Weld surfaces are generated through the thickness of the part. The 3D nature of this result requires a different method of calculation to the weld line result.
Weld line derivation
The angle flow fronts meet on the surface of the part will determine if a weld line will be formed.
When the angle the flow fronts meet is less than 135o, it is deemed that a weld line has occurred.
This is a surface only result.
Weld surface derivation
A weld surface is formed through the thickness of the part. Determining when the flow fronts will form a weld surface requires a complex set of conditions to be met.
While based on the same angle of incidence as the weld lines, the 3D nature of the result means that factors such as the number and aspect ratio of the tetrahedral elements feeding into a node, along with the parabolic nature of the flow front need to be considered before assigning a weld surface.