THE EFFECT OF FRICTION STIR PROCESSING AND SUBSEQUENT ROLLING ON THE SUPERPLASTIC BEHAVIOUR OF ALUMINIUM ALLOYS

Friction stir processing (FSP) can dramatically reduce grain size conferring excellent superplastic behaviour in certain aluminium alloys. FSP of thick plate followed by rolling to sheet could potentially be used as a method to improve performance of established superplastic alloys or to induce superplastic behaviour in alloys not normally associated with this phenomenon. An extruded Al-Mg-Zr alloy was FSP'ed prior to rolling to sheet. The development of microstructure and superplastic behaviour was characterised using a combination of light, scanning, orientation imaging, and transmission microscopy, coupled with hot uniaxial tensile testing. FSP transformed the coarse, highly textured extruded, structure into a very fine (approximately 600nm) randomly orientated, equiaxed material. The structure of the material after rolling and thermal treatment was complex and in certain cases significant grain coarsening resulted. A detailed investigation of factors responsible for this will be described.