What is the deepest point into the joint where fusion occurs between base metal and filler metal called?

• A. Toe
• B. Root
• C. Cap
• D. Fillet

Answer: (B)

The deepest point of fusion in a welded joint is called the root. In a groove or butt weld, the root is the interface at the bottom where base metal and filler metal actually fuse together along the joint line. It indicates how far penetration reaches through the joint thickness, so a proper root means the weld has fused from one side to the other at the deepest interior point. In practice, achieving a sound root is essential for full penetration and structural integrity; insufficient heat or improper technique can leave a shallow root with lack of fusion.

By contrast, the toe is where the weld meets the base metal at the surface along the sides, the cap is the final top surface of the weld bead, and a fillet weld is a triangular weld placed in a corner or between two pieces.