**1. Introduction**

Friction Stir Welding (FSW) is a solid-state welding technique, where similar [1–3] and dissimilar metals [2] are welded without reaching their melting point. FSW has gained significant attention due to its numerous advantages [4–6] over the traditional arc welding methods [7]. However, FSW has also some drawbacks [8–10] which are very little when compared with the traditional welding processes. Various joining configurations in terms of spot [3,11], butt [12–14], corner [15], lap [16–18] and fillet [19] joints have been studied and established for different metals and alloys using FSW technology through optimizing their welding process parameters. However, FSW of high strength to weight ratio aluminum alloys to produce T-joints are still demanding more studies and efforts to evaluate the joint efficiency, and to optimize the welding parameters. Arora et al. [20] reported that the hardness and tensile strength of the friction stir similar butt joint of AA2219 were lower than the base metal. However, a toughness value of the nugget zone enhanced compared to the base metal. Babu et al. [18] studied the effect of pin profiles on the friction stir

**Citation:** Ahmed, M.M.Z.; El-Sayed Seleman, M.M.; Zidan, Z.A.; Ramadan, R.M.; Ataya, S.; Alsaleh, N.A. Microstructure and Mechanical Properties of Dissimilar Friction Stir Welded AA2024-T4/AA7075-T6 T-Butt Joints. *Metals* **2021**, *11*, 128. https://doi.org/doi:10.3390/ met11010128

Received: 28 December 2020 Accepted: 6 January 2021 Published: 10 January 2021

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welded (FSWed) lap joint quality of the AA2014 aluminum alloy and reported that the strength and the failure mode of joints are related to the hook geometry. Sadeesh et al. [21] studied the influence of using five different tool designs in FSW of dissimilar AA2024 and AA6061aluminum alloys in butt joints and concluded that the joints efficiency is related to the varying welding process parameters and the shoulder to pin diameter ratio is the main dominant parameter. Manuel et al. [22] investigated the behavior of the friction stir welds of three dissimilar aluminum alloys in a T-joint using three base materials namely, AA2017-T4, AA5083-H111, and AA6082-T6. They reported that the arrangement of the skin materials with respect to the pin rotation direction influence the morphology and the mechanical properties of the joints [22]. In fact, the 2xxx and 7xxx aluminum alloys before FSW were known to be non-weldable by traditional fusion welding processes since number of problems such as porosity, solidification cracks and residual internal stresses are found in the weld. Moreover, the formed dendritic structure during solidification in the fusion zone is significantly deteriorating the mechanical properties of the joints [5].

AA2024 and AA7075 aluminum alloys are typically representing the 2xxx and 7xxx alloys series, respectively, which are usually used intensively to fabricate structural components of automotive and aircraft industries [23–25]. In many applications, especially the aircraft fuselage panels in the form of T-joints; where, the aluminum alloys AA2024 and AA7075 are used as skins and stingers, respectively. The stinger is the stiffening element which reinforces the section of the load carrying skin to avoid buckling and failure. In FSW of T-joints, Cui et al. [26] studied the effect of different rotational speeds on the microstructure and mechanical properties of the T-lap, T-butt-lap, and T-butt joints. They ascribed the formed defects such as tunnel defect, kissing bond, original joint line defect, and zigzag line to the initial matching modes of the blanks, material flow patterns, and insufficient heat input. Acerra et al. [15] investigated the dissimilar AA2024-T4 and AA7075-T6 T-joints and evaluated the design rules for the setup of FSW operations. They concluded that a large shoulder diameter is required to generate a sufficient heat input to fulfill the weld joint. Moreover, the presence of coating layers negatively affects the mechanical properties by a generation of macro and micro defects. In T-butt joints, especially in aerospace industry, the tool pin has been penetrated vertically into the materials setup surface to join three separate parts, two skin parts and vertical stringer element, to produce one-piece T-joint [15].

This work aims to conduct single pass FSW of T-butt joints between AA2024-T4 and AA7075-T6 aluminum alloys as skin and stringer; respectively, at different tool rotational rates of 400, 600 and 800 rpm and a constant welding speed of 50 mm/min. The role of FSW parameters and the welded materials on the microstructure, the strength and the fracture mode will be examined. Attention will be given to investigate the evolved texture using EBSD.
