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Non-isothermal Annealing of Severly Deformed Aluminum Alloy 2024

Khani Moghanaki, Saeed | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49889 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. In this study, non-isothermal annealing of multi-directionally forged (MDF) aluminum alloy 2024 has been investigated. Two initial heat treatments have been considered, the solution treated (without S′/S (Al2CuMg)) and the over-aged (with S′/S (Al2CuMg)) conditions. DSC curves have shown that GPB zones/Cu-Mg co-clusters have been dynamically formed during MDF. This dynamic formation of these meta-stable phases has been modeled and in the presented model volume fraction of meta-stable phases has been calculated and its effect on the dislocation density has been considered. Although, hardness of solution treated sample has been increased by two-fold after MDF, hardness of over-aged sample has not been increased. After performing 3 passes of MDF in solution treated alloy, the Copper {112}<111> texture component has been developed, while in over-aged alloy shear type texture component C {001}<110> has been developed. EBSD images, DSC curves, optical micrographs, and mechanical properties have demonstrated that there are several interactions between recrystallization and precipitation phenomena during non-isothermal annealing with different heating rates. The interaction between dissolution of GPB zones/ Cu-Mg co-clusters and static recovery has been characterized in solution treated sample during non-isothermal annealing. During non-isothermal annealing up to 250 oC with heating rate of 10 oC.min-1 some particle free zones (PFZ) with 3-6 μm in width have been formed concurrently with S′/S (Al2CuMg) formation. Furthermore, High angle grain boundaries (HAGBs) have been formed around large intermetallic particles (which is known as particle stimulated nucleation (PSN)). The controversy effect of heating rate has been characterized during non-isothermal annealing of solution treated sample. However increasing the heating rate decreases PSN during non-isothermal annealing up to 380 oC, it increases PSN during non-isothermal annealing up to 450 oC which has been explained by Zener pinning effect of S′/S (Al2CuMg) particles. Extended recovery has been observed during non-isothermal annealing with higher heating rates like 800 and 1600 oC.min-1 and increasing the heating rate activates PSN sites. PSN activation can decrease Goss texture intensity and it can develop randomly oriented grains. The microstructure, recrystallization behavior, and mechanical properties of solution treated and over-aged alloys have been compared during non-isothermal annealing. In over-aged alloy during non-isothermal annealing up to 450 oC with heating rate of 10 oC.min-1 partially recrystallized microstructure with strong γ-fiber have been developed by strain induced grain boundary migration (SIBM), while in solution treated alloy, fully recrystallized microstructure with randomly oriented grains have been developed by PSN. During non-isothermal annealing of over-aged alloy, increasing the heating rate leads to increasing in recrystallized grain fraction and decreasing in grain size which has been related to PSN activation at higher heating rates. Furthermore, the intensity of γ-fiber has been reduced. Mechanical properties of all samples which have been non-isothermally annealed up to 450 oC are same and similar to undeformed solution treated alloy (84 HV)
  9. Keywords:
  10. Multidirectional Forging ; Textures ; Precipitation ; Recrystallization ; Aluminum Alloy 2024 ; Severe Plastic Deformation

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