Toward a unified model to prevent humping defects in gas tungsten arc welding

During gas tungsten arc (GTA) welding, high welding speed and current can lead to a serious weld defect with a bead-like appearance known as humping. Currently, there is no unified model to predict the formation of humping defects in GTA welding. Here we propose and test a new comprehensive computational model that can predict and prevent the formation of humping defects considering the values of arc current, welding speed, nature of the shielding gas, electrode geometry, ambient pressure, torch angle, and external magnetic field during gas tungsten arc (GTA) welding. The model considers stability of the waves on the weld pool surface due to relative motion between the shielding gas and the liquid metal based on the Kelvin-Helmholtz instability theory. The main factors for the instability were found to be the velocities of the shielding gas and the weld metal, densities of the molten metal and shielding gas, weld pool size, and surface tension of the molten weld metal. The weld pool size and weld metal velocities were calculated by a numerical heat transfer and fluid flow model, and the shielding gas velocity was calculated from an analytical relation. Good agreement between the model predictions of humping and the independent experimental results from various sources show that the model can be used to prevent humping considering the effects of arc current, welding speed, nature of the shielding gas, electrode geometry, ambient pressure, torch angle, and external magnetic field during GTA welding. Recommendations are provided for the use of special electrodes and an external magnetic field and. where practical, controlled pressure and careful selection of shielding gas to prevent humping under conditions when high welding speed and current are needed to sustain productivity goals.