Numerical Investigation of Influencing Factors of Slag Transportation Process During Metal Active Gas Welding Using Particle Method

Metal active gas welding is a consumable electrode-type arc welding process in which mixture gas containing an active gas such as CO₂ and O₂ in addition to an inert gas such as Ar is used as a shielding gas. Since the active gas is cheaper than the inert gas, the metal active gas welding has the advantage of being less expensive than a metal inert gas welding in which pure inert gas is used as the shielding gas. On the other hand, one of the disadvantages of the metal active gas welding is that the active gas such as CO₂ and O₂ is dissociated in an arc plasma, and oxygen is generated. So, a weld part is oxidized and its toughness is reduced. As a solution to this problem, some elements such as Si and Mn are added to a wire for the metal active gas welding. Because these elements have a higher affinity for O than Fe which is the main component of base metal and wire, they adsorb O in a weld pool that consists of melted them, and suppress the oxidation of the weld part. Then, the O-adsorbed elements float to the weld pool surface as a slag. This slag is solidified on the weld bead after the arc plasma passes and causes the corrosion by the coating failure during an electrodeposition coating process and the slag entrapment which is one of the welding defects in a multi-pass welding. Therefore, it is necessary to remove slags after each welding to avoid these problems. However, this removal process is one of the reasons for the decrease in a production efficiency. In order to improve the efficiency of the slag removal, a guideline for the setting of welding conditions focusing on the control of the position of the solidified slag is required, and the clarification of the slag transportation mechanism is expected. The slag transportation process is considered to be affected by the weld pool convection which is determined by four driving forces: the Marangoni effect, the shearing force, the Lorentz force and the buoyancy. In addition to these forces, the slag floating on the weld pool surface may also be directly affected by the shielding gas flow. However, it is difficult to investigate these factors experimentally. Therefore, it is effective to use numerical simulations, but there is no report in which a computational model of the slag behavior being transported on the weld pool considering the four driving forces and the drag force by the shielding gas flow has been developed. In this study, to simulate the slag transportation process in a metal active gas welding considering the effects of the four driving forces and the drag force by shielding gas flow, a three-dimensional computational model was developed using an incompressible smoothed particle hydrodynamics method which is one of the particle methods. As a result, the slag generated near the center of a heat source was transported radially toward the edge of the weld pool, and a part of the slag transported to the backward of the pool was transported toward the center of the weld pool. This series of tendency of the slag transportation process was also observed in the experiment using a high-speed camera, showing the validity of this simulation. Moreover, in order to investigate the effects of driving forces on the slag transportation process, numerical experiments were also conducted in which each driving force was applied singly. As a result, the magnitude of each driving force indicated that the Marangoni effect force, the shearing force, and the drag force by shielding gas flow largely affected the slag transportation process. These force distributions also suggested that a slag generated near the center of the heat source was transported to the edge of the weld pool by the shearing force and the drag force by shielding gas flow, while a slag transported to the backward of the weld pool was transported to the center by the Marangoni effect force.