Microstructural modification and ductility enhancement of surfaces modified by FSP in aluminium alloys

Friction stir processing (FSP) is an emerging surface modification technology under exploitation, to evaluate its potentiality for improving both surface and bulk properties. Two types of material modification have been developed: the in-volume FSP (VFSP) consisting on the modification of the full thickness and the surface FSP (SFSP) consisting on the modification of the surface of processed materials up to 2 mm depth. This paper presents a comparative study of VFPS and SFSP for two aluminium alloys AA7072-T6 and AA5083-O with different tool geometries. A significant increase in materials formability was observed due to an increase in ductility resulting from grain size refinement, increasing the maximum bending angle by a factor of 12 for the VFSP treatment and of 4 for the SFSP treatment in the AA7022-T6 samples. In AA5083-O an increase of the maximum bending angle of about 2.5 times for VFSP treatment and of around 1.5 times for the SFSP treatment was observed.