Issue 48

R. Maciel et alii, Frattura ed Integrità Strutturale, 48 (2019) 269-285; DOI: 10.3221/IGF-ESIS.48.28 270 K EYWORDS . Friction stir welding, adhesive bonding, aluminium alloys, friction stir weld-bonding, mechanical behavior. I NTRODUCTION combination of regulatory requirements [1] and market demand [2, 3] have pushed for continuous improvements in energy efficiency and performance in transport solutions. Weight reduction through the use of new lightweight alloys and new structural designs is a way to achieve these goals [3]. In order to implement lighter materials and innovative structural designs new manufacturing processes are required. The aeronautical industry has been shy of welding processes in primary structures due to the related loss of mechanical properties from large heat inputs, weld quality control (process reliability) and the impossibility of welding precipitated hardened alloys (e.g. AA2024 aluminum alloy), in which cracks tend to form from the arc welding process. Solid state welding mitigates some of these concerns, as lower heat inputs result in improved mechanical performance and easier process control improves process reliability. Friction stir welding (FSW) FSW is a revolutionary joining method that allowed the welding of previously unwieldable alloys with excellent characteristics and has an enormous potential for application in a large array of industries . Friction stir welding has shown to produce sound quality, high performing joints making it the most appealing welding technology for aeronautical structures [4]. In its most basic form, FSW is performed with a tool composed of shoulder and pin, fractioning and mixing the material to weld. The tool is insert while in rotation into the pieces to be welded and transverses along the weld line, as shown in Fig. 1. The shoulder is mainly responsible for providing heat from friction on to the sheets or plates to be welded, while the pin’s main job is mixing the materials to be joined. Figure 1 : Principle of the FSW process [5]: a) butt-joint configuration; b) overlap configuration (SLJ) Figure 2 : Schematic of a butt-joint weld and correspondent keyhole cross-section [7]. The process starts with and initial plunge of the tool into the work piece at a controlled rate, followed by a pre-heating stage called “dwell time”. Preheating the material softens it along the joint line. When the suitable thermo-mechanical conditions are established, relative transverse movement, between the plates and the tool, starts, maintaining the rotation of the tool (welding phase). After welding, the tool is extracted from the plate leaving behind a characteristic keyhole, Fig. 2. A