Unified understanding and aggressive usage of deformation induced transformation in the stir zone of friction stir welding

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K14418
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 26030:Composite materials and interfaces-related
• Research Institution: Osaka University
• Principal Investigator: Yamashita Takayuki 大阪大学, 接合科学研究所, 助教 (20829080)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 中Mn鋼 / 摩擦攪拌接合 / 中性子回折 / TRIP効果 / その場観察 / 残留オーステナイト / 不均一変形 / 相安定性

Outline of Final Research Achievements

In this study, friction stir welding was performed on a medium-Mn steel at temperatures in the ferrite (α) and austenite (γ) dual-phase region. The joining region were subjected to in-situ neutron diffraction during tension to evaluate the phase stability of γ, deformation induced martensitic transformation behavior, and the role of each constituent phase to the mechanical properties. A large amount of γ remained on the bottom side of the joint, but its phase stability was low. Almost all of γ transformed to martensite (α’) before reaching tensile strength. The γ may contributed to the increase in work hardenability. In addition, it was suggested that the stress partitioning between flesh α’ and deformation induced α’ resulted in high work hardening rate at the later stage of deformation.

Free Research Field

接合科学、鉄鋼材料学、中性子科学

Academic Significance and Societal Importance of the Research Achievements

本研究成果は、不安定なγを有する鋼の変形挙動に関する重要な知見であり、より優れた鉄鋼材料を創生していく上での基礎的な知見として資することが期待される。また、中Mn鋼のFSW接合部の機械的特性をさらに改善するための指標として、より低温となる条件でFSWを実施する必要があること、FSWを前提とした合金設計をする上で相変態点温度を変化させることが有効であることを示唆する結果であり、学術的にも工業的にも大きな意義を有する。