TEM study of the microstructure evolution in a friction stir-welded AlCuMgAg alloy

The microstructure of a friction stir-welded Al-6.0Cu-0.75Mg-0.65Ag (wt.%) alloy in the peak-aged T6 temper was characterized by transmission electron microscopy. Strengthening precipitates found in the base alloys dissolved in the weld nugget, while those in the heat-affected zone coarsened considerably, causing softening inside the weld region. Precipitates of the Ω (Al2Cu) phase, being the main strengthening phase in base material, grew up to 200-300 nm in the heat-affected zone, but their density decreased. They co-existed with Θ' (Al2Cu), S' (Al2CuMg), Θ (Al2Cu) and σ (Al5Cu6Mg2) phases. The density of the Θ' and S' phases as well as their sizes increased compared to the base material. The high-resolution observation allowed to compare the morphology of the Ω phase plates in the heat-affected zone and in the base material