目的 揭示脉冲圆形超高斯（PW-SG）、脉冲横向椭圆高斯（PW-TE）和脉冲纵向椭圆高斯（PW-LE）三种模式对激光定向能量沉积（L-DED）过程中熔池行为的影响机制。方法 基于质量、动量和能量守恒定律,建立考虑氧和硫元素影响的脉冲激光空域整形定向能量沉积三维热流耦合模型,结合实验制备试样,并利用光学显微镜分析熔道形貌以验证模型准确性。结果 在三种模式中,PW-LE具有最大的峰值温度、熔池体积和熔道高度,且重熔最为严重;PW-TE则表现出最小的峰值温度、熔池体积、熔池表面积及熔道高度与深度,但熔道宽度最大,重熔最轻。所有模式的熔池均形成向内的Marangoni流动,其中PW-LE的峰值流速最高。在激光关闭的凝固阶段,三种模式的平均温度梯度、冷却速率和形态因子均逐渐降低,而平均凝固速率逐渐上升。此外,PW-SG和PW-TE诱导熔道表面形成鱼鳞纹结构,且PW-TE的鱼鳞纹最为显著。结论 周期性的热输入导致熔池行为呈现周期性时变特征,而不同的热分布和热累积（PW-LE> PW-SG>PW-TE）是引发熔池行为差异的主要原因。表面活性元素氧和硫促使热毛细系数始终为正,驱动熔池形成向内的Marangoni流动。不同的脉冲热源模式可诱导不同的晶粒结构和织构,其中PW-TE更有利于晶粒细化和CET,而PW-LE的高频重熔则倾向于柱状晶的外延生长和织构形成。
Abstract
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.
关键词
脉冲激光定向能量沉积 /
空域整形 /
数值模拟 /
热输运 /
流体流动 /
几何形貌
Key words
pulsed laser directed energy deposition /
spatial shaping /
numerical simulation /
heat transfer /
fluid flow /
geometric morphology
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基金
国家自然科学基金（52365041）; 贵州省教育厅资助项目（黔教合KY字[2021]315号）; 贵州省基础研究项目（QKHJC-ZK[2023]-017）
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