Laser arc hybrid process is actively researched as a new welding method since it has several advantages by the combination of laser beam and electric arc. By the coupling of two different heat sources, laser (CO2 or Nd:YAG) and arc (TIG or MIG) mutually assist and influence. The benefit or hybrid welding is improvement of welding speed, deep penetration and narrow fusion zone. When high power laser is irradiated on the metal surface, vaporized metal is discharged as a form of plume. This iaser induced metal vapor makes the path of electrons in arc plasma because it has lower ionization energy than surrounding gas such as argon or helium. In this way the laser irradiation affects the temperature distribution of are plasma. Although much attention has given to the laser arc hybrid welding, only few attempts have so far been made at the plasma of hybrid welding. The purpose of this paper is to understand the behavior of hybrid plasma and confirm the mutual interaction between laser and arc by analysis and monitoring of it. These are two parts of this paper.
First of all, the mechanism of interaction between arc plasma and laser-induced metal vapor is investigated. In order to analyze the hybrid welding plasma, simplifications based on physical phenomena should be adopted because of its complexity such as nonequilibrium state and vibration of laser-induced plasma and so on. All of the analysis could be divided into three steps. Conduction equation is solved to get the temperature distribution on the anode and diffusion equation is solved to obtain the distribution of metal vapor. Finally, the analysis for Ar-Fe mixture is conducted. Consequently the temperature distribution and flow pattern of hybrid plasma will be obtained. As the result of analysis temperature of plasma is increased in the vicinity of laser irradiation point. When 100A of welding current and 200W of CW Nd:YAG laser are used, the maximum value of temperature is about 15000K under the electrode and 14000K over the point of laser irradiation.
There is one other thing that is important for the combination of laser and arc. Laser light is absorbed in arc plasma. The absorption of laser energy in plasma is governed by inverse Bremsstrahlung which is determined by electron and ion density and frequency of laser. The density of plasma according to the temperature was calculated by Saha equation and the amount of absorption through the arc plasma could be obtained by the results of density calculations. As results of absorption analysis, when Nd:YAG laser was used, there was little absorption of laser light and shielding gas had no effect of absorption property. But when $CO_2$ laser is used, a different result is obtained. For the argon shielding gas, very high portion of energy is absorbed in plasma. It follows from these results that plasma temperature would be highly increased by absorption of laser light when $CO_2$ laser and argon shielding gas is used. In addition to this result of shielding gas effect, high power laser is considered by assumption of keyhole formation. When the deep keyhole is generated, as a result of analysis, the lower part of keyhole is heated by laser and the upper part of weld pool is heated by arc.
The remaining half part of this paper is concerned with temperature monitoring of plasma. A method is developed to measure the three dimensional temperature distributions of free buming arcs in order to monitor it in real time. Intensity distribution of arc plasma is obtained by a CCD and converted into temperature distribution by the equation of compensation. In the process of calculation fast algorithm of Abel inversion is adopted to enable the real time measurement. For currents from 50A to 150A, temperature distribution of argon arc plasma is obtained and verified by previous results. Abel inversion methods were developed for real-time calculations of emissions from circularly, elliptically and plane symmetric radiation sources. Using the geometrical relationship between the plasma and the measuring device, an upper triangular area matrix was introduced to determine the local plasma emissivity from the measured irradiation value, Fast calculations of Abel inversion could be achieved using this area matrix since here was no need to fit the data for inversion. Inversion correctness was checked by use of a given test function. This fast calculation algorithm could also be applied for the radiation source with the elliptical and plane symmetry. This technique was applied to the measurement of arc plasma intensity in the flat and V-grooved arc welding as well as the hybrid plasma generated irradiation of laser beam on the position separated from arc center.
In order to understand the characteristics of laser arc hybrid plasma, the welding variables were divided into three types. Not only variables depend on each laser and arc but also new welding variables such as laser-arc interspaces and angles were introduced and investigated. The temperature measurement technique developed in this paper is used for the monitoring of hybrid plasma. By the monitoring experiment, the effect of laser of arc plasma such as improvement of arc concentration and stabilization were confirmed and the possibility of arc control by pulsed laser was ascertained.
본 연구는 레이저 아크 하이브리드 플라즈마의 해석 및 모니터링을 통해 그 특성을 이해하고 응용하는데 있다. 해석을 위해 먼저 모재의 열전달 방정식을 풀어 온도 분포를 구하고 모재로부터 나오는 금속 증기의 해석을 한 후 Ar-Fe, He-Fe 혼합 플라즈마에 대한 해석을 수행하였다. 그 특성으로 레이저가 조사되는 지점에 플라즈마의 온도 상승이 이루어짐을 알 수 있고 CO2 레이저와 아르곤 보호가스를 사용하면 레이저 빔이 플라즈마를 통과 하면서 많은 양이 흡수 됨을 알 수 있었다. 또한 고출력 레이저에서 키홀 형상을 가정하여 플라즈마 해석을 수행하였다.
실시간으로 하이브리드 플라즈마의 온도를 측정하는 방법을 개발하였다. 먼저 CCD를 이용하여 빛의 강도를 얻고 아벨 역산을 통해 3차원으로 정보를 복원한 후 온도 결정식을 이용하여 플라즈마의 온도 분포를 얻었다. 이를 위해 실시간 아벨 역산 방법을 개발하였으며 레이저 아크 하이브리드 플라즈마의 온도를 측정하는데 사용되었다. 레이저가 아크에 주는 집중효과, 안정화 효과 등이 실험을 통해 확인 되었다.
