Microstructure evolutions after friction stir welding of severely deformed aluminum sheets

The microstructure evolutions of severely deformed aluminum sheets by Constrained Groove Pressing (CGP) after Friction Stir Welding (FSW) are investigated. To do so, the specimens are deformed in three different strains using CGP process which make different the initial microstructures. Then, the specimens are joined at various revolution pitches (traveling speed/rotation speed ratio) of 0.20, 0.25 and 0.33 mm/r. To understand the effect of initial strains on the properties of joints, the microhardness measurements and microstructure investigations of different areas from retreating to advancing sides of the joints are carried out. The results achieved from FSW of CGPed samples are compared with those achieved from FSW of as-annealed ones (0 pass). It is found that in contrast to 2 passes CGPed specimens, for specimens strained by 1 pass of CGP process after FSW at low revolution pitch, the grain size at the center of stir zone is finer than that at around of stir zone center. Also, it is revealed that for CGPed samples in contrast to as-annealed ones, some recrystallized grains are formed in HAZ. Moreover, the amount of recrystallized grains in HAZ is different at advancing and retreating sides.

► Increasing of heat input has different effects on the 1 and 2 passes CGPed samples.
► Recrystallization in HAZ is increased with increasing of heat input and strain.
► For CGPed samples both elongated and recrystallized grains are formed at TMAZ.
► Thermal instability is increased with increasing of strain imposed during CGP.