Microstructure and mechanical properties of friction spot welds of dissimilar AA5754 Al and AZ31 Mg alloys U. Suhuddin a,n , V. Fischer a,b , F. Kroeff a,b , J.F. dos Santos a a Helmholtz-Zentrum Geesthacht, Centre for Materials and Coastal Research, Institute of Materials Research, Materials Mechanics, Solid State Joining Processes Department, Max-Planck-Str.1, Geesthacht 21502, Germany b Federal University Rio Grande do Sul, Physical Metallurgy Laboratory, Av. Paulo Gama,110 Porto Alegre, RS 90040-060, Brazil article info Article history: Received 2 September 2013 Received in revised form 18 October 2013 Accepted 21 October 2013 Available online 28 October 2013 Keywords: Friction spot welding Dissimilar Intermetallics Diffusion abstract In the present study, friction spot welding or refill friction stir spot welding was performed to consolidate dissimilar AA5754 Al and AZ31 Mg alloys. The intermetallic compounds of Al 12 Mg 17 and Al 3 Mg 2 were primarily found in the weld, distributed at the interface between the base materials and in the Al top sheet. The distribution of the intermetallic compounds and the interfacial area between the base materials affect the lap shear strength of the weld. It is concluded that the material flow induced by tool movement plays an important role in both the distribution of the intermetallic compounds and the interfacial area between the base materials. & 2013 Elsevier B.V. All rights reserved. 1. Introduction To prevent global warming and save energy, industries have put extensive effort into replacing conventional materials with lightweight materials such aluminum (Al) and magnesium (Mg) alloys, e.g., in vehicle structural applications in the transportation sector. Consequently, reliable processes are needed to join such lightweight materials, not only for similar joint configurations but also for dissimilar joint configurations such as Al/Mg alloys, Al/steel alloys, and Mg/steel alloys. Friction-based joining is used in the welding of similar materials [1–5]. However, the process is also an attractive option for welding dissimilar materials [6–12]. Several friction-based joining processes have been used to pro- duce dissimilar joints, including friction stir welding (FSW) [6,8,9] and friction stir spot welding (FSSW) [10,12]. Some studies have shown that the dissimilar welding of Al/Mg alloys using friction-based joining processes [6–12] produces intermetallic compounds (IMCs) of Al 12 Mg 17 [6–12], Al 3 Mg 2 [6,10], and Mg 2 Si [10]. The formation of IMCs is detrimental to the mechanical properties of the joint. Friction spot welding (FSpW), also known as refill friction stir spot welding, is one of the spot FSW process variants that is used to weld two or more materials in a lap joint configuration. FSpW was developed and patented by GKSS Forschungszentrum Geesthacht (now Helmholtz Zentrum Geesthacht, HZG), Germany. The non-consumable tool used in FSpW consists of three inde- pendent moving parts: two rotating sleeve and pin, and a stationary clamping ring. A schematic illustration of the tool and the process is shown in Fig. 1. The stationary clamping ring holds the material against a baking bar in the lap joint configuration, while the rotating sleeve penetrates the materials and the pin moves in the reverse direction. The rotating sleeve introduces plastic deformation and generates frictional heating, which plas- ticizes the material. The sleeve squeezes the softened material, filling the cavity left by the pin. Then, the rotating sleeve and the pin move back to their initial positions, pushing the softened material back into the joint. Finally, the tool is retracted from the surface, leaving the weld without a keyhole. As a solid state welding process, FSpW offers many advantages, such as the ability to produce both a weld with good mechanical properties and a weld without a keyhole on the surface [3–5]. FSpW has been successfully used to join similar welds [3–5] and has been used to join dissimilar materials, such as Al and Mg [7]. However, relatively little effort has been made to develop this process, especially in the dissimilar joint configuration. Therefore, the objective of the present study was to investigate the relationship between the grain structure of the spot weld between dissimilar AA5754 Al and AZ31 Mg alloys and its mechanical properties. 2. Experimental procedure The FSpW process was used to join dissimilar AA5754-H24 Al alloy and AZ31 Mg alloy in the lap joint configuration. The sample Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.10.057 n Corresponding author. Tel.: þ49 4152 87 2070; fax: þ49 4152 87 2033. E-mail address: uceu.suhuddin@hzg.de (U. Suhuddin). Materials Science & Engineering A 590 (2014) 384–389