Precipitation sequence in friction stir weld of 6063 aluminum during aging

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.