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- Type of Document: M.Sc. Thesis
- Language: Farsi
- Document No: 50832 (07)
- University: Sharif University of Technology
- Department: Materials Science and Engineering
- Advisor(s): Kazeminezhad, Mohsen
- Abstract:
- Since the increase in the strength of metals resulting from the severe plastic deformation (SPD) leads to a reduction in their ductility, recently, several studies have been carried out on the deformation of metals. One if the goals has been increasing the ductility while maintaining the strength. In this project, copper is undergone severe plastic deformation in a constrained groove pressing (CGP) process. According to previous studies, it has been determined that with an optimal annealing process, not only the strength is not decreased, but also it is slightly increased and also the ductility is enhanced. Thus, in order to improve the surface of copper after CGP and also optimize its mechanical strength, warm rolling process is utilized. In order to determine deformation mechanisms, warm tension test is used for deformed copper with strain of 2.32 at temperatures of 175, 200, 225 and 250 ℃ and strain rates of 0.0001, 0.001, 0.01 and 0.1 s-1. Warm deformation behavior of copper confirms dynamic recrystallization (DRX). Increasing strain rate results in increase then decrease in stress. The decrease in stress at strain rates of more than 0.001 s-1 is due to formation of twinning and their effective role in DRX. Activation energy of DRX in CGP of copper is 161 kj/mol, which is the same as activation energy of twinning formation. Also, warm tension tests are conducted on non-deformed copper to compare their results with those of severely deformed ones. Warm rolling of severely deformed copper is performed with strain of 2.32 at 200℃ and area reduction of 10, 25, 40 and 55 percent and angular velocities of 35, 50 and 65 rpm. The microstructures of samples indicate DRX even at lowest rolling strain. Overall, increasing rolling strain results in an increase in recrystallized fraction and also volume fraction of twinning, which in turn makes it possible to achieve significantly higher strengths. Also, increasing strain rate results in decreasing grain size which results in increasing strength. This strength enhancement also comes with increase in fracture strain. The warm rolling is also performed in 250℃ with 40% area reduction through mentioned angular velocities. Increasing rolling temperature results in a decrease in strength due to higher grain size and reduction of fracture strain. In order to evaluate effects of warm rolling, cold rolling was performed in same conditions of warm rolling. Cold rolling of severely deformed copper with strain of 0.8 was unsuccessful, while warm rolling in 200℃ not only was successful, but also it is enhanced mechanical properties induced by SPD through DRX and formation of twinning. In angular velocity of 65 RPM, ultimate tensile strength reaches to 350 MPa which shows a 40% increase compared with that of severely deformed sample with strain of 2.32, and a 45% increase compared with sample severely deformed and annealed at 200℃. Also, fracture strain shows an increase of 100% and 53%, respectively. Overall, fracture strain of severely deformed and warm rolled copper is always higher than that of cold rolled samples. Also, effect of SPD on warm deformation is studied by using warm rolling with and without SPD. It is notable that not only cold rolling of severely deformed copper, but also warm rolling of non-preformed copper in one pass with strain of 0.8 was unsuccessful. In other words, the only situation in which imposing the strains of more than 0.5 in one rolling pass is possible, is warm rolling of severely deformed copper at 200℃. In all mentioned conditions, cold tension test for evaluation of mechanical properties is performed
- Keywords:
- Copper ; Severe Plastic Deformation ; Warm Rolling ; Mechanical Properties ; Dynamic Recrystallization ; Warm Tension Test
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