Abstract
Commercially pure aluminum alloy (A1050) with an ultra-fine grain size was produced by a friction stir process (FSP). The influences of a tool rotation speed on the temperature profile, microstructure and mechanical properties of a friction stir processed (FSPed) zone were also experimentally investigated. FSP was carried out at tool rotation speeds of 560∼1840 min−1. Although the maximum temperature in the FSPed zone increased almost linearly with the tool rotation speed, it was lower than the melting point of the starting material in every condition. The cooling rate of the FSPed zone increased from about 341 to 1473°C·min−1 with the tool rotation speed. In the FSPed zone, a dislocation density was very low and fine equiaxed grains were observed. This grain size increased with the tool rotation speed. It seems that these fine grains resulted from the inhibition of the growth of recrystallized grain, which was formed in the FSPed zone, by the high cooling rate after FSP. It is noteworthy that, for 560 min−1, the grain size decreased to even the submicron level with only a ‘single pass’ of FSP. Although, for 560 min−1, the average hardness of the FSPed zone increased up to about 37% as compared with the unprocessed zone, it decreased with the increase in the tool rotation speed. The tensile specimens, which were processed at 560, 980 and 1350 min−1, were fractured in the unprocessed zone. This result confirms that the tensile strength of the FSPed zone increased with the decrease in its grain size. From these results, it is concluded that FSP is very effective in producing ultra-fine grained materials with excellent mechanical properties. In addition, it is possible to control mechanical properties by varying the maximum temperature and grain size of the FSPed zone with the tool rotation speed.
