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Friction Stir Spot Welding via Consumable Pin

Baghdadchi, Amir | 2018

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51161 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Friction stir spot welding (FSSW) as a solid-state process is able to tackle some problems of fusion welding. On the contrary, FSSW has some drawbacks such as the low area of connection and weakness in fatigue behavior. In this study, an innovative method called friction stir spot welding via consumable pin (CP-FSSW) was used, which was able to eliminate the exit-hole. In this process, at the first stage, a hole is created in the center of the overlap area of the sheets; such that the hole is complete in the upper sheet and up to the half thickness of the lower sheet. Then, the rotating tool composed of a consumable pin with diameter length as much as the created hole and same type of the base metal comes down, enters the sheets, stirs the surrounding materials with rotating and friction and finally joins the sheets. The aim of this research is achieving the condition that the created hole is filled the created hole completely and the maximum strength of tensile-shear and cross-tension is achieved. Therefore, in this study, the friction stir spot welding of 6061 aluminum alloy sheets with 2mm thickness was performed using a consumable pin of the same alloy. The samples were welded by five rotational speeds of 700, 1000, 1300, 1600 and 2000 rpm. In order to observe the macrostructure and microstructure of the joint area and the fracture surfaces, the stereo, optical and scanning electron microscopes were used. The mechanical properties of the welds were investigated by tensile-shear, cross-tension, and microhardness tests. The results showed that in all the rotational speeds, the welding was successful so that, the created hole was filled completely and the welds showed good strength (higher than the values in the literature). Also, the connection between the consumable pin and the upper sheet was perfect while three regions (Complete bond, Kissing bond and Defect) were formed at the interface of the pin and the lower sheet. Furthermore, around the consumable pin at the interface of the upper and lower sheets, an area called swirly region was formed contributing to the strength of the welds. The most failure load in the tensile shear and cross tension tests were 12371N and 2814N, respectively. Generally, enhancement of the rotational speed and the related increment of the heat input promoted stirring of material around the pin and led to improve the bond strength in the swirly region. Hence, the tensile shear strength of the welds increased. On the other hand, by increasing the rotational speed and the heat input, the consumable pin was softened earlier, and its ability to fill the hole fell and cross-tension strength of the welds diminished. Also, the results of this process were compared with the conventional and pin-less friction stir spot welding
  9. Keywords:
  10. Aluminum Alloy 6061 ; Friction Stir Spot Welding ; Rotational Speed Stirrer ; Tensile-shear Strength ; Cross Tension Test ; Friction Stir Spot Welding Via Consumable Pin ; Cross Tention Strength

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