Microstructural properties of friction stir welded and post-weld heat-treated 7449 aluminium alloy thick plate

In order to improve the efficiency of the friction stir welding (FSW) manufacturing process a recently developed 7xxx series aluminium alloy has been welded in an underaged temper for age forming (TAF) condition and subsequently heat treated to the required T7 in service temper. Several complementary microstructural measurement techniques (SEM, TEM and SAXS) have been employed to capture a detailed view of the precipitate distribution, mean particle size, phase identity and fraction. Each friction stir weld zone has been treated separately and compared with the original underaged structure to capture the effect of the local thermal history. A comparison is made between microstructural measurements and predictions of the equilibrium fraction and solvus temperature for each phase using a CALPHAD approach. In contrast with welding thin sections the cooling rates obtained after friction stir welding thick plate are slow enough to allow precipitation of η′/η-phase within the nugget and TMAZ on cooling of the weld, at both grain boundaries and throughout the grain interior. Applying a subsequent post-weld heat treatment in effect replaces the fine η′/η-phase with coarse overaged precipitates in the nugget/TMAZ and reduces the strength contrast between nugget/TMAZ and HAZ.