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Weld nugget / stir zone microstructures often exhibit highly homogeneous distributions of non-deforming constituent phases as well as refinement of such phases and grain structures. The role of recrystallization in grain refinement during FSW/P is well understood. However, the processes involved in refinement of particle size and homogenization of particle distributions during FSW/P are less well understood.

FS of an as-cast AA356 alloy has been conducted over a wide range of RPM values using both threaded and smooth tools. The conversion of the primary - eutectic microstructure to a highly refined and homogeneous dispersion of Si particles in a finegrained Al matrix will be documented.

The roles of advective mixing, diffusional processes, and discontinuous shearing of material will be delineated and quantitative measures of homogeneity will be discussed. The potential roles of tool and process conditions in homogenization are also considered.