suck back

• A. caused when joint can't conduct heat away fast enough and causes a concave root surface (underneath)
• B. caused by excess shielding gas
• C. caused by low amperage
• D. caused by too fast travel speed

Answer: (A)

Suck-back shows up as a concave root on the underside of the joint, and it’s driven by how heat leaves the weld area. When the joint can’t conduct heat away fast enough, heat stays in the root region longer, and surface tension pulls the molten metal back into the joint as it starts to solidify. The result is a recessed, concave root face rather than a smooth, full root. In GTAW, controlling heat input and providing an effective heat sink helps keep the molten metal in place so the root fills properly instead of pulling back. The other factors listed influence the weld in different ways—shielding gas affects shielding quality, low amperage reduces heat input (often leading to underfill), and traveling too fast typically leads to a shallower weld—not the concave root caused by poor heat removal.