Characterization of the effects of friction stir processing on microstructural changes in DRA composites

The effect of friction stir processing (FSP) on the spatial homogeneity of SiC particles in discontinuously reinforced aluminum (DRA) metal matrix composites is investigated. DRA composites were produced with microstructures having controlled degrees of clustering, by carefully varying the particle size ratios (PSR) of the powders used in their fabrication. In this contribution, the spatial heterogeneity of these DRA microstructures is characterized by careful measurement of statistical two-point correlation functions and also via the multi-scalar analysis of area fractions (MSAAF) technique, at length scales up to 1 mm, by the use of high-resolution, large-area image montaging. These complimentary techniques yield similar results for the spatial heterogeneity of the DRA materials, both before and after FSP. The microstructural data suggest that significant microstructural modifications are imparted by FSP, including reductions in the measured spatial heterogeneity and microstructural anisotropy, as well as re-orientation of the reinforcement particles due to the stirring action. These microstructural modifications have a strong influence on the experimentally-measured tensile strengths and ductilities of the DRA composites. A general relationship exists between the measured tensile ductility and the microstructural parameters of homogeneous length scale (LH) and cluster size (dclus), independent of the DRA processing history.