Seal spot welding of steel and aluminium alloy by resistance spot welding: dissimilar metal joining of steel and aluminium alloy by Zn insertion

This research concerns a dissimilar metal joining of steel and aluminium (Al) alloys by means of zinc (Zn) insertion. The authors propose 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 uniform Al–Fe intermetallic compound (IMC) layer at the joint interface, leading to a strong bonded joint. The ultimate aim of this research was to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. As a practical issue characteristic to joints of dissimilar metals, anticorrosion measures against electrochemical corrosion must be undertaken. If there is moisture near a joint interface of dissimilar metals, electrochemical erosion will progress. Therefore, a sealing function that could prevent moisture intrusion is required. By applying the above-mentioned welding process to a set of metals with thermosetting resin spread in between, we realized seal spot welding, which not only prevented moisture intrusion but also retained high tensile strength. In this research, first, a cyclic corrosion test was performed on the seal spot-welded joint of galvanized (GI) steel, a steel grade widely distributed in Japan, and Al alloy was bonded by seal spot welding, and the following topics are discussed. Complete removal of sealant from the joint interface is the key to realizing the high tensile stress joint, because remaining sealant will lead to reduction in 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 the 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. © 2016 Informa UK Limited, trading as Taylor & Francis Group.