AbstractFriction stir welds in an Al-alloy AA6056 in T4 and T6 temper were investigated in terms of grain structure, dislocation density and second phase particles at the different zones of the welded joint. These results were subsequently compared with the unaffected base material. The aim of this investigation was to understand the metallurgical phenomena controlling the microstructural evolution associated with the friction stir weld
process for temper condition. Major emphasis was placed on the evaluation of second phase particle formation and evolution at weld zones. The stir zone of the T4 temper maintains the precipitation features of the base material, while for the T6 temper, dissolution of thin precipitates occurs. The
stir zone of post weld heat treated joints presented equiaxed grains fully covered of very thin precipitates randomly distributed along the matrix. These precipitates are about 2–10 nm in diameter. The remaining regions are characterised by partial dissolution and precipitate coarsening during the FSW process.