Which term best describes the condition of a weld that poorly joins the metal surfaces together?

The term that best describes the condition of a weld that poorly joins the metal surfaces together is "incomplete fusion." Incomplete fusion occurs when the molten weld metal does not adequately fuse with the base metal or with the previous layers of weld metal. This situation can arise due to insufficient heat, improper welding technique, or contamination of the weld area. The result is a weld that lacks strength and integrity, which can lead to potential failure under load.

In contrast, the other terms represent different welding defects that pose unique issues. Underfill refers to a weld that is too low, resulting in a concave appearance and potentially affecting the structural integrity, but it doesn't specifically denote a lack of fusion between the joint surfaces. Overlap is a defect where the weld metal rolls over the base metal surface without proper fusion, also not directly addressing the joining of surfaces. Misalignment relates to the incorrect positioning of components being welded, which can lead to an ineffective weld but does not pertain fundamentally to the fusion between the materials.