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Semi Solid Rheocasting of A356 Aluminum Alloy by High Speed Injection

Razavi, Mehran | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52268 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ashuri, Hossein
  7. Abstract:
  8. Aluminum is the lightest industrial element after magnesium. As a result, aluminum alloys have many industrial usages because of their high specific strength. Semisolid casting is one of the processes that is capable of producing aluminum alloys with high mechanical properties. In this process, there are a wide variety of methods to prepare semisolid slurry, one of the best ways of which to produce globular structure is electromagnetic stirrer. In this research, the semisolid slurry of A356 aluminum alloy is fabricated by electromagnetic stirrer under different conditions of stirrer intensity current, stirring time and pouring temperature. Then the fabricated semisolid slurry is injected into the step-die cavity with different thickness of 1.5, 3, 5, 7, 9 and 11 millimeter using ram speed of 1 m/s. In the next step, by preparing and analyzing metallographic images, effect of stirrer current intensity, stirring time, pouring temperature and preheating temperature of step-die on the parameter of shape factor, aspect ratio, particle size and fluidity is studied. Hardness test is done in order to study mechanical properties of samples in different conditions, and shear punch test is performed on the samples with the most globular structures.The results of metallographic pictures revealed that stirring semisolid slurry for a longer time increases crashing of primary dendrites and abrasion of these particles, and results in a more globular structure. Furthermore, these results showed that by increasing stirrer current, primary solid particles in the crucible clashed and joined together more. As a result, solid particles become coarse with rosette morphology which causes less sphericity and fluidity of semisolid slurry. While both of hardness test results and structural parameters showed a little difference in various preheating temperatures, fluidity of semisolid slurry was more suitable in high preheating temperatures. The optimum condition to produce globular structure and high fluidity is achieved with stirring under 40 A current, 90 s stirring time, pouring temperature of 640 ºC and preheating temperature of 540 ºC. Hardness test results and SEM images show that decrease in thickness of step-die increases not only cooling rate, but also volume fraction of eutectic phase and hard intermetallic compounds. These three factors cause increase in hardness in thinner steps. In addition, the results of shear punch test show that ductility decreases by reducing the thickness of steps. The reason is that the number of brittle phases, such as intermetallic compounds (Al5FeSi, Al15(Fe,Mn)3Si2) and silicon lathes, are more in thinner steps
  9. Keywords:
  10. Semi-Solid Casting ; Globular Structure ; Electromagnetic Stirrer ; Aluminum Alloy A356 ; Step-Die ; Rosette Morphology ; Fluidity

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