Mechanical Property Improvements in Aluminum Alloy through Grain Refinement using Friction Stir Process

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  • Kwon Yong-Jai
    Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST)
  • Shigematsu Ichinori
    Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST)
  • Saito Naobumi
    Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST)

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Ultra-fine grained specimens of 1050 Al alloy were produced by friction stir process (FSP), and also the influence of the tool rotation speed on the microstructure and mechanical properties was experimentally investigated. FSP was performed 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, around the FZ, no heat affected zone in which the hardness drops was formed. For 1840 min−1, the hardness distribution was roughly uniform within the FZ. The average hardness of the FZ was isotropic and was increased by 37% compared with the cold-rolled 1050 Al alloy (starting material) 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 stress increase resulting from strain strengthening. On the contrary, grain sizes of 2∼3 μm provided strain strengthening after a small stress drop. For grain sizes of 3∼4 μm, stress reached a maximum value with strain strengthening after a continuous transition from elastic to plastic deformation with no abrupt stress drop. When the grain size was below 3 μm, there was no severe loss of total elongation because of a large local elongation after the stress drop. The total elongation increased with the grain size. The tensile strength was inversely dependent on the grain size and increased remarkably to even about 46% greater than that of the starting material. Therefore, it is evident that FSP is very effective in enhancing hardness and tensile strength of materials through grain refinement.

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