Issue 46

H. Mekri et alii, Frattura ed Integrità Strutturale, 46 (2018) 62-72; DOI: 10.3221/IGF-ESIS.46.07 63 used for joining Al/steel, particularly in the automotive industry, are self-pierce riveting [24], mechanical clinching as well as adhesive bonding coupled with another joining technique [25]. By using solid state welding processes, such as friction welding [19, 20, 27], diffusion bonding [21] magnetic pressure/ pulse seam welding [22] and ultrasonic welding [23], sound Al/steel joints have been produced. However, these processes have several disadvantages such as longer joining times as well as geometry requirements, among others. Although the formation of the intermetallic phases is kept as minimum as possible due to the minimised energy input, a very thin layer of Fe n Al m phases is still formed at the Al/steel interface in all aforementioned processes. The Friction Stir Welding (FSW) process was developed by TWI in 1991 [10], as a novel method for joining Al-alloys and, since that time, the welding process has been employed when fabricating non-ferrous alloys (aluminum, titanium, magnesium, zinc and copper alloys), as well as steel and thermoplastic substrates having thicknesses from 1 to 50 mm. The FSSW process is a variant of the FSW technique [13, 28], in which the tool is not traversed and instead creates the joint by plunging into and retracting out the tool of the overlapping sheets [14, 15]. The influence of welding parameters on the failure load properties of Al/Al-alloy friction stir spot welds has been examined at length [5, 11, 12]. A schematic illustration of the FSSW process joining two metal sheets is shown in Fig.1. As shown in this figure, this process consists of three stages. First a rotating tool R S with a threaded probe is plunged into the upper sheet with a clockwise rotation rate R S of 1000, 1400, 2000 rpm and a plunge rate V p of 16 mm/min and with penetration depth of length D p (mm). The process control is a displacement (position) control. Second when a shoulder contact the upper sheet, the force control (load) is used, characterised by the tool being driven into the specimen at a rate controlled by the axial force on the tool. The plunging continues as the axial force F increases until a predetermined value F of 1400Kg is reached. The force is held constant for a 2 seconds dwell time to generate a frictional heat. The heated and soften material, is plastically deformed and a solid state bond is made between the sheets. After which, thirdly the tool is retracted with a withdrawal rate V w of 100 mm/min [4], each parameter in this study is characterised with the software MODDE 5.0 (Modeling and Design) [26] is used for the model elaboration and the statistical analysis of the experimental design. The present work concerns on the study of rupture of a FSSW assembly which consists of single lap of two different thin sheets of aluminum alloy 6061 T5 and galvanized steel having thickness 1.7mm. Figure 1: Schematic illustration of stages of FSSW process. M ATERIALS AND COMPOSITION ommercial grade AA 6061 alloy and galvanized steel sheets of 1.7mm thick were used as the starting materials. AA 6061 is in the T5 condition in the as received condition and a galvanized steel sheet. Their nominal chemical compositions are presented in Tab. 1. C