Microstructure and Thermomechanical Behaviour of Magnesium – C nanotubes Composites produced by Friction Stir Processing

Due to their exceptional mechanical and thermal characteristics, C nanotubes (CNTs) are attracting an ever increasing interest in view of tailoring the properties of metal matrix composites (MMCs) e.g. for applications at high service temperature or in thermal management… However, the poor wettability of CNTs by molten metals and their strong tendency to agglomerate are major obstacles to the large-scale production of CNTs-MMCs by classical ‘liquid-state’ processing routes such as squeeze casting. As an innovative ‘solid state’ process, Friction Stir Processing (FSP) hence appears as a very promising alternative for the production of CNTs-MMCs [1], although the method for inserting the reinforcing phase - in grooves or holes machined in the matrix material - remains time-consuming and labour-intensive.
More recently, Mertens et al. [2] 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 present work, the feasibility of extending this latter method to the production of CNTs-MMCs has been assessed. “Bucky papers” – 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, and the number of FSP passes, on the microstructure of the composites and particularly on the distribution of the CNTs in the Mg matrix has been carefully studied. Moreover, a particular attention has been given to the characterization of the thermomechanical behaviour of the FSPed AZ31B-CNTs composites.