POWDER SCALE MULTIPHYSICS NUMERICAL MODELLING OF LASER DIRECTED ENERGY DEPOSITION
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摘要: 激光定向能量沉积技术作为一种同轴送粉式金属增材制造技术, 以其制造效率高、成形尺寸大等优势在航空、航天、交通等领域具有广阔的应用前景. 然而, 该技术在金属零件的尺寸精度和形状精度控制方面存在诸如尺寸偏差大、表面不平整等控形问题, 亟需发展高效高精度预测熔覆层成形尺寸形貌的数值模拟方法. 针对该问题, 本文建立了考虑激光-粉末-熔池交互过程的高保真多物理场数值模型. 其中, 采用高斯面热源等效激光光束, 利用拉格朗日质点法求解粉末输送及其与激光交互的过程, 进一步结合有限体积法和流体体积法求解粉末-熔池的交互及其流动凝固过程, 并通过TC17合金单道熔覆层实验结果进行了验证. 基于该模型, 首先预测了不同工艺参数下单道熔覆层形貌尺寸, 并对熔覆层形貌的变化趋势及其内在的物理机理进行了深入分析. 结果表明, 依赖于工艺参数的粉末温度分布和粉末基板能量分配比例对熔池流场和熔覆层尺寸有显著的影响. 本文所建立的数值模型可辅助激光定向能量沉积增材制造技术控形工艺参数优化, 所得结论可为成形件尺寸和形状精度控制提供理论指导.Abstract: Laser-directed energy deposition (L-DED), as a coaxial powder feeding metal additive manufacturing process, has a broad application prospect in the fields such as aerospace and transportation for its' advantages of high deposition rate and fabrication of large parts. However, the L-DED suffers from process defects in the resolution of metal part size and shape, such as significant size deviation and surface unevenness, which requires high efficiency and accurate numerical model to predict the shape and size of the cladding track. In this work, we proposed a high-fidelity multi-physics numerical model that considers the interaction between powders, laser beam, and melt pool. In this model, the laser beam is modeled as a Gauss surface heat source, a Lagrangian particle-based model is used for the powders-laser beam interaction, and then the Lagrangian particle-based model is integrated to finite volume method and volume of fluid to simulate the interaction between powders and melt pool and the corresponding melting and solidification process. The proposed model is validated by the experimental data of single-track TC17 alloy fabricated using L-DED. Based on the validated numerical model, a set of single tracks with different combinations of process parameters are predicted, followed by an in-depth analysis of process parameters' effect on the sizes and shapes of the cladding tracks and the corresponding underlying physical mechanism. It is identified that the process parameters dependent temperature distribution of the injected powders and the ratio of energy absorbed by powders to that by the substrate play an essential role in the velocity field of the melt pool and the size and shape of the cladding track. We expect that the proposed numerical model is a powerful tool to aid the process parameters optimization for the L-DED additive manufacturing process. At the same time, the results of this study can provide theoretical guidance on the shape and size resolution control of the fabricated parts.
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图 1 (a)激光DED成形原理; (b)高保真多物理场模型
Figure 1. (a) Schematic diagram of laser DED. (b) High fidelity multi-physics model
图 2 同轴送粉激光直接能量沉积TC17单道扫描横截面视图, 其中黑色虚线为熔合线
Figure 2. Cross section view of the single track of TC17 by coaxial powder feeding laser directional energy deposition, where the black dashed line is the fusion line
图 5 不同激光功率工况下的数值模拟结果
Figure 5. Numerical results for the cases with different laser powers
图 6 不同送粉速率工况下数值模拟结果
Figure 6. Numerical results for the cases with different powder feed rates
图 7 不同激光扫描速度工况下的数值模拟结果
Figure 7. Numerical results for the cases with different laser scan speeds
图 8 不同粉末Z向速度工况下计算结果
Figure 8. Numerical results for the cases with different powder velocities along Z direction
图 9 不同粉末Z向速度工况下粉末(标有数值的黑色线)与熔池等温线对比
Figure 9. Comparison of isotherm for powders (the black line with values) and melt pool for the cases with different powder velocitiesalong Z direction
图 10 不同送粉模式工况下熔池流场、温度场, 其中标有数值的黑色线为粉末等温线
Figure 10. Flow field and temperature field of the melt pool with different powder feeding modes, where the black lines with values are the isotherm of the powders
表 1 工艺参数
Table 1. Process parameters
Parameters Units Value $P$ $ {\rm{kW}} $ 6 $v$ ${\rm{mm} }/\min$ 1000 $ {V_f} $ $ {\text{g}}/{\text{s}} $ 0.33 $ {r_0} $ $ {\rm{cm}} $ 0.3 ${r_{{\rm{focus}}} }$ $ {\rm{cm}} $ 0.2 ${l_{{\rm{focus}}} }$ $ {\rm{cm}} $ 1 ${z_{{\rm{focus}}} }$ $ {\rm{cm}} $ 0.5 
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表 2 TC17热物性参数
Table 2. Thermal properties of TC17
Properties Units Value $ {T_l} $ $ {\rm{K}} $ 1960 $ {T_s} $ $ {\rm{K}} $ 1770 $\rho $ $ {\rm{g}}/{\rm{c}}{{\rm{m}}^3} $ 4.69 $ k $ $ {\rm{W}}/({\rm{m}} \cdot {\rm{K}}) $ 6.5 $ c $ $ {\rm{J}}/({\rm{kg}} \cdot {\rm{K}}) $ 523 $ \sigma $ $ {\text{N/m}} $ 1.65 $\dfrac{ { {\rm{d} }\sigma } }{ { {\rm{d} }T} }$ $ {\rm{N}}/({\rm{m}} \cdot {\rm{K}}) $ 2.4$ \times {10^{-4}} $ $ \Delta H $ $ {\rm{J}}/{\rm{kg}} $ 3.03$ \times {10^5} $ $ {h_c} $ $ {\rm{W}}/({{\rm{m}}^2} \cdot {\rm{K}}) $ 10 $ \eta $ − 0.3 $ {\eta _p} $ − 0.7
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表 4 不同Z向粉末速度工况下熔池模拟结果
Table 4. Simulation results of melt pool for the cases with different powder velocities along Z direction
Powder Z-velocity/(cm·s−1) Width/mm Height/mm Depth/mm Max-velocity/(cm·s−1) Peak temperature/K 200 7 0.81 1 19.12 2690 400 6.75 0.68 1 16.22 2450 800 5.5 0.5 1 14.4 2350
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表 5 不同送粉模式下熔池模拟结果
Table 5. Simulation results of the melt pool for the cases with different powder feeding modes
Powder feeding mode Width/mm Height/mm Depth/mm Max-velocity/(cm·s−1) Peak temperature/K single-vertical nozzle 7 0.7 1 25.21 2920 four-lateral nozzles 6.75 0.68 1 16.22 2450
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