Tensile Strength of Friction Stir Spot Welded Dissimilar AA5754-to-AZ31B Alloys *X. Cao, **C. Garnier and P. Wanjara Aerospace Manufacturing Technology Centre, Institute for Aerospace Research, National Research Council Canada, 5145 Decelles Avenue, Montreal, Quebec, H3T 2B2, Canada *Presenting author, xinjin.cao@cnrc-nrc.gc.ca **Present address: St. Eloi plant, Airbus France, EADS, 316 route de Bayonne, 31060 Toulouse, France Abstract Friction stir welding (FSW) is a relatively new joining process. As a solid-state joining technique, FSW provides good potential for dissimilar materials, even for those considered to be “difficult” and "unweldable". As a variant, friction stir spot welding (FSSW) can have significant potential to replace riveting or resistant spot welding for aerospace and automotive applications. To date, limited work has been carried out on the FSSW of dissimilar material combinations. In this work, the tensile shear strength obtained is reported for a dissimilar 2-mm thick AA5754-to-AZ31 alloy system. The main process parameters investigated include tool rotation speed, tool plunge rate, dwell time and work-piece placement (i.e. either Al or Mg alloy on the top of the lap spot welds). The tensile shear strength is also compared with that obtained for similar AA5754-to-AA5754 and AZ31-to-AZ31 welds. Furthermore, the tensile shear strength is correlated with the joint geometrical dimensions and welding defects. Keywords Friction stir spot welding; Aluminum alloy; Magnesium alloy; Dissimilar joining; Tensile shear strength. Introduction Weight reduction in automotive and aerospace industries is one of the most crucial issues to improve the fuel economy and reduce the environment footprint. Therefore, lightweight materials such as aluminum and magnesium alloys have shown a lasting steady increase in industrial applications. However, the use of one material may not be optimum for the chemical, physical and mechanical properties in a certain application. The combination of dissimilar materials provides a potential for tailoring the properties to reduce weight, save cost, and provide new structural design concepts. For the manufacture of dissimilar materials assemblies, their differences in the optical, physical, chemical, mechanical, and thermal properties present significant technical challenges. In particular, fusion welding generally involves the formation of brittle intermetallic compounds in a dissimilar alloy combination, which may cause embrittlement (the root cause for dissimilar Al to Mg joints), cracking and degradation of the mechanical properties [1]. Therefore, the probability of achieving sound welds using fusion welding processes is rather low for the dissimilar Al to Mg alloy joints in industrial practice. As a relatively new solid-state joining process, FSW was first invented mainly for aluminum alloy joining in 1991 by TWI in England. Its principle is quite simple. A rotating tool is plunged into the work-piece to be joined and traverses along the joint line. The material to be joined becomes softened due to the frictional heating, plastic deformation and viscous dissipation, and the forging pressure from the shoulder reconsolidates the material behind the tool to obtain a sound joint. Friction stir spot welding is a variant of the linear lap FSW process since it involves only the plunge, stir and retraction of the FSW tool with no transverse movement, as schematically shown in Fig. 1 [2]. A crucial feature of FSW or FSSW is that the materials are joined in the plasticized state, considerably below the respective melting temperatures. Therefore, there exists a potential opportunity to avoid the formation of the undesirable brittle intermetallic compounds during FSW or FSSW of the dissimilar materials combinations. Due to its scientific and commercial importance, FSSW process has been well reviewed by, for instance, Pan [2] and Gerlich and North [3]. Fig. 1 Schematic diagrams indicating the friction stir spot welding (FSSW) process [2] FSSW is similar to resistance spot welding (RSW) or riveting and has been considered as a potential alternative to those processes as well as clinching for attaching/fastening two metallic work-pieces. To date, FSSW has been used in all- aluminum rear doors for the RX-8 sports cars by Mazda since 2003 as well as the engine hook and decklid for the Prius gasoline/electric hybrid vehicles by Toyota since 2004 [2, 3]. Kawasaki Heavy Industries (Japan) has also used FSSW to