This research concerns a dissimilar metals joining of steel and aluminum (Al) alloys by means of zinc (Zn) insertion. The authors proposed a joining concept for achieving strong bonded joints between Zn coated steel and Al alloys. 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.
As a practical issue characteristic to dissimilar metals joints, anticorrosion measures against electrochemical corrosion must be undertaken. If there is moisture near a dissimilar metals joint interface, electrochemical corrosion would progress. Therefore, a sealing function that could prevent moisture intrusion is required. By applying above mentioned welding process to a set of metals with thermosetting resin spread in between, we realized seal spot welding which would not only prevent moisture intrusion, but also retain high tensile strength.
In this research, first, a cyclic corrosion test was performed on the seal spot welded joint of galvanized (GI) steel and Al alloy, and the anticorrosion measures were found to retain tensile strength and also secure sufficient corrosion resistance. Then galvannealed (GA) steel, a steel grade widely distributed in Japan, and Al alloy was bonded by seal spot welding, and following topics have been discussed. Complete removal of sealant from the joint interface is the key to realize high tensile strength joint, because remaining sealant would lead to reduction of tensile strength. Therefore, heat generation at the interface was monitored by measuring electrical current and potential difference between the two electrodes, and a precise temperature control was performed. Moreover, the bonding process was clarified by stepwise analysis of the joint interface using optical microscopy, and a guideline for producing strong joints was proposed. And finally, a TEM observation also confirmed that the interface structure of seal spot welded joint was the same as joints without the resin; a thin and uniform Al-Fe IMC layer was formed and a strong metallurgical bonding was achieved.