Microstructural characteristics of joint line of friction stir welding of 2A97 Al-Li alloy

2A97-T851 aluminium lithium alloy with thickness of 25 mm was welded by friction stir welding. Using microhardness test, optical microscopy, transmission electron microscopy, the microhardness distribution and microstructure evolution of the friction stir welded joint were investigated. The results indicate that, the base material presents the highest hardness which decreases within the heat affected zone and heat mechanical affected zone, while in the nugget zone the hardness increases to certain extent. The lowest hardness is found in the heat affected zone. The refined grains observed within the weld nugget attributed to the dynamic recrystallization because the weld nugget experiences both high temperature and stirring rotation deformation. In the nugget, the S′ precipitates and most of the T1 phase dissolve. During nature aging after cooling, the G. P. zone and fine δ′ phase re-pricipitate. In the heat affected zone, S′ phase and partial T1 dissolve. θ′, δ′ and δ′/β′ composites are found in this zone.