AAZ

The arc attachment zone, or AAZ, is the region of base metal immediately adjacent to a weld bead where the arc has caused localized heating and metallurgical change, even though no fusion has occurred. It is distinct from the heat-affected zone (HAZ) by its proximity to the arc root and its exposure to intense radiant and conductive heat from the welding arc itself, rather than solely from the cooling weld pool.

In gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW), the AAZ forms where the arc spreads outward from its primary attachment point on the workpiece. The degree of damage depends on arc voltage, travel speed, and base metal composition. Steels with higher carbon content or alloy content are more susceptible to hardening and embrittlement in this zone. Aluminum alloys may experience grain growth and loss of strength. The AAZ typically extends only a few millimeters from the visible weld toe.

The practical significance of the AAZ lies in fatigue performance and corrosion resistance. Cracks often initiate at the toe of the weld, which marks the boundary of the AAZ, because of the combination of residual stress, metallurgical weakness, and stress concentration. In critical applications such as pressure vessels, pipelines, and structural steel where cyclic loading is present, the AAZ is a zone of concern. Post-weld heat treatment and shot peening are sometimes used to mitigate crack initiation in this region.

Arc attachment is the term used because the electrical arc is anchored or attached to the base metal surface at this point. The zone itself was named to distinguish it from the broader HAZ, which includes regions heated by conduction from the weld pool but not directly exposed to the arc. Modern welding metallurgy recognizes the AAZ as a separate phenomenon with its own mechanisms of damage, separate from conventional HAZ softening or hardening.