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Abstract
Five-axis laser scanning technology is an effective drilling method for special-shaped holes. Due to a gap in laser angle-of-incidence (AOI) control within a large scanning field, current technologies are challenging for fabricating large-size holes or special-shaped hole arrays. In this paper, a large scanning field five-axis laser concurrent drilling system was proposed. The laser AOI was independently controlled using two pairs of synchronous deflection mirrors. The laser control deviations under a large scanning field were investigated systematically by simulation and experiment. By establishing a complete correction method, the laser AOI control within a scanning field diameter of up to 35 mm was achieved. A series of special-shaped holes were fabricated concurrently on a 3.6 mm thick glass fiber reinforced plastic (GFRP), verifying that the AOI can be controlled by the five-axis laser scanning system. Our work provides a novel method to increase the scanning field of the five-axis laser scanning technology, expanding the application scope of the five-axis laser processing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Holes are widely used in industry. Small-size holes include injection nozzles in engines [1,2], cooling holes on turbine blades [3,4], etc. For fiber composite components, large-size rivet holes or screw holes are necessary [5,6]. Laser, as an effective tool, has been used for hole drilling for a long time. However, traditional laser processing technology, such as two/three-axis galvanometer drilling, cannot control the angle-of-incidence (AOI) of the conical-focused laser beam [7–9]. With the processing depth increase, the laser beam will be blocked by the sample surface and sidewall, which will cause the inevitable sidewall taper [10–14]. The drilling quality and precision will be affected. Special-shaped holes like cylindrical holes or inverted cone holes also could hardly be processed.
Several methods of laser AOI control have been proposed in response to the above challenges [15–17]. The five-axis laser scanning technology is one of the most effective methods. The ‘'five-axis'‘ include x, y, and z coordinates of the laser focus in 3D space and two projection angles (α and β) of the laser beam. Angles α and β jointly described the laser AOI. SCANLAB GmbH presented a five-axis micromachining system consisting of optical lenses and four deflectable mirrors [18]. The five-axis laser coordinates were jointly controlled by the deflection angles of the mirrors. The interaction between laser coordinates increased the control difficulty. The laser AOI was only controlled near the center of the scanning field, and the maximum scanning field was less than 5 mm in diameter [19,20]. ARGES GmbH proposed an independent control method for the five-axis laser coordinates [21]. Two deflectable transmissive parallel plates were adopted to move the laser beam parallel to the optical axis to control the laser AOI. Then, a 3D laser scanning was achieved by a three-axis galvanometer. The five-axis coordinates of the scanning laser did not interfere with each other, reducing the control difficulty. However, the scanning field was limited by the gap in the correction of the laser AOI deviation, and the maximum scanning field was less than 3 mm in diameter [22].
The five-axis laser scanning technology with a small scanning field solved the problem of small-size special-shaped holes. However, the laser drilling capability for special-shaped holes in a large scanning field is still limited. Large-size holes without sidewall taper cannot be processed [14,23]. The processing of special-shaped hole arrays within a large range [4,24] must be combined with a moving stage, restricting processing efficiency. Therefore, there is an urgent need to improve the scanning field of the five-axis laser scanning technology.
In this paper, a large scanning field five-axis laser concurrent drilling system was proposed. The laser AOI was independently controlled using two pairs of synchronous deflection mirrors. The laser control deviations under a large scanning field were systematically investigated by simulation and experiment. The laser AOI control within a scanning field diameter of up to 35 mm was achieved by establishing a complete correction method. A series of special-shaped holes were fabricated concurrently on a 3.6 mm thick glass fiber reinforced plastic (GFRP), verifying the laser AOI control capability. Our team has applied for a patent for this technology, and the authorized patent number is CN113319425B [25].
2. Optical principle of the five-axis laser drilling system
The optical principle of the laser AOI control is shown in Fig. 1(a). A parallel off-axis incident laser beam will be focused by the convex lens F at point O on the optical axis, and the focal length of F is f. The focused laser beam will form an angle α with the optical axis. The angle α is determined by the beam off-axis amount (ΔS) and off-axis direction [26]. According to the above principle, two parallel and synchronously deflectable mirrors (G0 and G1) can be used to control the angle α. The reflecting surfaces of G0 and G1 are opposite, parallel, and placed at 45° to the optical axis. The distance between the center of G0 and G1 is d. When G0 and G1 are synchronously deflected clockwise by θ, the laser beam will move upward parallel to the optical axis by ΔS. After passing through F, the focused laser beam will form an angle α with the optical axis. Similarly, when G0 and G1 are synchronously deflected counterclockwise by θ, the laser beam moves downward parallel to the optical axis. The center of the focused laser beam will form an angle - α with the optical axis. Therefore, the laser AOI can be controlled by the deflect angle and direction of G0 and G1.
Fig. 1. Schematic diagram of the optical principle of the five-axis laser drilling system: (a) principle of the laser AOI control, and (b) optical structure of the five-axis laser drilling system.
The optical structure of the five-axis laser drilling system is shown in Fig. 1(b). Two groups of orthogonal mirrors (Module I) were placed in front of a three-axis galvanometer (Module II and III) to control the laser AOI, where the angle α was controlled by Gα1 and Gα2, and the angle β was controlled by Gβ1 and Gβ2. Module II consisted of a concave lens F1 and a convex lens F2, and F1 could move back and forth along the X-axis driven by the voice coil motor. There were three functions of Module II: firstly, control the z coordinate of the laser focus through the movement of F1. Secondly, enlarge the beam waist radius for a smaller focus spot size. Third, enlarge the off-axis mount of the laser beam. A more extensive laser AOI adjustment range could be achieved at a smaller deflection angle of mirrors in the AOI controller, thereby improving the dynamic response performance of the drilling system. Module III consisted of deflectable mirrors Gx and Gy and a telecentric F-theta lens (F3), which could control the x and y coordinates of the laser focus. The laser AOI control example is shown in Fig. 1(b). When Gβ1 and Gβ2 were synchronously deflected counterclockwise along Y-axis by θβ and Gα1 and Gα2 were synchronously deflected counterclockwise along Z-axis by θα, the laser beam outputted from the point O1 of F2. Then, it was focused at point O by F3 through point O2. The focused laser beam formed an angle with the Z-axis, and the angle could be projected on the XOZ and YOZ planes into the angles α and β. With this optical structure, the five-axis coordinates (x, y, z, α, β) were controlled independently. The adjustment of AOI would not affect the three-axis laser scanning, reducing the control difficulty for a large scanning field.
3. Experiments
3.1 Experimental equipment
The experimental setup used to test the five-axis laser drilling system is shown in Fig. 2. A femtosecond laser (Axinite IR-30, Beilin) with a 1.26 mm beam diameter was adopted for the experiment. The pulse width was 436 fs, and the wavelength was 1030 nm. After passing through a 1.5 × beam expander, the linearly polarized laser was converted into round polarization by the quarter-wave plate, then transmitted into the five-axis laser drilling system. Gx and Gy were mirrors of a two-axis galvanometer (IntelliSCAN III 30, SCANLAB), and F3 was a telecentric F-theta lens (NJSL-591, Wavelength OE). Glass with an indium tin oxide (ITO) film was set as the tested sample, which was fixed on the XYZ stage. The experimental laser energy was 37 µJ, the frequency was 10 kHz, and the scanning speed was 10 mm/s. The radius of the laser spot was 16 µm. A laser confocal microscope (VK-X1000, Keyence) was adopted to observe the results.
3.2 Test method of laser AOI
The laser AOI test method consisted of three steps, as shown in Fig. 3(a):
Fig. 3. Schematic diagram of the laser AOI test method and definition of the laser AOI direction: (a) laser AOI test method, (b) the definition of the laser AOI direction for an open path, and (c) the definition of the laser AOI direction for a closed path.
Step 1: Place the ITO glass on plane B (focal plane). The drilling system scans a circle with a specific laser AOI. Then, a laser-etched circle path is left on the ITO glass, and the radius is RB.
Step 2: Move the ITO glass to plane A by the Z-axis stage, and the movement distance is d. The drilling system scans the same circle as Step 1 on the ITO glass surface. Then, another laser-etched circle path is left on the ITO glass, and the radius is RA.
Step 3: Calculate the laser AOI by Eq. (1):
The definition of the AOI direction was divided into two defined ways. The definition “for open path” was used to evaluate the tilt of the laser relative to the center of the scanning field at any point in the open path and was mostly used for system correction as well as for single-point scanning, as shown in Fig. 3(b). For the open scanning path, when the focused laser beam is incident obliquely toward the center of the scanning field, the angle between the laser beam and the Z-axis is positive. Otherwise, it is negative. The definition “for closed path” was used to evaluate the tilt of the laser relative to the center of the closed scanning path and was used for processing closed structures such as circles, squares, and ellipses, as shown in Fig. 3(c). For the closed scanning path, when the focused laser beam is incident obliquely toward the center of the closed scanning path, the angle between the laser beam and the Z-axis is positive. Otherwise, it is negative.
4. Results and discussions
4.1 Research on linear control of laser AOI
The design principle of the AOI control is shown in Fig. 4(a). Taking the angle α as an example, when Gα1 and Gα2 were synchronously deflected clockwise by an θα, the off-axis amount ΔSα1 could be expressed as:
Fig. 4. Schematic diagram of the control principle of the angle α: (a) optical structure for α control, and (b) relationship between deflection angle of mirrors (θα) and laser AOI (α).
In this work, dα were set to 40 mm, f1, f2, and f3 was 35.1 mm, 114.7 mm, and 100.0 mm, respectively. Under this parameter, the relationship between α and θα is shown in Fig. 4(b). The angle α increased with the increase of θα, and its growth rate gradually decreased. The maximum deflection angle used in this work was ± 2.50°. When θα was less than 2.50°, the angle α increased linearly with the increase of θα, and the relationship could be approximated as:
The R-squared value was adopted to evaluate the goodness of fit, and the R-squared value for Eq. (6) was 0.999995.
Similarly, the relationship between β and θβ could be expressed as:
The AOI of the focused laser beam in 3D space can be expressed as:
The study in this section demonstrated that the laser AOI could be linearly controlled by parallel and synchronously deflectable mirrors, simplifying the system control.
4.2 Optical design for laser independent control
Module II usually consisted of a concave lens F1 and a convex lens F2, as shown in Fig. 5. The lens parameters are shown in Table 1. Surface 1 and Surface 4 were spherical surfaces, and Surface 2 and Surface 3 were planes. However, when the incident laser was off-axis, a tilt angle ε would be caused by the field curvature of the spherical surfaces, as shown in Fig. 5(a). The tilt angle ε would cause an offset of the x and y coordinates of the focused laser. According to the simulation by Zemax, as the deflection angle θ of the mirrors in Module I increased from 0.00° to ± 2.50°, ε gradually increased from 0.00° to 0.10°, as shown by the black line in Fig. 5(c). The maximum offset of the x and y coordinates was 92 µm, as shown by the black line in Fig. 5(d). The control independence of the laser coordinates will be affected.
Fig. 5. The influence of lens field curvature on the laser control independence: (a) initial optical structure, (b) optimized optical structure, (c) relationship between θ and ε, and (d) relationship between θ and the offset of x and y coordinates.
Table 1. The parameters of the lens in Module II
The laser offset deviation was reduced by an elliptic surface (Surface 4) optimized by Zemax, as shown in Fig. 5(b). The radius of Surface 4 was - 52.104 mm, and the conic was - 0.398. The maximum ε decreased to 0.002°, as shown by the red line in Fig. 5(c). The maximum offset of x and y coordinates was 11 µm, as shown by the red line in Fig. 5(d). The optimized offset curve could be fitted as:
The R-squared value for Eq. (9) was 0.999797.
According to Eqs. (6–7), the relationship between α, β and xoffset, yoffset could be expressed as:
The independent control of the laser five-axis coordinates was basically achieved, and the remaining deviation would be corrected subsequently.
4.3 Research on laser AOI deviation in the large scanning field
According to the design principle of the telecentric F-theta lens, when the incident laser was deflected at its front focus (M) by angle θM, the focused laser would be parallel to the optical axis of F3, as shown in Fig. 6(a). The laser AOI tended to be zero. However, the two mirrors (Gx and Gy) of the two-axis galvanometer could not be placed at M simultaneously. The center of Gx was set before M by a distance of S3 - S2. The center of Gy was set behind M by a distance of S2 - S1. According to the optical principles [26], when the laser deflection angles θX and θY were equal to θM, the focused laser beam deflected by Gx was deflected clockwise along the optical axis by αX, and the focused laser beam deflected by Gy was deflected counterclockwise along the optical axis by αY. In the two/three-axis laser scanning system, the AOI deviation did not affect the processing result significantly, so it was usually ignored. However, the deviation must be corrected for the five-axis laser drilling system.
Fig. 6. Investigation of the laser AOI deviation: (a) schematic diagram of the laser beam focused by a telecentric F-theta lens, (b) simulation and experiment results of the laser AOI distribution on the X-axis and Y-axis, and (c) simulation results of the laser AOI distribution on the 2D plane (absolute value).
In this work, the distance between Gx and the front focus of F3 was 30.3 mm (S3 - S2), and the distance between Gy and the front focus of F3 was 5.2 mm (S2 - S1). S3 - S2 was 5.8 times that S2 - S1. The laser AOI distribution was simulated by Zemax, as shown in Fig. 6(b). The laser AOI increased linearly by 0.173 °/mm as the laser focus moved away from the scanning field center along the X-axis, and the laser AOI was positive. The laser AOI increased linearly by 0.030 °/mm as the laser focus moved away from the scanning field center along the Y-axis, and the laser AOI was negative. The laser AOI deviation on the X-axis was 5.8 times that on the Y-axis, which is consistent with the ratio between S3 - S2 and S2 - S1. The 2D distribution of laser AOI (absolute value) within the scanning field is shown in Fig. 6(c). The variation of the AOI with the movement of the focus along the X-axis was more significant than that of the Y-axis.
An experiment was proposed to test the laser AOI distribution. Several specific positions in the scanning field were chosen to scan test circles. The designated radius of the circle path was 250 µm, the designated laser AOI was 0.00°, and the distance d was 1 mm. Test results are shown in Fig. 7. The two laser-etched circle paths at the center of the scanning field (0.00, 0.00) coincided, which indicated a uniform distribution of laser AOI over the circumference, and the laser AOI was 0.00°. When the center of the circle path moved away from the scanning field center, the deviation of the laser AOI gradually increased. The yellow arrows in Fig. 7 represent the offset direction of the defocus laser. The defocus laser shifted away from the scanning field center along the X-axis. From point (- 17.50, 0.00) to point (17.50, 0.00), the laser AOI of the tested point was 2.99°, 2.91°, 1.46°, 1.39°, 1.40°, 1.48°, 2.92°, and 2.98°, respectively (refer to the definition of laser AOI direction for the open path). The defocus laser shifted closer to the scanning field center along the Y-axis. From point (0.00, - 17.50) to point (0.00, 17.50), the laser AOI of the tested point was - 0.56°, - 0.54°, - 0.26°, - 0.25°, - 0.24°, - 0.26°, - 0.53°, and - 0.55°, respectively. The experiment was consistent with the simulation in Fig. 6(b).
A preset deflection angle in the AOI controller was adopted to correct the AOI deviation, as shown in Fig. 8(a). The correction angle can be expressed as:
Fig. 8. Laser AOI correction: (a) correction deflection angle, and (b) laser AOI control range after correction.
Since the maximum deflection range of the mirrors in the AOI controller was ± 2.50°, the center of the laser AOI control range at the different positions would change after correction, as shown in Fig. 8(b). At the center of the scanning field, the laser AOI control center was 0.00°, and the control range was ± 6.51°. When the laser focus moved away from the scanning field center along the X-axis, the laser AOI control center moved linearly by 0.173 °/mm in the positive direction. Similarly, when the laser focus moved away from the scanning field center along the Y-axis, the laser AOI control center moved linearly by 0.030 °/mm in the negative direction. The move rates were consistent with the AOI increase rate in Fig. 6(b). Although the laser AOI control center moved, the laser AOI control range remained unchanged (∼ 13.05°).
4.4 Final correction of the five-axis laser drilling system
The final correction was performed by data conversion in the software, as shown in Fig. 9. Firstly, input the five-axis laser scanning data. Secondly, the laser AOI deviation would be corrected by Eq. (11). Third, the laser spot offset would be corrected by Eq. (10). Finally, output the corrected data to the five-axis laser drilling system.
The experiment results of the laser AOI after correction are shown in Fig. 10. The same positions as Fig. 7 were chosen to scan test circles. The designated radius of the circle path was 250 µm, the designated laser AOI was - 3.20°, and the distance d was 1 mm. The two laser-etched circle paths of all tested points were concentric, which indicated a uniform distribution of laser AOI within the scanning field. The laser AOI of all paths was about - 3.20° (refer to the definition of laser AOI direction for the closed path), and the control accuracy was better than ± 0.05°.
4.5 Concurrent drilling of special-shaped hole array
An experiment was carried out to verify the AOI control capacity of the five-axis laser drilling system. The tested sample was 3.6 mm thick GFRP. A series of round, square, and ellipse holes were fabricated concurrently within the 35 mm diameter scanning field without the movement of the drilling system and the sample. The material was removed layer by layer using a concentric path scanning method. The experimental laser energy was 120 µJ, the frequency was 20 kHz, the scanning speed was 100 mm/s, and the laser AOI was set to - 3.20°. After scanning a layer of all holes, the laser focus moved down 10 µm along the Z-axis, driven by Module II. The laser focus move range alone the Z-axis was ± 2.95 mm, which was enough for the 3.6 mm thick sample. The optical images of the processed holes are shown in Fig. 11. Round holes with a diameter of 1 mm were processed every 45° on the circle with a radius of 17 mm. In addition, three more round holes with a diameter of 1 mm were processed in the first quadrant, and the coordinates of hole centers were (0.00, 0.00), (4.00, 0.00), and (0.00, 4.00), respectively. Three ellipse holes were processed at (8.00, 0.00), (4.00, 4.00), and (0.00, 8.00). The long axis of the ellipse was 2 mm, and the short axis was 1 mm. Three square holes with a side length of 1 mm were processed at (12.00, 0.00), (8.00, 8.00), and (0.00, 12.00). The entrance optical image of the processed range is shown in Fig. 11(a), and the entrance optical images of the round, square, and ellipse holes are shown in Figs. 11(a-1), (a-2), and (a-3). The exit optical image of the processed range is shown in Fig. 11(b), and the exit optical images of the round, square, and ellipse holes are shown in Figs. 11(b-1), (b-2), and (b-3). The hole sidewalls were perpendicular to the material surface, and the dimension deviation between entrances and exits was less than 5 µm. The maximum aspect ratio was 3.6:1, as shown in Fig. 11(c). There was no visible material delamination or carbonization.
The results showed that the five-axis laser drilling system developed in this work could achieve high-precision concurrent processing of a special-shaped hole array within a large range. The processing did not require the movement of the drilling system or the sample, which improved the efficiency. It also showed that the system was capable of processing large-size special-shaped holes.
5. Conclusions
In this paper, a five-axis laser concurrent drilling system with a scanning field diameter of up to 35 mm was proposed and developed. By using two pairs of synchronous deflection mirrors, the independent control of laser AOI was achieved. The relationship between laser AOI and mirror deflection angles was completely linear, reducing system control and correction difficulty. The laser offset and AOI deviations caused by lens aberration were investigated by simulation and experiment. The deviations were corrected by an elliptic surface lens and control data conversion in the software. After correction, the laser AOI control accuracy could be better than ± 0.05°. The concurrent drilling function within a large scanning field was achieved and verified by the fabrication of a series of special-shaped holes on a 3.6 mm thick GFRP without the movement of the drilling system and the sample. The five-axis laser scanning technology with a large scanning field improved the capacity and application range of laser processing, making it possible to perform precision processing of special-shaped hole arrays and large-size holes.
Funding
National Natural Science Foundation of China (52175405).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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