Edinburgh Napier University

  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.