In welding automation, various sensors are applied to welding robot. Especially, the arc sensor is widely used due to relatively low price and reliable seam tracking performance. The arc sensor is that by weaving the weld torch, the welding current signal is used to reference signal for tracking the weld center line. However, pulsed gas metal arc welding (P-GMAW) technique was introduced recently which has several merits in welding process. It generates fewer spatters and gives less heat input to workpiece than a typical gas metal arc welding. Furthermore, with acute groove angle joint, it is known that it reduce the probability of fatigue failure at the welded position. For the V-groove automated welding, the torch weaving motion is necessary to change the welding current or the voltage signal. Many kinds of the mathematical modeling of a welding power circuit is proposed and used to predict the welding current signal for GMAW. During automated robot welding, the welding torch moves like a ‘zigzag’ motion and most of robot welding is perform on the V-groove or T-fillet joint. In this case the arc plasma continuously changes the arc shape due to the joint preparation so the arc plasma heat flux is not uniform as well.
This research proposes the mathematical model of a welding current for the P-GMAW to modifying the well known GMAW model. In addition, the arc plasma heat flux model at the V-groove is proposed to analyze the heat input to workpiece. Welding power circuit is simply modeled as a RL electric circuit and solved an ODE equation. For heat flux modeling, gray level of arc image was acquired by the high speed CCD camera for an Abel inversion to transform the 2D arc light intensity to 3D arc plasma irradiance. From the irradiance distribution final heat flux distribution is proposed as a 3D asymmetric Gaussian distribution.
Summarizing the welding current model and the heat flux model, the numerical analysis of molten pool is done by using a commercial CFD program tool. The governing equation and boundary condition is solved with included solver in the CFD package program and electromagnetic force, arc pressure, arc heat flux and droplet generation is programmed as a FORTRAN user subroutine and is added to CFD solver. In the numerical simulation of molten pool, six types of joint are chosen which have 3, 5, 7mm root gap and $45\deg$, $60\deg$ groove angle each other.
To verifying the simulation result, practical experiment is also conducted with same welding parameters which are used in the simulation. To compare the welding current waveform, the current signal from the simulation is acquired as an output text file from simulation and the signal from experiment is acquired DAQ system. And the cross section of the experiment results and simulation results are compared to verify the heat flux model.
용접 자동화에서 용접 로봇에는 다양한 방식의 센서가 적용된다. 그 중에서도 비용이 적고 비교적 신뢰성 있는 센서로 아크센서가 사용된다. 아크센서는 용접토치 위빙을 통해 용접전류 신호의 변화를 이용하여 용접선을 추적하게 된다.
P-GMAW 방식은 최근 널리 사용되기 시작하였으며 기존 용접방식에 비해 몇가지 장점을 가진다. 스페터가 작고 모재에 대한 열입력이 작다. 뿐만 아니라 작은 그루브 각을 가지는 맞대기 용접에 사용하는 경우 피로파괴에 확률을 낮춰 신회성 있는 용접을 할 수 있게 된다. V-그루브 맞대기 용접에서 토치 위빙이 필요하고 이러한 토치 위빙에 의해 용접전류 및 전압이 변화하게 된다. 이러한 지그재그 위빙에 의해 아크 플라즈마의 형상이 바뀌게 되고 용접부 형상에 따라 아크 플라즈마 열원의 모양도 변화한다.
본 연구에서는 기존에 알려진 GMAW에서의 동적 아크거동 모델을 이용하여 P-GMAW에서의 동적 아크 모델을 통해 용접공정변수와 용접부 형상정보를 통해 용접전류를 시뮬레이션 하였고 열원 모델을 수립하기 위해 CCD카메라를 이용하여 아크 플라즈마를 촬영하고 아벨 역산 방법을 이용하여 기존의 가우시안 분포의 아크 열원이 아닌 3차원 비대칭 가우시안 형태의 아크 열원을 제안하였다.
용접전류 모델과 열원모델을 바탕으로 용접공정 해석을 수행하여 CFD를 이용하여 용접공정에 관한 이해를 향상하고자 하였다. 해석결과는 동일한 조건의 실험결과와 비교하여 타당성을 검증하였다.
