抵抗スポット溶接による鋼と Al合金の異種材料接合

－亜鉛インサートによる鋼とAl合金の異種材料接合－

抄録

This research describes 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 alloy. A eutectic reaction between Zn in the Zn coating and Al in Al alloy is used to remove the dense, strong oxide film on Al alloy surface at low temperature, thereby resulting in the formation of a thin, uniform Al-Fe intermetallic compound (IMC) layer at the joint interface, leading to a strong bonded joint. The ultimate aim of this research is to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. This paper discusses the effectiveness of this joining concept by bonding galvanized (GI) steel sheet with Al alloy sheet, and galvannealed (GA) steel sheet with Al alloy sheet, by resistance spot welding.
A precise observation of the combination of GI steel sheet and Al alloy sheet joint interface confirmed that a thin, uniform Al-Fe IMC layer can be formed with this process as a result of enveloping the oxide layer on the Al alloy surface in the eutectic melt and effectively evacuating it from around the periphery of the joint area. An excellent joint property was achieved by the realization of such a metallurgical bonding. As for the combination of GA steel sheet and Al alloy sheet, oxide film on Al alloy surface was removed by liquid phase formed by diffusion between Al and Fe-Zn alloyed coating layer; the formation of Al-Fe IMC layer was confirmed by observation of the joint interface. The Al-Fe IMC layer was thick and non-uniform, compared with GI steel and Al alloy. The difference is believed to be based on the difference in bonding process caused by different coating types.
The region with Al-Fe IMC layer thickness of less than 2μm was defined as the effective bonded length. A clear correlation between the effective bonded length and the tensile strength was seen. An improvement of tensile strength was attempted by optimization of electrode shapes and welding conditions, considering differences in bonding processes of each metal combination, and a guideline for realizing high tensile strength was clarified.
From above considerations, dissimilar metals joining between steel and Al alloy by the resistance spot welding was realized.