- Email: [email protected]
Optical emission monitoring for defocusing laser percussion drilling
Accepted Manuscript Optical Emission Monitoring for Defocusing Laser percussion drilling Chao-Ching Ho, Yuan-Jen Chang, Jin-Chen Hsu, Chih-Mu Chiu, Chia-Lung Kuo PII: DOI: Reference:
S0263-2241(15)00565-5 http://dx.doi.org/10.1016/j.measurement.2015.10.031 MEASUR 3643
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
13 November 2014 12 October 2015 15 October 2015
Please cite this article as: C-C. Ho, Y-J. Chang, J-C. Hsu, C-M. Chiu, C-L. Kuo, Optical Emission Monitoring for Defocusing Laser percussion drilling, Measurement (2015), doi: http://dx.doi.org/10.1016/j.measurement. 2015.10.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Manuscript for Measurement
OPTICAL EMISSION MONITORING FOR DEFOCUSING LASER PERCUSSION DRILLING Chao-Ching Ho 1 , Yuan-Jen Chang1 , Jin-Chen Hsu 1, Chih-Mu Chiu 1 and Chia-Lung Kuo1 1 Department of Mechanical Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin, Taiwan, R.O.C., 64002, [email protected]
Abstract: This paper presents a novel method for controlling the laser-drilling process for a hole by monitoring induced plasma emission. The variat ion of light brightness from laser-induced plasma is used as an indicator to control laser percussion drilling. Th rough on-line plas ma emission acquisition and analysis, we obtain the positive association between the increased depth and the optical signal output. A coaxial photodiode is used to estimate the brightness levels of laser-induced plasma. The above constitute an inexpensive and practical on-line feedback system that can be easily imp lemented in the laser systems. All of the processing work is performed in air under standard atmospheric conditions without gas assist. The acquired signal for d rilling could also be used as an input to a focus point process control scheme. Moreover, the technology demonstrates the feasibility to develop an automated laser micro mach ining system. Experimental results show that drilling efficiency was increased 47% by applying the proposed defocusing laser percussion drilling. Keywords : Laser micro machining, laser-induced plasma, process monitoring, plas ma emission, defocusing laser micro mach ining. 1. INTRODUCTION Optimizing the focusing conditions for laser percussion drilling could improve productivity and drilling quality. The position of the focal spot relative to the workpiece surface determines whether the laser beam is converging or diverging. Drilling causes change in focal position, wh ich reduces laser power density and affects drilled-hole quality. An incorrect workpiece standoff or focal position that is due to the defocusing of the laser beam by the increasing depth can be a major cause of process deterioration [1]. As laser percussion drilling become more automated, process monitoring need to be applied to focusing conditions, rather than relying on experience-based control rules. Previous research has addressed the problems in automatic monitoring of material p rocessing using a laser. Kaebernick et al. [ 2 ] used a photodiode-based adaptive control system for improving the laser cutting surface quality by controlling the striation frequency. Fo x et a1. [1] developed a focus control system for closed-loop control of laser cutting and drilling, which uses the chromatic aberrat ions of the effector optics. However, the accuracy of the measurement will be reduced when with h igh peak power pulses. Stournaras et al. [3] used optical signals acquired by offaxial photodiodes that were positioned above the processing zone fo r real-time mon itoring of the d iameter and depth of
the drilled hole. Ho wever, the proposed configuration indicated a weak coefficient of confidence of the output optical signal with machin ing depth. Chang and Tu [4] proposed a closed-loop control for ablation depth in femtosecond pulse laser micro-mach ining by monitoring the brightness of the derived light. The ablation process was recorded by a CCD camera and analyzed frame by frame to obtain the brightness level of the plasma. However, this closed-loop method was not applicable fo r drilling a deep hole in which the plasma may be blocked by the sidewall of the hole. Ho et al. [5] showed that the cumulative size of the laserinduced plasma correlates with the depth of the hole. However, because of the radial arrangement of the camera, some of the detected plasma signal e mitted fro m a deep hole was blocked by the sidewall of the hole. Therefore, this system is a poor candidate to provide real-t ime control for laser drilling. Diego et al. [6] attempted to detect the working focus position during laser scribing by mon itoring the intensity of plasma emission. A linear correlat ion between plasma emission intensity and ablated mass per pulse was established. Our p revious work [7] showed that the intensity of the light emitted fro m the plasma p lu me correlates with the depth of the drilled hole. In the present work, we used a photodiode to detect the emitted light, and the focus position was adjusted accordingly. This increased quality while reducing process time.
2. ANALYS IS AND EXPERIMENTS A high-speed, silicon photodiode (Hamamatsu, S1223) with an active area of 3.6 × 3.6 mm2 was employed coaxially to measure the light emission of the plasma that appeared near the workpiece surface. It had a maximu m sensitivity of 960 n m and a response time of 50 ns, which makes it reliable for signal detection. The intensity of the light emitted fro m the plas ma plu me format ion passed back up through the nozzle, the focusing lens through a focused lens, a neutral density filters (transmittance 10%), and two 532 n m notch filters onto the photodiode. To distinguish between the ionization phenomena fo r the air and workpiece caused by the laser-induced plasma and to suppress the strong intensity of the light emitted fro m the plasma plu me to saturate the photodiode, a wavelength filter and neutral density filter are placed in front of the photodiode. An amp lifier with a peak detector circuit indicated when light
Manuscript for Measurement
Photodiode 532 nm notch filter
230 mm
level was maximal. The analog signal fro m the photodiode was periodically sampled by a dig ital signal processor (DSP) with a sampling rate of 1 M Hz. Fig. 1 shows a schematic of the experimental configuration. The experimental setup is shown in Fig. 2. The emp loyed laser source was a Q-switched Nd:YA G laser wh ich provides a Gaussian laser beam and delivers 200 mJ of radiation energy of the pulse laser in 6 ns at 532 n m. The output maximu m beam diameter was 6 mm, and the beam divergence was typically belo w 0.8 mrad. The beam was focused to a min imu m theoretical d iameter of 13.5 μm (1/e2 ) using a plano-convex lens of focal length 120 mm. It was focused on the surface of the workpiece, with the workp iece surface normal to the laser beam. The laser was operated at 15 Hz, and induced a plas ma that expanded normal to the wo rkp iece surface. The depth of focus (i.e., DOF) is ± 271 μm and given by DOF = (8λ/π)(f/D)2 , where λ = 0.532 μm is the wavelength, f = 120 mm is the lens of focal length and D = 6 mm is the beam diameter. The apparatus was set up on an electric discharge machine (Sodick, Model: A Q35L). For metallographic inves tigations, the depth of penetration was determined after a couple of hours of careful grinding along vertical planes of the workp iece until the drilled cavity became visible and the cross-sections of holes were carefully checked to determine the accurate depth of machining. The depth and the diameter of holes are investigated by optical microscopy (Oly mpus U-PM TVC 8C14561).
Workpiece
Fig. 2. Experimental setup Peak detector circuit
Table 1: Experimental conditions of on-line depth measurement.
DSP
230 mm
Amplifier
Photodiode 532 nm notch filter 532 nm mirror
Laser beam Nd:YAG Laser
Nozzle Plasma plume
Workpiece Computer
Fig. 1: Configuration fo r defocusing laser percussion drilling
Parameter
Value
Laser source Laser rad iation energy per pulse Laser wavelength Focusing lens Pulse time Pump pu lse duration Pulse frequency Ambient temperature Ambient mo isture
Nd:YA G 200 mJ 532 n m 120 mm 6 ns 80– 100 μs 15 Hz 25 °C 66 %
The first part o f our experiment used 1.0 mm-thick sheets of stainless steel (SUS 304) and an industrial Nd :YA G pulsed laser (Table 1). The power density of 2.30 MW/cm2 for focus on the surface of the workp iece was used in the experiment. Fig. 3 shows the drilling depth and the responding voltage of the coaxial photodiode BRES(N) for a given laser pulse nu mber (N ). At the beginning of the drilling process, a significant voltage variation occurs due to the limited amount of light emitted fro m the p lasma plu me was randomly scattered toward the photodiode. However, the drilled hole becomes deeper with increasing the shot
Manuscript for Measurement
number, which results in the light emitted fro m the p lasma plume is originating fro m the drilling zone and contributes to guiding the light toward the photodiode. Hence, the s mall scattering of values can be observed at the deeper drilled depth. Fig. 4 shows that the incremental drilling depth DINC(N) for different values of N decreased with cu mulative hole formation depth DAC(N), in part due to the laser defocusing effect stemming fro m the thickness of the part. The experimental results show good agreement with those presented in our previous work [7] and literature [8]. Our previous work [7] showed that the penetration depth is in inverse proportion to the peak plasma light emission measured for a given laser pulse number. Jiang et al. [8] hypothesized that the strength of the laser intensity on the axial d irect ion would be weakened as the defocusing distance increased.
Fig. 5: Correlat ion between increased drilling depth DINC(N) and responding voltage variation B VAR(N) of the coaxial photodiode
2.1 Correlation between Variation Brightness and Increased Depth
Fig. 3: The drilling depth and the responding voltage of the coaxial photodiode BRES(N) under different values of laser pulse number N
Fig. 4: Incremental drilling depth DINC(N) and cumulative hole format ion depth DAC(N) for different values of N
of
Plasma
When the laser strikes the workp iece surface repeatedly at the same spot (13.5 μm diameter at the 1/e2 level), the workp iece is heated, which effects drilling, and the laser intensity on the bottom surface of the d rilled cavity is reduced. As a result, plasma size reduces as focus position is more distant fro m the material surface [9], and its brightness also declines [7]. Hence, the brightness variation of the derived light fro m the plasma can be used as an indicator to control laser percussion drilling. Fig. 5 shows a correlation between the increased drilling depth DINC(N) and the responding voltage variation BVAR(N) of the coaxial photodiode due to induced plasma emission under different values of laser pulse nu mber (N). A linear relat ionship exists with a linear regression coefficient of 0.72.
Fig. 6: After initial laser percussion drilling at laser pulse number 100 at the drilled depth 400 μm, BRES(N) declines slowly as the result of BVAR(N) exceed ing a threshold KRE
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500 μm
500 μm
FD = 0 mm
Not defocused
0.4 mm
FD = 0.3 mm
Defocused
FD = 0.6 mm
Defocused
(e) FD = 0.4 mm Initial drilling
Defocus drilling
(f) FD = 0.5 mm
500 μm
Fig. 7: The defocusing experiment of increasing the defocusing distance FD under fixed laser pulse power
FD = 0.6 mm Fig. 9: Optical microscopy images (25×) of cross -sections of holes acquired under different defocusing settings
Fig. 8: The increased depth obtained under different defocusing distance settings 500 μm 500 μm
(a) FD = 0 mm 500 μm
(c) FD = 0.2 mm
(b) FD = 0.1 mm 500 μm
(d) FD = 0.3 mm
2.2 Relationship between the Increased Depth and the Defocusing Distance Fro m the previous sections, the increased depth can be correlated with a signal variation fro m the photodiode. The defocusing experiment was then employed to find the relationship between the increased depth and defocusing distance during drilling. The only changing parameter is the defocusing position. Fig. 6 shows that after initial laser percussion drilling, the responding voltage of the coaxial photodiode BRES(N) at laser pulse number 100 declines slowly as the result of the responding voltage variation BVAR(N) being increased beyond -0.1. The marked region on the curve in the left image shows the responding voltage variation B VAR(N), wh ich falls below the setting threshold KRES (i.e., -0.1) as the drilled workpiece surface is more distant from the focus position. The increment in the brightness of the plasma indicates the reduction in the drilled depth. Fig. 7 shows that the initial 100 pulse number is focused on the surface of the target, at which t ime the laser drilled depth is 400 μm. The focal point positioned accurately on the surface of the workp iece will ensure the optimu m drilling performance. After the in itial 100 laser pulse number were applied with focus fixed on the surface of the workp iece, the intensity in the beam was dropped rapidly. Hence, the laser drilling surface needed to be brought to within the DOF (i.e., ± 271 μm). When thicker materials are drilled, the depth of focus must be adapted to the material’s thickness in order to maintain intensity and drilling speed. Hence, the defocusing drilling depth on the target surface can be investigated by increasing the defocusing distance FD under fixed laser pulse power. The threshold value KRES is a good indicator to judge the progress of drilling. Since the brightness of the derived light is dominated by the defocusing, KRES can be used as the criterion to perform defocusing drilling by adjusting the
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focal position during the process. Fig. 8 shows the result of the defocusing experiment under fixed laser pulse power with addit ional 200 pulse nu mbers applied to measure the depth increment under different defocusing settings. The diagram shows that when defocus position in the range between 0.1 mm and 0.6 mm, the defocusing strategy has a positive effect on the average drilling depth. This informat ion can be used to monitor the brightness variation signal of the p lasma BVAR(N) to control feedback drilling. The maximu m increased depths at defocusing distance setting are FD = 0.3 mm and FD = 0.4 mm. Fig. 9 shows the resulting profile images for laser-drilled mach ining under different defocusing settings.
2.3 Defocusing Laser Percussion Drilling In the previous sections, the responding voltage variation BVAR(N) of the induced plasma was treated as the indicator to adjust the focal position. The derived light signal can be used as the indicator to adjust the defocus position according to the brightness variation of the induced plasma. Fig. 10 shows the experimental results in which the black line is the result of the focus fixed drilling, while the rest of data depict change in defocusing under fixed laser pulse power. The init ial focal position was just on the surface. When the responding voltage variation BVAR exceeded the threshold KRES, the defocus distance FD moved to 0.3 mm and then again to 0.4 mm according to the feedback of the brightness deviation induced by the plasma plu me. Three tests were carried out to confirm the reproducibility of the control process. Fig. 11 depicts the total final pulse nu mbers for focus-fixed drilling and defocusing drilling. Drilling efficiency was increased more than 47% by applying proposed defocusing laser percussion drilling than with no focus control. It is found the through holes with the in let diameter of 147 μm and aspect ratio around 7 was achieved for 1 mm thick stainless steel sheet. It was observed that the diameters of the inlet side of the percussion-drilled holes were considerably larger than the diameter of the laser spot on the material surface. Th is indicates that the high energy shock caused material to be ejected fro m the material surface [10]. The high degree of vaporizat ion in turn caused high vapor pressure to develop above the material surface, and this propelled molten material at the cavity to its rim. The hot plasma plu me fo rmed fro m the inside hole will lead to the sustaining outer ablation process. However, while drilling large aspect ratio holes, the volume of material removed per pulse is continuously decreasing with progressive shot due to the molten material is limited to eject fro m the entry-side and accu mulated inside the hole. While drilling large aspect ratio holes, the volume of material removed per pulse is continuously decreasing with progressive shot, which is resulted in the responding voltage of the coaxial photodiode declines slowly and finally the intensity of the light emitted fro m the plasma p lu me is too weak to be identified by the photodiode. Thus, the proposed method fails to pred ict the increased drilling depth when the responding voltage falls below the detection voltage level.
The material removal rate of nanosecond laser drilling can be affected by many factors such as laser fluence, irradiance, wavelength, duration, assisted gas and its pressure. In order to achieve high drilling rates in this work, the fluence was using 1.4×105 J/cm2 to co mpensate the low repetition rate (i.e., 15 Hz) and the irradiance was 2.10×106 W/cm2 . The material removal volu me was calculated based on the truncated cone shape with the thickness of 1000 μ m, and inlet diameter of 147 μm and outlet diameter of 60 μm (Fig. 12). The material removal rate was 0.12 mm3 / min by applying proposed defocusing laser percussion drilling. The material removal rate is similar with the work [11] at 10 kH z repetitive rate and the work [12] at 30 kHz repetitive rate. The laser irradiance of 1.50×109 W/cm2 for the work [11] and 6.30×107 W/cm2 for the work [12], respectively. However, the p roposed defocusing laser percussion drilling can achieve the similar drilling rate with lower laser irradiance.
Fig. 10: Experimental results of focus fixed drilling and defocusing laser percussion drilling
Fig. 11: Co mparison of breakthrough pulse numbers between defocusing laser percussion drilling and no focus control
Manuscript for Measurement
200 μm
[1]
(a) 100 μm
(b) Fig. 12: Optical microscopy images of laser-drilled holes with diameter measurements: (a) hole inlet and (b) hole outlet
3. CONCLUS IONS The paper demonstrates a strategy for monitoring and control of focus position during laser percussion drilling. It showed that a coaxial photodiode is capable of measuring deviation of plasma emission on-line, which provides the basic feedback for the imp rovement of drilling depth through defocus position control. The increment o f the brightness variation induced by plasma is associated with a decrease in drilling depth. The feasibility and improved efficiency of 47% with respect to focus fixed drilling for this defocusing feedback laser percussion drilling method was demonstrated. In the case of the ultrafast laser, the intensity was much higher than with the nanosecond laser. As a result, the brightness of the plasma induced by the ultrafast laser is much stronger than the nanosecond laser. Hence, the proposed method is applicable for ult rafast laser, and will be explored in the future. ACKNOWLEDGEMENTS The work was supported by the Ministry of Science and Technology, Taiwan, R.O.C. M OST 103-2221-E-224-030MY2 and 103-2622-E-224-001-CC2.
REFERENCES
M . D. Fox, P. French, C. Peters, D. P. Hand, and J. D. Jones, "Applications of optical sensing for laser cutting and drilling," Applied optics, vol. 41, pp. 4988-4995, 2002. [2] H. Kaebernick, A. Jeromin, and P. M athew, "Adaptive control for laser cutting using striation frequency analysis," CIRP Annals-Manufacturing Technology, vol. 47, pp. 137140, 1998. [3] A. Stournaras, K. Salonitis, and G. Chryssolouris, "Optical emissions for monitoring of the percussion laser drilling process," The International Journal of Advanced Manufacturing Technology, vol. 46, pp. 589-603, 2010. [4] G. Chang and Y. Tu, "Closed-loop control in ultrafast laser milling process using laser triggered plasma," International Journal of Machine Tools and Manufacture, vol. 60, pp. 3539, 2012. [5] C.-C. Ho, J.-J. He, and T.-Y. Liao, "On-line estimation of laser-drilled hole depth using a machine vision method," Sensors, vol. 12, pp. 10148-10162, 2012. [6] D. Diego-Vallejo, D. Ashkenasi, and H. Eichler, "M onitoring of focus position during laser processing based on plasma emission," Physics Procedia, vol. 41, pp. 904-911, 2013. [7] C.-C. Ho, C.-M . Chiu, Y.-J. Chang, J.-C. Hsu, and C.-L. Kuo, "On-line depth measurement for laser-drilled holes based on the intensity of plasma emission", M easurement Science and Technology, 2014 (To be appeared). [8] C. Y. Jiang, W. S. Lau, T. M . Yue, and L. Chiang, "On the maximum depth and profile of cut in pulsed Nd: YAG laser machining," CIRP Annals-Manufacturing Technology, vol. 42, pp. 223-226, 1993. [9] C.-C. Ho and J.-J. He, "On-line monitoring of laser-drilling process based on coaxial machine vision," International Journal of Precision Engineering and Manufacturing, vol. 15, pp. 671-678, 2014. [10] L. Tunna, W. O’Neill, A. Khan, and C. Sutcliffe, "Analysis of laser micro drilled holes through aluminium for micromanufacturing applications," Optics and Lasers in Engineering, vol. 43, pp. 937-950, 2005. [11] J. Wendland, P. M . Harrison, M . Henry, and M . Brownell, "Deep engraving of metals for the automotive sector using high average power diode pumped solid state lasers," in Proceedings of the 23rd International Conference on Applications of Lasers and Electro-Optics M iami, Florida, USA, 2005, pp. 1-7. [12] P. M . Harrison, M . Henry, I. Henderson, and M . F. Brownell, "Laser milling of metallic and nonmetallic substrates in the nanosecond regime with Q-switched diode pumped solid state lasers," in High-Power Laser Ablation, 2004, pp. 624-633.
S0263-2241(15)00565-5 http://dx.doi.org/10.1016/j.measurement.2015.10.031 MEASUR 3643
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
13 November 2014 12 October 2015 15 October 2015
Please cite this article as: C-C. Ho, Y-J. Chang, J-C. Hsu, C-M. Chiu, C-L. Kuo, Optical Emission Monitoring for Defocusing Laser percussion drilling, Measurement (2015), doi: http://dx.doi.org/10.1016/j.measurement. 2015.10.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Manuscript for Measurement
OPTICAL EMISSION MONITORING FOR DEFOCUSING LASER PERCUSSION DRILLING Chao-Ching Ho 1 , Yuan-Jen Chang1 , Jin-Chen Hsu 1, Chih-Mu Chiu 1 and Chia-Lung Kuo1 1 Department of Mechanical Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin, Taiwan, R.O.C., 64002, [email protected]
Abstract: This paper presents a novel method for controlling the laser-drilling process for a hole by monitoring induced plasma emission. The variat ion of light brightness from laser-induced plasma is used as an indicator to control laser percussion drilling. Th rough on-line plas ma emission acquisition and analysis, we obtain the positive association between the increased depth and the optical signal output. A coaxial photodiode is used to estimate the brightness levels of laser-induced plasma. The above constitute an inexpensive and practical on-line feedback system that can be easily imp lemented in the laser systems. All of the processing work is performed in air under standard atmospheric conditions without gas assist. The acquired signal for d rilling could also be used as an input to a focus point process control scheme. Moreover, the technology demonstrates the feasibility to develop an automated laser micro mach ining system. Experimental results show that drilling efficiency was increased 47% by applying the proposed defocusing laser percussion drilling. Keywords : Laser micro machining, laser-induced plasma, process monitoring, plas ma emission, defocusing laser micro mach ining. 1. INTRODUCTION Optimizing the focusing conditions for laser percussion drilling could improve productivity and drilling quality. The position of the focal spot relative to the workpiece surface determines whether the laser beam is converging or diverging. Drilling causes change in focal position, wh ich reduces laser power density and affects drilled-hole quality. An incorrect workpiece standoff or focal position that is due to the defocusing of the laser beam by the increasing depth can be a major cause of process deterioration [1]. As laser percussion drilling become more automated, process monitoring need to be applied to focusing conditions, rather than relying on experience-based control rules. Previous research has addressed the problems in automatic monitoring of material p rocessing using a laser. Kaebernick et al. [ 2 ] used a photodiode-based adaptive control system for improving the laser cutting surface quality by controlling the striation frequency. Fo x et a1. [1] developed a focus control system for closed-loop control of laser cutting and drilling, which uses the chromatic aberrat ions of the effector optics. However, the accuracy of the measurement will be reduced when with h igh peak power pulses. Stournaras et al. [3] used optical signals acquired by offaxial photodiodes that were positioned above the processing zone fo r real-time mon itoring of the d iameter and depth of
the drilled hole. Ho wever, the proposed configuration indicated a weak coefficient of confidence of the output optical signal with machin ing depth. Chang and Tu [4] proposed a closed-loop control for ablation depth in femtosecond pulse laser micro-mach ining by monitoring the brightness of the derived light. The ablation process was recorded by a CCD camera and analyzed frame by frame to obtain the brightness level of the plasma. However, this closed-loop method was not applicable fo r drilling a deep hole in which the plasma may be blocked by the sidewall of the hole. Ho et al. [5] showed that the cumulative size of the laserinduced plasma correlates with the depth of the hole. However, because of the radial arrangement of the camera, some of the detected plasma signal e mitted fro m a deep hole was blocked by the sidewall of the hole. Therefore, this system is a poor candidate to provide real-t ime control for laser drilling. Diego et al. [6] attempted to detect the working focus position during laser scribing by mon itoring the intensity of plasma emission. A linear correlat ion between plasma emission intensity and ablated mass per pulse was established. Our p revious work [7] showed that the intensity of the light emitted fro m the plasma p lu me correlates with the depth of the drilled hole. In the present work, we used a photodiode to detect the emitted light, and the focus position was adjusted accordingly. This increased quality while reducing process time.
2. ANALYS IS AND EXPERIMENTS A high-speed, silicon photodiode (Hamamatsu, S1223) with an active area of 3.6 × 3.6 mm2 was employed coaxially to measure the light emission of the plasma that appeared near the workpiece surface. It had a maximu m sensitivity of 960 n m and a response time of 50 ns, which makes it reliable for signal detection. The intensity of the light emitted fro m the plas ma plu me format ion passed back up through the nozzle, the focusing lens through a focused lens, a neutral density filters (transmittance 10%), and two 532 n m notch filters onto the photodiode. To distinguish between the ionization phenomena fo r the air and workpiece caused by the laser-induced plasma and to suppress the strong intensity of the light emitted fro m the plasma plu me to saturate the photodiode, a wavelength filter and neutral density filter are placed in front of the photodiode. An amp lifier with a peak detector circuit indicated when light
Manuscript for Measurement
Photodiode 532 nm notch filter
230 mm
level was maximal. The analog signal fro m the photodiode was periodically sampled by a dig ital signal processor (DSP) with a sampling rate of 1 M Hz. Fig. 1 shows a schematic of the experimental configuration. The experimental setup is shown in Fig. 2. The emp loyed laser source was a Q-switched Nd:YA G laser wh ich provides a Gaussian laser beam and delivers 200 mJ of radiation energy of the pulse laser in 6 ns at 532 n m. The output maximu m beam diameter was 6 mm, and the beam divergence was typically belo w 0.8 mrad. The beam was focused to a min imu m theoretical d iameter of 13.5 μm (1/e2 ) using a plano-convex lens of focal length 120 mm. It was focused on the surface of the workpiece, with the workp iece surface normal to the laser beam. The laser was operated at 15 Hz, and induced a plas ma that expanded normal to the wo rkp iece surface. The depth of focus (i.e., DOF) is ± 271 μm and given by DOF = (8λ/π)(f/D)2 , where λ = 0.532 μm is the wavelength, f = 120 mm is the lens of focal length and D = 6 mm is the beam diameter. The apparatus was set up on an electric discharge machine (Sodick, Model: A Q35L). For metallographic inves tigations, the depth of penetration was determined after a couple of hours of careful grinding along vertical planes of the workp iece until the drilled cavity became visible and the cross-sections of holes were carefully checked to determine the accurate depth of machining. The depth and the diameter of holes are investigated by optical microscopy (Oly mpus U-PM TVC 8C14561).
Workpiece
Fig. 2. Experimental setup Peak detector circuit
Table 1: Experimental conditions of on-line depth measurement.
DSP
230 mm
Amplifier
Photodiode 532 nm notch filter 532 nm mirror
Laser beam Nd:YAG Laser
Nozzle Plasma plume
Workpiece Computer
Fig. 1: Configuration fo r defocusing laser percussion drilling
Parameter
Value
Laser source Laser rad iation energy per pulse Laser wavelength Focusing lens Pulse time Pump pu lse duration Pulse frequency Ambient temperature Ambient mo isture
Nd:YA G 200 mJ 532 n m 120 mm 6 ns 80– 100 μs 15 Hz 25 °C 66 %
The first part o f our experiment used 1.0 mm-thick sheets of stainless steel (SUS 304) and an industrial Nd :YA G pulsed laser (Table 1). The power density of 2.30 MW/cm2 for focus on the surface of the workp iece was used in the experiment. Fig. 3 shows the drilling depth and the responding voltage of the coaxial photodiode BRES(N) for a given laser pulse nu mber (N ). At the beginning of the drilling process, a significant voltage variation occurs due to the limited amount of light emitted fro m the p lasma plu me was randomly scattered toward the photodiode. However, the drilled hole becomes deeper with increasing the shot
Manuscript for Measurement
number, which results in the light emitted fro m the p lasma plume is originating fro m the drilling zone and contributes to guiding the light toward the photodiode. Hence, the s mall scattering of values can be observed at the deeper drilled depth. Fig. 4 shows that the incremental drilling depth DINC(N) for different values of N decreased with cu mulative hole formation depth DAC(N), in part due to the laser defocusing effect stemming fro m the thickness of the part. The experimental results show good agreement with those presented in our previous work [7] and literature [8]. Our previous work [7] showed that the penetration depth is in inverse proportion to the peak plasma light emission measured for a given laser pulse number. Jiang et al. [8] hypothesized that the strength of the laser intensity on the axial d irect ion would be weakened as the defocusing distance increased.
Fig. 5: Correlat ion between increased drilling depth DINC(N) and responding voltage variation B VAR(N) of the coaxial photodiode
2.1 Correlation between Variation Brightness and Increased Depth
Fig. 3: The drilling depth and the responding voltage of the coaxial photodiode BRES(N) under different values of laser pulse number N
Fig. 4: Incremental drilling depth DINC(N) and cumulative hole format ion depth DAC(N) for different values of N
of
Plasma
When the laser strikes the workp iece surface repeatedly at the same spot (13.5 μm diameter at the 1/e2 level), the workp iece is heated, which effects drilling, and the laser intensity on the bottom surface of the d rilled cavity is reduced. As a result, plasma size reduces as focus position is more distant fro m the material surface [9], and its brightness also declines [7]. Hence, the brightness variation of the derived light fro m the plasma can be used as an indicator to control laser percussion drilling. Fig. 5 shows a correlation between the increased drilling depth DINC(N) and the responding voltage variation BVAR(N) of the coaxial photodiode due to induced plasma emission under different values of laser pulse nu mber (N). A linear relat ionship exists with a linear regression coefficient of 0.72.
Fig. 6: After initial laser percussion drilling at laser pulse number 100 at the drilled depth 400 μm, BRES(N) declines slowly as the result of BVAR(N) exceed ing a threshold KRE
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500 μm
500 μm
FD = 0 mm
Not defocused
0.4 mm
FD = 0.3 mm
Defocused
FD = 0.6 mm
Defocused
(e) FD = 0.4 mm Initial drilling
Defocus drilling
(f) FD = 0.5 mm
500 μm
Fig. 7: The defocusing experiment of increasing the defocusing distance FD under fixed laser pulse power
FD = 0.6 mm Fig. 9: Optical microscopy images (25×) of cross -sections of holes acquired under different defocusing settings
Fig. 8: The increased depth obtained under different defocusing distance settings 500 μm 500 μm
(a) FD = 0 mm 500 μm
(c) FD = 0.2 mm
(b) FD = 0.1 mm 500 μm
(d) FD = 0.3 mm
2.2 Relationship between the Increased Depth and the Defocusing Distance Fro m the previous sections, the increased depth can be correlated with a signal variation fro m the photodiode. The defocusing experiment was then employed to find the relationship between the increased depth and defocusing distance during drilling. The only changing parameter is the defocusing position. Fig. 6 shows that after initial laser percussion drilling, the responding voltage of the coaxial photodiode BRES(N) at laser pulse number 100 declines slowly as the result of the responding voltage variation BVAR(N) being increased beyond -0.1. The marked region on the curve in the left image shows the responding voltage variation B VAR(N), wh ich falls below the setting threshold KRES (i.e., -0.1) as the drilled workpiece surface is more distant from the focus position. The increment in the brightness of the plasma indicates the reduction in the drilled depth. Fig. 7 shows that the initial 100 pulse number is focused on the surface of the target, at which t ime the laser drilled depth is 400 μm. The focal point positioned accurately on the surface of the workp iece will ensure the optimu m drilling performance. After the in itial 100 laser pulse number were applied with focus fixed on the surface of the workp iece, the intensity in the beam was dropped rapidly. Hence, the laser drilling surface needed to be brought to within the DOF (i.e., ± 271 μm). When thicker materials are drilled, the depth of focus must be adapted to the material’s thickness in order to maintain intensity and drilling speed. Hence, the defocusing drilling depth on the target surface can be investigated by increasing the defocusing distance FD under fixed laser pulse power. The threshold value KRES is a good indicator to judge the progress of drilling. Since the brightness of the derived light is dominated by the defocusing, KRES can be used as the criterion to perform defocusing drilling by adjusting the
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focal position during the process. Fig. 8 shows the result of the defocusing experiment under fixed laser pulse power with addit ional 200 pulse nu mbers applied to measure the depth increment under different defocusing settings. The diagram shows that when defocus position in the range between 0.1 mm and 0.6 mm, the defocusing strategy has a positive effect on the average drilling depth. This informat ion can be used to monitor the brightness variation signal of the p lasma BVAR(N) to control feedback drilling. The maximu m increased depths at defocusing distance setting are FD = 0.3 mm and FD = 0.4 mm. Fig. 9 shows the resulting profile images for laser-drilled mach ining under different defocusing settings.
2.3 Defocusing Laser Percussion Drilling In the previous sections, the responding voltage variation BVAR(N) of the induced plasma was treated as the indicator to adjust the focal position. The derived light signal can be used as the indicator to adjust the defocus position according to the brightness variation of the induced plasma. Fig. 10 shows the experimental results in which the black line is the result of the focus fixed drilling, while the rest of data depict change in defocusing under fixed laser pulse power. The init ial focal position was just on the surface. When the responding voltage variation BVAR exceeded the threshold KRES, the defocus distance FD moved to 0.3 mm and then again to 0.4 mm according to the feedback of the brightness deviation induced by the plasma plu me. Three tests were carried out to confirm the reproducibility of the control process. Fig. 11 depicts the total final pulse nu mbers for focus-fixed drilling and defocusing drilling. Drilling efficiency was increased more than 47% by applying proposed defocusing laser percussion drilling than with no focus control. It is found the through holes with the in let diameter of 147 μm and aspect ratio around 7 was achieved for 1 mm thick stainless steel sheet. It was observed that the diameters of the inlet side of the percussion-drilled holes were considerably larger than the diameter of the laser spot on the material surface. Th is indicates that the high energy shock caused material to be ejected fro m the material surface [10]. The high degree of vaporizat ion in turn caused high vapor pressure to develop above the material surface, and this propelled molten material at the cavity to its rim. The hot plasma plu me fo rmed fro m the inside hole will lead to the sustaining outer ablation process. However, while drilling large aspect ratio holes, the volume of material removed per pulse is continuously decreasing with progressive shot due to the molten material is limited to eject fro m the entry-side and accu mulated inside the hole. While drilling large aspect ratio holes, the volume of material removed per pulse is continuously decreasing with progressive shot, which is resulted in the responding voltage of the coaxial photodiode declines slowly and finally the intensity of the light emitted fro m the plasma p lu me is too weak to be identified by the photodiode. Thus, the proposed method fails to pred ict the increased drilling depth when the responding voltage falls below the detection voltage level.
The material removal rate of nanosecond laser drilling can be affected by many factors such as laser fluence, irradiance, wavelength, duration, assisted gas and its pressure. In order to achieve high drilling rates in this work, the fluence was using 1.4×105 J/cm2 to co mpensate the low repetition rate (i.e., 15 Hz) and the irradiance was 2.10×106 W/cm2 . The material removal volu me was calculated based on the truncated cone shape with the thickness of 1000 μ m, and inlet diameter of 147 μm and outlet diameter of 60 μm (Fig. 12). The material removal rate was 0.12 mm3 / min by applying proposed defocusing laser percussion drilling. The material removal rate is similar with the work [11] at 10 kH z repetitive rate and the work [12] at 30 kHz repetitive rate. The laser irradiance of 1.50×109 W/cm2 for the work [11] and 6.30×107 W/cm2 for the work [12], respectively. However, the p roposed defocusing laser percussion drilling can achieve the similar drilling rate with lower laser irradiance.
Fig. 10: Experimental results of focus fixed drilling and defocusing laser percussion drilling
Fig. 11: Co mparison of breakthrough pulse numbers between defocusing laser percussion drilling and no focus control
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200 μm
[1]
(a) 100 μm
(b) Fig. 12: Optical microscopy images of laser-drilled holes with diameter measurements: (a) hole inlet and (b) hole outlet
3. CONCLUS IONS The paper demonstrates a strategy for monitoring and control of focus position during laser percussion drilling. It showed that a coaxial photodiode is capable of measuring deviation of plasma emission on-line, which provides the basic feedback for the imp rovement of drilling depth through defocus position control. The increment o f the brightness variation induced by plasma is associated with a decrease in drilling depth. The feasibility and improved efficiency of 47% with respect to focus fixed drilling for this defocusing feedback laser percussion drilling method was demonstrated. In the case of the ultrafast laser, the intensity was much higher than with the nanosecond laser. As a result, the brightness of the plasma induced by the ultrafast laser is much stronger than the nanosecond laser. Hence, the proposed method is applicable for ult rafast laser, and will be explored in the future. ACKNOWLEDGEMENTS The work was supported by the Ministry of Science and Technology, Taiwan, R.O.C. M OST 103-2221-E-224-030MY2 and 103-2622-E-224-001-CC2.
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