**1. Introduction**

Aluminum alloys have remained the prime selection in producing various components in many industries like aerospace, automotive, and shipbuilding because of their perfect strength to weight ratio [1–5]. AA5000-series alloys are characterized by a good strength-to-weight ratio and an appropriate corrosion resistance. However, they are difficult to join by conventional fusion welding techniques because of their dendritic structure, which seriously weakens the mechanical properties. Solid-state welding processes are

**Citation:** Ahmed, M.M.Z.; Ataya, S.; Seleman, M.M.E.; Allam, T.; Alsaleh, N.A.; Ahmed, E. Grain Structure, Crystallographic Texture, and Hardening Behavior of Dissimilar Friction Stir Welded AA5083-O and AA5754-H14. *Metals* **2021**, *11*, 181. https://doi.org/10.3390/met11020181

Academic Editors: João Pedro Oliveira and Zhi Zeng Received: 31 December 2020 Accepted: 18 January 2021 Published: 20 January 2021

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appropriate joining for either similar or dissimilar aluminum alloys [6]. Resistance spot welding is considered one of the dominant solid-state welding processes in automotive constructions [7,8]. However, the use of a continuous welding line process instead of weld spots leads to higher structural stiffness and better crash performance [9]. Friction stir welding (FSW) of the AA5000 series represents a promising technique to obtain defect-free and sound joints, either in similar [10] and dissimilar [11–13] welding combinations. FSW can also be used effectively for the welding of different types of materials [14–17], and the same principle of FSW can be used for the development of metal matrix composites [18–22]. In FSW, a non-consumable rotating tool induces a stirring action until the tool shoulder contacts the top surface of the sheets with a given plunge depth, generating a large amount of frictional heat [23]. As the tool moves along the welding line, the blanks are joined through a solid-state process, owing to the severe plastic strain and the metal mixing across the weld. The weld zone undergoes a solid-state process promoted by the frictional heat between the wear-resistant welding tool and the materials to be joined. The plasticized zone is further extruded from the tool advancing side to the retreating side during its steady traversing along the joint line [24]. FSW process parameters influence the final joint quality and performance, including traverse welding speed; tool rotational speed, geometry, and shape; blank thickness; heat input; applied force; tilt angle; specimen preparation; sheet-rolling direction; plates/sheets metallurgical history. It has been demonstrated that, among process parameters, the tool rotational speed and traverse welding speed have a strong effect on heat generation, heat dissipation, and cooling rate. Hence, the microstructure and texture, and mechanical properties evolution of the FSW joints are significantly affected by traverse welding speed and tool rotational speed values [6,11,24–26]. For this reason, an accurate choice of the FSW process parameters and of the tool material and geometry is required. In fact, the joint mechanical properties can be optimized by increasing the tool rotational speed or by decreasing the traverse welding speed [27,28]. The excessive agglomerations and joints defects are produced when the high strength aluminum alloy on the advancing side (AS) of AA5052/AA5J32 is placed because of material flow limitation [29]. Both material flow and joint quality are more dependent on the FSW conditions and their effects on heat input and temperature distribution in weld nugget, regardless of base material (BM) placement [30]. During FSW, the heat generation is controlled by tool rotation and welding speed due to the material plastic flow [30–32]. However, very high rotation speeds lead to macroscopic defects because of the excessive heat input [24,33]. Due to FSW, three different metallurgical zones are usually recognized, namely, nugget zone (NZ), thermomechanically affected zone (TMAZ), and heat-affected zone (HAZ) [34]. In the NZ, the metal is in direct contact with the pin being continuously stirred during the passage of the rotating tool, thus creating the necessary strong bond between the two metals under the welding. Fast thermomechanical heating (peak temperature may reach 0.6 to 0.95 TM) and cooling occur, and they favor the occurrence of dynamic recrystallization (DRX) phenomena, generating fine grain structures in the form of onion rings [34,35]. From a microstructural viewpoint, the NZ is generally characterized by a fine or even very-fine equiaxed grained structure, as mentioned in [34]. In the TMAZ, the microstructure experiences a significant grain morphology and size modification. Because of the insufficient deformation strain, DRX does not occur in the TMAZ. In the third zone, HAZ, the materials are subjected to thermal cycles with no plastic deformation, and the microstructure has the same grain structures as the parent material (BM) [6,25]. The transients and gradients in strain, strain rate, and temperature are inherent in the thermomechanical cycles of FSW, which control and shape the characteristic microstructural zones of a typical FSW joint. During FSW, material flows in a complex, vortex-like pattern around the pin from the advancing side to the retreating side [14]. The high stacking fault energy metallic materials, such as aluminum, enhance the dynamic recovery (DRV) to occur during the hot working process [36,37]. As the DRV rate is increased, low-angle grain boundaries (LABs) are formed to minimize the dislocation forest/multiplication by the rearrangement of most of the dislocations. In DRX, new, dislocation-free grains form at high energy sites, such as prior grain boundaries, deformation band interfaces, or boundaries of newly recrystallized grains [38,39]. All the herein mentioned mechanisms of formation for sub-grains and grains (TMAZ) and recrystallized fine grains (NZ) are always also dependent on the material's initial metallurgical conditions and are subject to different FSW process and tool parameters. Thus, the aim of this work was to examine the effect of FSW tool rotation rate and the welding speed on the grain structure, texture, and mechanical properties of AA5083/AA5754. In this work, three FSWed AA5083/AA5754 joints (J1: 600 rpm and 60 mm/min, J2: 400 rpm and 60 mm/min, and J3: 400 rpm and 20 mm/min) were produced. Through the thickness of the produced joints, the grain structure and texture were investigated using EBSD. In addition, both the hardness distribution and tensile properties measurements were investigated. A full description of materials and experimental procedures is in Section 2. The results and discussion are presented in Section 3. The conclusion drawn from this work is in Section 4.
