WELDING RESEARCH

In this project we examined the relative importance of the shear stress exerted by the plasma jet in GTA welding operations to that due to surface tension shear. This comparison, performed for a stainless steel workpiece, was possible because of previous modeling efforts in this group aimed at representing the coupling between the arc and the weld pool. The actual calculations involved the computation of the gas shear from the plasma model and the evaluation of the surface tension forces, using the predicted freesurface temperature profiles, from the pool model. The calculations have shown that for low-current operations (i.e., below 200 A) gas shear is unlikely to play an important role. At higher current levels the free surface of the weld pool is likely to deform and under these conditions gas shear could become important. However, this case was not studied explicitly. Introduction Figure 1 shows a schematic representation of a gas tungsten arc spot welding R. T C. CHOO and/. SZEKELY are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Mass. operation. The various driving forces for the weld pool circulation have been identified as buoyancy, electromagnetic, surface tension and gas shear stress. Of the four forces, the first three have been treated very extensively in the literature (Refs. 1-7). The reason is that these three forces can be calculated quite independently of the welding arc. Furthermore, the general conclusion is that the surface tension forces dominate. In order to consider the fourth driving force, the welding arc must be simulated to obtain the gas shear over the free surface. In addition, the coupling of the welding arc with the weld pool has only been carried out recently (Ref. 8). Virtually all previous modeling efforts simulate the weld pool KEY W O R D S Cas Shear Stress Marangoni Flows GTAW Arc Welding Weld Pool Mathematical Models Weld Arc Behavior Surface Tension Weld Pool Deformation Thermocapillary Force independently of the arc. The role gas shear stress plays at the free surface is actually quite complex. Most models assume that the momentum transferred by the plasma jet to the weld pool is negligible due to the small value of the argon gas viscosity relative to the liquid metal. However, if the magnitude of the gas shear stress approaches that of the Marangoni shear in the weld pool, then the effect of the gas shear can be important. The purpose of this investigation is to examine the role of gas shear on Marangoni flows for planar weld pool surfaces. There is a more subtle role that the gas shear plays and that is the gas shear may be responsible for surface instabilities such as waves and ripples, it is well known that when two fluids of different viscosities flow parallel to each other in the same direction, Kelvin-Helmholtz instabilities can form due to the mismatch in velocities at the interface (Refs. 9, 10). In this instance, perturbations in the free surface can result in waves that either grow and produce spray formations or damp out and oscillate. In the extreme situation, these phenomena are suspected to be responsible for the onset of deep penetrations found in high arc currents (Ref. 11). At low currents, surface oscillations are common (Ref. 12). WELDING RESEARCH SUPPLEMENT | 223-s