Top surface microstructure of friction-stir welded AA2524-T3 aluminum alloy joints

We characterized the microstructures on the top surface of friction-stir welding (FSW) joints of AA2524 aluminum alloys with Alclad. Types, distributions, dissolution and precipitation behaviors of the precipitates in different regions of the joints were investigated. The results show that the as-polished surface microstructures under the shoulder of the FSW tool exhibited “arc strip” feature. The Alclad on the outer surface of the plate was destroyed during FSW, and the residual Alclad was enriched in the retreating side of the FSW joint. The main precipitates in the FSW joint were composed of Guinier–Preston (GP) zones and θ′ (Al2Cu), S′ (Al2CuMg) and Fe-containing phases. In the heat-affected and thermal-mechanical-affected zones, coarsening occurred in some S′ phases. In the GP zone, some S″ phases were transformed into stable S′ or S phases. In the nugget zone, the density of the precipitates decreased due to the dissolution of some S′ or S phases back into the matrix during the weld-heating period, whereas during the weld-cooling period, the refined and dispersed unstable GP zones or S″ phases reprecipitated.