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Study On Fabricating And Rheoforming Of In-Situ Synthesized Magnesium Matrix Composites

Posted on:2010-07-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y HuFull Text:PDF
GTID:1101360278971343Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Magnesium alloy and its composites have a great potential to be applied in automobile and aerospace industries because of their light weight, high specific strength, excellent anti-shock and good machine ability compared to other structural metals.The fabrication of magnesium composites includes ex situ method and in situ method. Compared to the conventional composites fabricated by ex situ method, the in situ synthesized composites exhibit the following advantages, such as finer grains, cleaner reinforcements and matrix interface, better thermodynamics stability, better wet ability and lower cost and so on, which makes it become an improtant development direction of fabrication magnesium composites. Metallic matrix composites reduce the plasticity, thus making against the second plasticty processing, despite the increasing of the hardness and strength. And conventional casting is prone to have the defects of oxide inclusion and shrinkage cavity and porosity. Present, with the increasingly develop of the semi-solid forming technique and theory, it offers a new way to the shaping of the metallic matrix composites. The Mg2Si/AM60 magnesium matrix composites fabricated by in-situ synthesis and its semi-solid rheoforming were investigated in the dissertation. The main work and results are as follows:The mechanism of in situ synthesized Mg2Si was discussed from thermodynamics and kinetics, the kinetic model of in situ synthesized Mg2Si was established, the kinetics equation was deduced. And it was found that the major factors which influence reaction rate were: the system temperature, the size of Si particle and the thickness of diffusion middle layer.The effects of different content of crystal Si on the microstructures and mechanical properties of in-situ synthesized Mg2Si/AM60 magnesium matrix composites were studied, the best content of crystal Si was established and the strengthening mechanism of the composites was analyzed thoroughly. It is found that the reinforced phase Mg2Si with the morphology of the Chinese script type, polygonal type and dendritic type exit in the matrix microstructures. The effects of alloy elements on the morphology and distribution of reinforced phase Mg2Si and the mechanical properties of in-situ synthesized Mg2Si/AM60 magnesium matrix composites were investigated. The Chinese script type Mg2Si were changed to granular type and distributed uniformly into the matrix microstructures by adding in alloy elements. And there is a best adding contents for elements.Semi-solid specimens of Mg2Si/AM60 magnesium matrix composites were fabricated by mechanical stirring. The effects of different technological parameters such as shearing temperature, shearing time and shearing rate on the semi-solid microstructures of composites were investigated. The grain spheroidizing and fining mechanism of the composites was analyzed and discussed theoretically and technologically. The research indicates that mechanical stirring can restrict dendritic growth, reduce the length of free crystal and promote wall crystal to be free. According to M-S theory, it shows that it is favorabal to make the grains global by mechanical stirring.The steady apparent viscosity of semi-solid Mg2Si/AM60 magnesium matrix composites was tested. The effects of different technological parameters such as the volume fraction of Mg2Si, shearing temperature, shearing time and shearing rate on the steady apparent viscosity of semi-solid Mg2Si/AM60 magnesium matrix composites were investigated. And the rheological model of semi-solid Mg2Si/AM60 magnesium matrix composites was established.Based on the second developing platform, the steady apparent viscosity model was embedded in simulation software and the simulation of semi-solid rheoforming of Mg2Si/AM60 magnesium matrix composites disc part was sucessfully done. The filling and solidification process were analyzed, and the casting defects were predicted, which can provide theoretical guidance for rheological casting. The rheological casting test was carried out by using the composites and the microstructures of the cast were analyzed. Compared to the conventional casting, the semi-solid rheological casting can improve the microstructure of the cast.
Keywords/Search Tags:in-situ synthesis, Mg2Si reinforced particle, composites, semi-solid, apparent viscosity, rheoforming
PDF Full Text Request
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