Friction Stir Processing of Magnesium Matrix Composites Reinforced with Carbon Fibres or Carbon Nanotubes - A Comparative Study

The poor wettability of carbon substrates by liquid Mg and, in the case of carbon nanotubes (CNTs), their strong tendency to agglomerate, are major obstacles to the large-scale production of C-Mg composites by classical ‘liquid-state’ processing routes such as squeeze casting. As an innovative ‘solid state’ process, Friction Stir Processing (FSP) hence appeared as a very promising alternative for the production of C-Mg composites, although the method for inserting the reinforcing phase - in grooves or holes machined in the matrix material - remained time-consuming and labour-intensive.
More recently, the present authors proposed a new and easier technique for the insertion of C fibres in FSPed Mg-matrix composites i.e. FSP of a C fabric stacked between two metal sheets. In the current work, the feasibility of extending this latter method to the production of CNTs-Mg composites has been assessed. “Bucky papers” – thin disks made from agglomerated CNTs, thus ensuring for their safe handling – were stacked between two sheets of Mg alloy AZ31B, and the resulting sandwich was FSPed. The effect of FSP experimental parameters such as the rotational and advancing speeds or the number of passes on the microstructural changes occuring upon processing has been studied and compared with the case of C fibres-Mg composites. Moreover, a particular attention has been given to the distribution of the reinforcements in the Mg matrix and to the characterization of the resulting mechanical properties.