PhD Dissertations

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Browsing by Author "Bajilane, Isam Jabbar Ibrahim"

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    Development of a solid state spot welding technique for the manufacturing of detect free joints
    (2019-06) Bajilane, Isam Jabbar Ibrahim; Yapıcı, Güney Güven; Yapıcı, Güney Güven; Başol, Altuğ; Şendur, Polat; Tezel, Y. Ş.; İpekoğlu, M.; Department of Mechanical Engineering; Bajilane, Isam Jabbar Ibrahim
    Development of a Solid State Spot Welding Technique for the Manufacturing of Defect Free Joints.--Friction stir spot welding (FSSW) processing is a recently developed process for joining hard weldability materials, which has expanded into automotive applications by using the concept of light alloys. Thus, by reducing its fuel consumption, FSSW as a solid-state welding method does not need to melt the workpieces used, which has attracted the attention of automotive manufacturing companies around the world. Although considerable research has been conducted to observe the advantages of the FSSW process, rather more attention has been paid to solving the probe hole (keyhole) defect, which appears at the weld spot center in the welds as a result of the pin of the welding tool after joining process is complete. This study investigates the fabrication of flat friction stir spot welds without the keyhole by using a newly developed FSSW process, which uses the intermediate layer (IL) part, and tracks the mechanical properties of the fabricated welds. The welds produced by the Intermediate Layer FSSW (IL-FSSW) process showed an excellent appearance with no large distortion resulting from the welded sheets. The top surface of the spot weld showed a smooth, flat surface. Regardless of the use of different welding parameters, the appearances of all the spot welds were comparable. It is considerable that no keyhole is formed in comparison with the conventional FSSW welds. It was shown that using the IL part improved the lap shear failure force (LSFF). Some of the welded samples, which were about two folds relative to the maximum value of the American Welding Society (AWS) welds quality requirements. In order to understand the effect of the welding parameters on the tensile behavior, design of experiment (DOE) were utilized to optimize the results of the tensile test. The optimization work indicated that the LSFF increases linearly with the increasing plunging depth. The analysis of the variance (ANOVA) statistical method in respect to LSFF indicates that the tool rotation speed was the most significant parameter, whereas the plunging feed rate was the lower one in this regard. The fatigue tests were conducted under T6 and annealing (O) conditions and over Nf of the lap welded samples of the dissimilar Al 6061/Al 2024 and similar Al 6061/Al 2024 alloys, which were determined for all the conditions. In all the samples, the annealing treatment was observed to have a negative effect on the Nf under high applied loads. However, no heat treatment effect was clearly observed under the low load levels, except in Al 2024, in which the annealing treatment had a positive effect on the Nf. In terms of the Al/Cu welds, the flat weld spots without the keyhole were produced successfully. The results revealed that there was very little difference in the LSFF achieved with the IL-FSSW using the pinless tool when compared to the welds conducted with the conventional FSSW using a tool with a pin. The X-ray diffraction (XRD) analyses showed that the Al2Cu and Al4Cu9 phases formed as a result of the peritectic reactions at the interface of the sheets within the weld nugget. The Vickers examination showed distinctly different microhardness levels up to 575 Hv, which are superior to that of the base metal corresponding to the hard intermetallic compounds formed in the weld nugget. Two finite element models were built to simulate the IL-FSSW process, i.e. thermal and mechanical. The temperature distributions obtained from the thermal model were compared with the experimental measurements with decent agreement. The mechanical model is utilized to predict the strength of the joints in conjunction with the experimental values from shear-tensile tests, providing satisfactory results for demonstrating the trends in the mechanical behavior of various joints.