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Farshidi, Mohammad Hassan | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44022 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. The aim of this study was improvement of mechanical properties and grain refinem ent of aluminum 6061 alloy tubes by combining pre and post heat treatments with Tube Channel Pre ssing (TC P) as a new severe plastic deformation process. For this purpose , the effect of geometr i cal parameters of TCP on deformation behavior was studied at first. Then , the effect of pre and post heat treatments on mechanical properties improvement and micro structure was invest i gated. Finally, the effect of TCP process on mechanical properties improvement and grain refinement was considered. Results show that the mandrel design must follow die design in TCP. As an illustration , mandr el curvat ure radiu s must be equal or more than die curvature radius whil e element heights m u st be saved constant in design of mandrel cave depth. In addition , die design par ameter s m ust be chosen as a specific ratio of tube diameter in order to pr event increase of strain inhomogenity and wa ll thinning of tube during TCP . Supersatura tion of alloy before process causes highest mechanical properti es impro vement and grain refinement during TCP . More coarse precipitations appear during aging after TCP comparing with those during aging before TCP . Additionally, aging after TCP causes activat ion of dislocation anni hilation and recovery which result to relative decrease of m echanica l properties and increase in fracture strain. Imposing of 1 to 5 passes of TCP causes to increase strength, decrease in fracture strain and homog enous grain refinement due to activation of dislocation multiplication and migration mech anisms. Imposing of further TCP passes causes inhomogeneous grain refinement in which m ore coarse grains are surrounded by a chain of nano-grains . This implies overcoming of shear band intersection mechanism and saturation of dislocation multiplicat ion and mi gration mechanisms . After seventh pass of TCP , strength is decreased and fracture strain is increased due to increase of strain rate sensitivity and saturation of dislocation multipl ication and migration mechanisms
  9. Keywords:
  10. Heat Treatment ; Severe Plastic Deformation ; Mechanical Properties Improvement ; Nanograin ; Microstructure ; Aluminum Alloy 6061

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