Improving carbon nanotube dispersion in aluminum matrix composite powders by a repeating-deformation ball milling process

During high energy ball milling, nano-reinforcements can be gradually dispersed in metal powders with the severe plastic deformation of metal powder particles. However, prolonged deformation soon results in intrinsic cold-welding of metal powder particles, leading to unpleasant dispersion effect and great difficulty in dispersing high contents nano-reinforcements. In this study, a repeating deformation ball milling (RDBM) process was proposed to deal with the excessive cold-welding of aluminum (Al) powders to achieve homogeneously dispersed carbon nanotubes (CNTs). Results showed that the plastic deformation of cold-welded Al powders was re-activated by re-adding the process control agent of stearic acid in the CNTs/Al composite powder. Compared with the cold-welded powder, the composite powder fabricated by RDBM had improved CNT dispersion and finer matrix grains down to nanoscale, leading to noticeably enhanced elastic modulus and hardness. Microstructures during RDBM were examined in detail to understand the dispersion mechanism of CNTs. The reported RDBM strategy in this study can be a guidance for optimizing high energy ball milling process to acquire homogeneous dispersion of high contents of nano-reinforcements and high-performance metal matrix composites.