What is a weld nugget?

When spot welding or seam welding is applied, the joined area is melted and solidified. This melted and solidified part is called nugget.
Nugget can be observed by cutting and corroding the welded area.The hatched area shown in the illustration is a nugget, and the area around it is the heat-affected zone called HAZ.
For example, it may be expressed as “nugget diameter to be 5√t or more” as a quality standard for spot welding.(t indicates plate thickness.)
Nugget is generally not formed with projection welding, but if the part has projections embossed by a press (embossed projections), a nugget may be formed.