Which welding defect is characterized by a lack of fusion between weld metal and base materials?

The defect characterized by a lack of fusion between weld metal and base materials is indeed poor penetration. This defect occurs when the molten weld metal does not adequately fuse with the base material, resulting in a weak bond that can compromise the integrity of the weld joint. Proper penetration is essential for achieving a strong weld, as it ensures that the weld metal becomes an integral part of the base materials.

Poor penetration can be caused by several factors, including insufficient heat input, improper welding technique, or inadequate joint preparation. Identifying and correcting issues related to penetration during the welding process is crucial for maintaining the overall strength and performance of welded structures.

In contrast, brittle fracture pertains to the sudden, catastrophic failure of a material without significant plastic deformation, typically due to the presence of flaws or low toughness. Undercut refers to a groove that develops at the base of the weld metal due to excessive melting of the base metal, which is distinct from lack of fusion. Over-welding is a condition where too much weld metal is deposited, potentially leading to distorted structures, but it is unrelated to fusion issues.