PSI - Issue 28

Shayan Eslami et al. / Procedia Structural Integrity 28 (2020) 659–666 Shayan Eslami/ Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Joining different parts to produce a single component has always been an interesting engineering topic, and the necessity to cope with the fast-growing industrial requirements demanded more adaptable and environmental friendly joining methods for lightweight materials. High performance composites such as glass-fiber-reinforced polymer (GFRP) and carbon-fiber-reinforced polymer (CFRP), characterized by high strength to weight ratios, are more and more adopted for industrial applications. Glass fibers represent an estimated 95% of consumption, while the remaining 5% is mostly accounted by carbon and aramid fibers. The practical goals of fiber reinforcement are to increase modulus and strength, improve heat deflection temperature, reduce tendency to creep, and in some cases to save manufacturing costs (Biron, 2018). The main advantage of thermoplastics over thermosetting polymers is their recyclability and geometrical stability with temperature changes. In order to join thermosets, either mechanical fastening or adhesive bonding methods should be used since welding methods cannot be applied (Mishra et al., 2019). Thermoplastics are high-molecular weight polymers that can be reheated, which causes a weakening of secondary Van der Waals or hydrogen bonding forces enabling welding operations. Also, their processing time is significantly shorter than thermosets (Campbell, 2011). The weight of structures can be reduced by using lightweight metals such as magnesium and aluminum. However, thermoplastic based composites have a huge potential to be explored taking the weight reduction to an even higher optimization. The aerospace industries are driven mainly by the weight factor, and these high performing materials can minimize the structural weight, decreasing fuel consumption. For example, the Boeing aircraft, 777 Dreamliner had 12% components manufactured from composite and 50% from aluminum, while the more recent model, 787 Dreamliner, contains 50% of composite components (Mishra et al., 2019). This is an example of metallic materials being replaced by thermoplastic based composites due to their unique features. Despite the great design flexibility of polymers and composites, most components are made by joining different subparts. Therefore, in order to reliably join these materials, suitable joining methods are essential to fulfill the industrial and environmental demands. The available industrial joining solutions are not adequate to create new joint configurations due to the low melting point, thermal conductivity and surface energy of polymers, which restricts their applicability to particular joint: materials, geometry, thickness, and environmental conditions. With respect to the current industrial demands, and considering the lack of research in this area, the aim of this study is to develop technological solutions for joining short glass fiber thermoplastic composites. Friction Stir Welding (FSW) is selected as the based technology to develop a new technological solution to join composites. Since the invention of FSW by TWI (Midling et al., 1995), it has been developed and used to join metallic materials for different purposes (Infante et al., 2016). FSW is a highly adaptable welding technique with ability to weld a wide range of similar/dissimilar materials without the limitations of conventional fusion welding methods. Weldments produced by FSW show superior mechanical properties than those joined by fusion technologies, such as MIG and TIG (Salvati et al., 2019). Also, FSW is an inherently environmental-friendly process due to the absence of filler materials, toxic fumes or shielding gases (Eslami, 2019). Since the invention of FSW, the majority of the studies have been focused on metallic materials (Sakano, 2001), and scarce researches were focused on non-metallic joints. Up to now, only a few researchers attempted to adapt FSW to weld polymers and thermoplastic composites (Kumar et al., 2019; Lambiase et al., 2019). Using FSW to join thermoplastic based materials can significantly reduce the final weight of the vehicles, which is one of the main concerns in the transportation industries. Nevertheless, due to the low melting point, hardness and thermal conductivity of thermoplastics, they behave differently from metallic ones, which demands for new technological solutions that are still not commercially available. The advantages of FSW, naturally led to studying the possibility of using FSW for welding polymers (Eslami et al., 2016). However, in this case it is difficult to obtain sound welds using conventional FSW tools (Nelson et al., 2004). Recently, a new tool made of PEEK (Eslami et al., 2017) and a sensitized clamping device (Eslami et al., 2018) were developed, which produced welds with 97% of the parent material’s tensile strength (Eslami et al., 2018). FSW proved to be a reliable method for welding non-metallic materials and further studies regarding thermoplastic based materials are required, especially concerning tool design and process optimization.