Nugget
The small area where the base metal is melted and hardened during spot welding. The nugget’s cross-section is oval shaped and arched. The nugget diameter is the actual welded/fused area between the parts. The strength of a spot weld is largely dependent on nugget size.

Corona Bond
The area where electrode pressure was applied the weld nugget. It prevents the melted metal to spatter. If the weld nugget and corona bond sizes become similar, the pressurized area won’t be able to hold the melted metal and spattering will occur.

Heat Affected Zone
The heated area around the weld nugget. The heat changes the metal structure of this area, but there is no melting.