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Investigations on Microstructure and Mechanical Properties of Mg Based Nanocomposite Fabricated by Friction Stir Processing

Azizieh, Mahdi | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42107 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein
  7. Abstract:
  8. The aim of this research is to fabricate Mg based nanocomposite by friction stir processing and investigation of the process parameters on the microstructure and mechanical properties of the nanocomposite. In order to this aim, alumina nanoparticles as reinforcement were poured in the groove, with 5mm depth, and due to rotation of friction stir tool, these nanoparticles were mixed with the matrix and nanocomposite was produced. To reach to the best condition in nanocomposite fabrication, the effect of friction stir processing parameters such as, tool rotation speed, tool design, number of friction stir passes, cooling media and reinforcement percent on microstructure, hardness, tensile properties, texture and wear resistance were investigated. It is found that addition of nanoparticles in friction stirred matrix reduces the grain size effectively. In addition, with decrease in rotation speed, the grain size reduces due to lower heat input. However, with increase in rotation speed, nanoparticles distribution was improved. Furthermore, with increase in friction stir passes, due to high material flow, nanoparticles distribution was enhanced. The threaded pin had the best results in compare with non-threaded and three fluted pin. In high cooling condition, due to low temperature, the smallest grain size was achieved and the best strength and wear resistance was reached. Texture of friction stirred region in compared to the initial material was changed significantly. In the initial material, due to rolling manufacturing procedure, the {0002} plane was parallel to rolling plane. However, in center of friction stirred region, the normal vector of this plane was oriented in the process direction. Due to dynamic recrystallization, the grain size and dislocation density, in compared with initial material, was reduced. Furthermore, with addition of nanoparticles, because of increase of the obstacles due to nanoparticles interface and severe grain refining, tensile strength was increased. On the other hand due to texture changes in friction stir process, the Schmid factor was uncertain in this region and deformation occurred heterogeneous. Also with increase in rotation speed, due to grain growth, ductility increases and tensile strength decreases. Similar to tensile strength, the hardness number decrease with increase of rotation speed. As a result of higher strength with lower rotation speed, the wear resistance was decreased with increase of rotation speed of tool.
  9. Keywords:
  10. Nanocomposite ; Magnesium Alloy ; Crystal Texture ; Microstructure ; Friction Stir Welding

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