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ALUMINUM alloys are well known for their high ratio of strength to weight, which results in extensive applications of these alloys in various industries. However, aluminum alloys present low weldability when joined by conventional fusion welding. The friction stir welding (FSW) process was invented for the first time in 1991 in The Welding Institute as a solid-state welding method.[1] Since its development, FSW has been widely used as an alternative welding procedure to join different materials.[2,3] During FSW, the severe plastic deformation along with the very high friction heat result in a sound weld. Besides, the refining of the weld microstructure leads to excellent mechanical properties compared to conventional welding procedures. During the FSW process, the extensive local heat generated leads to plasticizing and softening the materials in the stir zone. As FSW is carried out in a solid state, there are many advantages over conventional fusion welding. Among these advantages are the lower solubility of hydrogen, finer grain size of weldment, and the absence of general weld defects such as porosities and discontinuities. In addition, joining by FSW requires neither the grooves nor the surface preparation such as de-oxidizing.

MORTEZA GHAFFARPOUR, Master’s Student, and BIJAN MOLLAEI DARIANI, Associate Professor, are with the Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran. Contact e-mail: ghaff[email protected] SINA KOLAHGAR, Master’s Student, is with the Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran. KAMRAN DEHGHANI, Associate Professor, is with the Department of Materials Science and Engineering, Amirkabir University of Technology. Manuscript submitted October 4, 2012. Article published online April 19, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

Because of the unique properties of the welds produced by FSW, this technique is widely used to join the aluminum alloys that suffer from insufficient weldability. As the conventional fusion welding is not desirable for welding aluminum alloys, there are many works conducted on welding the aluminum alloys by FSW. These works are in regard to the effects of FSW parameters on weld quality,[4,5] sheet formability after FSW,[6,8] and optimization of the FSW process.[9,12] On the other hand, FSW is also an excellent technique for joining the dissimilar materials. This is especially very practical in joining the dissimilar aluminum alloys to take advantage of different properties of different alloys. For example, it is well known that 5000 and 6000 aluminum series exhibit different properties, including their formability and strengthening mechanisms. These 5000 and 6000 aluminum series are extensively used in the transportation industry and in aerospace. Thus, welding the dissimilar aluminum alloys by FSW can be very beneficial in terms of their industrial applications. Therefore, the aim of the present work was first to weld the dissimilar 5083-H12 and 6061-T6 aluminum alloys