Departmental Bulletin Paper Friction Stir welding of Ultrafine Grained TWIP Steel

Ueji, Rintaro  ,  Fujii, Hidetoshi  ,  Kunishige, Kazutoshi

44 ( 1 )  , pp.27 - 30 , 2015-06 , 大阪大学接合科学研究所 , オオサカ ダイガク セツゴウ カガク ケンキュウジョ , Joining and Welding Research Institute, Osaka University
Microstructures and mechanical properties of ultrafine grained twinning induced plasticity (TWIP) steels, which were welded by both friction stir (FS) welding and conventional tungsten inert gas (TIG) welding, were studied. The ultrafine grained microstructure was successfully obtained by cold-rolling to a reduction in thickness of 88% and subsequent annealing at a warm temperature (620℃). Kikuchi line analysis in Transmission Electron Microscope (TEM) clarified that most of the grain boundaries in the annealed specimen are high angle grain boundaries whose misorientation angle is larger than 15°.The TIG welding of the ultrafine grained TWIP steel produces coarse grains whose mean grain size is 70 μm. on the other hand, the joint of the FS welded sample exhibits ultrafine grains with a high density of dislocations, and retains high hardness of the base metel. These results suggest that FS welding is an appropriate process for the joining of the ultrafine grained TWIP steels. The misrostructure evolution during the FSW was discussed with focus on the temperature dependence of the stacking fault energy of the TWIP steel. The tensile deformation of the parts of these joints were also examined at room temperature.

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