Abstract: 6005 A-T6 aluminum alloy was butt-welded by bobbin tool friction stir welding,and the effect of heat input on formation and mechanical properties were investigated. The results showed that the welded joints was consisted of heat affected zone（ HAZ）,thermal mechanically affected zone（ TMAZ） and weld nugget zone（ WNZ）. The total amount of plasticized materials and volume of squeezing die determined the morphology of the welded joint. If WP≤1,the amount of plasticized materials is the main influence factor,and the morphology is hourglass shaped. With the decreasing of the heat input,the interface of weld seam and the BM becomes flat gradually,and tunnel defect forms at the advancing side（ AS）. The tensile strength increases initially and then decreases with the decrease of heat input. When WP＞1,The volume of the squeezing die became the main influence factor,and the interface of weld seam and the BM keeps flatness. No tunnel defect is observed.The tensile strength increases with the decrease of heat input. Lazy S feature forms at the intersection of plasticized materials flow. Heat input affects the morphology of the lazy S through influencing the materials flow. Lazy S feature has little influences on the tensile properties of the joints.