2023 Fiscal Year Final Research Report
Simultaneous control of surface microstructure and residual stress using solid solution of tool elements during friction stir processing
| Project/Area Number |
21K14438
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| Research Category |
Grant-in-Aid for Early-Career Scientists
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| Allocation Type | Multi-year Fund |
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| Review Section |
Basic Section 26050:Material processing and microstructure control-related
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| Research Institution | Osaka University |
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Principal Investigator |
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| Project Period (FY) |
2021-04-01 – 2024-03-31
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| Keywords | 摩擦攪拌プロセス / 残留応力 / 表面合金化 / 残留オーステナイト / 超硬合金 / 摩耗 / マルテンサイト |
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| Outline of Final Research Achievements |
Friction stir processing (FSP) with the WC tool wear was conducted on the low-carbon steel plate to elucidate the mechanism of compressive residual stress applied to the FSPed steel surface, alloyed with the tool constituent elements. The alloyed topmost layers in the stir zone consisted of martensite structures with small amount of retained austenite grains. The residual stresses applied to the stir zone surface depended on the alloying contents and the related martensite start temperature (Ms). The compressive residual stresses can be maximized at the Ms of about 150°C due to the martensitic transformation expansion near room temperature. Further decreasing Ms below about 150°C resulted in applying tensile residual stresses and increasing volume fraction of retained austenite. This suggests that martensitic transformation expansion near room temperature is insufficient for applying compressive residual stress.
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| Free Research Field |
溶接・接合冶金学
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| Academic Significance and Societal Importance of the Research Achievements |
FSPによる疲労強度改善の事例として，き裂発生起点となる欠陥や粗大析出物の除去を目的とした鋳造合金や積層造形部材への適用は多数みられるが，ツール摩耗による元素供給を利用した改善は本研究が初である．当該プロセスでは基板表面への元素添加と焼入れが同時に施されるため，炭素量や合金元素の少ない加工性を重視した軟鋼に対しても，部分的に疲労特性を強化でき，構造設計上の材料選択における自由度の大幅な向上に貢献できると考えている．
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