The Marangoni effect is the mass transfer along an interface between two fluids due to the surface tension gradient. In the case of temperature dependence this phenomenon may be called thermo-capillary convection.
Mechanism: Since a liquid with a high surface tension pulls more strongly on the surrounding liquid than one with a low surface tension. The presence of a gradient in surface tension will naturally cause the liquid to flow away from regions of low surface tension. The surface tension gradient can be caused by concentration gradient or by a temperature gradient (Surface tension is a function of temperature).
Fluid flow in the weld pool determines GTA weld pool shape and force driving the fluid flow is surface tension gradient. This is called Marangoni convection.
Pure metals (and low sulfur materials) have a negative temperature coefficient of surface tension. When the area under the arc is heated up, the surface tension drops in the center of the weld pool and the direction of the fluid flow is from the center towards the edges of the pool. When sulfur, oxygen, selenium, or other surface active elements are introduced to the weld pool at very low levels they cause the surface tension to rise with an increase in temperature. In this case, the direction of fluid flow is from the outside of the pool, where the surface tension is lower, towards the center, thus increasing penetration.
There are many other factors which are responsible for fluid flow in the weld, different forces creating weld pool circulation,etc. which will be discussed in coming articles. Till then,
Keep reading, Happy welding…