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The Effect of Warm Multi -directional Forging on Microstructure and Strength of 2024 Aluminum Alloy

Nasrollahnejad, Farzaneh | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53712 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. The 2024 aluminum alloy is deformed by two passes of multi-directional forging at room temperature, 180 oC, 250 oC, 320 oC and 380 oC with initial state of solid solution. Microstructure, mechanical properties and the state of precipitates are investigated via optical and scanning electron microscopies, hardness test, shear punch test and differential scanning calorimetry. Fragmentation and distribution of the precipitates are observed after straining at different temperatures. Dynamic precipitation of GPB zones at room temperature and 180 oC are studied by DSC analysis. The increase of both hardness and strength and the formation of shear bands are related to the presence of GPB zones. After deformation at 250 oC, a reduction in hardness and strength is detected which is related to dissolution of GPB zones and formation of S precipitates. DSC results illustrate that dynamic dissolution of S precipitates are observed during second pass of MDF at 380 oC. This is attributed to the elevated temperature and higher strain level. In this research, the probability of restoration phenomena is examined. Fully recrystallized microstructure is not found in any specimen. However, observation of subgrains in SEM results of the specimens deformed at higher temperatures confirms the occurrence of dynamic recovery. The presence of S precipitates on the low-angle grain boundaries is verified according to the SEM results. The incidence of continuous dynamic recrystallization is postponed due to the strong Zener pinning effect of the S precipitates on the low-angle grain boundaries migration. Fine recrystallized grains in the vicinity of the coarse second phase particles are shown in the optical micrographs. This partially recrystallized microstructure can confirm the occurrence of particle stimulated nucleation (PSN)
  9. Keywords:
  10. Dynamic Recovery ; Multidirectional Forging ; Aluminum Alloy 2024 ; Dynamic Precipitation ; Dynamic Recrystallization ; Zener-Pinning Effect ; Dynamic Dissolution

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