A6061-T6 aluminum alloy sheet with three millimeter thick was friction thermomecanical processed under processing conditions at various rotating speeds and feeding speeds from 800 to 2000rpm and 50 to 400mm/min, respectively. Microstructure on cross section of the friction thermomechanical processed sheet was investigated by mean of an optical microscopy and scanning electron microscopy method in order to characterize the metallurgical plastic flow pattern in the stir zone and evolution in size and morphology of the matrix grain structure from original parent sheet. Microstructure with good quality was obtained at the optimum FTMP conditions of the 1200〜1500rpm of tool rotating speeds and 100〜200mm/min of tool feeding speeds using a tool with 10mm of shoulder diameter, and 3mm of pin diameter. Stir zone in a central part consisted very fine recrystallized grain structure with 3.5μm in mean grain size. However, the recrystallization in stir zone can lead to drastic degrease of its impact toughness comparing with the base material. Many oxide inclusions are distributed at microstructure. Fracture mechanical analysis reveals that the toughness degradation is attributed to premature damage evolution such as microcrack initiations into the coarse oxide inclusions.
