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The Effect of Friction Stir Processing with SiC Nanoparticles on the Stability of Grain Boundaries and Mechanical Properties of Severely Deformed Aluminum

Sarkari Khorrami, Mahmoud | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47784 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Kazeminezhad, Mohsen
  7. Abstract:
  8. Severe plastic deformation (SPD) is known as an efficient route for grain refinement in the metals. However, the size of SPD products is almost small due to the limitations associated with the SPD processes. Hence, the growing attention is paid to the welding of severely deformed metals. Friction stir welding, as a solid state welding process, seems to be suitable for this purpose. But, considering the significant amount of stored strain within the SPDed metals, appreciable grain growth after recrystallization occurs not only in the stir zone, but also in its surrounded areas. In this study, the 1050-aluminum sheets severely deformed through constrained groove pressing (CGP) were processed by friction stir processing (FSP). The microstructural assessments manifested the formation of relatively large grains (~15 m) in the stir zone. The grain structure of stir zone was found to be much larger than that of 2-pass CGPed base aluminum signifying the instability of CGPed aluminum upon FSP. It was observed that the grain structure of stir zone became finer when SiC nanoparticles was used during FSP. In addition, using SiC nanoparticles led to the improvement in the mechanical properties of stir zone; however, it was not able to prevent the heat affected zone (HAZ) from the softening as a result of recrystallization and subsequent grain growth. Performing FSP in the liquid Nitrogen environment reduced this difficulty. A simultaneous use of SiC nanoparticles and the liquid Nitrogen environment resulted in the formation of ultrafine grain structure in the stir zone as well. The recrystallization mechanism of the stir zone during FSP was also investigated through examining the microstructure and texture evolutions at regions ahead of the FSP tool. It was found that the static recrystallization and subsequent grain growth took place at this area before reaching the FSP tool. These phenomena caused the main part of stored strain within the CGPed base aluminum to be released. The remained part of stored strain affected the microstructural evolutions in the stir zone. In approximately 200 m distance from the position of FSP tool at regions located ahead of the FSP tool, the grain subdivision procedure via dynamic recovery and continuous dynamic recrystallization (CDRX) as well as geometric kind of CDRX was gradually initiated. This procedure was completed at regions placed beneath the FSP tool where the material was entirely stirred by the tool. According to the results, it can be stated that the use of SiC nanoparticles and the application of FSP in the liquid Nitrogen environment did not alter the recrystallization mechanism or texture development. The uniform distribution of SiC nanoparticles in the stir zone enhanced not only the mechanical properties of stir zone, but also its thermal stability even up to 400 C
  9. Keywords:
  10. Severe Plastic Deformation ; Nanoparticles ; Friction Stir Welding ; Commercial Pure Aluminum ; Grain Boundary Stability

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