Friction stir welding (FSW) is a widely used solid-state joining process for metals and alloys developed at The Welding Institute (TWI) in 1991. During the FSW process, a rotating tool is inserted into the interface between two workpieces, which is then traversed along the welding line. The rotating tool is usually composed of a pin and a shoulder. The applied plastic deformation and heat-induced by friction between the rotating tool and workpiece lead to the formation of a solid-state weld [1,2,3,4,5]. Although FSW is applicable for various metals [6,7,8,9,10], it is mostly used in the joining of Al alloys [1,11,12,13,14]. FSW of Al alloys has many advantages compared with conventional fusion welding processes (e.g., MIG and TIG). These include finer microstructure, better dimensional stability, lower processing defects, reduced residual stresses, and superior mechanical properties [3,15,16].

Non-heat-treatable Al alloys, which are also called strain-hardened or work-hardened alloys, include a group of alloys that are usually strengthened via cold work and/or solid solution hardening. The various combinations of additions used for these alloys are shown in Table 1 [3,17,18]. Temper designations for the alloys strengthened by strain hardening consist of an H followed by two or more digits (e.g., AA3103-H13). The first digit following the H indicates whether the strain-hardened alloy has been thermally treated, and the digit following the H1, H2, and H3 (e.g., H1xx, H2xx, or H3xx conditions) indicates the degree of the applied strain hardening. Furthermore, the letter “O” is used to present the annealed conditions by which the lowest strength is achieved [19,20].

FSW is extensively used in many industries (e.g., shipbuilding, marine, aerospace, railway vehicles, and automotive sectors) in joining non-heat-treatable Al-based products [13,67,68,69,70,71]. Although the FSW process is mostly used in butt welds, other joint designs such as spot welds and T-joint welds are also being performed. In most cases, FSW is applied for large-scale products that are welded by setting the workpiece on a worktable. Even though the applications are in a one-dimensional form, the facility is under development to conduct FSW in a three-dimensional form [2,72,73]. The following are some common examples of FSW, which imply the strong need for FSW use in various industrial sectors.

One of the main applications of FSW is in shipbuilding, where it is often used for joining the boat’s hulls and its stiffeners, decking, bulkheads, and superstructure made from corrosion resistant AA5XXX aluminum alloys, such as AA5086, AA5454, AA5456, AA5059, and AA5383 AA5083 products [74,75,76]. In addition, the FSW process is used to join honeycomb panels which have been developed with a high noise-absorbing coefficient for the walls of the ship cabin. In shipbuilding construction, prefabricated panels fabricated by FSW lead to reduced problems for retaining highly skilled welders, thus reducing labor costs. [67,68,77,78]. Furthermore, a portable prototype FSW machine has been recently used in manufacturing the bow section of a new type of ocean viewer vessel made from the AA5083-H321 alloy [67,78].

The various advantages of the FSW process, such as the weight of the structure, the strength of the joint (particularly fatigue performance), and finished cost, led to interest from the aerospace industry [2,78,79,80]. Various joining processes, especially FSW, are performed to join the main structural areas in a transport aircraft, namely fuselage and pressure cabins, wings, and empennage (horizontal and vertical stabilizers). The engineering properties required for these structures are strength, stiffness, fatigue crack growth, fracture toughness, and corrosion [81]. FSW is used in the manufacturing of aircraft (A3xxx Airbus series) for the production of longitudinal joints in the fuselage, links, and central container of the wing [68].

The use of FSW is also increasing in the railway vehicle industry and is an ideal process for butt welding of lengthy longitudinal extruded section profiles for high-speed trains [82,83]. In train and tram structures, FSW is now used for roof panels, car-body, and railway wagons, which are made from longitudinal hollow Al extrusions [78,84,85].

The FSW of different Al alloys has been extensively used in the automotive industry for the high-volume production of vehicle components for years due to the high integrity of the technique [51,72]. Among the innovations used in joining aluminum alloy body panels, prototype frames, and structural components in automobiles are FSW and friction stir spot welding (FSSW) [86].