Fusion zone (FZ): In fusion welding, a pool of molten metal is created, with the molten metal coming from two different parts and the filler material, the process can be viewed as casting a small amount of molten metal into a metal mold. The resultant structure and its properities can best be understood by analyzing the casting and then considering the effects of the associated heat treatment on the adjacent base metal.

The pools of molten metal created by fusion welding are prone to all of the prolems and defects associated with metal casting, such as gas porosity, inclusions, blowholes, cracks, and shankage.

Heat-affected zone (HAZ): is the region adjacent to the fusion zone, which the metal is not melted but is subjected to elevated temperatures in the welding process.

In heat-affected zone, the structures and properities of the welded materials can be extremely complex and varied. The changes include phase transformations, recrystallization, grain growth, embrittlement, or even cracking.

HAZ is often the weakest area in the as-welded joint, except where there are obvious defects in the weld deposit, most welding failures originate in the heat-affected zone.

Residual Stress: is the result of restraint to thermal expansion and contraction offered by the pieces being welded.