Abstract: In order to save the fuel consumption and reduce the pollution, aluminum alloy is one of the most acceptable materials for weight reduction in vehicle body assembly. However, how to join aluminum alloys and steels is an unsolved problem. Friction stir welding is tried to join aluminum alloy and zinc coated steel. The steel-aluminum dissimilar materials lap joints are manufactured by friction stir welding in two ways. One is that the pin is pressed into the steel material to cause a mechanical and metallurgical bonding between the dissimilar materials. Another is that only a metallurgical bonding between the dissimilar materials is formed by the heat flow from the friction of the tool and the aluminum material. The lap joints are shear tested and the interface between steel and aluminum materials are observed. It is shown that the zinc coating on the steel plate has an obvious effect on the joint forming between steel and aluminum and there is an obvious difference in shear strength and failure mode between metallurgical and mechanical-metallurgical bonding joints. In mechanical-,metallurgical bonding joint, the interface layer between steel and aluminum materials is thick and composed of aluminum, zinc phase while in metallurgical bonding joint, the thickness of interface layer is about 2-3 μm and it is mainly composed of the intermetallic compound of Fe-Al. And the factors influencing joint strength and failure mode of steel-aluminum friction stir welding joint are also investigated.

Key words: failure mode, friction stir welding, interface layer, joint strength, steel/Al dissimilar materials joint