2026-07-30 オークリッジ国立研究所（ORNL）
＜関連情報＞
- https://www.ornl.gov/news/neutrons-reveal-how-friction-stir-welding-could-strengthen-steel-armor
- https://www.sciencedirect.com/science/article/pii/S0924013626000038
摩擦攪拌接合された装甲鋼における微細構造と残留応力形成の相関関係を中性子回折によって解析した Correlation between microstructure and residual stress formation in friction stir welded armor steels characterized by neutron diffraction
Jhoan Guzman, Kaue C. Riffel, Martin McDonnell, Jeffrey Bunn, Andrew Payzant, Doug Kyle, Antonio J. Ramirez
Journal of Materials Processing Technology Available online: 5 January 2026
DOI:https://doi.org/10.1016/j.jmatprotec.2026.119198
Highlights
- Residual stresses in FSW result from competition between transformation-induced expansion and tempering relaxation.
- The width of the softened heat-affected zone determines the extent of tensile residual stresses, and heat input can be adjusted to control them.
- HHA steel retains fresh martensite with compressive stir-zone cores, while RHA steel overtempers and forms wider tensile regions.
- Maximum longitudinal tensile stresses in the softened heat-affected zone reach about 70 % of the yield strength.
- Results establish a framework for transformation-hardening steels to optimize solid-state joining and tailor residual stress.
Abstract
Friction stir welding (FSW) is a solid-state joining process that minimizes the heat-affected zone (HAZ) compared with fusion-based arc welding, making it well suited for joining martensitic armor steels where hardness and ballistic resistance are critical. This study investigates residual stress formation in three defect-free FSW butt-joint configurations relevant to armored-vehicle fabrication: similar rolled homogeneous armor (RHA–RHA, Case 1), similar high-hardness armor (HHA–HHA, Case 2), and dissimilar HHA–RHA (Case 3) joints produced under temperature-controlled conditions (770 °C). Neutron diffraction was employed to quantify the magnitude and spatial distribution of residual stresses in the longitudinal, transverse, and normal directions and to correlate them with weld microstructure and hardness. Tensile residual stresses were concentrated in the softened HAZ, reaching approximately 300 MPa for Case 2 and 400 MPa for Case 1 (≈50–70 % of the base-metal yield strength; ∼581 MPa for RHA and ∼566 MPa for HHA), while compressive residual stresses dominated the stir zone. The spatial extent of tensile stresses scaled with the width of the softened HAZ, which was largest in the dissimilar HHA–RHA joint and smallest in the HHA–HHA joint. Full-width-at-half-maximum (FWHM) analysis revealed low microstrain in overtempered HAZ regions and high microstrain in the stir zone associated with severe plastic deformation and fresh martensite formation. This work demonstrates that residual stress evolution in FSW of martensitic armor steels is governed not primarily by peak temperature or thermal contraction, as inferred from fusion-welding analogies, but by the competition between transformation-induced volumetric expansion and tempering-induced stress relaxation. The relative dominance of these mechanisms is shown to depend on alloy hardenability and local thermal history, leading to more extensive HAZ softening and broader tensile stress regions in the lower-hardenability RHA steel. These findings establish a transferable mechanistic framework for optimizing solid-state joining strategies in high-strength steels and other transformation-hardening alloys beyond armor applications.
