Study On Carbon Nanotubes Reinforced Aluminum Matrix Composites Fabrication By Combining Friction Stir Processing And Subsequent Rolling

The CNTs were distributed chaotically in the Al matrix as they were experiencedintense stirring during the process of friction stir processed carbon nanotubesreinforced aluminum matrix composites, and the advantage of one-dimensionalnanomaterials of CNTs can not be displayed. Therefore, based on CNTs/Alcomposites by FSP, the composites were fabricated further by hot rolling in order tomake CNTs oriented arrangement in the Al matrix which can improve the propertiesof composites. The main results can be followed:The microstructures of CNTs/Al composites by FSP were studied further, theresults show that: the equiaxed nanocrystals are prone to form in the friction stir zoneduring FSP, the interfaces between the CNTs and Al matrix are clean and combinedwell. The strengthening mechanisms of composites are analyzed based on themicrostructure, the performances of CNTs/Al composites are improved significantlydue to the combined action of bridging strengthening, dislocation strengthening,dispersion strengthening and grain refinement strengthening. The tensile strength ofthe composites with4.7vol%CNTs is152.7MPa and increased1.74times comparedto the FSPed Al matrix without the CNTs. However, the CNTs are distributedchaotically in the matrix, the effect of bridging strengthening is weaked.The CNTs/Al composites by FSP are hot rolled subsequently, the effect ofprocess parameters on properties and surface formation of composites was analyzed tooptimize the process of hot rolling. It is found that the composites with3.9vol%CNTshave better formation and higher tensile strength of221MPa under the hot-rollingtemperature of350℃,80%total deformation and four passes hot-rolling.The microstructures, physical and mechanical properties of the composites bysubsequent hot rolling were researched. The results show that: the extraction, clusterand arrangement of the CNTs in the dimples of the tensile fracture along thehot-rolling direction are improved, the CNTs are observed along the vertical directionof hot rolling seen from the TEM images, which show brittle fracture. Meanwhile, thegrains are strip along the direction of the hot rolling, and the mean grains sizedecreases. Lots of dislocations are found around the CNTs, and the lamellar Al4C3phase are formed in the CNTs/Al interfaces. The performance of composite material strength improved obviously for thecompound preparation. When CNTs content is4.7vol.%, the tensile strength ofcomposite is221MPa, increased44.7%for which preparated by FSP. What is more,the compound preparation can improve the conductivity and thermal stability ofcomposite, as CNTs content of composite is4.7vol.%, the electrical conductivity andthe thermal expansion coefficient during100~400℃, are increased by24.06%andreduced by3.0%for which preparated by FSP respectively.The strength mechanism of composite preparation by FSP and Hot rolling isdiscussed, and which includes bridge strengthening, dislocation strengthening,dispersion strengthening and fine-grain strengthening. The deformation reinforcementis caused due to the increase of dislocation density. The effect of bridge reinforcementincreases because of the number of CNTs which reached the critical length increasing,and the generation of the flake Al4C3layer between the CNTs and aluminum matrix.The reason of the improving of the conductivity and thermal stability of composite isthat the hot rolling processing made the the CNTs directio nal align and the conditionsof CNTs/Al interface conditions improve.