Papers by Author: Yutaka Matsuda

Abstract: Friction stir processing (FSP) causes fine-equiaxed microstructure[1]. In this study, microstructure and mechanical properties of a 7075 aluminum alloy subjected to multipass FSP, MP-FSP, are assessed. A new zone, PBZ, has been discovered between stir zones, SZs. The SZs are composed of fine-equiaxed grains, while PBZs are composed of two types of (fine-equiaxed and coarse-elongated) grains, both of which are still finer than those of base metal. Elongation at 773K of MP-FSPed specimen becomes larger than that of base metal, based on superplastic deformation due to the finer microstructure. Local elongation is smaller in PBZ than in SZ.

Abstract: Friction stir processing (FSP) is a method for controlling the microstructure that has been proposed by applying friction stir welding, FSW. In this study, microstructure and mechanical properties of a 7075 aluminum alloy subjected to multi-pass FSP, MP-FSP, are assessed to obtain fundamental knowledge for improving the plasticity of aluminum alloys. The MP-FSP has been applied to 7075 alloy plates with T6 and O tempers, and microstructural characterization has been made by means of optical and scanning electron microscopies together with EDX and EBSD analyses, while mechanical properties were measured by means of micro hardness and tensile tests at room and high temperatures. From microstructural observation, a new zone, PBZ, has been discovered between stir zones, SZs. The PBZ is composed of two types of (fine and coarse) grains, where the coarse grain contains many sub-grains. Hardness in PBZ is intermediate between that in BM and SZ both in T6 and O specimens; hardness generally decreases and increases in T6 and O specimens, respectively, by MP-FSP. In accord to the hardness change, strength at room temperature is decreased by MP-FSP in T6 specimen, and increased in O specimen. Elongation at 773K is increased both in T6 and O specimens because of superplastic deformation. However, local elongation is smaller in PBZ than in SZ, which can be attributed to the microstructural change by the deformation: grain shape remains equiaxed in SZ while it becomes elongated in the tensile direction in PBZ.