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

The principal objectives of the research reported in this thesis are: to determine theeffect that sinter time has on the metallurgical behaviour of alumina-reinforcedaluminium-606lmatrix composites; compare and assess the wear resistance ofalumina and silicon carbide reinforced aluminium 6061-matrix composites, togetherwith monolithic aluminium 6061 alloy; determine the effect that reinforcementparticle size has on the wear resistance of aluminium 6061-matrix composites;identify the relative merits of two techniques for depositing copper coatings on toalumina reinforcements.Through investigation, a successful method of processing silicon carbide and aluminaparticulate-reinforced AA6061 composites, fabricated by cold uniaxial pressing withvacuum sintering, has been determined. The processing route is as follows: pressing at400 MPa; vacuum sinter at 600°C for 30 minutes; solution heat treat for 30 minutes at530°C then water quench; precipitation (ageing) heat treat for 7 hours at I 75°C, thenair cool.Metallurgical examination of composites revealed that magnesium was found tocollect at interface regions around alumina particulates, resulting in the depletion ofmagnesium from the aluminium 6061 matrix. The severe depletion of magnesiumfrom the AA6061 matrix when alumina is used as a reinforcement was found to occurduring long (greater than 30 minutes) sintering times using a sintering temperature of600°C. It is postulated that the formation of spinel (MgA12O4) formed from thereaction of magnesium with alumina is a probable cause for the Mg migration. Thecomposites containing alumina particulates were found to have lower hardness valuesthan the monolithic alloy and composites containing silicon carbide, when sinteringtook place for longer than 30 minutes. Adding 5 wt% silicon to the AA6061 matrix incomposites reinforced with alumina particulates was found to reduce the magnesiumdepletion for sinter times up to one hour at 600°C and give improved composite bulkhardness.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 differentapplied loads if required) to be tested simultaneously without interference betweentest pieces, was designed and commissioned.Two electroless methods for copper coating alumina particulates were alsoinvestigated. One method used formaldehyde as the reducing agent, while the otheremployed hydrazine-hydrate as the reducing agent. The latter method has proven to bequicker, and with improved results, compared to the traditional method usingformaldehyde as the reducing agent