Grain Refinement and Mechanical Property Improvements in Aluminum Alloys using the Friction Stir Process

Abstract

The influence of tool rotation speed and microstructure on hardness and tensile properties of ultra-fine grained 1050 aluminum alloys produced by friction stir process (FSP) was experimentally investigated. FSP was carried out with only a single pass at tool rotation speeds ranging from 560 to 1840 min-1 . For 1350 min-1 and below, the hardness within friction stir processed zone (FZ) was higher on the advancing side than on the retreating side. However, heat affected zone in which hardness drops was not formed near the FZ. For 1840 min-1, the hardness distribution was roughly uniform within the FZ. The average hardness of the FZ was isotropic and was elevated even to 37% compared with the starting material by increasing with decreasing the tool rotation speed, i.e. with decreasing the grain size. During tensile deformation, the friction stir processed materials with grain sizes of 2 μm and below exhibited an abrupt stress drop phenomenon at very early stage, which was not accompanied by a further increase in stress by strain hardening. In contrast, grain sizes of 2-3 μm provided strain hardening after stress drop. On the other hand, for grain sizes of 3-4 μm, stress reached a maximum value with strain hardening after a continuous transition from elastic to plastic deformation without the abrupt stress drop. The total elongation increased with the grain size. For the grain sizes of 3 μm and below, there was no severe loss of total elongation because of a large local elongation after the stress drop. Tensile strength was inversely dependent on the grain size and FSP increased the tensile strength of the cold-rolled 1050 aluminum alloy by 46% through the grain refinement. Hence, it is evident that FSP is very effective in enhancing hardness and tensile strength of materials through grain refinement.