**1. Introduction**

The continuing scientific and technological attention to reduce vehicle weight and emissions resulted in a diffuse use of Al alloys as a substituting metal with respect to steel, in many applications that were formerly dominated by steel. This change of technology metallic material has strongly promoted tailor-welded blanks new solutions especially in the vehicle applica‐

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tions [1, 2]. Laser-beam welding and friction-stir welding (FSW) are currently considered to be the most prospective welding processes.

One of the main welding defects, from which FSW is likely to be affected, is the oxide layer formation on the butt surface ("kissing-bond" phenomenon). The kissing-bond generally means a partial remnant of the unwelded butt surface below the stir zone. Its formation is mainly attributed to insufficient plunging of the welding tool during FSW [17], and it is usually

New Approaches to the Friction Stir Welding of Aluminum Alloys

http://dx.doi.org/10.5772/64523

9

In this context, the present contribution shows the effect of the process parameters, tool geometry, and size on macromechanical and micromechanical properties of FSWed joints by using a conventional pin and a nonconventional pinless tool configuration. The potential advantages offered by the pinless tool configuration can be fully exploited only as thin sheets are welded since, as the thickness increases, the shoulder influence becomes ever more

A new FSW approach is here presented. This was developed to promote a better joint forma‐ bility and it consists of carrying out the FSW process on both the sheet surfaces. In this process, the first welding operation is followed by a second welding performed at the plate opposite surface. Such an innovative methodology has been defined by these authors as double-side friction stir welding (DS-FSW) [19, 20]. This new FSW methodology has proven to be able to seal the geometric discontinuities, possibly produced by the first welding process, by means of the second welding operation performed at the opposite surface at the same experimental conditions. In addition, this new approach allows more uniform hardness values across the NZ. Moreover, the recrystallized grain size across the NZ is more homogeneous with respect to the surrounding FSW zones, compared to the conventional FSW, as shown by Cabibbo et al. [20]. Such improvement in the joint quality is very attractive, especially in those cases where the joint materials are meant to be subjected to post-welding forming operations. The hardness and local Young's modulus, determined by nanoindentation, were used to probe the overall weld joint strength. Nanoindentation profiles are also used to correlate the sub-micrometer hardness values to the corresponding FSW microstructure, and finally to properly correlate the welded plate formability with the welded sheet microstructure and micromechanical

A further novel approach to the FSW process (defined by authors as RT-type [21]) is also reported. This new configuration consists of a combination of different plate-to-pin motions. In one configuration, the pin axial spin rotation is set perpendicularly to the sheet blanks travelling along the welding line, with a lateral rotation radius *R* = 0, 0.5 and 1 mm. In a second configuration, the pin translation along the welding plate is set parallel to the welding line. Both these new welding approaches were compared with the conventional FSW practice, in which the welding motion occurs linearly along the welding line (and this conventional configuration is here defined by authors as T-type). With this respect, the two here proposed new configurations were also characterized using tools with different pin heights. These involved different sinking values during FSW. The study of the new setup also includes plate

responsible of the formation of small geometric discontinuities into the NZ [18].

**2. Novel approach to the FSW process**

localized to the top sheet surface.

response.

Anyhow, the common difficulties involved in laser welding of aluminum alloys include porosity, hot cracking, poor coupling (due to the high reflectivity of the metal), and degrada‐ tion of the material properties in the heat-affected zone (HAZ) [3–5]. Despite several advan‐ tages offered by laser beam welding applied to aluminum alloys, this welding technology usually suffers from seam imperfections such as notches, which reduce the mechanical properties of the joint [5]. In order to overcome such drawbacks, the friction stir welding (FSW) has strong potentials against the laser beam welding, as it is a solid-state welding technology [6, 7]. In this sense, FSW is surely considered to be the most significant development in the metal joining techniques over the past two-to-three decades. The nonwelded nugget zone (NZ) makes this welding technology an energy-effective one. It is also an environmentally friend and a versatility welding technique often considered as a "green" technology. In fact, com‐ pared to the fusion welding processes, FSW consumes less energy with very low fraction of wasted material and a drastic reduction of dangerous fumes production [3–7].

Moreover, FSW produces a high-quality joint, compared to other conventional fusion welding processes. It is also a welding process particularly suited for joining nonmetal materials to metals, especially in those cases where it is not possible by using conventional fusion methods [8, 9]. Its key factors and main properties consist of the welding nature of the FSW metals. The weld zone undergoes a solid-state process promoted by the frictional heat between a rotating tool and the welding metal. The plasticized zone, induced in the material by the rotating tool, is further extruded from the leading side (advancing side, AS) to the trailing side (retreating side, RS) of the tool during its steady translation along the joint line [10]. Neither filler material nor shielding gas is required. The temperature involved is typically some 50–100°C below the metal melting point, and thus there is no volume change during joining. Moreover, it is generally agreed that FSW, compared to the fusion welding techniques, induces rather low residual stresses after welding. This also implies process-reduced manufacturing costs [11].

As for the welded alloy mechanical properties acquired after the welding process, the FSW generally guarantees better tensile, bend, and fatigue properties than fusion welds. Taking advantages of these positive factors, this process has already been applied to a great variety of aluminum alloys, other than many other metallic materials. In the case of the aluminum alloys, the FSW technique has found many applications, such as external fuel tank of rock‐ ets, stock of railways, bridges [12, 13], to cite but few. Other interesting applications of FSW in the aerospace industry include fuselage, structural parts, cryogenic tanks, etc. [10]. Other interesting applications also include the marine applications (like offshore industry) [10, 14].

The microstructure modifications occurring at the central FSW zone (i.e., NZ) most usually consists of dynamic recrystallization resulting in the formation of fine equiaxed grains [8, 16]. This recrystallized zone can slightly reduce the welded alloy mechanical properties. For this reason, an accurate choice of the process parameters (rotational speed, welding speed, tilt angle, and sinking) and of the tool geometry (pin and shoulder geometry and size) is required. In fact, by increasing the pin rotational speed or by decreasing the welding line progression, the alloy mechanical properties can usually be optimized [15, 16].

One of the main welding defects, from which FSW is likely to be affected, is the oxide layer formation on the butt surface ("kissing-bond" phenomenon). The kissing-bond generally means a partial remnant of the unwelded butt surface below the stir zone. Its formation is mainly attributed to insufficient plunging of the welding tool during FSW [17], and it is usually responsible of the formation of small geometric discontinuities into the NZ [18].
