Effect of friction stir processing on the microstructure of cast A356 aluminum

Friction stir processing (FSP) was applied to cast A356 Al to modify the as-cast microstructure. FSP homogenizes and refines the cast microstructure, completely eliminates porosity, and creates a microstructure with fine Si particles (0.25-0.42 μm) distributed in a fine grain aluminum matrix (3-4 μm). Further, FSP parameters significantly influence microstructural development in the processed zone of cast A356 Al. Generally, higher tool rotation rate creates a more homogeneous microstructure. At lower tool rotation rates, a macroscopically visible banded structure characterized by a low density of coarse particles, was detected in the nugget zone. However, FSP parameters did not significantly influence the Al matrix grain size. The varied distribution pattern, size, and volume fraction of Si particles at different locations within the FSP zone indicates inhomogeneous material flow. A post-FSP T6 heat treatment did not alter the Si particle distribution, but did significantly coarsen the Si particles.