Mechanism for minor element effect on GTA fusion zone geometry

The presence of certain impurities in the base metal has been known for some time to affect the fusion zone geometry of CTA bead-on-plate welds, sometimes dramatically. Highspeed motion pictures of the weld pool during and after the addition of aluminum and sulfur suggested that the change in pool shape occurs because of a change in fluid flow patterns in the weld pool. It is proposed that surface tension driven fluid flow (Marangoni convection) is responsible in many cases for the fluid flow pattern produced in weld pools and that minor elements change fluid flow patterns by altering the surface tension gradient. A surface tension gradient (and thus surface tension driven fluid flow) exists because the surface tension is temperature dependent, and large temperature gradients occur on the weld pool surface. The presence of low concentrations of surface active impurities can substantially alter the temperature dependence of the surface tension (including changing its sign) as well as changing the magnitude of the surface tension. The change in magnitude and direction of surface tension gradients on the weld pool surface changes fluid flow patterns and changes the fusion zone geometry. Evidence in support of the model is presented.