Aluminum matrix composites with significantly enhanced mechanical and physical properties are expected by using the carbon nanotube (CNT) as the reinforcement, due to its good mechanical properties (extremely high strength ~30 GPa, modulus ~ 1 TPa) as well as good electrical and thermal conductivity. However, it is a challenging task to individually disperse CNTs into the metal matrix due to the entangling or bundling of CNTs resulting from large aspect ratio and the strong Vander force. The CNT clusters in the CNTs reinforced metal matrix (CNT/metal) composites would reduce either mechanical or physical properties of the resultant composites. Individual dispersion of CNTs, relatively long CNT length and good CNT-metal interface bonding are the keys to obtain high-performance CNT/metal matrix composites.
In this work, 1.5-4.5vol.% CNTs reinforced 2009Al (CNT/2009Al) composites were fabricated by means of friction stir processing (FSP) technique and subsequent rolling, and subjected to detailed microstructural investigation and property evaluation. Firstly, the CNT/2009Al composites were fabricated by multi-pass FSP. Microstructural observations indicated that the CNTs were uniformly and randomly dispersed in the aluminum matrix after 3-pass FSP. The CNTs were cut shorter as the FSP pass increased and it was found that the reciprocal of CNT length exhibited a linear relationship with the number of FSP passes. The grains of the matrix were significantly refined due to the effective pinning of CNTs on the growth of recrystallized grains, and the grain size tended to stable after 3-pass FSP. The maximum strength of the composites was obtained with 3-pass FSP. This is attributed to the combined effect of CNT cluster reduction, grain refinement and CNT shortening.
Secondly, in order to align CNTs in the aluminum matrix, the FSP CNT/2009Al composites with randomly arranged CNTs were subjected to hot-rolling with a reduction of 80%. Microstructural observations indicated that CNTs were directionally aligned along the rolling direction in the composites after hot-rolling. The tube structure of the CNTs was retained and the CNT-Al interface was well bonded without pores after FSP and rolling. As a result, both the strength and modulus of the composites were significantly enhanced compared to the 2009Al and increased with increasing the CNT volume fraction. In particular, 3vol.% CNT/2009Al composite exhibited an ultimate tensile strength of 600 MPa and elongation of 10%, much higher than the corresponding values for CNT/Al composites fabricated by other processes.