A combined approach to microstructure mapping of an Al–Li AA2199 friction stir weld

A wide range of complementary techniques are used to build up a detailed picture of the microstructural zones found in friction stir welds (FSW) in an advanced AA2199 Al–Li alloy. Neutron and synchrotron X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, small angle X-ray scattering, scanning electron microscopy, electron backscatter diffraction (EBSD) and hardness mapping are brought together to build up a detailed two-dimensional picture of the grain morphology, precipitate type, size, volume fraction and matrix solute content across the weld cross section and to explain the general lack of a W-shaped hardness profile across FSW in third-generation Al–Li–Cu–Mg alloys. Dissolution of the age-hardening phases occurred in different regions of the weld, depending on their respective solvus temperatures, with δ′ dissolving within the heat-affected zone and T1 coarsening in the thermo-mechanically affected zone before going into solution in the weld nugget. Changes to the precipitate distribution, and especially to the T1 phase, are linked to a significant reduction in hardness (strength) and unstrained lattice parameter across the weld zone. It was also possible to show that the low recovery of nugget zone hardness is primarily due to its poor natural ageing response.

► We characterised the weld microstructure in a novel Al-Li alloy. ► We use a range of techniques to study precipitation around friction stir welds. ► Using synchrotron small angle X-ray scattering provides unique insight into precipitation in high-strength aluminium alloys.