Study On Hot Pressing Sintering Of Carbon Nanotubes Reinforced Magnesium Matrix Composites

Carbon nanotubes(CNTs) are considered as one of the most superior nanomaterials for its excellent physical properties and menhanical properties. And study on carbon nanotube/magnesium matrix composites have attracted widespread attention; The current research focus on overcoming the carbon nanotube reunited and improving the wettability of carbon nanotubes with the matrix. The CNTs/Mg/AZ91 D composite was fabricated by hot pressing under argon atmosphere, using the mixed powder of CNTs/Mg/AZ91 D and CNTs/Mg/AZ91D/MgH2 composites in this thesis. The effect of mixing power crafts, composition ratio and sintering process on the composites were investigated. The microstructure, phase composition and fracture surface of the composites were studied by means of optical microscope, X-ray diffraction and scanning electron microscope and the related property of the composites were tested.The mixed processes of the CNTs/Mg/AZ91 D powders showed that the dry ball milling process had a certain degree of damage to the tubular structure of the CNTs. Although hand lapping can reserved CNTs, but the dispersion of CNTs is poor in the matrix and reunion phenomenon obviously.The CNTs/Mg/AZ91 D composite was fabricated by hot pressing sintering have showed that the composite had good formability by using hand lapping process.The main phase of the composites for several content of CNTs was magnesium, and a small amount of Mg17Al12, MgO and C was also detected by XRD. Microhardness of the composite material is related to the content of CNTs. When CNTs by adding 1.0wt.%, the composite have maximum microhardness and achieved 57.95 HV, compared to matrix the microhardness of composites improved 17%. Cross-sectional morphology of the composite material can be found magnesium alloy was melted and was filled in the interface between grains during the extrusion process, and has improved denseness of the composite material; The CNTs were bonded well with the matrix in the fracture surface of the composites, presenting the state of pull out or bridging, serving as the purpose of bridge strengthening and load transfer mechanism.CNTs/Mg/AZ91D/MgH2 composites were prepared by ball-milling under hydrogen atmosphere, then the surface of AZ91 D and Mg produces a certain amount of MgH2.Then MgH2 would be decompose and release activity magnesium to improve the wettability of CNTs and the matrix in the pressing sintering process. XRD analysis show that the powder prepaerd by ball-milling under hydrogen atmosphere was found probably containing about 33 wt.% of MgH2. Microhardness of CNTs/Mg/AZ91 D composites increased with the increase of content of carbon nanotubes are presented a trend of first increased and then decreased. When the addition of CNTs were 1.5wt.%, the microhardness reached the maximum(reached 75.67 HV), which is 22%more than the value of the matrix respectively; and increased by nearly 23%, compared to maximum microhardness of CNTs/Mg/AZ91 D by mechanical ball milling; The composites appeared tiny cracks becasue of the CNTs would have emerged reunion when high levels of CNTs in the matrix. and reduced the properties of composites.For the CNTs/Mg/AZ91 D composite was fabricated by two ball milling process.