ORIGINAL ARTICLE The effect of preheating on mechanical properties of friction stir welded dissimilar joints of pure copper and AA7075 aluminum alloy sheets Seyed Vahid Safi 1 & Hossein Amirabadi 1 & Mohammad Kazem Besharati Givi 2 & Seyed Majid Safi 3 Received: 7 February 2015 /Accepted: 21 September 2015 # Springer-Verlag London 2015 Abstract In this paper, apart from introducing brand-new warm friction stir welding (WFSW) method, the effect of preheating on the mechanical properties of dissimilar friction stir welded copper/aluminum alloy sheets was investigated. Sheets of aluminum alloy 7075 and copper both with the thickness of 5 mm were used. The utilized welding tool was made of a H-13 tool steel with a threaded cone shape. Rota- tional speeds (ω) of 1200–1400 rpm and travel speeds (v) of 50–100 mm/min were used for better understanding of the effect of welding parameters on heat input. The sheets were kept in furnace with temperatures of 75 and 125 °C, and welding was done afterward. At last, tensile test was done to compare the mechanical properties of the welds. The tensile test results showed 100 % increase in the strength of preheated joints in comparison with the non-preheated joints which were welded at room temperature. Considering the high thermal conductivity of both copper and aluminum, the reason of in- crease in strength of the joints could be related to the low temperature gradient between the weld zone and base metal because the heat gets out of the stir zone with lower steep. The maximum strength occurred for the specimen with the preheating of 75 °C, rotational speed of 1300 rpm, and travel speed of 75 mm/min. Keywords Friction stir welding . Mechanical properties . Preheating . Pure copper . Aluminum . Alloy 1 Introduction Friction stir welding (FSW) is a process including many ac- tions and interactions between a series of simultaneous ther- modynamic processes. These reactions are the results of heat input rate, cooling rate, flow and deformation, dynamic re- crystallization, and integration of mechanical joints [1]. This method of welding was invented in 1991 by Thomas Wayne et al. in TWI in Cambridge, UK [2]. FSW is a relatively new method in metal joining because of its solid state nature (no melting happens at joining) and has some advantages over other fusion welding methods. Low distortion and shrinkage, good mechanical properties, fewer defects, and the ability to weld some metals that cannot be welded with fusion welding methods are among the most important advantages of this method [3]. In last few years, this process has been used in aerospace and aviation industries, automotive industries, fuel tanks, etc. in most developed countries. Copper has a wide range of applications because of its great thermal and electrical conductivity, corrosion and fatigue resistance, and good flex- ibility. Copper alloys have various sets of properties which are dependent on the addition of elements and heat treatment [4]. Today, aluminum and its alloys have a wide range of applica- tions in defense industry, aerospace, transportation, marine industry, construction industry, packaging, and containers. Some of the properties which have made aluminum and its alloys as one of the most economical and popular group of metals are as follows: light weight, flexibility, physical prop- erties, mechanical properties, and corrosion resistance [5]. Sound and flawless joining of dissimilar materials for chemical, nuclear, aerospace, transportation, power genera- tion, and electronics applications enjoys great importance. However, joining of dissimilar materials by fusion welding processes is very difficult due to the weak weldability resulting from different chemical, mechanical, and thermal * Seyed Vahid Safi vahid.safi@gmail.com 1 Department of Engineering, University of Birjand, Birjand, Iran 2 Department of Engineering, University of Tehran, Tehran, Iran 3 Department of Engineering, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Int J Adv Manuf Technol DOI 10.1007/s00170-015-7877-5