a friction stir welding method includes providing a joining tool including a friction surface , a first workpiece , and a second workpiece , and arranging the first workpiece and second workpiece in position with a first joining surface abutting a second joining surface . a friction surface of a joining tool resists at least one of the first treating surface and the second treating surface , and the first workpiece and the second workpiece cooperatively define a slot . then positioning , rotating , and moving the joining tool to rub and stir at least one of the first workpiece and the second workpiece , thus plasticizing at least part of the first workpiece and the second workpiece to join the first workpiece and the second workpiece .