Corrosion and protection of friction stir welds in aluminium alloys

Friction stir welding (FSW) is a new joining method that offers considerable advantages in weightsaving and ease of manufacture of aluminium structures for aerospace applications. The resulting welds have excellent mechanical properties but can be vulnerable to corrosion. FSWs in the aerospace alloy AA2024 show susceptibility to localised attack in both the heat-affected zone (HAZ) and nugget. The severity of attack in each region is affected by the welding processing parameters, which determine the heat input into the weld. The morphology of attack, revealed by synchrotron microtomography, is predominantly along grain boundaries parallel to the rolling direction of the parent plate. The rotation speed of the toolpiece is the primary factor determining corrosion susceptibility. For a high rotation speed weld, attack is predominantly in the HAZ, and the nugget acts as a net cathode. In contrast, for a low rotation speed weld, attack is predominantly in the nugget region. Laser surface melting (LSM) improves the corrosion resistance of both parent material and welds. The LSM layer is highly homogeneous with high resistance to anodic attack owing to the absence of consitituent intermetallic particles, which are typical pit initiation sites, and the high solute level in the layer. The cathodic reactivity is also decreased as a result of the absence of intermetallic particles. Further improvements are achieved by addition of alloying elements to the LSM layer. Chromium is particularly effective as it lowers the cathodic reactivity of the surface. LSM of AA7449 leads to a drop in the cathodic reactivity of the surface, but the anodic reactivity remains high. However, as the corrosion is uniform rather than localised, it appears that the layer may have sacrificial properties in protecting the underlying weld.