Welding is a particularly energetic manufacturing process where a small volume of metal goes from room temperature to melting point and back in a matter of milliseconds.  Once the weld metal reaches a molten state, it begins to flow and swirls around like the wind in a storm.  Movement of molten metal is governed by gradients in surface tension.  In pure metals and alloys, surface tension decreases as temperature increases.  So for a welding process where the exposed surface of the molten metal is smooth and continuous, as is typical on TIG welding and conduction-mode laser welding and electron beam welding, the surface tension is lower at the center of the weld and higher at the edges.  The surface tension gradient sets up movement of the liquid from center to the edges, from low to high surface tension.  Such an action produces a wide and shallow weld.

Mixing of metals in molten state is not completely homogenous, and impurity elements can segregate to the surface and lower the surface tension.  But some impurity elements also change the effect of temperature on surface tension, causing surface tension to actually increase with temperature.  High surface tension at the center of the weld pulls in liquid from the outer edges as the liquid in the center sinks producing a deeper and narrower weld.  Surface tension driven fluid flow is commonly called Marangoni convection from the work of Carlo Marangoni.

Fluid flow in melt pool can be controlled by controlling impurity element concentrations and has been well documented for stainless-steels containing sulfur, oxygen, selenium, and tellurium.  For example, under identical welding conditions, the weld depth-to-width ratio in TIG welding of 304 SS can increase from 0.2 at <20 ppm sulfur to nearly 0.4 at 160 ppm of sulfur.  These ratios will depend on many factors including power density which will be much higher for conduction mode laser beam and electron beam welding.