“…Conventional fusion welding methods, such as arc and laser heating have been used to join aluminum and steel [2,5,6]; however, the poor seam surface of the weld and the formation of porosity in the weld hinder the practical applications of these two techniques. Friction welding has been proven practical to eliminate the formation of the intermetallic phases and to form a sound weld [3,7,8], but this method is usually used for joining cylindrical parts. As a newly emerging technology, friction stir welding (FSW), which has been widely applied in the industry for joining aluminum alloys and is expanding its application in joining steels [9,10], is expected to provide a practical solution for joining aluminum-steel.…”