Many computational models and experiments have demonstrated that Marangoni convection dominates other forces in weld pools under the practical gas tungsten arc welding conditions. The Marangoni convection in the weld pools is important because it has a decisive effect on the reproducibility of weld geometry and is responsible for other welding problems, including arc wander, humped beads, and porosity.;A flow visualization technique was developed to observe the Marangoni convection in simulated weld pool of sodium nitrate (NaNO3) subject to a defocused CO2 laser beam. The intent was to provide a better understanding of Marangoni convection in weld pools, which is impossible to conduct in opaque metal pools, and to understand the effect of the power and diameter of the heat source on the Marangoni convection.;The study of the effect of the Marangoni convection on the weld pool shape was conducted in stationary welds of NaNO3 and gallium (Ga), a high and low Prandtl number material, respectively. It is proposed that, in the absence of a surface-active agent and a significant electromagnetic force, pool bottom convexity increases with increasing value of Peclet number. It was shown that for a given material composition and welding process, the weld shape often reveals a good deal about the nature of weld pool convection.;Finally, a transparent pool of NaNO3 with potassium propionate (C2H5COOK) as a surface-active agent was used for the flow visualization. The reversed Marangoni convection was clearly observed. The stationary laser weldings showed deeper weld pools in NaNO3 containing 1 mole % C2H5COOK than in pure NaNO 3. This is consistent with the effect Of C2H5COOK on the convection pattern observed in flow visualization.;It is believed that the results of this dissertation are the valuable tools to explain the significance of the Marangoni convection in weld pools and to explain why small differences in some specific chemical composition have such a profound effect on the weld geometry.
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