The 6005A-T5 aluminum alloy welded joints are prepared by use of friction stir welding (FSW) at different welding speeds. The microstructure and mechanical properties of these joints have been investigated. The relationship between evolution of precipitates in different regions and the mechanical properties of welded joints is established. The β″ phases completely dissolve back into the aluminum matrix due to enough welding heat input in nugget zone (NZ) during the welding processing. GP zones are formed during the subsequent natural aging, which results in the hardness recovery of NZ. Incomplete recrystallization occurs in the ther-mo-mechanically affected zone (TMAZ), and the grains are elongated with high dislocation density. The heat affected zone (HAZ) contains Q" and β″ phases. With the decrease of welding speed, β″ phase gradually disappears and Q" phase is formed, which leads to a decrease in the strength of welded joints. The average value of longitudinal residual stress is higher than that of transverse residual stress. With the increase of welding speed, the peak longitudinal residual tensile stress increases, but the effect on the transverse residual tensile stress is negligible.