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

Abstract

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.

Enhanced Long-Term Reliability of Seal DeltaSpot Welded Dissimilar Joint between 6061 Aluminum Alloy and Galvannealed Steel via Excimer Laser Irradiation

“…The chemical compositions at positions A and B in Figure 10c were 60.8 wt% Al/39.2 wt% Fe and 54.1 wt% Al/45.9 wt% Fe, indicating that the reaction layers near the Al alloy and steel were FeAl 3 and Fe 2 Al 5 , respectively. The lack of Zn and oxide elements (≤0.1wt%) meant that the liquid Zn (melted by joule heating) was discharged to the edges of the nugget by the electrode force after acting as a flux for the liquid Al-Zn eutectic reaction that aided in removing the oxide layer from the Al alloy surface [44,45]. The average IMC thicknesses of the WO, CWB, and PWB samples were 4.2, 6.1, and 6.2 µm, respectively.…”

Joining Technology Innovations at the Macro, Micro, and Nano Levels

“…Joining of steel with aluminum is the most important and challenging technical issue in the automotive industry, because brittle Fe-Al intermetallic compounds (IMCs) formed between steel and aluminum decrease the mechanical properties of the joint. Controlling the IMCs is the key factor to achieve a strong steel-aluminum joint [16][17][18][19]. Yang et al investigated the microstructure and mechanical properties of dissimilar Al/steel joints with and without nickel coating [20].…”

Critical assessment 27: Dissimilar resistance spot welding of aluminium/steel: Challenges and opportunities

“…This factor is a function of heat input, electrode force and electrode geometry. • Defects at joint interface: Defects includes formation of porosity/void in Al nugget adjacent to the joint interface, caused mainly by Al solidification shrinkage and melt expulsion and steel vaporisation, and presence of lack of wetting regions, where the IMC layer is discontinuous, cased primarily due to the uneven Al oxide removal [46,47]. It is of note that the formation of Kirkendall-porosity is minimised during RSW due to rapid cooling as welding as well as the presence of a compressive loading of joint [28].…”

“…Mechanical properties of Al/steel RSW depend on how aforementioned stages can be controlled. The factors controlling the mechanical properties can be summarised as follows [40,[47][48][49][50][51][52][53][54][55][56][57]:…”

“…absence of reaction layer) due to presence of Al oxide. Removing the oxide layer needs a high heat input, resulting in the formation of a thick Fe-Al IMC layer at the joint interface, making it impossible to obtain satisfactory joint strength [46,47]. The presence of zinc, as a protective coating on the steel surface, can be regarded as beneficial due to two distinct features:…”