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Manufacturing of Composite and Nanostructured Thin Walled Tubes by ASB Process

Mohebbi, Mohammad Sadegh | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39872 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. In this study, utilizing the so-called “flow forming”, a cold-bonding process is developed to produce multi-layered thin-walled tubes and cylinders. In this process, which is entitled as “spin-bonding”, two layers of tubes are bonded by thickness reduction. Al/Al and Cu/Al tubes were bonded at various temperatures, thickness reductions, feed rates and roller attack angles. The bonding quality was examined by peel test, metallography and SEM fractography. Moreover, spin-bonding was accumulatively applied to aluminum tubes up to 4 cycles as a severe plastic deformation (SPD) for thin-walled tubes and cylinders entitled as “Accumulative Spin-Bonding” (ASB). Beside of experimental works, an analytic model is developed to study the strain and strain rate histories of the process. The results show that the successful bond is formed after a threshold thickness reduction. As a result of enhancement of the area fraction of bond spots, the bond strength increases by increasing the thickness reduction. Because of the effects of feed rate and roller attack angle on the reaction stresses, as obstacles for initialing the stable bond, the threshold deformation for bonding is affected by these two parameters. The results of various process temperatures demonstrate that as a result of facility of exposing the virgin metal and enhancement of the bond spots in high temperatures, the bond strength of Al/Al is increased by increasing the process temperature. The bond strength of Cu/Al, however, is reduced by increase of the process temperature, as a result of formation of the brittle intermetallic phases. TEM micrographs of the ASBed tubes illustrate the grain refinement by the mechanism of establishment of subgrains and strain induced transition of low angle to high angle grain boundaries. This leads to a nanostructure with average grain size of 150 nm after fourth cycle of ASB. Mechanical properties of these tubes indicate that because of redundant strains the hardness of outer layer is more than inner one. As a result of continous entering of the outer layers into the inner ones, the hardness and its homogenity increase by increase of the ASB cycles. This scheme of hardness development leads to a decrease in the difference between the yield strength and ultimate tensile strength and consequently on the uniform elongation.

  9. Keywords:
  10. Flow Forming ; Severe Plastic Deformation ; Cladding ; Thin Walled Tube ; Cold Bonding of Multilayered Tubes ; Accumulative Spin Bonding (ASB)Process

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