metals Article Friction Stir Spot Butt Welding of Dissimilar S45C Steel and 6061-T6 Aluminum Alloy Kun Gao 1 , Shengwei Zhang 1 , Mounarik Mondal 1 , Soumyabrata Basak 1 , Sung-Tae Hong 1, * and Heechan Shim 2   Citation: Gao, K.; Zhang, S.; Mondal, M.; Basak, S.; Hong, S.-T.; Shim, H. Friction Stir Spot Butt Welding of Dissimilar S45C Steel and 6061-T6 Aluminum Alloy. Metals 2021, 11, 1252. https://doi.org/10.3390/ met11081252 Academic Editor: Aleksander Lisiecki Received: 15 July 2021 Accepted: 5 August 2021 Published: 7 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Mechanical Engineering, University of Ulsan, Ulsan 44610, Korea; 18217603739@163.com (K.G.); zsw19900619@gmail.com (S.Z.); mondal.mounarik3@gmail.com (M.M.); sb34@iitbbs.ac.in (S.B.) 2 Materials R&D Center, ILJIN Global Co. R&D Center, Seoul 06157, Korea; shinheechan@iljin.com * Correspondence: sthong@ulsan.ac.kr; Tel.: +82-052-259-2148 Abstract: Friction stir spot welding (FSSW) of dissimilar S45C steel and 6061-T6 aluminum alloy in a butt configuration is experimentally investigated. Butt spot welding is performed using a convex scrolled shoulder tool at different tool rotational speeds. FSSW butt joints are successfully fabricated by offsetting the tool to the steel side. The microstructures of the joints fabricated at three different tool rotational speeds are characterized using scanning electron microscopy and energy dispersive spectrometry. Microstructural analysis shows the presence of intermetallic compounds (IMCs) along the steel/aluminum interface. The thickness of the IMC layer and the tensile strength of the joint increase with increasing the tool rotational speed. The results of tensile tests and microstructural analysis show that the joint performance is closely related to the IMCs at the joint interface. Keywords: friction stir spot butt welding; aluminum alloy; steel; dissimilar joint; intermetallic compounds 1. Introduction Incorporation of lightweight materials for manufacturing various automotive compo- nents has been increasing to meet regulations of lower emissions and better fuel efficiency, while simultaneously resolving safety issues [1]. Among various lightweight materials, aluminum alloys are widely used for their formability and cost-effectiveness [2]. Although aluminum alloys provide various advantages, they are not able to replace steels completely. In many industrial applications, it is very difficult or nearly impossible for lightweight aluminum alloys alone to fulfill imposed structural or mechanical requirements. As a re- sult, steels, which have superior mechanical properties and cheaper prices than aluminum alloys, still have wide applications in the automobile, aerospace, and railway industries. Therefore, to achieve weight reduction while satisfying the structural or mechanical re- quirements, the joining of dissimilar steel and aluminum alloys is unavoidable in many industrial applications [3]. However, the joining of these two alloys imposes complications due to the vast differ- ences in their thermomechanical properties and their tendency to form brittle intermetallic compounds (IMCs) [4]. Researchers have attempted to join steels and aluminum alloys using two different joining methods: conventional fusion welding and solid-state joining. Conventional fusion welding methods, such as resistance spot welding [5], tungsten inert gas brazing [6], and laser welding [7], have been utilized to join steels and aluminum alloys. However, technical difficulties, including the formation of complex weld pool structures, inhomogeneous solidification microstructures, and segregation, hinder their practical ap- plications. Moreover, most conventional fusing welding techniques involve relatively high heat input, resulting in the formation of a thick layer of brittle IMCs [4,8,9]. Since fatigue cracks generally originate inside the brittle IMC layer, researchers have recommended limiting the thickness of the IMC layer to less than 10 μm to achieve mechanically sound joints [10–12]. Metals 2021, 11, 1252. https://doi.org/10.3390/met11081252 https://www.mdpi.com/journal/metals