Which condition is indicated by a concave root surface (suck back)?

• A. Suck back
• B. Undercut
• C. Porosity
• D. Overlap

Answer: (A)

A concave root surface is a sign of suck back. In TIG welding, the molten pool at the root can be drawn back or pulled away from the joint as it solidifies, leaving a hollowed or cupped root rather than a full, rounded root bead. This happens when the pool isn’t maintained at the root during solidification or when the arc is momentarily pulling away, causing the metal to retreat rather than fill the root properly. This differs from undercut (a groove along the weld toe), porosity (gas pockets within the weld metal), and overlap (bead overhang beyond the joint); those issues affect the toe, internal quality, or surface overfill, not the concave root profile caused by suck back.