Effect of the ageing on precipitation spatial distribution in stationary shoulder friction stir welded AA2050 alloys

The spatial distribution of the strengthening precipitates in stationary shoulder friction stir welds (SSFSW) of the AA2050 Al-Cu-Li alloy has been characterized by small-angle X-ray scattering (SAXS), combined with microhardness mapping and transmission electron microscopy (TEM). It is shown that welding the alloy before ageing (T3WT8) leads to a more homogeneous distribution of precipitates along the weld zones than welding after ageing (T8W). Further in-situ SAXS ageing (24 h at 155 degrees C) has been performed on the nuggets of both welds, to better evaluate the subsequent precipitate evolution. The nugget zone is shown to be very inhomogeneous, which can be explained by the 2 passes sequence of SSFSW. The mid-depth zone shows no fine precipitation, because of the formation of coarser phase consuming the solutes. Irrespective of the welding sequence, further ageing does not yield homogeneous precipitation in the welds.