In some procedures, what type of weld may be used in the backing to connect to the top pieces?

• A. Fillet weld
• B. Seam weld
• C. Grooved weld
• D. Plug weld

Answer: (D)

Plug welding is used when you need to join two pieces with a backing plate by welding through a hole in the top sheet. Drilling or punching a hole in the upper panel and filling it with weld metal fuses the top piece to the backing plate, creating a strong connection while keeping the outer surface flush and tidy. This method is common when you’re attaching panels to a backing plate or tying internal supports to a surface where a through-penetrating, discrete weld is preferred.

Fillet welds are for edge-to-face or corner joins and don’t provide the through-hole bonding to a backing plate. Seam welds create a continuous weld along a seam, not a single through-hole connection to backing. Grooved welds are used for butt joints with groove preparation and aren’t the typical method for attaching a top sheet to a backing plate via a hole.