Which weld is primarily used for filling gaps in joints?

The choice of fillet weld as the correct answer is based on its specific application in welding processes. Fillet welds are commonly used to join two pieces of metal at a corner or an edge, making them particularly effective for filling gaps in joints where two surfaces don't perfectly align.

In situations where components are not flush or there is a slight misalignment, a fillet weld provides a way to bridge that gap, ensuring a strong connection. The shape of the weld, which resembles a right triangle in cross-section, enables it to accommodate some degree of variation in joint fit-up.

While groove welds are designed to join two pieces of metal along their edges and provide a strong joint, they are not typically used for filling gaps, as they require a more precise fit. Spot welds are more suited for joining thin sheets in a quick, localized manner, and do not effectively fill gaps. Plug welds involve creating holes in one piece and welding to the other, which again is not primarily focused on gap-filling.

Ultimately, fillet welds are versatile and specifically tailored to address the needs of joining parts with gaps, making them suitable for a wide range of welding applications.