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Feasibility Study on Fabrication of Copper-Graphite and Copper-Babbitt Composites by Friction Stir Processing (FSP)

Sarmadi, Hassan | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41365 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Seyed Reihani, Morteza
  7. Abstract:
  8. Because of low friction coefficient, Copper-Graphite and Copper-Babbitt composites can be used as brushes and bearing materials instead of materials containing lead which cause environmental problems. So far, many methods such as powder metallurgy and centrifugal casting have been used to fabricate Copper-Graphite composites. The most important problem of these methods is agglomeration of Graphite particles. Simultaneous stirring of samples during fabrication process which is one of the features of friction stir processing (FSP), can solve the problem. Recently, FSP has been used for fabricating surface composite for the purpose of producing hard and wear resistance layers on ductile substrate used in different industries such as aerospace and automotive. Since the basis of FSP method is existence of a minimum coefficient of friction, at first the objective of this research was optimization of process parameters (such as rotating speed, transverse speed and specially tool pin profile) in order to fabricate composite and then study of friction and wear properties of these composites. Results show that using of tool with a triangular pin leads to better dispersion of particles. Using FSP process on the surface of annealed pure copper leads to decrease in grain size and increase in hardness. This results in reduction of coefficient of friction and increase in wear resistance. Friction coefficient decreases significantly with increase in graphite content so that friction coefficient of 4-pass sample containing 22vol.% graphite, is 79% lower than annealed copper. Friction coefficient of these composites decreases with increase in graphite content to a critical content and in this point, friction coefficient of composite becomes independent of graphite content and nearly equal to friction coefficient of pure graphite and more increase in graphite content has no effect on friction coefficient. Critical graphite content in this work was estimated about 25vol.% graphite. Amount of wear rate of 4-pass sample containing 22vol.% graphite decreases 65% in comparison with annealed copper. Friction coefficient of Copper-Babbitt composite decreases with Increase in Tin content according to Babbitt content and then increases with more increase in tin content. Changes in friction coefficient are explained with two different elements one results in decrease and another one increase in friction coefficient. Here, uncombined Babbitt with copper results in decrease in friction coefficient and hard intermetallic phases results in increase in friction coefficient. Wear loss of Copper-Babbitt composites decreases slightly to third pass and then increases significantly in fourth pass. Significant increase in wear loss in fourth pass can be related to increase in intermetallic phases especially hard phase Cu3Sn and change in wear mechanism because of these phases. In the case of Copper-Babbitt-graphite composites, friction coefficient decreases with increase in graphite to Babbitt ratio. This shows that ability of graphite in reduction of friction coefficient is higher in comparison with Babbitt
  9. Keywords:
  10. Surface Composite ; Friction Coefficient ; Graphite ; Friction Stir Welding ; Babbit Alloy

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