Chang-Je Lee

Characteristics of Plasma Emission Signals in Fiber Laser Welding of API Steel (II) -The Relationship between Welding Conditions and Emission Signals-

Laser welding by fiber laser accompanied by a lot of spatter and humping bead. This is because the deep and narrow keyhole usually form due to high beam quality. So the weld bead is formed defects, because the plasma jet with a high vapor pressure make the molten pool on keyhole wall scattered. For such a reason, unstable behavior of keyhole is difficult to monitor laser welding by using the laser induced plasma. Mostly, fiber laser welding of thick plates most be influenced by this effect. Therefore, fiber laser welding has been difficult to apply the sole. Thus, laser welding monitoring based on plasma measurements have much difficulty in measurements and analysis of signal. In this study, influence of the plasma emission signal according to welding speed and laser power in fiber laser welding analysed by using RMS and FFT analysis. We can verify that RMS value of the plasma emission signal changes with welding parameters in fiber laser welding, and aspect ratio greater than 1, the peak of FFT frequency had been moved in accordance with welding parameter.

Characteristics of Plasma Emission Signals in Fiber Laser Welding of API Steel (IV) - Correlation of Keyhole`s Periodic Motion and FFT Analysis Results -

The effects of laser welding beam quality is very large. However, not an analysing case was found for the difference on the plasma emission signal during laser welding according to the beam quality. Therefore, in this study, we compared and evaluated penetration and signal change according to the beam quality at the a similar wavelength band by using a fiber laser and Nd:YAG laser. In addition, we took high speed videography in order to make sure that FFT analysis reflects the actual motion period of keyhole and found the period of video analysis and FFT mostly matched. As a result, it is expected to secure higher reliability than evaluating signal intensity when appling FFT to monitoring.

Technology on Arc Welding Machine of Aluminum by Digital Control

Recently, welding technology of nonferrous metals which were difficult to implement by arc welder has become available through digitalization of arc welding machine. Among them, the welding quality improvement of aluminum welding is very noticeable. These results increase the arc stability by controlling arc current and voltage waveform precisely, and control wire feed speed by synchronizing with arc current which the feed rate of filler wire is controlled by a precise motor control of servomotor and not by a simple constant speed feeding. Not only through the hardware digitalization of arc welding machine but also through advance of software of arc welding, it became possible to implement a certain level of welding quality by a simple operation. These led to CMT welding process implementation which requires low heat input than current arc welding and highly increased the applicability of the aluminum welding.

Characteristics of Plasma Emission Signals in Fiber Laser Welding of API Steel (III) -The Effect on Plasma Emission Signals by Shield Gas-

Ar, , and He are the conventional kind of shield gas that are used for laser welding. Many researches on the impact of laser welding shield gas have been done, and it is on going until now. However, there are few studies that analyze the changes and differences of the plasma emission signal. Therefore, in this study, we evaluated the change in the penetration characteristics according to the type of shield gas during fiber laser welding impacts to the plasma signal. As a result, if was checked that the difference in molecular weight of Ar, , and He affects to the amount of spatter, and also found that the measured plasma radiation signal changes similar to the order of the molecular weight of the gases. Especially, clear change on the signal intensity per each shield gas was measured through RMS, and found that the shield gas was nothing to do with the FFT analyzed result.

Characteristics of Plasma Emission Signals in Fiber Laser Welding for API Steel (I) - Variation of Signals by Measuring Conditions -

Fiber laser is a heat source which is introduced recently, and so has a little researched data compare with conventional laser processing. Moreover basic data for welding monitoring are also insufficient. Therefore, in this study, the change of signal with measuring position and angle of plasma emission signals were analysed as a basic experiment for real time monitoring in fiber laser welding. As a result, the signals measured from the side, front and rear had the biggest intensity at , and frequency peak to reflect the behavior of keyhole and swing of plasma by shield gas was detected at . However, both intensity of signal and the result of FFT for monitoring were satisfied at the angle of from the side.

A Study on Characteristics of Plasma Emission Signals with Welding Conditions in CO2Laser

Today, implementation of the defect detection in laser welding has been researched for a long time, but most studies have been made around thin plate within 1~3mm t . Therefore, this study was measured and analyzed light emission signals of the induced plasma in CO2 laser lap welding of 6mm t Zn primer+coated steel, and based on this analysis, research made an investigation into possibility of monitoring in thick plate welding. It was been analyzing the measured signals by RMS and FFT processing, as a results, we were able to confirm definite difference of two signals between humping bead and sound bead. Thus, possibility of real time monitoring in CO2 laser lap welding verified experimentally.

Study on the Relationship Between Emission Signals and Weld Defect for In-Process Monitoring in CO2Laser Welding of Zn-Coated Steel

In this study, the plasma induced by CO2 laser lap welding of 6t Zn coated steel used for ship building was measured using photodiodes and a microphone. Then, the welding phenomenon with gap clearance of lap joint was compared with RMS-treated signal. Thus, we found that intensity of the RMS-treated signal increased with Zn vaporization; further, the presence of defects results in rapid variations with the RMS value as a function of lap-joint parameters. Besides, the FFT value of the raw signal with variations of changing welding parameters was calculated, and then the calculated FFT frequency value was set as the bandwidth of digital filter for a more accurate in-process monitoring. The RMS values were acquired by filtering the raw signal. By matching the weld beads and the calculated RMS values, we confirmed that there is a strong relationship between the signals and the defects.

A Study on Correlationship between the Induced Plasma and Emission Signals for In-process Monitoring in Stainless Steel Welding of Fiber Laser (II) - Properties Changes of the Measured Signals in a Thick Plate Welding -

A Study on Correlationship between the Induced Plasma and Emission Signals for In-process Monitoring in Stainless Steel Welding of Fiber Laser (I) Properties Changes of the Measured Signals in a Thin Plate Welding Chang-Je Lee* and Jong-Do Kim**,† *R&D Institute, BEST F.A Co., Ltd., ChangWon 642-290, Korea **Division of Marine Engineering, Korea maritime and Ocean University, Busan 606-791, Korea