It is well known that the heat transfer experienced by the weldment during welding can alter the microstructure and thus the properties of the weldment. So the heat transfer and fluid flow in the molten pool can significantly influence the weld pool geometry, the temperature gradients, the local cooling rates and the solidification structure.
In this project we have been studied the heat transfer and fluid flow of the molten pool during stationary gas tungsten arc welding operation using argon shielding gas. The transporting phenomena from the welding arc to the surface of base material, such as current density, heat flux, arc pressure and shear stress acting on the weld pool surface were taken from the simulation results of the corresponding welding arc. The various driving forces for the weld pool the convection, that is the self-induced electromagnetic force, surface tension due to temperature gradient at the surface of molten pool, shear stress exerted by the arc plasma jet and buoyancy force due to the large temperature gradients created in the pool by the arc heating were considered. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Heat and mass transfer equations, including the generalized Navier-Stokes equation, were solved by a finite difference method. Because fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation.
In the present investigation, d$\gamma$/dT was calculated as a function of temperature and sulfur content. This allows for a realistic simulation of the effect of the concentration of surface active elements on the fluid flow and weld pool geometry.
A computational and experimental study was carried out to quantitatively understand the influence of the heat flow and fluid flow in the transient development of the weld pool during GTA welding operations. At this first phase of this project were conducted considering the above four driving forces. The numerical model was applied to the SUS 430 and SUS 304 stainless steel and compared with the experimental results for the different welding currents and time. Good agreement was found between the predictions of the mathematical model and experimental observations with respect to fusion zone size and shape, thereby verifying the predictions of the mathematical model employed.
In the second phase of this project it was examined the relative effect of each driving forces in the weld pool. It has been shown that the surface tension force and shear stress exerted by the plasma jet dominates the fluid flow and controls the development of the weld pool for this welding process.
In the third phase of this project, a computational model has been developed to describe the influence of sulfur in the base metal and constant in surface tension coefficient, Furthermore, it has been developed the transient melting and collapse of weld pool in a stationary GTA welding.
In the last phase of this project, it is to investigate the heat transfer and fluid flow of the weld pool in pulsed current GTA operation. A mathematical model was conducted to determine what effects pulsed current GTA pulsation parameters such as frequency, pulse time ratio, and peak pulse duration have on weld penetration, bead width, weld shape, and thermal cycles.
In most cases it was found that convection played a major role in affecting the weld pool shape and size.
용접 공정 중에 용접부가 받는 열 전달로 인해 그 조직이 변경되며 결국 용접부의 특성을 변경 시킬 수 있다는 것은 잘 알려진 사실이다. 따라서 용융 풀(pool)의 열 전달 및 물질 유동은 용접 풀의 형상, 온도 구배, 국부적인 냉각 속도 및 응고 조직에 매우 큰 영향을 끼친다. 본 연구 보고서는 아르곤 차폐 가스를 사용한 정지된 불활성 가스 텅스텐 아크 용접(GTAW : gas tungsten arc welding)공정에 대한 용융 풀의 열 전달 및 물질 유동에 관하여 연구하고 있다. 용접 아크로부터 모재 금속의 표면으로 전달되는 현상들 즉 전류 밀도, 열 플럭스, 아크 압력 및 용접 풀에 작용하는 전단 응력은 용접 아크의 수치해석 결과로부터 얻은 자료가 적용되었다. 용접 풀의 대류 효과를 일으키는 여러 종류의 구동력 즉 전자기력, 표면 장력, 아크 플라즈마 젯트에의한 전단 응력 및 부력이 고려되었다. 용융 경계는 용접에 하나의 곡선 형태이며 또 미지의 형상을 가지기 때문에 boundary-fitted 좌표계를 도입하여 운동량 방정식에 대한 경계를 정밀하게 묘사하였다. 수치 해석 모델을 SUS 430과 SUS 304 스테인레스 강에 적용하였으며, 서로 다른 용접 전류 및 시간에 대해 실험 결과와 비교하였다. 또 각 구동력에의한 효과를 보기 위해 이에 따른 용접 풀의 형상에 대한 수치 해석이 수행되었다. 그리고 표면 장력에서 황의 함량 및 표면 장력 계수의 영향과 용융 후 응고 모델의 수치 해석이 수행되었다. 특히 펄스 전류 GTA 용접 공정의 용접 풀의 수치 해석 모델이 제시되었다.