The aim of this work is the study of the effect of the weld travel speed on the defect formation and on the changes in the microstructures and mechanical properties of welds in aluminium alloys. Welds were produced on aluminium. alloys 5083-0, 2024-T3 and 6063-T6, in thick 3 mm, using weld travel speeds between 150 and 600mm/min in the case of alloy 5083-0 and between 200 and 400mm/min in alloy 2024-T3. Welds on alloy 6063-T6 were all produced with the travel speed of 550mm/min. Defects tend to form manly in aluminium alloys 5083-0 and 2024-T3 and are less usual in alloy 6063-T6, on the advancing side of the welds. In this last alloy hardness drastically decreases in the weld-deformed zone contrarily to the other two alloys where a small decrease of hardness (2024-T3) or even an increase of hardness is observed in the advancing zone (5083-0). These defects are mainly cracks or voids and are originated by large plastic deformation and hardening of the material, as well as on its complex flow behaviour in this zone. The increase of the weld travel speed increases the frequency of voids formation, though other welding parameters may contribute to the phenomenon.