Bi-directional scanning strategies for residual stress management by tailoring microstructural evolution in directed energy deposition of 9Cr-1Mo steel

Abstract

The transient heating and cooling cycles in arc-based directed energy deposition (DED) processes play the critical role in defining the microstructure and mechanical performance of nuclear-grade steel (9Cr-1Mo) deposition. The current study establishes a comprehensive framework that advances the underlining physics of thermal gradients, phase transformations, and residual stress evolution during the DED process. By combining finite element (FE) simulation and microstructural analysis, the study demonstrates a ∼17 % reduction in tensile residual stress along the build direction following optimized bi-directional scanning strategies. Thermodynamic modeling using the Scheil-Gulliver approach elucidates the formation of key phases, such as δ-ferrite, martensite, and precipitates (M₂₃C₆ and MX), under non-equilibrium conditions inherent to DED process. These findings reveal that non-equilibrium cooling suppresses δ-phase formation, promotes a martensitic matrix, and tailors cellular or dendritic morphologies decisive for mechanical performance. Transmission electron microscopy (TEM) analysis highlights that bi-directional scanning refines martensitic laths, reduces dislocation tangles, and promotes uniform carbide precipitation, significantly enhance microstructural stability and material performance. The current work not only shows the ways of mitigating Type IV cracking through stress reduction and microstructural control but also provides a transferable framework for tailoring deposition strategies in ferritic/martensitic steels and other high-performance materials suitable for industrial applications.