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Effect of Intermediate Annealing on the Microstructure and Mechanical Properties of AA2024-O during Multi-Directional Forging

Rezaei, Mahboubeh | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53715 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. Severe plastic deformation (SPD) processes, in addition to increasing the strength of alloys, reduce their ductility. Aluminum alloy 2024 in the initial annealing state (420°C, 150 minutes) is failed after 5 passes if subjected to the multi-directional forging (MDF) process at room temperature. Therefore, in this project, with initial effective strains of 1 and 2, intermediate annealing was used at temperatures of 150°C, 250°C and 350°C (20 minutes) between process passes. This not only maintains the major strength created in this alloy after the initial passes but also provides the capability of applying more passes in this process. After each step, the microstructural examination was performed by optical and scanning electron microscopy. Also, mechanical properties were evaluated by Vickers hardness and compression test. The results showed that grain refining occurs mainly by applying multidirectional forging to an effective strain of approximately 1 and increases the hardness and strength by 1.64 times. However, the rate of increase in strength and hardness decreases and becomes saturated due to the dynamic recovery and the fine particles are placed along the grain boundaries. Intermediate and final annealing at 150°C, under strains of 1 and 2, showed thermal stability due to the occurrence of static recovery. Between the two groups (initial strains 1 and 2) at the mentioned temperature, the maximum strength of 294 MPa was achieved in 5 passes. Intermediate annealing at 250°C (under initial strain 2), while maintaining hardness (85 Vickers) caused 8 severe deformation passes. By observing the flow softening phenomena (extended recovery, grain growth) at temperatures of 250°C and 350°C, these two temperatures were chosen in addition to 150°C ,as the intermediate annealing, to increase the hardening strain rate and strength. Evidence showed that using the temperature of 350°C then 150°C for intermediate annealing led to a good combination of strength (325 MPa) and ductility (7 passes) at the same time. The interaction of intermetallic particles with grain boundaries, grain refinement, and grain structure modification are responsible for ductility, grain growth, and Particle-Stimulated Nucleation (PSN), respectively. Finally, by comparing the two groups (initial strains 1 and 2), it was understood that at the two temperatures of 150°C and 350°C, the specimen with a higher number of initial passes showed a greater tendency to restore the properties. In fact, applying annealing between passes in several stages and dividing the strain into smaller strains has an effective role in maintaining the properties of the material. However, at 250°C, the difference due to strain-induced grain growth in the initial strain 1 and the high density of grain boundaries in the initial strain 2 was not significant.According to the scanning electron microscopy studies, precipitations and particles already present in the microstructure of 2024-O aluminum alloys, were crushed and scattered while undergoing severe plastic deformation unless partially dissolved at temperatures above 300°C. While severe deformation, by increasing annealing temperature from 150°C to 350°C, due to high diffusion rate, particle size increases and by continuing the process at higher strains i.e introducing high density of dislocations to the material, a narrow particle size distribution is obtained. EDS and elemental mapping showed that intermetallic coarse particles (>1μm), Al7Cu2Fe, AlFeCuMnSi and Al6Fe were responsible for the occurrence of the PSN mechanism at 350°C. Metastable precipitates (θ/θ') or fragmented particles caused by crushing the coarse brittle particles are responsible for locking the grain boundaries (Zener drag) and preventing them from moving
  9. Keywords:
  10. Multidirectional Forging ; Ductility ; Intermediate Annealing ; Aluminum Alloy 2024 ; Flexibility ; Strength Mechanism ; Intermetallic Particles

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