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A study of the powder processing, tribological performance and metallurgy of Aluminium-based, discontinuously reinforced metal matrix composites.

Mitchell, Colin A (2002) A study of the powder processing, tribological performance and metallurgy of Aluminium-based, discontinuously reinforced metal matrix composites. PhD thesis, Edinburgh Napier University.

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    Abstract/Description

    The principal objectives of the research reported in this thesis are: to determine the
    effect that sinter time has on the metallurgical behaviour of alumina-reinforced
    aluminium-606lmatrix composites; compare and assess the wear resistance of
    alumina and silicon carbide reinforced aluminium 6061-matrix composites, together
    with monolithic aluminium 6061 alloy; determine the effect that reinforcement
    particle size has on the wear resistance of aluminium 6061-matrix composites;
    identify the relative merits of two techniques for depositing copper coatings on to
    alumina reinforcements.
    Through investigation, a successful method of processing silicon carbide and alumina
    particulate-reinforced AA6061 composites, fabricated by cold uniaxial pressing with
    vacuum sintering, has been determined. The processing route is as follows: pressing at
    400 MPa; vacuum sinter at 600°C for 30 minutes; solution heat treat for 30 minutes at
    530°C then water quench; precipitation (ageing) heat treat for 7 hours at I 75°C, then
    air cool.
    Metallurgical examination of composites revealed that magnesium was found to
    collect at interface regions around alumina particulates, resulting in the depletion of
    magnesium from the aluminium 6061 matrix. The severe depletion of magnesium
    from the AA6061 matrix when alumina is used as a reinforcement was found to occur
    during long (greater than 30 minutes) sintering times using a sintering temperature of
    600°C. It is postulated that the formation of spinel (MgA12O4) formed from the
    reaction of magnesium with alumina is a probable cause for the Mg migration. The
    composites containing alumina particulates were found to have lower hardness values
    than the monolithic alloy and composites containing silicon carbide, when sintering
    took place for longer than 30 minutes. Adding 5 wt% silicon to the AA6061 matrix in
    composites reinforced with alumina particulates was found to reduce the magnesium
    depletion for sinter times up to one hour at 600°C and give improved composite bulk
    hardness.
    During the research, a need for an improved wear testing machine was identified.
    Therefore a wear test rig, which allows samples of different materials (under different
    applied loads if required) to be tested simultaneously without interference between
    test pieces, was designed and commissioned.
    Two electroless methods for copper coating alumina particulates were also
    investigated. One method used formaldehyde as the reducing agent, while the other
    employed hydrazine-hydrate as the reducing agent. The latter method has proven to be
    quicker, and with improved results, compared to the traditional method using
    formaldehyde as the reducing agent.

    Item Type: Thesis (PhD)
    Uncontrolled Keywords: sinter time; metallurgical behaviour; alumina-reinforced aluminium-6061 matrix composites; wear resistance; copper coatings;
    University Divisions/Research Centres: Faculty of Engineering, Computing and Creative Industries > School of Engineering and the Built Environment
    Dewey Decimal Subjects: 600 Technology > 620 Engineering > 620 Engineering & allied operations
    Library of Congress Subjects: T Technology > TN Mining engineering. Metallurgy
    Item ID: 3807
    Depositing User: Mrs Lyn Gibson
    Date Deposited: 22 Jul 2010 15:54
    Last Modified: 12 Jan 2011 04:55
    URI: http://researchrepository.napier.ac.uk/id/eprint/3807

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