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On The Solution Treatment of Severely Deformed AA 2024-O and its Post-Severe Plastic Deformation

Forouzesh, Milad | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53562 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. In this study, the solution treatment of severely deformed AA 2024-O and its post-severe plastic deformation is investigated. The investigation was performed in two cases of pre-multi directional forging (pre-MDF) and post-multi directional forging (post-MDF) on the solution treatment. After applying pre-MDF, finding kinetics of temperature and solution time caused that natural aging to be studied as a treatment after solid solution. By measuring the difference in hardness before and after natural aging in pre-MDFed and un-MDFed samples, 500℃ and 5 minutes were found to be the most desirable temperature and time in terms of mechanical properties and microstructure. Continuous recrystallization (CRX) with a grain size of 47µm occurs during the solution annealing of the pre-MDFed (4 passes) sample that it is 220% less than that in un-MDF (150µm); also, rich presence of second phase particles after its aging by field-emission scanning electron microscope (FE-SEM) was analyzed. In the sample with pre-MDF, the highest aging hardness (100 HV) was reported compared to without pre-MDF; additionally, the percentage of increase in hardness before and after aging (∆H%) for samples with pre-MDF and without pre-MDF were obtained 43% and 23%, respectively. Using the DSC test, it was found that in the pre-MDFed sample, the peak of S′/S (Al2CuMg) phase formation spreads and shifts from 262°C to a lower temperature of 235°C. Besides, activation energy by Johnson-Mehl-Avrami equations for pre-MDFed and un-MDFed was calculated 103 kJ/mol and 108 kJ/mol respectively. Lower activation energy means that pre-severe plastic deformation generates a high density of dislocations and vacancies that accelerates the dissolution of solute atoms and provides S′/S -phase nucleation sites. In the second part of this research, post-MDF was applied on pre-MDFed samples and resulted in the production of continuous dynamic recrystallization (CDRX) with a hardness of (170 HV), that from solid solution (70 HV) to post-MDF, a hardness increase of 143% has been achieved, this value has significantly increased compared to the sample without pre-MDF (86%). Non-isothermal annealing by differential scanning calorimetric analysis (at a constant heating rate of 10 min/°C) on the CDRX microstructure shows that the temperature of static recovery of the sample with pre-MDF increased from 140°C to 160°C. pre-MDF also transmits S'/S phase formation temperature (Al2CuMg) and occurrence of Particle Stimulated Nucleation (PSN) from 230 °C to 210 °C. The formation of the recrystallization peak of the post-MDFed sample in the DSC test, for the sample with pre-MDF, was transferred from 400°C to 385°C and appeared stronger. The activation energies calculated for post-MDFed samples with pre-MDF and without pre-MDF were 128 kJ/mol and 133 kJ/mol, respectively. The decrease of activation energy in the sample with pre-MDF signifies that the pre-SPD has been implicated as an important factor in accelerating the formation of the S'/S phase and the occurrence of the PSN phenomenon
  9. Keywords:
  10. Aluminum Alloy 2024 ; Hardness ; Multidirectional Forging ; Aging ; Solution Treatment ; Continuous Recrystallization ; Severe Plastic Deformation

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