Yoshiro Ito

Detection technique for transition between deep penetration mode and shallow penetration mode in CO2 laser welding of metals

In metal-welding, there are two types of welding: deep penetration mode (keyhole-type) and shallow penetration mode (heat conduction-type). In this paper, we propose a technique to detect the transition between these two types in CO2 laser welding by monitoring optical and acoustic emissions from a laser-irradiated point. Stainless steel, mild steel and an aluminium alloy were used for the work piece. Fast Fourier transform analysis was performed to obtain the frequency spectra of the optical and acoustic emissions. With deep penetration mode welding the acoustic emissions around 1-3?kHz appeared strongly. When the welding changed to shallow penetration mode, the main frequency components appeared in a higher frequency range. The main frequency components of the emission shifted from a lower frequency range to a higher frequency range according to the decrease in penetration depth. This phenomenon was observed for all three materials examined. Representative bands of both higher and lower frequency ranges were searched. As a result, the ratio of the emission intensity measured in the range of 12-15?kHz to that measured in the range of 0-3?kHz showed a good correlation with the aspect ratio. We could evaluate the transition between the two types of welding by taking the emission intensity ratio.

Micromachining through silicon substrates by ultrafast laser at 1552 nm

Micromachining of silicon (Si) using lasers is attracting much interest in the field of modern microfabrications. Because Si absorbs majority of industrial lasers in near-IR to uv region, laser processing of Si is usually performed using linear absorption processes of the laser radiation. Micromachining by non-linear absorption processes induced by short pulse lasers has many unique features and has been applied to transparent materials in many fields. The application of these non-linear processes to the micromachining of Si would be very advantageous. In this study, ultrafast pulses from a fiber-based infrared laser at 1552.5 nm were irradiated through silicon substrates such that they were focused at the front or the rare surfaces of the substrates under an infrared microscope. Focused outputs of the laser made sharp and deep trenches on the front surface. When the laser was focused on a gold (Au) film on the surface of the second substrate placed at the back of the first Si substrate, the Au film was ablated by irradiation through the Si substrate. This was applied successfully to an up-conversion of the frequency of crystal oscillator in a Si package. When the focus was placed at rear surface of a Si substrate and aberration of the laser light caused by large refractive index of Si was compensated, an Au film on the rear surface was ablated and deposited on another substrate placed at the back of the substrate.Micromachining of silicon (Si) using lasers is attracting much interest in the field of modern microfabrications. Because Si absorbs majority of industrial lasers in near-IR to uv region, laser processing of Si is usually performed using linear absorption processes of the laser radiation. Micromachining by non-linear absorption processes induced by short pulse lasers has many unique features and has been applied to transparent materials in many fields. The application of these non-linear processes to the micromachining of Si would be very advantageous. In this study, ultrafast pulses from a fiber-based infrared laser at 1552.5 nm were irradiated through silicon substrates such that they were focused at the front or the rare surfaces of the substrates under an infrared microscope. Focused outputs of the laser made sharp and deep trenches on the front surface. When the laser was focused on a gold (Au) film on the surface of the second substrate placed at the back of the first Si substrate, the Au film was a...

Monitoring of cw YAG laser welding using optical and acoustic signals

In this study, we propose a technique to detect the weld morphology between deep penetration mode and shallow penetration mode in CW 4kW YAG laser welding by monitoring an optical and acoustic emissions. Both signals were analyzed by Fast Fourier Transform (FFT) program and their frequency spectra were obtained. Mild steel, stainless steel and aluminum alloy were used for workpiece. When the weld morphology changed from deep penetration mode to shallow penetration mode in mild steel, main frequency components of the acoustic spectrum, which appeared in 1 to 10 kHz, became smaller and a flat spectrum was obtained. The similar change of the main frequency components was observed for FFT spectra of optical emission intensity. This phenomenon was also observed for other two materials examined. We found that the intensity ratio of lower frequency components versus higher frequency components of the spectrum correlated well to the aspect ratio of welding bead. The ratio of the optical emission intensity measured in the range of 23 - 25 kHz to that measured in the range of 2 - 4 kHz showed a good correlation with the aspect ratio, and the ratio of 32 - 35 kHz to 2 - 4 kHz correlated well to the aspect ratio for the acoustic emission. Thus we could evaluate whether the welding was deep penetration mode or shallow penetration mode by analyzing the frequency components of the signals in YAG laser welding.In this study, we propose a technique to detect the weld morphology between deep penetration mode and shallow penetration mode in CW 4kW YAG laser welding by monitoring an optical and acoustic emissions. Both signals were analyzed by Fast Fourier Transform (FFT) program and their frequency spectra were obtained. Mild steel, stainless steel and aluminum alloy were used for workpiece. When the weld morphology changed from deep penetration mode to shallow penetration mode in mild steel, main frequency components of the acoustic spectrum, which appeared in 1 to 10 kHz, became smaller and a flat spectrum was obtained. The similar change of the main frequency components was observed for FFT spectra of optical emission intensity. This phenomenon was also observed for other two materials examined. We found that the intensity ratio of lower frequency components versus higher frequency components of the spectrum correlated well to the aspect ratio of welding bead. The ratio of the optical emission intensity measure...

Micro-patterning of organic thin-film electronic devices by ultrashort laser

Micro-patterning of two types of organic thin-film electronic devices, an organic light emitting diode (OLED) display and an organic photovoltaic cell (OPV), are carried out using an ultrashort fiber laser. Selective machining technique of aluminium (Al) electrode of the OLED by an ultrashort fiber laser was developed and test pieces of the OLED display were fabricated successfully. It is found that the Al layer on ITO is removed by only one pulse so long as it is above its threshold energy. Similar process was applied to machining of OPV to fabricate modules in planer structure connected in series. The results indicate that selective machining of different part of OPV can be achievable using single ultrashort fiber laser. Machining conditions need to be optimized for each layer.Micro-patterning of two types of organic thin-film electronic devices, an organic light emitting diode (OLED) display and an organic photovoltaic cell (OPV), are carried out using an ultrashort fiber laser. Selective machining technique of aluminium (Al) electrode of the OLED by an ultrashort fiber laser was developed and test pieces of the OLED display were fabricated successfully. It is found that the Al layer on ITO is removed by only one pulse so long as it is above its threshold energy. Similar process was applied to machining of OPV to fabricate modules in planer structure connected in series. The results indicate that selective machining of different part of OPV can be achievable using single ultrashort fiber laser. Machining conditions need to be optimized for each layer.

Micro-hole drilling on cemented tungsten carbide by ultrashort laser pulses

Micro-holes of 100 to 300 μm diameters have been drilled on plates of cemented tungsten carbide of 0.5 to 1mm thick using ultrashort laser pulses. Laser was scanned by a galvano-scanner with scan speed of 0.035 to 15 mm/s and focused by lens with 80 mm focal length. Effects of scan speed, focus position and polarization of the laser, linear vs. circular, were examined. It is found that the polarization of the laser has profound effects on the shape of the drilled hole: circularly polarized laser results in more straight, less tapered holes than the linearly polarized one in shorter times. Position of the focus relative to the sample surface affects to the hole shape, too. Periodic surface structures with 260 to 300 nm periods are formed on the side of drilled holes regardless the polarization of the laser.

Detection of transition from keyhole-type to heat conduction-type welding in CO2 laser welding of metals

In welding of metals, there are two welding types: keyhole-type welding and heat conduction-type welding. In this paper, we propose a technique to detect the transition between these two welding types in CO2 laser welding by monitoring optical emission of a laser-induced plasma and acoustic emission from a laser irradiated point.SUS304, SS400 and A15083 were used for workpiece. Frequency analysis of the optical emission and acoustic emission was carried out using FFT program. When the welding is the keyhole-type, the frequency components of the acoustic signal at around 2 - 4 kHz appear strongly. When the welding changes to the heat conduction-type, the main frequency components appear at higher frequency range. Main frequency components of the signal shifts from lower frequency range to higher frequency range according to the decrease in the penetration depth. This phenomenon is observed for all three materials examined. Ratio of emission intensity at the lower frequency range to that at the higher frequency range is correlated to the aspect ratio. From this, we can evaluate the transition between the keyhole-type welding and heat conduction-type welding. The similar shift of the main frequency components is observed for the optical signal.In welding of metals, there are two welding types: keyhole-type welding and heat conduction-type welding. In this paper, we propose a technique to detect the transition between these two welding types in CO2 laser welding by monitoring optical emission of a laser-induced plasma and acoustic emission from a laser irradiated point.SUS304, SS400 and A15083 were used for workpiece. Frequency analysis of the optical emission and acoustic emission was carried out using FFT program. When the welding is the keyhole-type, the frequency components of the acoustic signal at around 2 - 4 kHz appear strongly. When the welding changes to the heat conduction-type, the main frequency components appear at higher frequency range. Main frequency components of the signal shifts from lower frequency range to higher frequency range according to the decrease in the penetration depth. This phenomenon is observed for all three materials examined. Ratio of emission intensity at the lower frequency range to that at the higher frequ...

Photoacoustic measurement of energy absorption in pulsed Nd:YAG laser processing

Amount of laser energy absorbed by metal plates were measured by a photoacoustic method as a function of energy fluence. A Q-switched Nd:YAG pulse laser was used as an excitation source. Photoacoustic signal generated in the specimen was detected by a piezoelectric transducer attached to it. Time integrated intensity of the photoacoustic signal corresponded to the amount of absorbed energy at the surface of the metal plates, and was separated into three stages as a function of the energy fluence. The photoacoustic signal intensity began to increase strongly when the energy fluence became larger than a threshold value, and reached the maximum at a certain energy fluence, then decreased with further increase in the energy fluence. The threshold corresponded to a damage threshold of the specimen.Amount of laser energy absorbed by metal plates were measured by a photoacoustic method as a function of energy fluence. A Q-switched Nd:YAG pulse laser was used as an excitation source. Photoacoustic signal generated in the specimen was detected by a piezoelectric transducer attached to it. Time integrated intensity of the photoacoustic signal corresponded to the amount of absorbed energy at the surface of the metal plates, and was separated into three stages as a function of the energy fluence. The photoacoustic signal intensity began to increase strongly when the energy fluence became larger than a threshold value, and reached the maximum at a certain energy fluence, then decreased with further increase in the energy fluence. The threshold corresponded to a damage threshold of the specimen.