Friction stir butt welding between A6061 and SPC270 sheets with a thickness of 1 mm was investigated in this study. The welding conditions, including the clearance, offset, welding speed, and tool rotation speed, were optimized and the highest tensile strength of 235.0 MPa can be achieved using the welding parameters proposed by this study. In addition, the influence of the end-face state of the SPC270 specimen was also studied. The results show that the tensile strength of the welding joints can be improved to 240.7 MPa by increasing the contact area between A6061/SPC270. Moreover, the input heat can be decreased with end-face milling of the SPC270 specimen, and a thin IMC layer with 150 nm in thickness was confirmed. In contrast, the IMC layer obtained using the welding specimen without end-face milling was 1130 nm in thickness. The temperature at the welding interface and the load added to the works during welding were also measured to reveal the influence of end-face milling.