In-situ grain refinement of super martensitic stainless steel by synchronous laser remelting during wire-arc direct energy depositing

Super martensitic stainless steels offer great potential for wire-arc direct energy depositing (DED) of large, complex steel structures, but their as-deposited coarse and uneven microstructure limits mechanical performance. To overcome this, a synchronous laser remelting DED approach is introduced, using an auxiliary laser to remelt the mushy zone and enhance molten pool convection. The influence of laser remelting on molten pool thermodynamics was studied using high-speed infrared thermography techniques and finite element simulations of the temperature field. By stirring the mushy zone, the synchronized laser enhances molten pool convection, suppresses columnar grain growth, and promotes the fragmentation of growing columnar dendrites, thereby increasing the density of heterogeneous nucleation sites. As a result, the laser remelting strategy effectively promotes a columnar-to-equiaxed grain transition, leading to a fully and uniformly equiaxed microstructure with prior austenite grain sizes refined from 14.03 μm to 3.25 μm. The refined microstructure leads to a synergistic improvement in both strength and ductility of the specimens. The longitudinal tensile strength increases from 862 MPa to 1003 MPa, while the elongation improves from 16.5 % to 19.5 %. In addition, the more uniform grain size distribution in the laser remelting specimens leads to a smoother and more linear microhardness profile along the build direction, despite the persistent influence of tempering effects. However, the laser remelting specimens exhibit slightly reduced corrosion resistance, likely because of higher densities of dislocations and grain boundaries that undermine passive film stability. This work comprehensively explains the interplay between process parameters, solidification dynamics, microstructural evolution, and material performance.