脉冲激光空域整形定向能量沉积316L不锈钢过程中的熔池行为OA

Pulsed laser spatial shaping directed energy deposition(PLSS-DED),as a novel process strategy,demonstrates distinctive advantages and potential in regulating microstructures and macroscopic properties.However,the formation of microstructures and properties depends on melt pool behaviors,including heat transfer,fluid flow,and morphological characteristics.Therefore,the work aims to investigate the evolution of melt pool behaviors under pulsed circular super-Gaussian(PW-SG),pulsed transverse elliptical Gaussian(PW-TE),and pulsed longitudinal elliptical Gaussian(PW-LE)modes during the PLSS-DED process,thus establishing a theoretical foundation for customizing microstructures and enhancing macroscopic performance. The PLSS-DED process is inherently accompanied by complex and dynamic physical events,posing significant challenges for real-time monitoring of melt pool behaviors,thus making numerical simulation a critical tool for studying melt pool evolution.In this study,a three-dimensional thermo-fluid coupled transport numerical model was developed with the COMSOL Multiphysics platform,incorporating heat transfer,fluid flow,solid/liquid phase transition,and gas/liquid interface dynamics.Considering the effects of active elements such as oxygen introduced by oxidation-deoxidation reactions in the melt pool and sulfur in 316L stainless steel powder,a three-dimensional thermal-fluid coupled transport model for pulsed laser spatial shaping directed energy deposition was constructed based on mass,momentum,and energy conservation laws.Specimens were prepared with a pulsed laser-directed energy deposition experimental platform,and then the melt track morphology was analyzed with an optical microscope to verify the model accuracy,with maximum and average errors of 7.17%and 3.87%,respectively. The study revealed that melt pool behaviors(temperature field,velocity field,and geometric morphology)under all three modes exhibited periodic oscillations synchronized with the thermal input frequency.Thermal-flow analysis demonstrated that the PW-LE mode exhibited the highest peak temperature and fluctuation amplitude,while the PW-SG and PW-TE modes showed the minimum fluctuation amplitude and peak temperature,respectively.Notably,complete melt pool solidification within a single pulse cycle occurred exclusively under the PW-TE mode.Periodic thermal input induced multiple remelting phenomena in deposited tracks,with the PW-LE mode demonstrating the most severe remelting and the PW-TE mode exhibiting the mildest.All modes generated inward Marangoni flow within the melt pool,with the PW-LE mode achieving the maximum peak flow velocity.During the solidification phase after laser-off,the average temperature gradient(G),cooling rate(G×R),and morphology factor(G/R)at solidification interfaces progressively decreased across all modes,while the average solidification rate(R)gradually increased.The PW-TE mode exhibited notably lower G and G/R values but higher R and G×R values compared to other modes.Geometric characterization showed that the PW-LE mode produced the largest melt pool volume and track height,whereas the PW-TE mode demonstrated the smallest melt pool volume,surface area,track height,and depth,but achieved the maximum track width.Furthermore,both PW-SG and PW-TE modes induced fish-scale structures on track surfaces,with the PW-TE mode generating more pronounced pattern protrusions than the PW-SG mode. In summary,the periodic thermal input induces temporally periodic characteristics in the thermodynamic and dynamic behaviors of the melt pool,while distinct thermal distribution and accumulation patterns(PW-LE>PW-SG>PW-TE)serve as the primary factors governing variations in heat transfer,fluid flow,and geometric morphology.Surface-active elements such as oxygen and sulfur maintain a consistently positive thermocapillary coefficient,thereby driving the formation of inward Marangoni flow within the melt pool.The laser-off phase facilitates the transition of solidification microstructure from columnar to equiaxed grains.Different pulsed heat sources induce distinct grain structures and crystallographic textures:the PW-TE mode promotes grain refinement and columnar-to-equiaxed transition(CET)behavior,whereas the high-frequency remelting characteristic of the PW-LE mode favors epitaxial growth of columnar grains and texture formation.