Production of Ultra-Fine Grained Aluminum Alloy using Friction Stir Process

Abstract

1050 aluminum alloy with an ultra-fine grain size was produced through friction stir process (FSP). The influence of tool rotation speed on the temperature profile, microstructure and mechanical properties of the friction stir processed zone (FZ) was also experimentally investigated. FSP was carried out with only a single pass at tool rotation speeds ranging from 560 to 1840 min � 1 under a constant tool traverse speed of 155 mmmin � 1 . The maximum temperature of the FZ was lower than the melting point of the workpiece material, although increasing linearly with the tool rotation speed. The cooling rate of the FZ also increased linearly from 341 to 1473 � Cmin � 1 with the tool rotation speed. The FZ had very low dislocation density and was composed of fine equiaxed grains. These fine grains would result from the growth inhibition of dynamically recrystallized grains by the high cooling rate of the FZ. The grain size of the FZ increased with the tool rotation speed. However, it is noteworthy that, for 560 min � 1 , the grain size decreased to even the submicron level in spite of only the single pass of FPS. The average hardness of the FZ increased significantly for 560 min � 1 to about 37% as compared with the unprocessed zone (UZ), although decreasing with the increase in the tool rotation speed. For 560, 980 and 1350 min � 1 , the tensile test specimens were fractured in the UZ. This result indicates that the tensile strength of the FZ increased more than that of the UZ by the grain refinement through FSP. Hence, it is concluded that FSP is very effective in producing an ultra-fine grained material with excellent mechanical properties. In addition, it is possible to control its grain size resulting in the mechanical properties by varying the maximum temperature with the tool rotation speed.