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Abstract The technique of vertical compensation friction stir welding (VCFSW) has advantages in welding huge sheets of aluminum alloys, especially in structural industries like aerospace, automotive body, shipbuilding, and rail transportation. The friction stir welding (FSW) process has advantages over traditional fusion welding processes such as the reduction of energy usage, no need to consumables, high quality of weldments with no porosity and fumes, reduced bad environmental effects, reduced waste, and minimized impact on the safety and health. The VCFSW technique has benefits over traditional FSW: reinforcement the joints, elimination grooves, tunnels and gap defects, enhanced microstructures, improved mechanical properties and hence obtaining high-quality joints. The industrial requirements for this work grow from the wish to apply the technology described in this present work in airplane manufacturing in Arab Organization for Industrialization, AOI located in Cairo, Egypt. The adoption of a new manufacturing process e.g. airplane structures creates many challenges, as in the case dealt with in the present thesis, repercussions that require a nearly full product developments affecting the primary structure design, with many or at least, reengineering of the different parts. The main obstacles of conventional welding processes in huge structures were related to the lack of mechanical properties due to the increase heat input, lack of defect control and the difficulty to iii weld precipitated hardened alloys (as AA2024 and AA7075). The main obstacle in VCFSW technique is the use of an additional material in the gap between two workpieces of the joint; this thin plate (compensation material) of the substrate which results greatly increases the difficulties of the joining process. It’s known to be particularly difficult. That creates a number of challenges. It also creates several opportunities. PURPOSE: the present work is motivated by two main issues. First is the industry need to adapt the technology to the welding of aluminum alloys in applying the industries of motor car, the transportation of rail, shipbuilding, and aircraft. The second is a need to better understand the metallurgical characterization and mechanical properties of VCFSW process and how they affect the joint efficiency. APPROACH: In this study, an extensive experimental program was undertaken in order to identify the parameters of variables and their influences. In this respect, AA2024 aluminum alloy was chosen as base metal (BM). On the other hand, AA7075 aluminum alloy was chosen as compensation material at two heat treatment conditions solution heat treatment with artificial aging (T6) and annealing heat treatment (O). Three variables of process parameters in VCFSW technique were investigated like tool traverse and rotational speed and tool inclination angle in order to obtained best parameters. Nevertheless, three main parameters have been studied in this study; the first case is FSW without addition interlayer strip iv width and VCFSW with different addition interlayer strip width values at 1, 1.5, 2, 2.5 and 3 mm in case of solution heat treatment with artificial aged (T6), while the second case is FSW without addition material and VCFSW with different addition interlayer strip width values at 1, 1.5, 2, 2.5 and 3 mm in case of annealed heat treatment (O) and finally third case is FSW of T-joints with different combinations geometries e.g. T-butt-lap joint, T-doublebutt joint and T-lap joint. FINDINGS: The obtained results from this study in VCFSW joints revealed that an increasing rotational speed at 2000 rpm led to increase tensile strength and elongation. On the other hand, decreasing traverse speed at 20 mm/min resulted higher tensile strength and elongation of VCFSW joints. While best tool tilt angle was at 2.5°. With the technique of VCFSW in producing welded joints revealed high homogeneity without any defects if it compared to conventional FSW process in case of T6. The material flow in VCFSW joints was balanced material flow while in FSW joints were insufficient material flow in case of T6. The optimum mechanical characteristics (hardness, tensile and bending) of the fabricated-welded joints were gotten during the using of the width of compensation strip at 1.5 mm in case of T6. The fracture surface of the welded joint of the width of compensation strip at 1.5 mm revealed the typical ductile fracture in case of T6. While the obtained results from this VCFSW in case of O revealed that the quality of the welded joints in VCFSW technique are based on the capacity of compensation material in filling in and mixing with the v BM. The material transfer in the pin-driven region takes place layer by layer. On the other side, the shoulder driven material flow can be described as the effectiveness of the shoulder to keep the compensation material in the weld cavity. The optimum mechanical hardness, tensile and bending characteristics of the fabricated welded joints were gained during the using of the compensation strip at 3 mm was used in case of O. The fracture mode in VCFSW with interlayer strip width at 3 mm showed a ductile fracture mode which happens at 45° in BM without clear decreasing in area. Finally, the results in case of FSW of T-joints showed that the joint efficiencies of all produced T-joints at different geometries along the stringer are lower than those of all joints along the skin. IMPLICATIONS: the compensation or additional material has crucial role in excluding the cavities, enhancing joints efficiency and producing extra sound joints by inserting strip of compensation material between both edges of the base metal (BM). The effect is acute when found a huge gap between two aluminum plates before welding process of the joint. To adopt this industry successfully, the process needs strong control before welding process of type of materials and thickness (base metal and compensation material), width of compensation material, heat treatment of materials, type of joints and feature fabrication. Additionally process parameters in VCFSW technique such as spindle speed, travel speed and inclination angle need to be adjusted during welding process according to type of materials, thickness and width of compensation |