Study On Microstructure And Properties Of Aluminum Matrix Composites By Friction Stir Processing

Aluminum matrix composites reinforced with0~6vol%carbon nanotubes (inshort CNTs) were fabricated by friction stir processing(FSP). The effect of CNTscontent on Macro morphology and microstructure was investigated. The mechanicsproperties of the composite were tested, and the influence of content of CNTs on themechanics properties was analyzed. The microstructures of composite were observedby means of SEM and TEM. The wearing and strengthening mechanism ofcomposites were studied. It provided a theoretical basis for extend application ofFSP.The results show that: CNTs/Al composites of different content CNTs weresuccessfully prepared by FSP. In Center of friction stir zone, the microstructuretransform from original elongated microstructure to the fine equiaxed grain after FSP.There are black points in the microstructure which are hollow hole within muchparticles. They are segregation of CNTs in this region. With the increase of thecontent of CNTs, the black points gradually increase.The results of mechanics properties show that: with the increasing of thecontent of CNTs, the hardness and tensile strength of composites are improvingwhile their elongation is reducing. Fracture tests’ results show that the macro fractureof the composites distributes great fibrous when content of CNTs is1.6%. And thecharacteristics of the fracture is microporous aggregation. The fractures distributeless fibrous in macro while much dimple in microscopic when CNTs content is6%.The fractures of composite materials show brittle fracture on macro and ductilefracture on micro. The fracture mechanism dominated by Al-CNTs interfacedebonding and CNTs Pulling out. CNTs play role in bridging fiber. When CNTsadded in Al matrix, self-lubricating film is formed on the surface, it can preventeddirectly contact between base material and friction pair, and can effectively reducewear vale of composites. The wear value of composites is reduced and tended to bestable with the increase of the CNTs content. The wear value was increased withfriction pressure raised. The wear value of CNTs6/Al and CNTs4.4/Al also increasedslowly when friction pressure is below40N. But the wear value of CNTs6/Al washigher than that of CNTs4.4/Al when friction pressure up to40N. The wear mechanism of pure aluminum is mainly abrasive wear while wear mechanism ofcomposites is mainly abrasive wear and self-lubrication film fallen off.TEM observations show that microstructure of composites is fine equiaxedgrains while the grain sizes are uneven. The grain sizes is about100-200nm. TheGrain refinement process was performed by dynamic recrystallization of which issub-crystal nucleating mode. CNTs embedded in aluminum matrix metal are uniformdistribute in the composites, of which local area have CNTs of aggregation andwreckage. Good combination can be obtained between the CNTs and aluminummatrix. There are large numbers of dislocations and dislocation tangles distributed inthe interface. CNTs mainly distributed within the aluminum grains. There are notobserved CNTs in grain boundary.