The phase changing problem at the liquid-solid interface that occurs during the stationary GTA welding has been studied, by considering the four driving forces for weld pool convection, i.e., the electromagnetic force, the buoyancy force, the aerodynamic drag force, and the surface tension force at the weld pool surface. It was found that the effect of the buoyancy force was small, but that the aerodynamic drag force and the electromagnetic force might have a strong effect on the weld pool convection. Particularly when dσ/dT is positive, as can occur in the presence of minor elements, both the surface tension force and the electromagnetic force promote heat transfer from the heat source to the weld pool bottom, and thus produce a deep weld penetration. But when the aerodynamic drag force is included, the surface tension effects of the minor elements were negligibly small.
This study is also concerned with the effect of weld pool surface deformation on the weld penetration under the arc pressure. For the welding current of 100 A the shape of liquid-solid interface is little affected by the weld surface deformation. However, when the weld pool surface is deformed for the welding current of 200 or 300 A, it encourages the conductive heat transfer from the arc to the liquid-solid interface. But, as the weld surface deformation is serious, the aerodynamic drag force becomes dominant in the weld pool convection, thus impeding the flow induced by the electromagnetic force, and producing a shallow weld penetration.
In the numerical simulation, difficulties associated with the irregular shape of the deformed weld pool surface and the moving liquid-solid interface have been successfully overcome by adopting a Boundary-Fitted Coordinate system. This method also can be applied effectively to this transient weld pool development problem with the phase change condition.
To provide a basis for comparison and to guide the development of the numerical model, experiments were...