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

doi:10.1016/j.jmatprotec.2026.119198

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

Research output: Contribution to journal › Article › peer-review

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.

Original language	English
Article number	119198
Journal	Journal of Materials Processing Technology
Volume	349
DOIs	https://doi.org/10.1016/j.jmatprotec.2026.119198
State	Published - Mar 2026

Funding

The authors thank the Welding Engineering Program of The Ohio State University. Thanks to the Manufacturing and Materials Joining Innovation Center ( Ma2JIC ) and the National Science Foundation (NSF) award numbers: 1822144 , 1539992 , and 2052747 . We extend our gratitude to the Center for Design and Manufacturing Excellence (CDME) at The Ohio State University for providing the FSW machine. Thanks to the Center for Electron Microscopy and Analysis (CEMAS), the place where SEM images were taken. Thanks to Michael Eff (EWI) and Jeffrey Rodelas (Sandia National Laboratories) for the discussions. Jhoan Guzman expresses his appreciation to the Fulbright Program (cohort 2021), the Ministry of Science, Technology, and Innovation of Colombia, and Ma2JIC for funding his PhD. This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The beam time was allocated to the HIDRA instrument on proposal number IPTS-32327. We thank The United States Army Combat Capabilities Development Command (DEVCOM) Ground Vehicle Systems Center (GVSC). Distribution Statement A. Approved for public release; distribution is unlimited. OPSEC9670. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

Keywords

Friction stir welding
High-hardness armor steel
Neutron diffraction
Residual stress
Rolled homogeneous armor steel

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10.1016/j.jmatprotec.2026.119198

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title = "Correlation between microstructure and residual stress formation in friction stir welded armor steels characterized by neutron diffraction",

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.",

keywords = "Friction stir welding, High-hardness armor steel, Neutron diffraction, Residual stress, Rolled homogeneous armor steel",

author = "Jhoan Guzman and Riffel, \{Kaue C.\} and Martin McDonnell and Jeffrey Bunn and Andrew Payzant and Doug Kyle and Ramirez, \{Antonio J.\}",

note = "Publisher Copyright: {\textcopyright} 2026 The Authors",

year = "2026",

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doi = "10.1016/j.jmatprotec.2026.119198",

language = "English",

volume = "349",

journal = "Journal of Materials Processing Technology",

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T1 - Correlation between microstructure and residual stress formation in friction stir welded armor steels characterized by neutron diffraction

AU - Guzman, Jhoan

AU - Riffel, Kaue C.

AU - McDonnell, Martin

AU - Bunn, Jeffrey

AU - Payzant, Andrew

AU - Kyle, Doug

AU - Ramirez, Antonio J.

PY - 2026/3

Y1 - 2026/3

N2 - 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.

AB - 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.

KW - Friction stir welding

KW - High-hardness armor steel

KW - Neutron diffraction

KW - Residual stress

KW - Rolled homogeneous armor steel

UR - https://www.scopus.com/pages/publications/105027169349

U2 - 10.1016/j.jmatprotec.2026.119198

DO - 10.1016/j.jmatprotec.2026.119198

M3 - Article

AN - SCOPUS:105027169349

SN - 0924-0136

VL - 349

JO - Journal of Materials Processing Technology

JF - Journal of Materials Processing Technology

M1 - 119198

ER -

Correlation between microstructure and residual stress formation in friction stir welded armor steels characterized by neutron diffraction

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