Effect of friction stir processing on microstructure and mechanical properties of aluminium

Commercially pure aluminium was subjected to friction stir processing (FSP) to study the microstructure developed and its effects on the mechanical properties. Friction stir processing refined the grain size to 3 μm in a single pass from the starting coarse grain size of 84 μm. Electron backscattered diffraction (EBSD) results showed occurrence of dynamic recrystallization and also revealed existence of different orientations within the stir zone and across the transition zone. Transmission electron microscopy (TEM) revealed fine grains with well defined boundaries. The arrangement and absorption of dislocation into the sub-grain boundaries, formed by dynamic recovery, was also revealed by TEM. The yield strength of the material was improved by a factor of 2.4 after FSP owing to grain refinement. The most important feature of the friction stir processed material was that even after this significant improvement in strength there was little loss of ductility. The hardness also improved by 34% with the peak hardness being observed towards the advancing side.

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► FSP of Al refined the grain size significantly in one pass.
► Occurrence of dynamic recrystallization was revealed by EBSD.
► Orientation variation was observed at different locations across the stir zone.
► TEM revealed the dislocation structures developed.
► Hardness and strength improved substantially and ductility was retained after FSP.