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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 156707, 10 pages
http://dx.doi.org/10.1155/2013/156707
Research Article

External Electric Field-Assisted Laser Percussion Drilling for Highly Reflective Metals

National Yunlin University of Science and Technology, No. 123, Section 3, University Road, Douliou, Yunlin 640, Taiwan

Received 29 August 2013; Accepted 4 October 2013

Academic Editor: Chien Hung Liu

Copyright © 2013 Chao-Ching Ho et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, an external electric field was employed during the laser percussion drilling on highly reflective materials. The laser-produced plasma was sputtered substantially, and the charged ions in the plasma plume were drawn by the electrodes. Different configurations of plate electrodes were proposed and investigated in this work to provide a simple, low-cost method that allows expelling the laser-induced plasma during the percussion drilling process. The electric field resulted from the potential that was applied across the two electrodes. This electrical perturbation produced a uniform electric field when the laser-generated plasma was created in the plane plate-charged capacitor. The electric field with different electrode configurations applied to the charged particles that are carrying the electrons was also simulated in this work. All processing work was performed in air under standard atmospheric conditions and in the absence of assisting process gas. The depth of the holes drilled when various types of electrode configurations were used was measured, and the results were used to evaluate the percussion drilling rate. Results show that vaporized debris is expelled by the applied electric field; hence, in optimal configuration the penetration depth can be increased by up to 91.1%.

1. Introduction

Laser percussion drilling has gained great attention in the industry due to its wide industrial applicability and usage in processing of various materials, such as metals, glass, and ceramics. Laser percussion drilling is also characterized by a noncontact machining process, smaller beam spot size, high operating speeds, great flexibility, and accuracy. However, it is difficult to apply laser percussion drilling to highly reflective target surfaces, such as aluminum, which reflect the optical energy and dramatically reduce the processing efficiency [1, 2]. This drawback lengthens the drilling time, increasing the cost of the process and decreasing the yield.

Several solutions have been proposed to overcome this issue. Thawari et al. used high power pulsed Nd:YAG laser to provide enough energy to penetrate the highly reflective surface without reducing the yield of the process, but the processing efficiency remained low [3]. Gu et al. reported that shorter wavelength lasers improved absorption by the reflective material, thereby providing a higher cutting speed [4]. Zhu et al. utilized an ultrafast pulsed laser, such as a femtosecond laser, to provide high cutting speed that allowed avoiding the effect of surface reflectivity [5]. Both methods require expensive short-pulsed laser equipment, which increases the cost of machining.

External electric fields have been developed for the status monitoring based on the detection of phenomena from laser interaction with materials during the process. A simple and low-cost monitoring method was provided by using plasma detection with an external electric field [6]. Different configurations of electrodes were proposed to determine the optimal detection of hole-like penetration [7]. In our previous work [8], we proposed a new design of electrodes to increase the signal’s level and the detection sensitivity of laser-induced plasma. Meanwhile, the same setup was utilized to determine the focal position of laser processing.

Studies utilizing the electric field techniques for laser machining have been reported in the literature [912]. In [9, 10], the authors explored the feasibility in using the electric field as plasma control tool to assist the laser welding by a CO2 laser to provide better machining performance. In [11], the authors have evaluated electrostatic fields and laser-induced discharges to enhance both the removal of debris and the rate of ablation during ultrashort pulse laser drilling of copper. Results showed that debris is expelled to a much greater distance when an electric field is applied. However, those studies did not investigate the effect of the different configurations of the electrodes and did not perform quantitative analysis of penetration depth. In [12], the authors employed an external electric field during the femtosecond laser micromachining. It was found that the applied external electric field’s strength had a significant effect as the field prevented the particles from redepositing onto the machined silicon wafer surface due to the charged ions in the plasma plume that were drawn to the electrodes.

In this paper, we explored the effect of the electric field and the influence of different electrode configurations on laser percussion drilling. The electric field simulations were performed to determine the field strength in different electrode configurations. The depth of penetration and the inlet diameter were measured and examined.

2. Methodology

2.1. Electric Field Simulation

Laser percussion drilling with nanosecond pulsed lasers is achieved through evaporation, dissociation, atomization, ionization, and excitation of machined material and surrounding gases. Therefore, laser-induced plasma plume is produced during the drilling process [13]. When laser-induced plasma plume is generated between two metallic charged plate electrodes and the separation distance between the two plate electrodes is close to the size of the plasma plume, the drawn charged ions or electrodes would reach the electrode. Moreover, the strong electric field draws the charged ions in the plasma. Accordingly, the ions and the electrons of the laser-induced plasma reach the charged patterns. Copper plate electrodes with dimensions of  mm3 were selected for all experiments. Finite element (FE) method is a numerical simulation technique that is most often used to study the electric field’s distribution during the laser processing. To understand how the electric field of the plate electrodes is distributed, a finite element simulation was conducted using the COMSOL software. As shown in Figure 1, a mesh model was constructed in order to simulate the distribution of electric field lines, and the simulated results are shown in Figure 2. In order to determine the relation between the effect of the electric field and the distance between the two plate electrodes, the corresponding FE-simulated electric field was applied along the -direction with the  mm movement from the origin , as shown in Figure 3.

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Figure 1: The mesh model and environment for the simulation of the electric field were constructed.
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Figure 2: The simulated result of the electric field.
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Figure 3: The effect of electric field and the distance between the two plane electrodes.

As illustrated in Figure 4, eight possible electrode configurations were explored and investigated in the present simulation. These were (1) the configuration without the electric field applied (type Normal), (2) the horizontal configuration (type 1), (3) the modified horizontal configuration (type 2-1), (4) the modified horizontal configuration obtained by swapping the polarity (type 2-2), (5) the vertical configuration (type 3-1), (6) the vertical configuration obtained by swapping the polarity (type 3-2), (7) the modified vertical configuration (type 4-1), and (8) the modified vertical configuration obtained by swapping the polarity (type 4-2).

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Figure 4: Four electrode configurations employed to assist laser percussion drilling.

In the horizontal configuration (type 1), two electrodes that were made of plate electrodes separated by 7 mm were placed, in parallel, at a distance of 1 mm above the sample surface. Thus, the electric field was applied horizontally to the produced plasma. For the modified horizontal configuration (types 2-1 and 2-2), the electrodes had the same polarity and the same relative distances as those in type 1 setup, but the workpiece was connected to the opposite polarity. In the vertical configurations (types 3-1 and 3-2), the electric field was applied normally to the produced plasma, and the work-piece was connected to the polarity of corresponding electrodes. The electrodes were placed 3 mm above the sample surface. For the modified vertical configurations (types 4-1 and 4-2), the two electrodes were separated by 6 mm and the samples were placed in the middle between the two plate electrodes. Preliminary simulation was performed to predict the strength of the electric field. Results of the ssimulations that were performed to determine the electric field’s strength in different electrode configurations (type 1, type 2-1 and type 2-2, type 3-1 and type 3-2, and type 4-1 and type 4-2) are shown in Figures 5, 6, 7, and 8, respectively. In all of the above configurations, the electrodes were supplied with a high DC voltage of 300 volts. Figure 9 shows the relationship between the strength of the electric field and the distance from the origin along the -direction, for different configurations of the electrodes. The maximal strength of the electric field at the origin was 91,884 V/m, and it was obtained for type 3-1 and type 3-2 configurations. For type 2-1 and type 2-2 configurations, the strength of the electric field at the origin was 80,424 V/m. For type 1 configuration, the strength of the electric field at the origin was 40,681 V/m. The weakest electric field strength at the origin was obtained for type 4-1 and type 4-2 configurations, and attained the value of 1,760 V/m.

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Figure 5: Results of the simulations of the electrical field strength by employing electrode configuration type 1.
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Figure 6: Results of the simulations of the electrical field strength by employing electrode configurations of type 2-1 and 2-2.
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Figure 7: Results of the simulations of the electrical field strength by employing electrode configurations of type 3-1 and 3-2.
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Figure 8: Result of the simulations of the electrical field strength by employing electrode configurations of type 4-1 and 3-2.
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Figure 9: The electrical field strength by employing electrode configurations under the different electrode configurations of type 1, type 2-1, type 2-2, type 3-1 and type 3-2, and type 4-1 and type 4-2 supplied a high DC voltage of 300 V.
2.2. Experimental Setup

The setup for studying the effect of the electric field is shown in Figure 10, and the isometric view is shown in Figure 11. The entire experimental setup was established on a Sodick AP1L Micro Precision EDM machine. A fixture connected the holders of the optical components, a mirror, and a lens, to the column of the EDM machine. The mirror and the focal lens were placed on each precision stage to tune the laser direction and adjust the focal point of the laser. The mirror reflected the laser to the sample while the lens with a -axis-stage focused the laser onto the work-piece. The work-piece was made of aluminum 5052 with a thickness of 0.6 mm. A 120 mm focal length lens was used for focusing. The laser beam was focused on the surface of the work-piece. Eight electrode configurations (normal, type 1, type 2-1, type 2-2, type 3-1 and type 3-2, and type 4-1 and type 4-2) were employed in the present experiment. A power supply (GPR-30H10D, GWinstek) was connected to the electrodes through a detection circuit board to apply voltage to one of the electrodes and ground the other. The range of the applied voltages was 300 volts. The laser source was a Nd:YAG laser (LOTIS, LS-2134UTF) that provided four harmonic modes, that is, four different wavelengths. The 532 nm wavelength with pulse mode was used in all of our experiments. In the experiments, the maximal energy of a single pulse was 200 mJ, while the maximal frequency was 15 Hz. The pulse width was around 6 ns. Aluminum 5052 (Al5052) has a high reflectivity (over 90%) for Nd:YAG lasers. Therefore, the machining efficiency for this material is low. At the same time, the specific heat of Al5052 (0.88 J/g°C) is higher than the specific heats of other metals such as Au (0.129 J/g°C) and Cu (0.385 J/g°C). Thus, the machining efficiency is deteriorated.

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Figure 10: Setup for studying the effect of electric field.
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Figure 11: Isometric view of the setup for studying the effect of the electric field.

The laser drilling parameters are listed in Table 1. In these experiments, the depth of penetration was determined after a couple of hours of careful grinding along vertical planes of the work-piece until the drilled cavity became visible. The depth of the penetration, the diameters of the holes, and the thickness of the recast were investigated by optical microscopy (OM). All experiments were performed in air without additional process gas. The depth of the penetration and the diameter were determined by averaging over results from ten separate drilling holes of the same work-piece.

tab1
Table 1: The laser machining parameters.

3. Results and Discussion

Different types of electrode configurations were used to investigate the coupling effect of the electric field’s strength on percussion drilling. The profiles, the penetration depths, and the inlet diameters of the drilled holes were all recorded, and the reasons for the obtained results were discussed for each case. The radiation energy of the laser was set to 200 mJ, and the applied voltage was 300 volts. For each electrode configuration, ten spots on the same work-piece were examined with the same processing parameters. The separation between the adjacent spots was 0.5 mm in order to reduce the thermal influence across the spots. Normal type of electrode configuration was employed in order to further examine the depth of penetration in the absence of electric field, and these results were also used for comparison purposes. The experimentally obtained curves of applied electric fields and inlet diameters and penetration depths, determined for different electrode configurations, are shown in Figures 12, 13, 14, 15, 16, 17, and 18.

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Figure 12: The effect of the electric field on inlet diameter.
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Figure 13: The effect of the electric field on penetration depth.
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Figure 14: The effect of the electric field on inlet diameter.
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Figure 15: The effect of the electric field on penetration depth.
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Figure 16: The effect of the electric field on inlet diameter.
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Figure 17: The effect of the electric field on penetration depth.
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Figure 18: The effect of the electric field on inlet diameter.

Experimental results show that the depth of the penetration increased when electric fields were applied. The reason was attributed to the fact that the movement of the drilled material was accelerated by the electric field, and the charged particles were removed by the electric force. Comparison of the results of type 2-1 and type 1 configurations suggests that the penetration depth is deeper for type 2-1 configuration due to its much stronger electric field (as compared to the electric field of type 1), as shown in Figures 13 and 15.

As shown in Figures 15 and 17, at pulse number 30, the penetration depth increased from 287 μm for type 2-1 to 361 μm for type 2-2. This occurred because the mass of the laser-produced positive ion (i.e.,  kg) was heavier than that of the electron (i.e.,  kg) and thus it caused attraction to the plate’s surface in type 2-2 configuration. As shown in Figure 19, at pulse number 20, the depth of the penetration increased from 293 μm for normal type to 560 μm for type 3-1 configuration. The maximal increase in the drilling depth was around 91% due to the self-induced electrical discharges that occurred between the electrode and the surface of the work-piece and contributed to the expulsion of the plasma-produced plume, as shown in Figure 20. For type 3-2 configuration, the laser-produced plasma plume was redeposited onto the sidewalls due to the electric field that was introduced by the removed electrons that were drawn back to the target, as shown in Figure 21. For type 4-1 configuration, the penetration depth could not be trivially compared to the normal type due to the weaker electric field, as shown in Figure 23. Using type 4-2 configuration at pulse number 20, the penetration depth could be increased from 313 μm (for type 4-1) to 361 μm (for type 4-2) (see Figures 22, 24, and 25). This was attributed to the fact that the electric field accelerated the electrons to improve the material melted, but the plume redeposit ions were prevented by the weaker electric field.

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Figure 19: The effect of the electric field on penetration depth.
fig20
Figure 20: The profile images and measured depths of the drilled holes of (a) normal type and (b) type 3-1.
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Figure 21: Vaporized material was redeposited onto the sidewalls under type 3-2.
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Figure 22: The effect of the electric field on inlet diameter.
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Figure 23: The effect of the electric field on penetration depth.
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Figure 24: The effect of the electric field on inlet diameter.
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Figure 25: The effect of the electric field on penetration depth.

4. Conclusions

This work showed the feasibility of using the electric field to influence the behavior of the plasma plume during laser percussion drilling. The depths of the drilled holes, obtained in the presence of applied electric field, were all deeper than those obtained in the normal case. These results were due to the effect of the electric force by which the plume particle could be accelerated in the electric field. A significant improvement of 91% in penetration depth was achieved using the vertical configuration of electrodes, in which the plate electrodes were connected to the positive potential voltage, but the workpiece was connected to the grounding line.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was supported by the National Science Council, Taiwan, R.O.C. NSC 102-2221-E-224-039.

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