to 0.17°. For the laser processing with scanning speed of 1,000 mm/s, the increased power enhances the hole taper angle from 0.28° to 0.43° and then fluctuates a little. When the laser scanning speed increases to 1,200 mm/s, the relative low power obtains the higher hole taper angle of about 0.3°, but the increased power leads to the drop of value to 0.13° and then jump to 0.26°. In general, the hole processed with laser scanning speed of 1,000 mm/s and power of 24.5 W obtains the highest hole taper angle of 0.43°. Such a phenomenon indicates that the relative low laser scanning speed and high power would be more beneficial to decreasing the hole taper angle. Generally, the tapered hole should be ascribed to the thermal dissipation effect. In the entrance position, the laser energy is almost absorbed by CFRP. However, with the increase in the depth of drilled hole, some laser energy will be absorbed by the hole wall and transmitted, which decreases the processing efficiency. Therefore, the hole diameter decreases a little with the hole depth and produces the tapered morphology.

Figure 7

Variation in hole taper with the laser scanning speed and power.

3.2 Surface morphology of laser processed CFRP and HAZ

During laser drilling, most of the laser energy is utilized to vaporize CFRP composite, but some laser energy heats the composite, which influences the surface morphology and adjacent region. According to the measurement of hole dimension, it can be summarized that the present laser parameters exert little influence on the hole processing. In general, the high laser scanning speed and power would benefit the processing efficiency. Considering the highest hole taper angle at laser speed of 1,000 mm/s and power 28 W, the processed CFRP was chosen for further studying. The surface morphology of typical laser drill hole wall was observed by SEM and the results are shown in Figure 8. Due to the rapid ablation by nanosecond short-pulsed laser, the carbon fiber and epoxy resin have been well vaporized, which leads to the smooth surface, as shown in Figure 8(a). However, the differed vaporization temperatures of carbon fiber and epoxy resin result in more vaporized epoxy resin, which produces some protruded carbon fibers. Especially for the carbon fibers cut along radial direction, such a feature becomes more prominent. While for the carbon fibers cut along axial direction, they exhibit the partly ablated feature. The enlarged image on carbon fibers with axial direction demonstrates that the laser ablated fibers have flat surface, as shown in Figure 8(b). This indicates that the nanosecond short-pulsed laser with rotary method could realize the precision cutting. The debris also could be observed on the processed surface, which should be the remelted slags of CFRP. Certainly, there are still some carbon fibers delaminated integrally, which leaves vacant slot. The observation on the interface of intersected carbon fibers shows that more epoxy resin matrix vaporize, leaving gap between the carbon fibers, as shown in Figure 8(c). The observation on the carbon fibers with radial direction exhibits relative flat cutting surface and some gaps exist among some carbon fibers, as shown in Figure 8(d). Such a phenomenon implies that all the carbon fibers with different directions could be well cut by the nanosecond short-pulsed laser with rotary drilling method.

Figure 8

Typical microstructure of the laser processed CFRP at a laser power of 28 W and laser scanning speed of 1,000 mm/s: (a) Morphology of laser processed CFRP on hole wall; (b) morphology of laser processed carbon fibers along axial orientation; (c) morphology of laser processed carbon fibers with mixed orientation; and (d) morphology of laser processed carbon fibers along radial orientation.

The typical HAZ adjacent to laser processed hole in CFRP is shown in Figure 9. Clearly, there is some difference between hole entrance and exit. Based on the LSCM observation, the HAZ could be observed on the top surface of laser processed hole, as shown in Figure 9(a) and (b). The width of HAZ layer is approximately 3.8 μm along the hole edge, which should be primarily attributed to the heating effect caused by laser irradiation. Initially, the heat is transferred into CFRP via thermal conduction. Due to the different thermal conductivity, the heat could be dissipated faster in carbon fibers than that in epoxy resin. While the great difference in thermal stability leads to the carbonization of epoxy resin in certain scope and formation of HAZ. Due to the heat dissipation, the bottom of laser processed hole exhibits less heat affected phenomenon, as shown in Figure 9(c) and (d). Such result should be mainly ascribed to the changed structure of laser ablation region. For the hole entrance, the laser irradiation is mainly absorbed by the surface layer, which causes the thermal transmission and temperature rising in adjacent region. For the inner region, the higher thermal conductivity of carbon fibers contributes so much. Thus, the hole wall would dissipate most of the transmitted heat, which restrains the thermal transmission downward. Consequently, the hole exit region has low temperature and almost no HAZ.

Figure 9

LSCM observation on interface of the laser processed hole at laser power of 28 W and laser scanning speed of 1,000 mm/s: (a) Low-magnification image showing top hole margin; (b) high-magnification image showing HAZ; (c) low-magnification image showing bottom hole margin; and (d) high-magnification image showing HAZ.

3.3 Mechanical properties of CFRP after laser rotary cutting

To analyze the effect of laser processed hole on CFRP composite, the tensile test was performed, and the results are shown in Figure 10. The CFRP composite plate was laser drilled with the typical parameters of laser power of 28 W and laser scanning speed of 1,000 mm/s. The typical stress load–displacement curves of tensile test for CFRP composite specimens with and without laser drilled holes are shown in Figure 10(a). Clearly, the CFRP composite specimens with and without laser drilled hole are almost fractured without any plastic deformation. That should be mainly ascribed to the low ductility of carbon fibers. The inset image shows morphology of fractured specimens. It can be seen that the CFRP composite specimens without laser drilled hole mainly fracture at the clamping region. Such a fracture phenomenon indicates that the vertical stress action would induce the local stress concentration and promote the initiation of microcrack. On the contrary, the CFRP composite specimens with laser drilled hole mainly fracture at the hole region. This implies that the decreased stress loading area would result in the crack initiation locally. The detailed tensile properties are given in Figure 10(b). The fracture load of CFRP composite without laser drilling is 32.15 kN, with a corresponding fracture stress of 803.66 MPa and a displacement of 5.94 mm. The fracture load of CFRP composite with laser drilling is 20.3 kN, with a corresponding fracture stress of 732.97 MPa and a displacement of 3.29 mm. Clearly, the laser drilling decreases the fracture strength of the CFRP composite by about 9% and the displacement by 44.6%. Such a decreasing should be ascribed to the damage on the continuous carbon fibers. According to the previous research works [22,23,24,25], the laser processing would mainly influences the processed surface and adjacent matrix. Except the HAZ, the rapid heating and cooling would lead to phase transformation and interfacial stress. The laser drilling on CFRP composite would produce a high-temperature environment on the hole wall, which causes the interface cracking. Such a damage is detrimental to the structural stability of the CFRP composite, as it can serve as the cracking initiation during loading. Generally speaking, the corner in the laser drilled shape plays an important role, because of higher stress concentration here. That would decrease mechanical properties of the CFRP composite with such shape. In comparison, the circular shape is the optimal choice for CFRP composites due to its smooth transitional surface. Therefore, the decreasing ratio of tensile strength of the CFRP composite with laser drilled hole is relatively acceptable [26].

Figure 10

Tensile test of the CFRP specimens with and without laser processed hole in the center (laser power of 28 W and laser scanning speed of 1,000 mm/s): (a) Typical tensile curves (inset image showing the morphology of failed CFRP); and (b) tensile properties.

3.4 Surface roughness after laser processing

The surface roughness of the CFRP composite processed by typical laser parameter was characterized by LSCM, and the results are shown in Figure 11. For the CFRP composite processed by laser scanning speed of 600 mm/s and laser power of 28 W, the surface displays a characteristic morphology of grooves and ridges, as shown in Figure 11(a). Moreover, there is slope between ridge and groove but the height of ridge differs greatly, which is caused by the different remelting and vaporization temperatures of carbon fiber and epoxy resin. Comparatively, the epoxy resin is easier to be ablated. The high laser power is beneficial to ablation of CFRP, while the low laser scanning speed would reinforce the ablation effect further. The overlapped effects lead to more ablation of epoxy resin and cause the highest roughness of 4.266 μm. For the CFRP composite processed by laser scanning speed of 800 mm/s and laser power of 28 W, it exhibits similar groove and ridge morphology, but their heights differ significantly, as shown in Figure 11(b). Compared with the CFRP composite processed with lower laser scanning speed, the width of groove increases a little. Such a morphology also confirms the above deduction that the increased laser scanning speed benefit the relative homogeneous ablation, because of its restrained local overheating. Thus, the surface roughness of CFRP composite processed by laser scanning speed of 800 mm/s and laser power of 28 W decreases a little to 3.74 μm. When the laser parameters change to laser scanning speed of 1,000 mm/s and laser power of 31.5 W, the surface morphology of CFRP composite changes obviously, as shown in Figure 11(c). Although ridges and grooves remain the predominant features, there is a noticeable slope transitioning to the groove bottom. Additionally, the width of the groove decreases further and some grooves are covered by slopes. That implies the simultaneously increased laser scanning speed and laser power could realize the rapid ablation of carbon fibers and epoxy resin, which well contributes to surface smoothing. Therefore, the CFRP composite processed at laser scanning speed of 1,000 mm/s and laser power of 31.5 W obtains the lowest surface roughness of 2.925 μm. When the laser parameters are adjusted to laser scanning speed of 1,200 mm/s and laser power of 31.5 W, the surface morphology reverts back to typical groove and ridge morphology, as shown in Figure 11(d). Clearly, the ridges are sharp with small slope and the width of groove is increased again. The regular and homogeneous ridges indicate the laser ablation is highly consistent, which benefits the surface quality. Then, the CFRP processed by laser scanning speed of 1,200 mm/s and laser power of 31.5 W obtains the surface roughness of 3.108 μm.

Figure 11

Surface roughness analyses on the CFRP with different laser processing parameters: (a) laser scanning speed of 600 mm/s and laser power of 28 W; (b) laser scanning speed of 800 mm/s and laser power of 28 W; (c) laser scanning speed of 1,000 mm/s and laser power of 31.5 W; and (d) laser scanning speed of 1,200 mm/s and laser power of 31.5 W.

The surface roughness of the CFRP composite with different laser processing parameters are shown in Figure 12. Obviously, the laser scanning speed and power both exert influence on the surface roughness. When the laser scanning speed is set at 600 mm/s, the surface roughness increases with higher laser power, ranging from 3.6 to 4.25 μm. If the laser scanning speed is increased to 800 mm/s, the surface roughness tends to decrease as the laser power increases, with values ranging from 4.0 to 3.75 μm. When the laser scanning speed is increased to 1,000 mm/s, the increased power promotes the increase in surface roughness first from 3.3 to 3.7 μm, but the surface roughness drops to 3.0 μm when power is 31.5 W. When the laser scanning speed increases to 1,200 mm/s, the surface roughness mainly exhibits decreasing tendency with value ranging from 4.1 to 3.2 μm.

Figure 12

Variation in surface roughness with the laser scanning speed and power.

According to the recent research [27,28,29,30], the quality of laser processed CFRP is mainly influenced by the input energy, heat dissipation, scanning speed, pulse frequency, etc. Due to the greatly differed physical properties of carbon fiber and epoxy resin, the laser ablation of CFRP would demonstrate diversified behavior. For the epoxy resin, it would experience more rapid ablation, because of its low melting and vaporization temperatures. Comparatively, the carbon fiber needs more heat to vaporize, which causes the non-synchronous ablation in CFRP composite. Then, the instantaneous high-density laser energy could play an important role, because it could vaporize the carbon fibers and epoxy resin simultaneously with less heat dissipation [31,32]. However, the laser power and laser scanning speed also exert influence on the processed surface. The increased laser power would contribute to the ablation efficiency, while the increased laser scanning speed demonstrates different effect. When the laser power is sufficient to ablate CFRP composite, the increased laser scanning speed would accelerate the processing efficiency and quality. If the laser power is insufficient, the laser scanning speed should be decreased to meet the requirement of surface morphology. In the present research, the laser power between 21 and 31.5 W and laser scanning speed between 600 and 1,200 mm/s could well realize the drilling of hole in CFRP composite. Comparatively, the laser scanning speed between 800 and 1,000 mm/s and laser power around 28 W would be the optimal choice, which could realize the best balance between surface quality and efficiency.

Actually, the industrial application scenario of CFRP should be fully taken into account for the design of laser processing parameters. As demonstrated above, the different laser processing parameters could produce surfaces with some difference, which might exert diversified influence on the mechanical properties. Especially, the damage generated in the processed surface layer would decrease the strength, because it acts as cracking source. The delamination between carbon fiber and epoxy resin would accelerate the failure of CFRP during the long-term service [33]. Moreover, the laser irradiation could lead to the degradation of epoxy resin. If such reaction occurs in the layer adjacent to the surface, voids would be generated, which is difficult to be observed by conventional method [34,35,36]. However, the void defects would induce the decreasing of peak strength, which is detrimental to service life. If the CFRP component is used in aircraft or automobile, the fatigue properties should be a primary consideration. If the CFRP component is used in architecture or sport application, the strength should be carefully evaluated. To achieve the specific properties, the reasonable laser processing parameters should be designed. In addition, the appropriate detection method should be applied to evaluate the quality of the processed CFRP. In general, the CFRP processing strategy, including specific laser processing procedure and detection technology, would be the future research topic, which helps to improve the efficiency and effectiveness. That would promote the development and application of CFRP in more industrial fields.