The aim of this research is to study the effect of the welding process on the microstructure and mechanical properties of friction stir welded joints in aluminium alloys 2024-T3, 5083-O and 6063-T6. A small loss of hardness and strength was obtained in welds in alloys 2024-T3 and 5083-0 as opposed to welds in alloy 6063-T6, where a substantial softening and a drop of strength were observed. In alloy 6063-T6 a strength efficiency of only 45 to 47% was obtained.