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Friction Stir Welding Hidetoshi Fujii

Abstract Several material designs using friction stir welding (FSW) are demonstrated. The FSW is solid-state processing techniques, which can be used for many processes such as welding and surface modification. In particular, this method is very useful for the material design of transformable materials such as steel and Ti alloys. When the FSW of steel is performed below the A1 point, the optimal microstructure consisting of very fine ferrite and globular cementite is obtained, regardless of the carbon content. When the FSW of Ti alloys is performed below the b transus, equiaxed grains of approximately 1–2 lm are obtained, and thus the mechanical properties, such as fatigue and toughness, are expected to improve.













Keywords Recrystallization Transformation Severe deformation FSW Steel

11.1

Features of the Friction Stir Welding

During friction stir welding (FSW), the materials are maintained in the solid state [1]. Accordingly, this new method has a variety of excellent advantages that have already been used for various industrial applications such as trains, ships, automobiles, and civil engineering structures for Al alloys [2–9]. In FSW, an approximate /10–20 cylindrical tool rotating at a high speed is brought into contact with the materials which generates heat, as shown in Fig. 11.1a; thus, the two materials are welded using the frictional heat due to the friction and deformation of the materials [3, 4]. As shown in Fig. 11.1b, the tool consists of a large diameter part (Shoulder) and a small tip part (Probe). Only the probe is inserted into the materials, and the tool is moved along the butt interface, the materials are welded through plastic flow and recrystallization caused by the tool. The maximum temperature is below the melting point, which indicates that the welding is performed in the solid state; accordingly, the reduction in the joint H. Fujii (&) Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 Y. Setsuhara et al. (eds.), Novel Structured Metallic and Inorganic Materials, https://doi.org/10.1007/978-981-13-7611-5_11

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(b)

(a) Retreating side

φ 15

Welding direction

Probe

Shoulder

Advancing side

M6

Fig. 11.1 Principles of FSW and tool shape

strength is much lower than that for conventional welding. In some cases, the joint strength is higher than the base metal. In particular, in the case of steel, the heat-affected zone (HAZ) is not generally softened, thus a 100% joint efficiency can be easily obtained [10–17], except for some special steels, such as high-strength steel [18]. Figure 11.2 shows a schematic illustration of a joint, and Fig. 11.3 shows actual microstructures of a 1080 aluminum alloy joint at a cross section [4, 19]. An equiaxial and recrystallized structure of several microns is formed at the center of the joint, and this area is called the stir zone (SZ). Outside of the stir zone, a thermomechanically af