This research concerns a dissimilar metals joining of steel and aluminum (Al) alloy by means of zinc (Zn) insertion. The authors proposed a joining concept for achieving strong bonded joints between Zn coated steel and Al alloys. The ultimate aim of this research is to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. This paper presents the results of fundamental investigations concerning the effect of Zn insertion on the properties of diffusion bonded joints.
Bonded joints of a combination of galvanized (GI) steel and Al alloy were subjected to joint interface observations; joint strength tests were followed by fracture surface observations of the test specimens. The results confirmed that the Al-Zn eutectic reaction effectively removed the oxide film on the Al alloy surface at low temperature in air, enabling the formation of a uniform Al-Fe intermetallic compound (IMC) layer. The formation of this Al-Fe IMC layer facilitated metallurgical bonding that lead to strong joints.
In addition, considering an actual application for car body production, bonded joints of not only a combination of GI steel and Al alloy but also of a combination of galvannealed (GA) steel, a steel grade widely distributed in Japan, and the Al alloy were prepared by diffusion bonding and rapid cooling. Detailed observations were performed, and the influence of different types of Zn coating on the bonding process has been discussed. During the bonding process of GI steel and Al alloy, the oxide film on Al alloy surface and the Zn coating on steel are then removed together with the Al-Zn eutectic liquid phase, as bonding pressure is applied, resulting in the formation of an Al-Fe (Zn) IMC layer. On the other hand, during the bonding process of GA steel and Al alloy, aluminum and the Fe-Zn alloy coating layer react to form an Al-Fe (Zn) IMC and a mixed layer consisting of Al (Zn) and a Zn liquid phase. Therefore it is more difficult to push them out around the periphery of the joint. As a result, a thick Al-Fe (Zn) IMC layer is more easily formed, and the bondability is lower in comparison with GI steel and the Al alloy.
