By developing mathematical models for the arc and the weld pool in the GTAW process, the effect of the electrode tip angle on both arc and weld pool was studied. The present paper is concerned with the model for the arc. By applying a variable cathode surface area, the effect of the electrode tip angle (in the range of 10 to ) on the arc properties, especially on the anode current density, heat flux and gas shear stress over the weld pool, was investigated. Comparison of the calculated results with the available experimental data for 200 A arcs of different lengths showed that the model predictions for temperatures higher than 10 000 K are in very good agreement. For temperatures less than 10 000 K, some modifications were necessary to take into account the absorption of heat by the cooler parts of the arc. It was found that by increasing the electrode tip angle, the anode spot at the weld pool surface tended to be more localized. This led to a higher maximum heat flux and anode current density. On the other hand, the gas shear stress increased on decreasing the electrode tip angle.
In part I, the effect of the electrode tip angle on the arc properties was investigated. In part II, a mathematical model for the weld pool is developed in order to study the effect of the electrode tip angle on the weld pool properties. The information required to simulate the flow in the weld pool including the heat flux to the workpiece, the input current density and the gas shear stress, was derived from the arc model. By individually examining the various driving forces in the weld pool, it is found that the buoyancy and electromagnetic forces do not play major roles in determining the flow pattern in the weld pool for a 200 A arc. Instead, the relative magnitude of the gas shear stress and the surface tension and also the sign of the surface tension determine the flow pattern in the weld pool. The electrode tip angle which alters the gas shear stress and especially the heat flux to the workpiece can produce a significant change in the overall shape and size of the weld pool. With a very sharp tungsten electrode, the heat-flux distribution over the weld pool tends to be flatter. In addition, there is a very strong shear stress due to gas flow at the top of the workpiece. The overall result of these two effects is a wider and shallower weld pool.
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