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Effect of Artificial Aging after Multi Directional Forging of Super Saturated AA 2024

Rashidi, Mohadeseh | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56680 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. So far, much research has been done on severe plastic deformation of aluminum alloy 2024, but there is limited information on the effect of artificial aging for this alloy in the supersaturated, post-MDF state. As one of the methods of extreme plastic deformation process, multidirectional forging process (single pass) has been performed on different alloys including aluminum alloy 2024 at ambient temperature. Alloy 2024 was subjected to solution treatment at 500°C for 1 hour and multi-directional forging process was immediately performed on it. Then, at temperatures of 100, 140, 190, 240, and 290 °C, the aging process was continued for one hour until artificial aging. In order to investigate the microstructure and state of the sediments, optical microscope (OM) and scanning electron microscope (SEM) images were prepared from the cross-sectional surface of the resulting samples, and to investigate the mechanical properties, the samples were subjected to hardness and pressure tests. Due to two reasons, the strength of multidirectional forging supersaturated alloy increases from HV82 to HV143, one is due to the applied hard work and the other is due to dynamic aging during the deformation process. The increase in hardness and yield stress and the formation of shear bands were attributed to the presence of GPB areas and sediments. Also, the obtained results show that by increasing the temperature of artificial aging, an increase in mechanical properties including hardness and yield stress and silane occurred. By continuing the heat treatment, artificial aging causes the occurrence of recovery, recrystallization and grain growth phenomena, which is associated with a decrease in hardness and silane stress and yield strength. At the beginning of the artificial aging heat treatment at 100°C, in addition to the recovery phenomenon, the formation of S'/S (Al2CuMg) deposits and the dissolution of GPB regions also occur, as shown in pictures 4-3 and 4-7 (a) and (b). It also confirmed the presence of S sediments on the sub-borders and areas free of sediments were also observed. The interaction of the recovery phenomenon with the dissolution of GPB regions in the dissolved alloy was detected. S deposits have a locking effect on the sub-boundaries and by preventing the mobility of the boundaries, it delays the occurrence of recrystallization. As the temperature increases up to 140°C, more sediments are formed. The sedimentation is due to the instability and supersaturation of the network, resulting from the dissolution of quasi-stable sediments and the partial dissolution of S'/S due to the application of severe plastic deformation and temperature increase. At this aging temperature, the sample has the highest degree of hardness and silane tension and yield strength. At the temperature of 140 °C, the increase in hardness due to the increase in the volume fraction of sediments and the crushing of grains has a more dominant effect than the decrease in hardness due to the rebound phenomena and the reduction of dislocations, and finally, according to the OM images at the temperature of 190 °C, small recrystallized grains (high-angle boundaries) are found in the vicinity of the particles. The coarseness of the second phase showed that in addition to the loss of hardness and strength in this sample, it can confirm the occurrence of a fully recrystallized microstructure with the particle stimulated nucleation (PSN) mechanism. At the temperatures of 240°C and 290°C, the growth of the seeds was revealed, and the hardness reached 128 and 97, respectively, and the yield and yield stress reached 505, 451 and 386, 326, respectively. In addition, the effect of different primary solutions was also investigated, because this value is effective on the dynamic aging rate and consequently affects the subsequent artificial aging. For this purpose, the samples were dissolved at temperatures of 480°C, 500°C, and 520°C for one hour and then subjected to MDF and aged at the most optimal aging temperature in terms of mechanical properties and microstructure, i.e. 140°C for one hour. The sample dissolved at 520°C reaches a level of dissolution that the size of the deposits due to its aging has a significant effect on the zener locking and causes it to have the highest hardness and yield stress and silane after artificial aging compared to the other two solution temperatures
  9. Keywords:
  10. Aluminum Alloy 2024 ; Multidirectional Forging ; Artificial Aging ; Severe Plastic Deformation ; Yield Stress

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