The precipitation sequence in friction stir weld of 6063 aluminum during po
stweld aging, associated with Vickers hardness profiles, has been examined
by transmission electron microscopy. Friction stir welding produces a softe
ned region in the weld, which is characterized by dissolution and growth of
the precipitates. The precipitate-dissolved region contains a minimum hard
ness region in the as-welded condition. In the precipitate-dissolved region
, postweld aging markedly increases the density of strengthening precipitat
es and leads to a large increase in hardness. On the other hand, aging form
s few new precipitates in the precipitate-coarsened region, which shows a s
light increase in hardness. The postweld aging at 443 K for 43.2 ks (12 hou
rs) gives greater hardness in the overall weld than in the as-received base
material and shifts the minimum hardness from the as-welded minimum hardne
ss region to the precipitate-coarsened region. These hardness changes are c
onsistent with the subsequent precipitation behavior during postweld aging.
The postweld solution heat treatment (SHT) and aging achieve a high densit
y of strengthening precipitates and bring a high hardness homogeneously in
the overall weld.