Surface Composites by Friction Stir Processing

Abstract

Surface metal matrix composites (MMCs) are a class of modern engineered materials specifically developed to alter material properties at the surface itself without altering the chemical and physical properties of the substrate. Several processing routes involve melting the substrate in order to incorporate dispersing phases in the form of particles or fibers. Oxidation of the liquid, formation of intermetallics between the dispersing phase and the molten substrate and coarse microstructure due to solidification are the important limitations associated with liquid state processing routes. On the other hand, several solid state processing routes have been developed to produce surface composites without melting the substrate. Friction stir processing (FSP) is one of the best examples of such techniques which facilitate to develop surface MMCs without melting the substrate. Several pure metals and alloys including aluminum, magnesium, titanium, and steels have been used as matrix materials and surface MMCs were developed by incorporating wide range of dispersing phases such as SiC, TiC, Al2O3, TiO2, SiO2, B4C, CNT, Graphene, Hydroxyapatite, etc. This article provides the fundamental mechanisms behind composite formation within the solid state during FSP. Furthermore, the role tool design and processing factors on successful fabrication of composite is explained. A brief summary of different material properties which can be altered by FSP is presented along with the merits and limitations of FSP in developing surface composites.