  Two different precipitate hardening aluminium alloys processed by friction
stir welding were investigated. The microstructure and the hardness of the as
delivered materials were compared to that of the weld nugget. Transmission
electron microscopy observations combined with three-dimensional atom probe
analyses clearly show that \beta;" precipitates dissolved in the nugget of the
Al-Mg-Si giving rise to some supersaturated solid solution. It is shown that
the dramatic softening of the weld could be partly recovered by post-welding
ageing treatments. In the Al-Mg-Sc alloy, Al3Sc precipitate size and density
are unchanged in the nugget comparing to the base metal. These precipitates
strongly reduce the boundary mobility of recrystallised grains, leading to a
grain size in the nugget much smaller than in the Al-Mg-Si alloy. Both coherent
and incoherent precipitates were detected. This feature may indicate that a
combination of continuous and discontinuous recrystallisation occurred in the
weld nugget.
