Fabrication Of CNTs/Al Composites By Hot Extrusion And Their Mechanical Properties

In this paper, the powder mixing and hot extrusion process during the fabrication processing of CNTs/Al composites were studied and optimized. The optimized synthesis parameters were adopted to fabricate CNTs/Al composites with volume fractions of 2% and 4.5%. The microstructure of the composites was investigated with SEM and TEM and the strengthening mechanism and fracture mode of CNTs/Al composites were investigated by tensile tests and fracture surface analysis.The mixing process of the CNTs and Al powders were investigated by microstructure observation and sedimentation experiment. Detailed experiments were carried out to investigate the factors that affect the mixing of composite powders. These factors include the mixing agent, the surfactant (SDS), the mixing time, content of CNTs and the dry manner. The results showed that CNTs deposited along with aluminum powder in ethanol, but cannot in water. When the content of CNTs is less than 4.5%, uniformly dispersed CNTs in aluminum powders can be achieved. But CNTs agglomerates can be found when the CNTs content is higher than 8%. When the wet powders are compacted to a relative density of 60%, the CNTs may keep the uniformly dispersed state during the drying processing. The optimized mixing parameters are as followed: ethanol without SDS as mixing reagent, mixing time 1h and dry after compact.CNTs/Al composites powders with volume fractions of 2% and 4.5% were obtained by the optimized mixing technique. And then they are vacuum degassed and hydraulic pressed at a high pressure of 270MPa, thus obtaining the preform with relative density of 92.5% and 93%. The preforms are hot extruded to get composites with low porosity. When the powders with CNTs content of 4.5% are hot extruded at 350℃, 450℃and 550℃, respectively, high quality composites with smooth surface, no macro-cracks and low micro-porosity can be obtained at 450℃. So the optimized extrusion temperature is 450℃. The composite matrix contains fine sub-grains with CNTs mainly dispersing along the sub-grain surface. In addition the CNTs keep intact and show good interfacial bonds with aluminum matrix. Extruded at 450℃, 4.5%CNTs/Al composites show a higher tensile strength than extruded at 350℃and 550℃. With the increase of CNTs content, the tensile strength of the composites increase, but the elongation decreases. Fracture analysis showed that the interfacial bonding of CNTs/Al composites extruded at 350℃and 550℃were weak, with crack initiating at the CNTs rich regions and then spreading along aluminum powder interface. However, when the billet was hot extruded at 450℃, interfacial bond strength between aluminum powders was good, resulting in the cracks spreading across the aluminum powder rather than the aluminum powder interface. As a result, much dimples and ridges are formed in this composite, proving that CNTs play a role in bridging and strengthening the matrix alloy.