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Study on the relationship between laser processing sound and material removal characteristics
Journal of Materials Processing Technology 97 (2000) 168±173
Study on the relationship between laser processing sound and material removal characteristics T. Kuritaa, T. Onob,*, N. Moritac a
Graduate School, Tokyo Metropolitan Institute of Technology, 6-6 Asahigaoka, Hino, Tokyo, Japan b Department of Mechanical Systems Engineering, Faculty of Engineering, Tokyo Metropolitan Institute of Technology, 6-6 Asahigaoka, Hino, Tokyo, Japan c Department of Mechanical Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,Chiba-shi, Chiba, Japan Received 13 August 1998
Abstract Laser processing is an important manufacturing technology in machining dif®cult-to-cut materials. It is known that a sound is generated when laser processing is carried out, the intensity of the sound changing according to the processing conditions. The purpose of this research is to clarify experimentally the relationship between the material removal characteristics and the pressure level of the processing sound when a low and a high frequency laser beam are applied to the processing of ceramic materials. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Low and high frequency Q-switched YAG laser; Ceramic material; Material removal volume; Sound pressure level
1. Introduction Laser processing is characterized by various types of materials, such as a hard metal and a ceramic, which can be processed under high-speed conditions, because it is possible to concentrate the input laser energy to a small area. On the other hand, there are energy losses due to plasma and the re¯ection of the laser beam on the surface of the work material, so only a few percent of the input energy is actually utilized for the material processing. In laser processing, heating, melting and vaporization of material occur continuously when a high power laser energy is applied, such phenomena making it dif®cult to control the laser processing. In the case of a material removal process, for instance, it is very dif®cult to keep the removal depth of the material and the processing accuracy at a constant level without applying laser process control. When considering these technological aspects, it may be necessary to realize a high accuracy control system for laser processing, in which the processing information is monitored and is transferred to a laser machine controller to activate a feed-back system. It * Corresponding author. Tel/Fax.:81-42-585-8629 E-mail address: [email protected] (T. Ono)
is known that sound is generated during laser processing and that the intensity of the sound changes according to the processing conditions [1±7]. It has been clari®ed experimentally that there exists a relationship between the focusing position of a laser beam and the processing sound, and the auto-focusing system has been developed in the case of laser drilling [3] based on the results of this feature. In order to utilize the detected laser processing sound to control the accuracy of laser processing, it is essential to clarify the relationship between the generated sound and the laser processing characteristics such as the material removal volume and the surface roughness. The purpose of the present research is to clarify experimentally the relationship between the material removal volume and the pressure level of the processing sound for a small material removal volume by utilizing a low frequency Q-switched YAG laser beam, and also to con®rm the application of these experimental results to laser grooving when a high frequency Q-switched YAG laser beam is applied. An AE (acoustic emission) sensor is usually used to monitor the state of laser processing, because the generated sound of laser processing contains high-frequency components up to 100 kHz [3]. However, in this research, the processing sound was monitored with a microphone from the stand-point of practical
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 3 7 1 - 4
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
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application, and the characteristics of the sound frequency up to 20 kHz were analyzed with an FFT (fast fourier transform). 2. Experimental set-up and experimental conditions 2.1. Experimental set-up Fig. 1 shows the experimental set-up for the sensing and for the frequency analysis of the laser processing sound. The work material was set on the numerically controlled table, and a laser beam was irradiated to the pre-determined position. The sound generated was detected by a condenser microphone which was set at a distance of 40 mm from the incident position of the laser beam with an inclination of 308, and was recorded by a DAT (digital audio tape) data recorder. The frequency characteristics and sound pressure levels of the detected sound were analyzed by means of an FFT. The variation in the pressure level at the speci®ed frequency versus laser processing time was analyzed using the time tracking function of an FFT. 2.2. Experimental conditions A continuous excitation YAG laser oscillator, to which an AO-Q switch is attached, was used. The oscillated laser beam pattern was TEM00 single mode. Fig. 2(a) and (b) show two types of laser beam used for the experiments into laser processing, as described below. (1) A low frequency Q-switched YAG laser beam: Laser drilling was carried out with a low frequency Q-switched YAG laser beam. In this case, a single laser beam, which was controlled with the outer electric circuit, was irradiated to the surface of the work material. In this report, a laser beam irradiated by this method is described as ``a single pulse laser beam''. The work material was set on a numerically controlled table, and a single pulse laser beam was irradiated repeatedly every second. (Al2O3 TiC) ceramic and
Fig. 2. Two types of laser beam used for the experiments.
(WC Co) sintered carbide were used as the work materials, their dimensions being 12.7 mm 12.7 mm 4.76 mm. The diameter of the focused beam was 35 mm and the applied laser energy was 0.78 mJ/pulse. The maximum incidence times of the single pulse laser beam were 500. (2) A high frequency Q-switched YAG laser beam: Laser grooving was carried out using a high frequency Q-switched YAG laser beam. The irradiation mode of this type of laser beam is described as ``a continuous pulse laser beam'' in this report. The work material used was (Al2O3 TiC) ceramic with the dimension of 12.7 mm 12.7 mm 4.76 mm, and the processing conditions were set as follows: the processing speed was kept constant at 1 mm/s; the applied average laser energy was 1.10 W; the Q-switch frequency was 2 kHz; and the focused diameter of the laser beam was set at 35 and 90 mm. 3. Experimental analysis of laser processing sound The distribution pattern of the pressure level of a detected laser processing sound differs greatly according to the characteristics of the irradiated beam. 3.1. The distribution pattern of the sound pressure level in the case of a single pulse laser beam
Fig. 1. Experimental set-up for the detection and an analysis of laser processing sound.
Fig. 3 shows the relationship between the sound pressure levels and frequencies of a detected laser processing sound when a single pulse laser beam was irradiated to the surface of the work material. A distinctive phenomenon shown in
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T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
latter is called a time tracking diagram of the sound pressure level. Peak values of the sound pressure level were detected at the integer folds of a selected Q-switch frequency when a continuous pulse laser beam was irradiated for grooving. In order to investigate the relationship between the depth of the groove and the sound pressure level, the value of the sound pressure level at the frequency of 2 and 18 kHz was used. 4. Experimental results and discussion 4.1. The relationship between the material removal volume when a single pulse laser beam is used for laser drilling Fig. 3. Relationship between the detected frequencies and sound pressure level for a single pulse laser beam (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Work material: (Al2O3 TiC) ceramic.
this ®gure is that the value of the sound pressure level is distributed over a wide range of frequency, so it is dif®cult to identify exactly the frequency showing the maximum sound pressure level. In this report, the sound pressure level at a frequency of 14 kHz was selected as the maximum value in order to investigate the relationship between the material removal volume and the sound pressure level. 3.2. The distribution of sound pressure level in the case of a continuous pulse laser beam Fig. 4 shows 3D diagram of the characteristics of the laser processing sound when a continuous pulse laser beam of 2 kHz was applied to the work material, which was moved with a processing speed at 1 mm/s for laser grooving. It may be possible to clarify the relationship between frequencies and sound pressure level, and the variation of sound pressure level versus processing time at a speci®c frequency. The
In order to clarify the relationship between a small material removal volume and the sound pressure level, a single pulse laser beam (1 Hz) was irradiated for laser drilling. The processed work material was ground by #1000 diamond grinding wheel until the maximum depth of the processed hole could be observed. Fig. 5(a) and (b) show the cross-sections of processed holes when the incident times of the single pulse laser beam were changed from 10 to 300 for (a) an (Al2O3 TiC) ceramic and (b) a (WC Co) sintered carbide. It could be observed that the depth of processed hole becomes greater with the increase of the incident times of a single pulse laser beam. The processed cross-section of an (Al2O3 TiC) ceramic was triangular in shape, but rectangular in the case of a (WC TiC) sintered carbide. The material removal volume is represented by using the area of cross-section of the processed holes, because the removed volume differed even if the depth of the groove had the same value. Fig. 6 shows the relationships between the accumulated area of the cross-section of processed hole calculated using SEM images and the maximum sound pressure level versus the number of incidences for an (Al2O3 TiC) ceramic. The accumulated cross-section
Fig. 4. 3D diagram of the characteristic of the laser processing sound for a continuous pulse laser beam (Laser energy (average): 1.10 W; Diameter of focus: 35 mm; Processing speed: 1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic.
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
Fig. 5. SEM images of processed holes (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm).
171
relationship between the material removal volume for each laser beam incidence and the sound pressure level. However, only a small amount of work material was removed when a single pulsed laser beam was applied only once, so the accuracy of the calculation of the cross-section from the SEM images shown in Fig. 5 could not be ensured. In this experiment, the following procedures were taken to deduce the removed cross-section for the one-time incidence of a single pulse laser beam by referring to Fig. 6. It is assumed that the accumulated cross-section is S when a single pulse laser beam is irradiated N times, and S0 for N0 times. Then, the removed cross-section becomes (S ÿ S0) at (N ÿ N0) times incidence, and f SÿS0 = NÿN0 gN6N0 can be de®ned as the removed cross-section at the Nth incidence. Based on this concept, the removed cross-section at an arbitrary incident time that corresponds to N was measured using Fig. 6. The same experiment was carried out for (WC Co) sintered carbide as a work material. Fig. 7 shows the relationship between the removed cross-section per one incidence of a single pulsed laser beam and the maximum sound pressure level at the frequency of 14 kHz for both an (Al2O3 TiC) ceramic and (WC Co) sintered carbide. It was clari®ed experimentally that the sound pressure level increased as the increase of removed cross-section, and that the relationship between them can be expressed by straight lines approximately. 4.2. The relationship between the depth of groove and the sound pressure level when a continuous pulse laser beam is used for laser grooving
Fig. 6. Sound pressure level and accumulated area of cross-section versus incident times for a single pulse laser beam (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Q-sw. frequency: 1 Hz; Work material: (Al2O3 TiC) ceramic).
increased as the increase of the incident times of a single pulse laser beam, but the maximum sound pressure level decreased monotonously. The purpose of the application of a single pulse laser beam for laser drilling is to clarify the
It is possible to process a groove by applying a continuous pulse laser beam and by traversing a work material at a speci®c processing speed. Experiments were carried out to con®rm the relationship between the depth of the groove and the pressure level of the processing sound when a continuous pulse laser beam was applied for laser grooving. Fig. 8 shows the time tracking of the values of sound pressure level at the frequencies of 2 and 18 kHz of the processing sound
Fig. 7. Relationship between the meterial removal area per pulse and the sound pressure level (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Q-sw. frequency: 1 Hz).
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T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
level at the frequencies of 2 and 18 kHz of the laser processing sound was measured with the time tracking function of an FFT. Both the diagrams of the bottom roughness of the groove and the variation of sound pressure level were processed with an image processor and are presented versus the processing time in Fig. 8. The one-to-one relationship between the value of sound pressure level and the depth of the groove could be measure using this ®gure. Fig. 9 shows the relationship between the depth of the groove and the sound pressure level at the same processing time. The value of the sound pressure level increased when a deeper groove was processed, and the relationship between the depth of the groove and the sound pressure level could also be indicated approximately by a straight line when a continuous pulse laser beam was applied. 5. Conclusions Experiments were carried out in order to clarify the relationship between the pressure level of a laser processing sound and the material removal characteristics when both (Al2O3 TiC) ceramic and a (WC Co) sintered carbide were processed by two types of YAG laser beam. The main conclusions were as follows. Fig. 8. Fluctuation of the sound pressure level and the depth of the groove versus the processing time (Laser energy (average): 1.10W; Processing speed: 0.1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic).
and the bottom roughness of the processed groove versus processing time. The black-colored part shown in Fig. 8 is the cross-section of a processed groove that was taken with SEM. This section was generated by grinding with a #1000 diamond wheel parallel to the scanning direction of the laser beam until the deepest position of the groove was observed. On the other hand, the variation diagram of sound pressure
1. The sound pressure level was distributed continuously from around 8 to 20 kHz frequencies when laser drilling was carried out by utilizing a low frequency pulse (1 Hz) laser beam. 2. When a single pulse laser beam was used, the relationship between the small material removal volume per one incidence of the laser beam and the value of the sound pressure level at around 14 kHz could be represented by a straight line for a constant initial diameter of hole. This phenomenon was verified experimentally for both (Al2O3 TiC) ceramic and (WC Co) sintered carbide.
Fig. 9. Relationship between the depth of the groove and the sound pressure level (Laser energy (average):1.10 W; Diameter of focus: 90 mm; Processing speed: 0.1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic).
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
3. The peak values of the sound pressure level were detected at the integer folds of a selected Q-switch frequency when a continuous pulse laser beam was applied for laser grooving, and the values of the sound pressure level fluctuated with the progress of the laser processing. 4. In the case of laser grooving by a continuous pulse laser beam under constant diameter of laser beam focus and constant processing speed, the relationship between the depth of the groove and the value of the sound pressure level can be expressed approximately by a straight line.
173
References [1] G.V. Arbach, R.L. Melcher, C.E. Scranton, IBM Tech. Disclosure Bull. 25 (1983) 5092. [2] C.E. Yeack, R.L. Melcher, H.E. Klauser, App. Phys. Lett. 44 (1982) 1043. [3] T. Miyazaki, T. Uyemura, Y. Yamamoto, Ann. CIRP. 22 (1973) 67. [4] M.T. Brienza, A.J. DeMaria, Appl. Phys. Lett. 11 (1967) 44. [5] W.W. Duley, Y.L. Mao, Appl. Phys. 27 (1994) 1379. [6] R.D. Rawlings, W.M. Steen, Optics and Laser Eng. 2 (1981) 173. [7] P. Sheng, G. Chryssolouris, J. Mater. Proc. Technol. 43 (1994) 145.
Study on the relationship between laser processing sound and material removal characteristics T. Kuritaa, T. Onob,*, N. Moritac a
Graduate School, Tokyo Metropolitan Institute of Technology, 6-6 Asahigaoka, Hino, Tokyo, Japan b Department of Mechanical Systems Engineering, Faculty of Engineering, Tokyo Metropolitan Institute of Technology, 6-6 Asahigaoka, Hino, Tokyo, Japan c Department of Mechanical Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,Chiba-shi, Chiba, Japan Received 13 August 1998
Abstract Laser processing is an important manufacturing technology in machining dif®cult-to-cut materials. It is known that a sound is generated when laser processing is carried out, the intensity of the sound changing according to the processing conditions. The purpose of this research is to clarify experimentally the relationship between the material removal characteristics and the pressure level of the processing sound when a low and a high frequency laser beam are applied to the processing of ceramic materials. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Low and high frequency Q-switched YAG laser; Ceramic material; Material removal volume; Sound pressure level
1. Introduction Laser processing is characterized by various types of materials, such as a hard metal and a ceramic, which can be processed under high-speed conditions, because it is possible to concentrate the input laser energy to a small area. On the other hand, there are energy losses due to plasma and the re¯ection of the laser beam on the surface of the work material, so only a few percent of the input energy is actually utilized for the material processing. In laser processing, heating, melting and vaporization of material occur continuously when a high power laser energy is applied, such phenomena making it dif®cult to control the laser processing. In the case of a material removal process, for instance, it is very dif®cult to keep the removal depth of the material and the processing accuracy at a constant level without applying laser process control. When considering these technological aspects, it may be necessary to realize a high accuracy control system for laser processing, in which the processing information is monitored and is transferred to a laser machine controller to activate a feed-back system. It * Corresponding author. Tel/Fax.:81-42-585-8629 E-mail address: [email protected] (T. Ono)
is known that sound is generated during laser processing and that the intensity of the sound changes according to the processing conditions [1±7]. It has been clari®ed experimentally that there exists a relationship between the focusing position of a laser beam and the processing sound, and the auto-focusing system has been developed in the case of laser drilling [3] based on the results of this feature. In order to utilize the detected laser processing sound to control the accuracy of laser processing, it is essential to clarify the relationship between the generated sound and the laser processing characteristics such as the material removal volume and the surface roughness. The purpose of the present research is to clarify experimentally the relationship between the material removal volume and the pressure level of the processing sound for a small material removal volume by utilizing a low frequency Q-switched YAG laser beam, and also to con®rm the application of these experimental results to laser grooving when a high frequency Q-switched YAG laser beam is applied. An AE (acoustic emission) sensor is usually used to monitor the state of laser processing, because the generated sound of laser processing contains high-frequency components up to 100 kHz [3]. However, in this research, the processing sound was monitored with a microphone from the stand-point of practical
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 3 7 1 - 4
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
169
application, and the characteristics of the sound frequency up to 20 kHz were analyzed with an FFT (fast fourier transform). 2. Experimental set-up and experimental conditions 2.1. Experimental set-up Fig. 1 shows the experimental set-up for the sensing and for the frequency analysis of the laser processing sound. The work material was set on the numerically controlled table, and a laser beam was irradiated to the pre-determined position. The sound generated was detected by a condenser microphone which was set at a distance of 40 mm from the incident position of the laser beam with an inclination of 308, and was recorded by a DAT (digital audio tape) data recorder. The frequency characteristics and sound pressure levels of the detected sound were analyzed by means of an FFT. The variation in the pressure level at the speci®ed frequency versus laser processing time was analyzed using the time tracking function of an FFT. 2.2. Experimental conditions A continuous excitation YAG laser oscillator, to which an AO-Q switch is attached, was used. The oscillated laser beam pattern was TEM00 single mode. Fig. 2(a) and (b) show two types of laser beam used for the experiments into laser processing, as described below. (1) A low frequency Q-switched YAG laser beam: Laser drilling was carried out with a low frequency Q-switched YAG laser beam. In this case, a single laser beam, which was controlled with the outer electric circuit, was irradiated to the surface of the work material. In this report, a laser beam irradiated by this method is described as ``a single pulse laser beam''. The work material was set on a numerically controlled table, and a single pulse laser beam was irradiated repeatedly every second. (Al2O3 TiC) ceramic and
Fig. 2. Two types of laser beam used for the experiments.
(WC Co) sintered carbide were used as the work materials, their dimensions being 12.7 mm 12.7 mm 4.76 mm. The diameter of the focused beam was 35 mm and the applied laser energy was 0.78 mJ/pulse. The maximum incidence times of the single pulse laser beam were 500. (2) A high frequency Q-switched YAG laser beam: Laser grooving was carried out using a high frequency Q-switched YAG laser beam. The irradiation mode of this type of laser beam is described as ``a continuous pulse laser beam'' in this report. The work material used was (Al2O3 TiC) ceramic with the dimension of 12.7 mm 12.7 mm 4.76 mm, and the processing conditions were set as follows: the processing speed was kept constant at 1 mm/s; the applied average laser energy was 1.10 W; the Q-switch frequency was 2 kHz; and the focused diameter of the laser beam was set at 35 and 90 mm. 3. Experimental analysis of laser processing sound The distribution pattern of the pressure level of a detected laser processing sound differs greatly according to the characteristics of the irradiated beam. 3.1. The distribution pattern of the sound pressure level in the case of a single pulse laser beam
Fig. 1. Experimental set-up for the detection and an analysis of laser processing sound.
Fig. 3 shows the relationship between the sound pressure levels and frequencies of a detected laser processing sound when a single pulse laser beam was irradiated to the surface of the work material. A distinctive phenomenon shown in
170
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
latter is called a time tracking diagram of the sound pressure level. Peak values of the sound pressure level were detected at the integer folds of a selected Q-switch frequency when a continuous pulse laser beam was irradiated for grooving. In order to investigate the relationship between the depth of the groove and the sound pressure level, the value of the sound pressure level at the frequency of 2 and 18 kHz was used. 4. Experimental results and discussion 4.1. The relationship between the material removal volume when a single pulse laser beam is used for laser drilling Fig. 3. Relationship between the detected frequencies and sound pressure level for a single pulse laser beam (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Work material: (Al2O3 TiC) ceramic.
this ®gure is that the value of the sound pressure level is distributed over a wide range of frequency, so it is dif®cult to identify exactly the frequency showing the maximum sound pressure level. In this report, the sound pressure level at a frequency of 14 kHz was selected as the maximum value in order to investigate the relationship between the material removal volume and the sound pressure level. 3.2. The distribution of sound pressure level in the case of a continuous pulse laser beam Fig. 4 shows 3D diagram of the characteristics of the laser processing sound when a continuous pulse laser beam of 2 kHz was applied to the work material, which was moved with a processing speed at 1 mm/s for laser grooving. It may be possible to clarify the relationship between frequencies and sound pressure level, and the variation of sound pressure level versus processing time at a speci®c frequency. The
In order to clarify the relationship between a small material removal volume and the sound pressure level, a single pulse laser beam (1 Hz) was irradiated for laser drilling. The processed work material was ground by #1000 diamond grinding wheel until the maximum depth of the processed hole could be observed. Fig. 5(a) and (b) show the cross-sections of processed holes when the incident times of the single pulse laser beam were changed from 10 to 300 for (a) an (Al2O3 TiC) ceramic and (b) a (WC Co) sintered carbide. It could be observed that the depth of processed hole becomes greater with the increase of the incident times of a single pulse laser beam. The processed cross-section of an (Al2O3 TiC) ceramic was triangular in shape, but rectangular in the case of a (WC TiC) sintered carbide. The material removal volume is represented by using the area of cross-section of the processed holes, because the removed volume differed even if the depth of the groove had the same value. Fig. 6 shows the relationships between the accumulated area of the cross-section of processed hole calculated using SEM images and the maximum sound pressure level versus the number of incidences for an (Al2O3 TiC) ceramic. The accumulated cross-section
Fig. 4. 3D diagram of the characteristic of the laser processing sound for a continuous pulse laser beam (Laser energy (average): 1.10 W; Diameter of focus: 35 mm; Processing speed: 1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic.
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
Fig. 5. SEM images of processed holes (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm).
171
relationship between the material removal volume for each laser beam incidence and the sound pressure level. However, only a small amount of work material was removed when a single pulsed laser beam was applied only once, so the accuracy of the calculation of the cross-section from the SEM images shown in Fig. 5 could not be ensured. In this experiment, the following procedures were taken to deduce the removed cross-section for the one-time incidence of a single pulse laser beam by referring to Fig. 6. It is assumed that the accumulated cross-section is S when a single pulse laser beam is irradiated N times, and S0 for N0 times. Then, the removed cross-section becomes (S ÿ S0) at (N ÿ N0) times incidence, and f SÿS0 = NÿN0 gN6N0 can be de®ned as the removed cross-section at the Nth incidence. Based on this concept, the removed cross-section at an arbitrary incident time that corresponds to N was measured using Fig. 6. The same experiment was carried out for (WC Co) sintered carbide as a work material. Fig. 7 shows the relationship between the removed cross-section per one incidence of a single pulsed laser beam and the maximum sound pressure level at the frequency of 14 kHz for both an (Al2O3 TiC) ceramic and (WC Co) sintered carbide. It was clari®ed experimentally that the sound pressure level increased as the increase of removed cross-section, and that the relationship between them can be expressed by straight lines approximately. 4.2. The relationship between the depth of groove and the sound pressure level when a continuous pulse laser beam is used for laser grooving
Fig. 6. Sound pressure level and accumulated area of cross-section versus incident times for a single pulse laser beam (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Q-sw. frequency: 1 Hz; Work material: (Al2O3 TiC) ceramic).
increased as the increase of the incident times of a single pulse laser beam, but the maximum sound pressure level decreased monotonously. The purpose of the application of a single pulse laser beam for laser drilling is to clarify the
It is possible to process a groove by applying a continuous pulse laser beam and by traversing a work material at a speci®c processing speed. Experiments were carried out to con®rm the relationship between the depth of the groove and the pressure level of the processing sound when a continuous pulse laser beam was applied for laser grooving. Fig. 8 shows the time tracking of the values of sound pressure level at the frequencies of 2 and 18 kHz of the processing sound
Fig. 7. Relationship between the meterial removal area per pulse and the sound pressure level (Pulse energy: 0.78 mJ; Diameter of focus: 35 mm; Q-sw. frequency: 1 Hz).
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T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
level at the frequencies of 2 and 18 kHz of the laser processing sound was measured with the time tracking function of an FFT. Both the diagrams of the bottom roughness of the groove and the variation of sound pressure level were processed with an image processor and are presented versus the processing time in Fig. 8. The one-to-one relationship between the value of sound pressure level and the depth of the groove could be measure using this ®gure. Fig. 9 shows the relationship between the depth of the groove and the sound pressure level at the same processing time. The value of the sound pressure level increased when a deeper groove was processed, and the relationship between the depth of the groove and the sound pressure level could also be indicated approximately by a straight line when a continuous pulse laser beam was applied. 5. Conclusions Experiments were carried out in order to clarify the relationship between the pressure level of a laser processing sound and the material removal characteristics when both (Al2O3 TiC) ceramic and a (WC Co) sintered carbide were processed by two types of YAG laser beam. The main conclusions were as follows. Fig. 8. Fluctuation of the sound pressure level and the depth of the groove versus the processing time (Laser energy (average): 1.10W; Processing speed: 0.1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic).
and the bottom roughness of the processed groove versus processing time. The black-colored part shown in Fig. 8 is the cross-section of a processed groove that was taken with SEM. This section was generated by grinding with a #1000 diamond wheel parallel to the scanning direction of the laser beam until the deepest position of the groove was observed. On the other hand, the variation diagram of sound pressure
1. The sound pressure level was distributed continuously from around 8 to 20 kHz frequencies when laser drilling was carried out by utilizing a low frequency pulse (1 Hz) laser beam. 2. When a single pulse laser beam was used, the relationship between the small material removal volume per one incidence of the laser beam and the value of the sound pressure level at around 14 kHz could be represented by a straight line for a constant initial diameter of hole. This phenomenon was verified experimentally for both (Al2O3 TiC) ceramic and (WC Co) sintered carbide.
Fig. 9. Relationship between the depth of the groove and the sound pressure level (Laser energy (average):1.10 W; Diameter of focus: 90 mm; Processing speed: 0.1 mm/s; Q-sw. frequency: 2 kHz; Work material: (Al2O3 TiC) ceramic).
T. Kurita et al. / Journal of Materials Processing Technology 97 (2000) 168±173
3. The peak values of the sound pressure level were detected at the integer folds of a selected Q-switch frequency when a continuous pulse laser beam was applied for laser grooving, and the values of the sound pressure level fluctuated with the progress of the laser processing. 4. In the case of laser grooving by a continuous pulse laser beam under constant diameter of laser beam focus and constant processing speed, the relationship between the depth of the groove and the value of the sound pressure level can be expressed approximately by a straight line.
173
References [1] G.V. Arbach, R.L. Melcher, C.E. Scranton, IBM Tech. Disclosure Bull. 25 (1983) 5092. [2] C.E. Yeack, R.L. Melcher, H.E. Klauser, App. Phys. Lett. 44 (1982) 1043. [3] T. Miyazaki, T. Uyemura, Y. Yamamoto, Ann. CIRP. 22 (1973) 67. [4] M.T. Brienza, A.J. DeMaria, Appl. Phys. Lett. 11 (1967) 44. [5] W.W. Duley, Y.L. Mao, Appl. Phys. 27 (1994) 1379. [6] R.D. Rawlings, W.M. Steen, Optics and Laser Eng. 2 (1981) 173. [7] P. Sheng, G. Chryssolouris, J. Mater. Proc. Technol. 43 (1994) 145.