How does the fusion zone differ from the heat-affected zone in a welded joint?

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point. — welding Shielded metal arc welding.

In a welded joint, the fusion zone and the heat-affected zone (HAZ) are distinguished by the extent of thermal impact on the material's state:

Fusion Zone (Weld Metal): This is the portion of the joint that has undergone complete melting during the welding process. It typically consists of a mixture of filler material and the melted base metal. Upon cooling, it develops a cast structure, often characterized by columnar grains.1
Heat-Affected Zone (HAZ): This region is located adjacent to the fusion zone. Unlike the fusion zone, the base metal in the HAZ does not melt. However, it is subjected to thermal cycles that alter its microstructure or mechanical properties. The specific changes depend on the peak temperature reached, the duration of exposure, and the cooling rates.1

While the fusion zone represents the area of phase change from solid to liquid and back to solid, the HAZ represents a solid-state transformation caused by the heat conducted away from the weld pool.1