Friction Stir Channel Pressing of Carbon Nanotubes Reinforced 7075 Aluminum Alloy Composites

Friction stir channel pressing (FSCP) is a new solid-state method for producing metal matrix composite, which was invented by the authors based on the principles of friction stir welding and equal channel angular pressing. The carbon nanotubes (CNTs) reinforced 7075 aluminum alloy composites (CNTs/Al-7075) were fabricated by FSCP, with different volume percentages of CNTs (0%, 2% and 4%). The distribution of CNTs in Al-7075 matrix and the microstructures including fine grains and second-phase particles were analyzed by optical microscope, scanning electron microscope and transmission electron microscope. The solution and aging treatments were used for improving the microstructures and the mechanical properties of the CNTs/Al-7075 composites. The results show that the CNTs/Al-7075 composite with a uniform distribution of CNTs is fabricated by FSCP. The grain refinement of Al-7075 is realized by FSCP, and further finer grains are obtained by introduction of CNTs. The grains of CNTs/Al-7075 composite become finer with increasing the volume percentage of CNTs. The precipitation behavior of second-phase particles of the Al-7075 produced by FSCP and the CNTs/Al-7075 composite is improved by the solution and aging treatments, resulting in an increase in micro-hardness. The strengthening mechanisms of the CNTs/Al-7075 composite include fine-grain strengthening, dislocation strengthening, load transfer mechanism and second-phase strengthening, among which the second-phase strengthening plays a leading role.