Microstructural characterisation of friction stir processed aluminium

Abstract

Friction stir processing was carried out on commercially pure aluminium, and a detailed microstructural characterisation was performed by electron backscattered diffraction and transmission electron microscopy. Friction stir processing resulted in significant grain refinement with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with some ultrafine grains being also observed. Electron backscattered diffraction studies showed majority of the boundaries to be high angle, confirming the occurrence of dynamic recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation arrangement into subgrain boundaries, grains having different dislocation densities and in different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a progressive transformation of sub-grain boundaries into high angle grain boundaries. A multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be operating during the process. The microstructure is not affected by changing the rotation speed from 640 to 800 rev min⁻¹, except that the grain size was marginally larger for higher rotational speed.

Keywords:

• Friction stir processing
• Microstructure
• Grain refinement
• Electron backscattered diffraction
• Transmission electron microscopy
• Dynamic recrystallisation

The authors would like to thank Professor K. Prasad Rao, IIT Madras, for providing the NRB supported FSP facility. Thanks are also due to Professor I. Samajdar, IIT Bombay, for providing access to the DST supported national facility for texture and OIM for EBSD studies.