What vocabulary word describes a groove melted into the base metal that is left unfilled by weld metal?

The term that accurately describes a groove melted into the base metal and left unfilled by weld metal is "undercut." In MIG welding, an undercut refers to this specific defect that occurs when the weld metal does not fuse completely to the base metal, resulting in a depression or groove. This can weaken the overall integrity of the weld, as it creates areas that are not adequately filled with the weld material, thereby affecting the strength and durability of the weld joint.

Understanding undercut is crucial for welders because it highlights the importance of proper welding techniques, including the correct settings for speed, heat input, and filler material. By minimizing undercut, welders can ensure stronger, more reliable joints that meet industry standards. Other terms, such as channel, kerf, and slit, do not convey the specific issue of an unfilled groove resulting from the welding process, making "undercut" the appropriate vocabulary choice in this context.