Study On Precipitates And Thermal Structure Stability Of CNTs Reinforced 7075 Aluminum Matrix Composite Fabricated By FSP

Carbon Nanotubes reinforced 7075 aluminum matrix composites were prepared by friction stir processing. The microstructure, alloy elements, evolution and distribution of second phases were observed by scanning electron microscope(SEM), energy spectrum(EDS) and electronic probe(EPMA). The effect of solution treatment to redissolution of second phases and aging treatment to separating out of precipitates were studied by SEM, X-Ray diffraction(XRD) and transmission electron microscope(TEM). And the influence of CNTs on precipitates was analyzed. Besides, the effect of heat treatment on grain size of composites was tested by electron backscattered diffraction(EBSD), and the grain growth index was calculated. The effect of CNTs and heat treatment on the coefficient of thermal expansion were tested and compared.The results show that the grain of 7075 aluminum alloy was refined by strong mechanical action of tool. The microstructure is mainly on dynamic recrystallization organization. The addition of CNTs can further refine grain. Elements are uniformly distributed. The more addition of CNTs, it is more likely to lead to entanglement. The best solution temperature was 470℃, with the increase of solution time, the second phases of composites gradually reduce. And in the first 8h solution time, it reduces quickly, and then becomes flat. After same solution and aging of different time, the amounts of precipitates(MgZn2) increase with the extension of aging time. Compared with coarse grain 7075 aluminum alloy, the amounts of precipitates reduce. Meantime, the addition of CNTs can promote precipitates forming nucleus at the broken place of CNTs, and accelerate precipitation of composites.At the experiment of different solution time of FSPed 7075 and 5vol.% CNTs/7075 composites, with the increase of solution time, the average grain size of composites becomes bigger inordinately. The comparison shows the addition of CNTs can restrain the growth of grain sizes, and it affects more at earlier stage, weaken at later period. At the experiment of thermal expansion analysis, the coefficient of thermal expansion of composites increase with the rise of temperature. At the same test, the coefficient of thermal expansion of composites decrease with the increase of CNTs content. Besides, the T6 and annealing heat treatment can effectively reduce the coefficient of thermal expansion of composites.