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Friction Stir Sot Welding of Aluminum to Steel Using Consumable Pin

Rafiei, Hamid Reza | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54880 (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 as a solid state process is able to solve some of the disadvantages of fusion welding processes. On the other hand, this welding method has disadvantages such as low connection surface due to the hole left by the pin and poor resistance to fatigue loading. In this study, a method called friction stir spot welding via consumable pin was used so that the hole caused by the pin no longer remains at the welding site. The process is done in such a way that first a hole is made in the center of the overlap of the sheets. The rotating tool, in which the consumable pin is made of 6061 aluminum alloy with a diameter equal to the diameter of the hole is lowered at a certain speed, enters the sheets and mixes the surrounding materials with the rotationa action leading to welding of two sheets to each other. The aim of this study is to achieve the condition that the hole that is created on the sheets, is filled with pins and achieves maximum shear tensile and cross-tension strengths. Therefore, in this research, friction stir spot welding of 6061 aluminum alloy sheet and St12 sheet steel with a thickness of 2 mm with consumable pin of 6061 aluminum alloy was used. The variables considered in this study were the rotational speed of the tool and the downing speed of the tool. Welding of samples was performed at four rotational speeds of 700, 1000, 1500 and 2000 rpm and three downing speeds of tools 5, 15 and 25 mm / min. Stereo microscope, optical and scanning electron microscopes were used to observe the macrostructure and microstructure of the joint area, the fracture site of the specimens and their fracture surfaces. Shear-tensile, cross-tension and microhardness tests were used to evaluate the mechanical properties of welds. The results showed that welding at all rotational speeds was successful so that the connection between the consumable pin and the upper sheet as well as the lower sheet was complete and a small amount of cavity defect was observed in the samples. Friction and turbulence of the consumable pin with the steel sheet walls cause the intermetallic compounds. However in this study, the intermetallic compounds were observed to be partial and insignificant. the maximum weld fracture force in shear-tensile and cross-tension tests were 8864 and 1172 N, respectively. In general, at high heat input compared to low heat input, the amount of weld strength in both shear-tensile and cross-tension states was lower. It was related to the pasty state of the consumable pin at high heat input and reduction of pin. On the other hand, in the samples with higher weld strength, fracture occurred from the interface of the consumable pin and the floor of the steel sheet cavity. The hardness test results also showed that due to the low heat of this process compared to fusion welding, the hardness changes in the minimum and maximum heat input are not significant
  9. Keywords:
  10. Steels ; Friction Stir Spot Welding ; Cross Tension Test ; Intermetallic Compounds ; Aluminum Alloy 6061 ; Tensile Strength Test ; Rotational Speed Stirrer ; Downing Speed ; Consumable Pin

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