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Abstract <p>Metal additive manufacturing (AM) enables the fabrication of functionally graded materials (FGMs) with tailored local properties; however, establishing the link between <i>in&#xa0;situ</i> measured thermal history and microstructure evolution remains a challenge for process understanding. This study investigates the microstructure genesis in a graded austenitic-to-martensitic steel joint fabricated by laser-based directed energy deposition with powder (DED-LB-p), focusing on the use of experimentally measured thermal cycles to understand and control the microstructure evolution in the junction. An infrared thermal camera was employed to monitor the process and extract local thermal histories across the compositional gradient. Transformation-relevant parameters, including cooling rates and time above critical temperatures (Ac<sub>1</sub>/Ac<sub>3</sub>), combined with composition-dependent martensite start temperature (Ms) estimates, were used to interpret the observed microstructures. In the 316L region, a typical AM columnar austenitic microstructure was observed, while the 9Cr region displayed heterogeneous structures (from tempered to fresh martensite, including ferrite), resulting in non-monotonic hardness variations (220–360 HV0.1). In the gradient zone, increasing alloying content initially promoted martensitic transformation, producing a hardness peak (400 HV0.1), followed by a drop when the predicted Ms fell below ambient, stabilizing austenite. These results demonstrate that linking <i>in&#xa0;situ</i> thermal history with transformation criteria provides a physically grounded methodology to relate process conditions to microstructure evolution in graded DED steels, supporting process understanding and potential control without post-process heat treatments.</p> Graphical Abstract <p></p>

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Linking In Situ Thermal History to Microstructure Evolution in Graded Austenitic-to-Martensitic Steels Produced by Laser-Based Directed Energy Deposition

  • 期刊论文

  • 发表日期：

    2026年8月1日

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  • Matthieu Bossy,
  • Xavier Boulnat,
  • Sabrina Marcelin,
  • Geneviève Foray,
  • Mady Guillemot,
  • Flore Villaret,
  • Thorsten Marlaud,
  • Nicolas Tardif

摘要

Abstract

Metal additive manufacturing (AM) enables the fabrication of functionally graded materials (FGMs) with tailored local properties; however, establishing the link between in situ measured thermal history and microstructure evolution remains a challenge for process understanding. This study investigates the microstructure genesis in a graded austenitic-to-martensitic steel joint fabricated by laser-based directed energy deposition with powder (DED-LB-p), focusing on the use of experimentally measured thermal cycles to understand and control the microstructure evolution in the junction. An infrared thermal camera was employed to monitor the process and extract local thermal histories across the compositional gradient. Transformation-relevant parameters, including cooling rates and time above critical temperatures (Ac1/Ac3), combined with composition-dependent martensite start temperature (Ms) estimates, were used to interpret the observed microstructures. In the 316L region, a typical AM columnar austenitic microstructure was observed, while the 9Cr region displayed heterogeneous structures (from tempered to fresh martensite, including ferrite), resulting in non-monotonic hardness variations (220–360 HV0.1). In the gradient zone, increasing alloying content initially promoted martensitic transformation, producing a hardness peak (400 HV0.1), followed by a drop when the predicted Ms fell below ambient, stabilizing austenite. These results demonstrate that linking in situ thermal history with transformation criteria provides a physically grounded methodology to relate process conditions to microstructure evolution in graded DED steels, supporting process understanding and potential control without post-process heat treatments.

Graphical Abstract