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A preliminary study on the effect of external magnetic fields on Laser-Induced Plasma Micromachining (LIPMM)
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ScienceDirect Manufacturing Letters 2 (2014) 54–59 www.elsevier.com/locate/mfglet
Research Letters
A preliminary study on the effect of external magnetic fields on Laser-Induced Plasma Micromachining (LIPMM) Sarah Wolff, Ishan Saxena ⇑ Department of Mechanical Engineering, Northwestern University, Evanston, IL 60201, USA Received 2 November 2013; received in revised form 19 February 2014; accepted 19 February 2014 Available online 27 February 2014
Abstract Laser Induced Plasma Micromachining (LIPMM) is a novel, tool-less micromachining process which offers machining characteristics superior to conventional laser ablation, such as multi-material capability, higher machined depth and better wall geometries. This study utilizes highly empirical methods for the purpose of a proof of concept and demonstrates the viability of using external magnetic fields in modifying the geometry and improving the aspect ratio of machined spots (up to 6) in LIPMM, which is accomplished by pulling the plasma spatially downward to machine spots with greater depth and consistent diameters, and to achieve horizontal squeezing of the plasma to create channels. Ó 2014 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
Keywords: Laser Induced Plasma Micromachining; Laser ablation; Magnetically controlled plasma
Laser-induced plasma micromachining (LIPMM) is a relatively new method of micro-manufacturing that utilizes the plasma generated in a dielectric to physically interact with a workpiece. LIPMM has demonstrated its ability to machine a variety of materials such as ceramics, metals, polymers and materials with special surface characteristics such as transparency and high reflectivity [1,2]. However, one of the potential advantages of LIPMM which has remained unexplored is the capability to manipulate plasma plume spatially using an external magnetic field, in order to add more flexibility to the process in terms of produced feature geometry and higher aspect ratios which would otherwise be impossible to achieve using regular LIPMM. This paper explores the method of magnetically-controlled LIPMM (MC-LIPMM) with the aim of producing higher aspect-ratio micro-features and modified feature geometries. Table 1 lists several of the advantages ⇑ Corresponding author. Tel.: +1 2245655792.
MC-LIPMM has over more conventional methods, including spot-based LIPMM [3–8]. During plasma generation, dielectric breakdown occurs inside the dielectric media, which may be distilled water, kerosene, mineral oil or another transparent dielectric liquid. Fig. 1 shows an example of plasma generated in a dielectric. When the peak irradiance of the ultra-short pulsed laser beam is greater than the dielectric ionization threshold potential, there is a generation of free electrons leading to the formation of a highly localized plasma zone through optical breakdown of the dielectric [9–11]. The laser induced plasma is then brought in physical contact with the workpiece, which results in vaporization of the material near surface. A key difference with this machining from others is that the mechanical energy in the plasma gives rise to shock waves and cavitation, replacing the dielectric and applying pressure to the workpiece. At the end of the pulse, the dielectric fills back into the void and flushes the debris off the surface of the workpiece, thereby facilitating material removal [12].
E-mail address: [email protected] (I. Saxena). http://dx.doi.org/10.1016/j.mfglet.2014.02.003 2213-8463/Ó 2014 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
55
Table 1 Advantages of MC-LIPMM over other existing processes. Existing process
Advantages of MC-LIPMM
Micro electrical discharge machining (micro-EDM)
Tool-less No need for electrical mechanism for plasma generation Capable of machining nonconductive materials More energy efficient material removal Improved material removal rate (MRR) Decreased heat affected zones (HAZ) Better geometric characteristics of machined features Better at machining materials with high reflectivity or low absorptivity Improved MRR Decreased HAZ Less dependence on process parameters Improved MRR In-process manipulation of the micro-feature without changing lens optics
Ultra-short laser ablation
Spot laser-induced plasma micromachining (S-LIPMM)
Figure 1. Plasma generation in dielectric within a petri dish under a laser beam.
The question of feasibility of MC-LIPMM [13] arises from analyses of the effects of magnetically-controlled micro-EDM, which shows that an external magnetic field led to relatively higher debris transportation and helped with flushing during micro-machining of micro-features
such as deep channels and holes [14]. Magnetic fields can change the shape of plasma; plasma expands along the direction of magnetic force and compresses in the direction normal to the force. In addition, a surrounding magnetic field can increase oscillating frequency of the plasma and therefore, increase its temperature and energy density [15]. The spatial position of plasma relative to the surrounding magnetic field is a crucial parameter. This study observes the plasma and machining characteristics as a result of various magnetic field configurations around the plasma plume. In addition to observing plasma behavior, more specific objectives include maximizing the aspect ratio of machined spots from the plasma; that is, to move the plasma downward to machine spots with more depth and with consistent diameters, and to achieve maximum squeezing of plasma to create machined channels on the workpiece. 1. Material and methods The laser system used in this study is a commercial (RAPID PS, Lumera) Nd-YVO4 solid-state ultra-short
Figure 2. Laser and positioning system.
56
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
Table 2 Magnetic field configurations.
12 neodymium magnets
6000
Distilled water
AA5052
Squeeze plasma for L-LIPMM within a repellent field.
pulsed laser that emits linearly polarized laser pulses of 8 ps pulse duration, at 532 nm wavelength (second harmonic). The beam is redirected to a substrate through a beam delivery system, which consists of a 5 beam expander and a diffraction limited focusing lens of 25 mm focal length. The beam has a Gaussian (TEM00) profile at laser exit. The workpiece is mounted on a positioning system, attached to the laser system. The layout of the laser and positioning systems are in Fig. 2. Four different configurations of substrate and magnetic field combinations were tested in addition to machining without an external magnetic field, as shown in Table 1. Polyamide (PA 66) and Aluminum Alloy AA5052 were used as substrates. Kerosene or distilled water were used as dielectric media for each setup. The ultra-high-pull neodymium rectangular magnets had dimensions of 1/400 in length, 1/400 in width and 1/800 in thickness and had 2.7 lb pull each, or an individual surface field of about 4400 Gauss (Table 2).
(d)
At the focal point of the laser, the plasma is in physical contact with the work piece, resulting in the vaporization of the material. At this point for each setup, the shutter of the system opened and the laser-induced plasma machined the substrate for five seconds, and the plasma plume was translated down (into the substrate) by 30 lm at a feed rate of 5 lm/s. Subsequently, the shutter was opened to translate the plasma about 200 lm in either the x or y direction. All experiments were completed with a pulse repetition frequency of 10 kHz and average beam power of 0.13 W. Measurements and surface visualization were carried out with an ALICONA Infinite Focus-Optical 3D white light interferometer with 50 nm resolution, as well as with an optical microscope. 2. Results and discussion Table 3 compares and details the measurements and aspect ratios of the micro-channels and craters for each
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
57
Table 3 Comparison of resulting machined shape dimensions with different magnetic configurations. Magnet configuration type
Machined shapes
Approximate shape dimensions
Maximum Aspect Ratio
8 neodymium magnets
Channel
1
12 neodymium magnets
Elliptic crater
1 neodymium magnet
Crater Elliptic crater
12 neodymium magnets
Crater Elliptic crater
100–200 lm in length 30 lm width 30 lm depth Nearest proximity (750 lm) to field: 100 lm length 20 lm width 40 lm depth Farthest away (450 lm) from magnetic field: 20 lm diameter 30 lm depth Nearest proximity (150 lm) to field: 50 lm length 15 lm width 90 lm depth Farthest away (450 lm) from magnetic field: 200 lm diameter 150 lm depth Nearest proximity (150 lm) to field: 175 lm length 160 lm width 140 lm depth
2
6
0.875
Figure 3. Cross-section profiles of features near the magnetic field in the magnetic configurations (a) to (d), with Z depth on the y-axis and path length on the x-axis.
setup. The key observations are that external magnetic fields surrounding the spot plasma uniaxially squeeze the plasma horizontally to machine a channel or an elliptic crater, whereas a magnetic field underneath the substrate attracts the plasma downward to create a machined crater with a larger aspect ratio. Fig. 3 presents cross-section
profile views of features corresponding to dimensions in Table 3. Micro-channels were up to about 100–200 lm long, 10–30 lm wide and 30–85 lm in depth. The craters exhibited aspect ratios of up to about 6. Features machined on the surface of the workpiece exhibited channel-like
58
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
Figure 4. Elliptic craters are effects of a magnetic field on LIP-MM on an AA5052 workpiece.
characteristics closer to the surface field of the surrounding magnets and more spot-like characteristics where the surface field was closer to 0 Gauss at the center. A pattern of channel-like plasma machining (L-LIPMM) to spot-like machining (S-LIPMM) could be observed with a variety of the empirical setups, as seen in Fig. 4. Fig. 4 corresponds to configuration (b), with 12 neodymium magnets and kerosene dielectric where elliptic craters were machined 250 lm apart with the left-most crater 750 lm away from the surface of the magnet. Only heat affected zones (HAZ) and no machining were observed with laser-induced plasma without an external magnetic field at the same settings and focal point as the trials with external magnetic fields. Some factors to keep in consideration include the temperature threshold of the rare earth magnets. Above certain temperatures, the permanent magnets may lose a fraction of their magnetic strength. However, a key factor in MCLIPMM is the amount of control in steadying the magnetic fields surrounding the workpiece and comparing the topology and machining characteristics of resulting channels and spots to those machined without external fields. There is a large amount of variability with the strength of the magnetic field, which is largely dependent on the location of plasma relative to the field and the feed rate of the plasma during machining. More control in the location of the plasma as well as the laser beam relative to the different areas of surrounding magnetic fields can lead to more informed machining and improved aspect ratios. Testing with electromagnets and repeated trials can possibly alleviate the control issue. 3. Conclusions Preliminary results of the MC-LIPMM process show that larger external magnetic fields relative to the region of machining can squeeze or stretch the plasma, leading to channels and spots with larger aspect ratios compared
to LIPMM without external magnetic fields. Suggested future work includes multiple trials with a larger variety of magnet setups and configurations, including electromagnets, substrate materials and controls to help fulfill this proof of concept. Model formulation and optimization methods to control and test specific parameters such as field strength, frequency and power of the laser and feed rate are necessary for more predictable and efficient LIPMM process. In-situ manipulation of the process can ultimately lead to complex surface texture geometries and faster machining times. References [1] Pallav K, Han P, Ramkumar J, Nagahanumaiah, Ehmann KF. Comparative assessment of the laser induced plasma micromachining and the micro-EDM processes. J. Manufact. Sci. Eng. 2013;136:011001. [2] Pallav K, Saxena I, Ehmann K. Laser induced plasma micromachining process – principles and performance. Int. J. Mach. Tools Manufact. 2014 (To Appear). [3] Pallav K. Laser Induced Plasma Micro-machining Process (LIPMM). Ph.D. Dissertation. Evanston, IL: Mechanical Engineering, Northwestern University; 2013. [4] Pallav K, Ehmann KF. Laser induced plasma micro-machining. ASME Conf. Proc. 2010;2010:363–9. [5] Pallav K, Ehmann KF. Feasibility of laser induced plasma micromachining (LIP-MM). In: Ratchev S, editor. Precision assembly technologies and systems. Springer; 2010. p. 73–80. [6] Pallav K, Saxena I, Ehmann K. Comparative assessment of the laser induced plasma micro-machining (LIP-MM) and the ultra-short pulsed laser ablation processes. J. Micro Nano-Manuf. 2014 (To appear). [7] Pallav K, Ehmann K. Numerical simulation of the laser induced plasma micro-machining process (LIP-MM). In: Proceedings of the International Workshop on Micro-factories (IWMF2010), Daejeon, Korea; 2010. [8] Pallav K, Ehmann K. Laser-Induced Plasma Micro-machining. In: Proceedings of the 2010 ISFA – International Symposium on Flexible Automation, Tokyo, Japan; 2010. [9] Docchio F. Lifetimes of plasmas induced in liquids and ocular media by single Nd: YAG laser pulses of different duration. EPL (Europhys. Lett.) 2007;6:407.
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59 [10] Sacchi C. Laser-induced electric breakdown in water. JOSA B 1991;8:337–45. [11] Nahen K, Vogel A. Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. II. Transmission, scattering, and reflection. IEEE J Selected Topics Quant Electron 1996;2:861–71. [12] Zhang J, Sugioka K, Midorikawa K. High-speed machining of glass materials by laser-induced plasma-assisted ablation using a 532-nm laser. Appl Phys A 1998;67:499–501. [13] Malhotra R, Saxena I, Ehmann K, Cao J. Laser-induced plasma micro-machining (LIPMM) for enhanced productivity and flexibility
59
in laser-based micro-machining processes. CIRP Ann Manufact Technol 2013. [14] Yeo S, Murali M, Cheah H. Magnetic field assisted micro electrodischarge machining. J Micromech Microeng 2004;14:1526. [15] Harilal S, Tillack M, O’shay B, Bindhu C, Najmabadi F. Confinement and dynamics of laser-produced plasma expanding across a transverse magnetic field. Phys Rev E 2004;69:026413.
ScienceDirect Manufacturing Letters 2 (2014) 54–59 www.elsevier.com/locate/mfglet
Research Letters
A preliminary study on the effect of external magnetic fields on Laser-Induced Plasma Micromachining (LIPMM) Sarah Wolff, Ishan Saxena ⇑ Department of Mechanical Engineering, Northwestern University, Evanston, IL 60201, USA Received 2 November 2013; received in revised form 19 February 2014; accepted 19 February 2014 Available online 27 February 2014
Abstract Laser Induced Plasma Micromachining (LIPMM) is a novel, tool-less micromachining process which offers machining characteristics superior to conventional laser ablation, such as multi-material capability, higher machined depth and better wall geometries. This study utilizes highly empirical methods for the purpose of a proof of concept and demonstrates the viability of using external magnetic fields in modifying the geometry and improving the aspect ratio of machined spots (up to 6) in LIPMM, which is accomplished by pulling the plasma spatially downward to machine spots with greater depth and consistent diameters, and to achieve horizontal squeezing of the plasma to create channels. Ó 2014 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
Keywords: Laser Induced Plasma Micromachining; Laser ablation; Magnetically controlled plasma
Laser-induced plasma micromachining (LIPMM) is a relatively new method of micro-manufacturing that utilizes the plasma generated in a dielectric to physically interact with a workpiece. LIPMM has demonstrated its ability to machine a variety of materials such as ceramics, metals, polymers and materials with special surface characteristics such as transparency and high reflectivity [1,2]. However, one of the potential advantages of LIPMM which has remained unexplored is the capability to manipulate plasma plume spatially using an external magnetic field, in order to add more flexibility to the process in terms of produced feature geometry and higher aspect ratios which would otherwise be impossible to achieve using regular LIPMM. This paper explores the method of magnetically-controlled LIPMM (MC-LIPMM) with the aim of producing higher aspect-ratio micro-features and modified feature geometries. Table 1 lists several of the advantages ⇑ Corresponding author. Tel.: +1 2245655792.
MC-LIPMM has over more conventional methods, including spot-based LIPMM [3–8]. During plasma generation, dielectric breakdown occurs inside the dielectric media, which may be distilled water, kerosene, mineral oil or another transparent dielectric liquid. Fig. 1 shows an example of plasma generated in a dielectric. When the peak irradiance of the ultra-short pulsed laser beam is greater than the dielectric ionization threshold potential, there is a generation of free electrons leading to the formation of a highly localized plasma zone through optical breakdown of the dielectric [9–11]. The laser induced plasma is then brought in physical contact with the workpiece, which results in vaporization of the material near surface. A key difference with this machining from others is that the mechanical energy in the plasma gives rise to shock waves and cavitation, replacing the dielectric and applying pressure to the workpiece. At the end of the pulse, the dielectric fills back into the void and flushes the debris off the surface of the workpiece, thereby facilitating material removal [12].
E-mail address: [email protected] (I. Saxena). http://dx.doi.org/10.1016/j.mfglet.2014.02.003 2213-8463/Ó 2014 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
55
Table 1 Advantages of MC-LIPMM over other existing processes. Existing process
Advantages of MC-LIPMM
Micro electrical discharge machining (micro-EDM)
Tool-less No need for electrical mechanism for plasma generation Capable of machining nonconductive materials More energy efficient material removal Improved material removal rate (MRR) Decreased heat affected zones (HAZ) Better geometric characteristics of machined features Better at machining materials with high reflectivity or low absorptivity Improved MRR Decreased HAZ Less dependence on process parameters Improved MRR In-process manipulation of the micro-feature without changing lens optics
Ultra-short laser ablation
Spot laser-induced plasma micromachining (S-LIPMM)
Figure 1. Plasma generation in dielectric within a petri dish under a laser beam.
The question of feasibility of MC-LIPMM [13] arises from analyses of the effects of magnetically-controlled micro-EDM, which shows that an external magnetic field led to relatively higher debris transportation and helped with flushing during micro-machining of micro-features
such as deep channels and holes [14]. Magnetic fields can change the shape of plasma; plasma expands along the direction of magnetic force and compresses in the direction normal to the force. In addition, a surrounding magnetic field can increase oscillating frequency of the plasma and therefore, increase its temperature and energy density [15]. The spatial position of plasma relative to the surrounding magnetic field is a crucial parameter. This study observes the plasma and machining characteristics as a result of various magnetic field configurations around the plasma plume. In addition to observing plasma behavior, more specific objectives include maximizing the aspect ratio of machined spots from the plasma; that is, to move the plasma downward to machine spots with more depth and with consistent diameters, and to achieve maximum squeezing of plasma to create machined channels on the workpiece. 1. Material and methods The laser system used in this study is a commercial (RAPID PS, Lumera) Nd-YVO4 solid-state ultra-short
Figure 2. Laser and positioning system.
56
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
Table 2 Magnetic field configurations.
12 neodymium magnets
6000
Distilled water
AA5052
Squeeze plasma for L-LIPMM within a repellent field.
pulsed laser that emits linearly polarized laser pulses of 8 ps pulse duration, at 532 nm wavelength (second harmonic). The beam is redirected to a substrate through a beam delivery system, which consists of a 5 beam expander and a diffraction limited focusing lens of 25 mm focal length. The beam has a Gaussian (TEM00) profile at laser exit. The workpiece is mounted on a positioning system, attached to the laser system. The layout of the laser and positioning systems are in Fig. 2. Four different configurations of substrate and magnetic field combinations were tested in addition to machining without an external magnetic field, as shown in Table 1. Polyamide (PA 66) and Aluminum Alloy AA5052 were used as substrates. Kerosene or distilled water were used as dielectric media for each setup. The ultra-high-pull neodymium rectangular magnets had dimensions of 1/400 in length, 1/400 in width and 1/800 in thickness and had 2.7 lb pull each, or an individual surface field of about 4400 Gauss (Table 2).
(d)
At the focal point of the laser, the plasma is in physical contact with the work piece, resulting in the vaporization of the material. At this point for each setup, the shutter of the system opened and the laser-induced plasma machined the substrate for five seconds, and the plasma plume was translated down (into the substrate) by 30 lm at a feed rate of 5 lm/s. Subsequently, the shutter was opened to translate the plasma about 200 lm in either the x or y direction. All experiments were completed with a pulse repetition frequency of 10 kHz and average beam power of 0.13 W. Measurements and surface visualization were carried out with an ALICONA Infinite Focus-Optical 3D white light interferometer with 50 nm resolution, as well as with an optical microscope. 2. Results and discussion Table 3 compares and details the measurements and aspect ratios of the micro-channels and craters for each
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
57
Table 3 Comparison of resulting machined shape dimensions with different magnetic configurations. Magnet configuration type
Machined shapes
Approximate shape dimensions
Maximum Aspect Ratio
8 neodymium magnets
Channel
1
12 neodymium magnets
Elliptic crater
1 neodymium magnet
Crater Elliptic crater
12 neodymium magnets
Crater Elliptic crater
100–200 lm in length 30 lm width 30 lm depth Nearest proximity (750 lm) to field: 100 lm length 20 lm width 40 lm depth Farthest away (450 lm) from magnetic field: 20 lm diameter 30 lm depth Nearest proximity (150 lm) to field: 50 lm length 15 lm width 90 lm depth Farthest away (450 lm) from magnetic field: 200 lm diameter 150 lm depth Nearest proximity (150 lm) to field: 175 lm length 160 lm width 140 lm depth
2
6
0.875
Figure 3. Cross-section profiles of features near the magnetic field in the magnetic configurations (a) to (d), with Z depth on the y-axis and path length on the x-axis.
setup. The key observations are that external magnetic fields surrounding the spot plasma uniaxially squeeze the plasma horizontally to machine a channel or an elliptic crater, whereas a magnetic field underneath the substrate attracts the plasma downward to create a machined crater with a larger aspect ratio. Fig. 3 presents cross-section
profile views of features corresponding to dimensions in Table 3. Micro-channels were up to about 100–200 lm long, 10–30 lm wide and 30–85 lm in depth. The craters exhibited aspect ratios of up to about 6. Features machined on the surface of the workpiece exhibited channel-like
58
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59
Figure 4. Elliptic craters are effects of a magnetic field on LIP-MM on an AA5052 workpiece.
characteristics closer to the surface field of the surrounding magnets and more spot-like characteristics where the surface field was closer to 0 Gauss at the center. A pattern of channel-like plasma machining (L-LIPMM) to spot-like machining (S-LIPMM) could be observed with a variety of the empirical setups, as seen in Fig. 4. Fig. 4 corresponds to configuration (b), with 12 neodymium magnets and kerosene dielectric where elliptic craters were machined 250 lm apart with the left-most crater 750 lm away from the surface of the magnet. Only heat affected zones (HAZ) and no machining were observed with laser-induced plasma without an external magnetic field at the same settings and focal point as the trials with external magnetic fields. Some factors to keep in consideration include the temperature threshold of the rare earth magnets. Above certain temperatures, the permanent magnets may lose a fraction of their magnetic strength. However, a key factor in MCLIPMM is the amount of control in steadying the magnetic fields surrounding the workpiece and comparing the topology and machining characteristics of resulting channels and spots to those machined without external fields. There is a large amount of variability with the strength of the magnetic field, which is largely dependent on the location of plasma relative to the field and the feed rate of the plasma during machining. More control in the location of the plasma as well as the laser beam relative to the different areas of surrounding magnetic fields can lead to more informed machining and improved aspect ratios. Testing with electromagnets and repeated trials can possibly alleviate the control issue. 3. Conclusions Preliminary results of the MC-LIPMM process show that larger external magnetic fields relative to the region of machining can squeeze or stretch the plasma, leading to channels and spots with larger aspect ratios compared
to LIPMM without external magnetic fields. Suggested future work includes multiple trials with a larger variety of magnet setups and configurations, including electromagnets, substrate materials and controls to help fulfill this proof of concept. Model formulation and optimization methods to control and test specific parameters such as field strength, frequency and power of the laser and feed rate are necessary for more predictable and efficient LIPMM process. In-situ manipulation of the process can ultimately lead to complex surface texture geometries and faster machining times. References [1] Pallav K, Han P, Ramkumar J, Nagahanumaiah, Ehmann KF. Comparative assessment of the laser induced plasma micromachining and the micro-EDM processes. J. Manufact. Sci. Eng. 2013;136:011001. [2] Pallav K, Saxena I, Ehmann K. Laser induced plasma micromachining process – principles and performance. Int. J. Mach. Tools Manufact. 2014 (To Appear). [3] Pallav K. Laser Induced Plasma Micro-machining Process (LIPMM). Ph.D. Dissertation. Evanston, IL: Mechanical Engineering, Northwestern University; 2013. [4] Pallav K, Ehmann KF. Laser induced plasma micro-machining. ASME Conf. Proc. 2010;2010:363–9. [5] Pallav K, Ehmann KF. Feasibility of laser induced plasma micromachining (LIP-MM). In: Ratchev S, editor. Precision assembly technologies and systems. Springer; 2010. p. 73–80. [6] Pallav K, Saxena I, Ehmann K. Comparative assessment of the laser induced plasma micro-machining (LIP-MM) and the ultra-short pulsed laser ablation processes. J. Micro Nano-Manuf. 2014 (To appear). [7] Pallav K, Ehmann K. Numerical simulation of the laser induced plasma micro-machining process (LIP-MM). In: Proceedings of the International Workshop on Micro-factories (IWMF2010), Daejeon, Korea; 2010. [8] Pallav K, Ehmann K. Laser-Induced Plasma Micro-machining. In: Proceedings of the 2010 ISFA – International Symposium on Flexible Automation, Tokyo, Japan; 2010. [9] Docchio F. Lifetimes of plasmas induced in liquids and ocular media by single Nd: YAG laser pulses of different duration. EPL (Europhys. Lett.) 2007;6:407.
S. Wolff, I. Saxena / Manufacturing Letters 2 (2014) 54–59 [10] Sacchi C. Laser-induced electric breakdown in water. JOSA B 1991;8:337–45. [11] Nahen K, Vogel A. Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. II. Transmission, scattering, and reflection. IEEE J Selected Topics Quant Electron 1996;2:861–71. [12] Zhang J, Sugioka K, Midorikawa K. High-speed machining of glass materials by laser-induced plasma-assisted ablation using a 532-nm laser. Appl Phys A 1998;67:499–501. [13] Malhotra R, Saxena I, Ehmann K, Cao J. Laser-induced plasma micro-machining (LIPMM) for enhanced productivity and flexibility
59
in laser-based micro-machining processes. CIRP Ann Manufact Technol 2013. [14] Yeo S, Murali M, Cheah H. Magnetic field assisted micro electrodischarge machining. J Micromech Microeng 2004;14:1526. [15] Harilal S, Tillack M, O’shay B, Bindhu C, Najmabadi F. Confinement and dynamics of laser-produced plasma expanding across a transverse magnetic field. Phys Rev E 2004;69:026413.