Numerical and experimental investigations of variable polarity gas tungsten arc welding

Variable polarity gas tungsten arc welding (VP-GTAW) has been suggested as a feasible method for welding aluminum alloys due to the enhanced surface cleaning of the oxide film. However, the principles governing the correlations between the electrode positive (EP) ratio, weld geometry, and weld microstructure are not well understood. Therefore, experiments with different EP ratios but same welding current were conducted. A three-dimensional heat transfer and fluid flow model of VP-GTAW was developed with validation against experimental results. Solidification parameters such as the temperature gradient, solidification growth rate, and cooling rate were computed from the model. The results indicate that VP-GTAW produces nearly defect-free joints. Both the top weld width and bottom weld width decrease with increasing EP ratio due to the decreased heat input. The bottom weld width is greater than the minimum weld profile width in the middle of the plate, which indicates that the Marangoni stress has a significant effect on the convective heat transfer and weld geometry. Variable grain dimensions are produced by different EP ratios at the same welding current. Weld pool oscillation originated from the alternating arc force plays a dominant role in the grain dimension.