PHASE-FIELD STUDY ON THE EVOLUTION OF MICROSTRUCTURE OF THE MOLTEN POOL FOR ADDITIVE MANUFACTURING
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摘要: 激光增材制造(laser additive manufacturing, LAM)技术极适合复杂整体构件的近净成形和高附值损伤件的快速修复. 然而, 激光增材制造熔池内部复杂的动态凝固过程显著影响成形件的终态组织, 进而制约其服役性能. 本文针对激光直接能量沉积(direct energy deposition by laser, DED-L) Inconel 718过程, 构建宏观传热传质与多相场耦合的多尺度数学模型, 解决了熔池宏−微观温度场的直接耦合, 并基于MPI并行程序设计实现了熔池二维的全域定量模拟, 研究了凝固过程中的晶粒演变过程. 结果表明, 模拟的熔池尺寸、凝固界面与实验结果吻合较好. 熔池凝固界面形态和晶体择优取向是影响晶粒演变的重要因素. 在熔池横截面上, 凝固过程主要受温度梯度方向的驱使, 取向与温度梯度方向夹角越小的晶粒占优生长. 在纵截面上, 晶粒的生长表现出弯曲生长以及“上三角”的晶粒特征, 温度梯度方向的渐变导致了晶粒弯曲, 相邻晶粒的竞争行为决定了晶粒形貌. 本文阐明了金属激光增材制造晶粒演变的机理, 有助于厘清增材制造热物理、化学、冶金过程, 为凝固组织的预测和调控提供理论指导. 此外, 该多尺度数学模型也适用于其他金属材料的激光增材制造过程.Abstract: Laser Additive Manufacturing (LAM) technology is very suitable for the near net forming of complex integral components and the rapid repair of high value-added damaged parts. However, the complex dynamic solidification process in the molten pool of LAM significantly affects the final microstructure of the formed parts, thereby restricting its service performance. A multi-scale mathematical model that integrates a macro heat and mass transfer and a multi-phase fields was established for the direct energy deposition by laser (DED-L) process of Inconel 718. The direct coupling of the macro-micro temperature field of the molten pool is solved. The two-dimensional global quantitative microstructure simulation of the molten pool is realized based on MPI parallel program design. The grain evolution process in the solidification of the molten pool is studied. The results show that the simulated molten pool size and solidification interface morphology are in good agreement with the experimental results. The morphology of solidification interface and the preferred orientation of crystal are important factors affecting the grain evolution. On the cross-section of the molten pool, the smaller the angle between the preferred orientation and the direction of temperature gradient, the more dominant the grain growth, because the solidification process is mainly driven by the direction of temperature gradient. On the longitudinal-section of the molten pool, the grain growth shows the characteristics of bending growth and "upper triangle". The gradual change of temperature gradient leads to the grain bending, and the competition behavior of adjacent grains determines the grain morphology. In this work, the mechanism of grain evolution in metal LAM is elucidated, which helps to clarify the thermophysical, chemical and metallurgical processes of additive manufacturing, and provides theoretical guidance for the prediction and control of microstructure. In addition, the multi-scale mathematical model can also be applied to the LAM process of other metal materials.
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图 2 横截面温度场数据拟合和插值过程示意图
Figure 2. The schematic diagram of temperature field data fitting and interpolation process
图 4 MPI工具的计算效率和结果可靠性分析
Figure 4. The analysis of MPI computational efficiency and result reliability
图 5 实验的熔池形貌与模拟的对比
Figure 5. Comparison of the molten pool morphology between the experiment and the simulation
图 6 激光增材制造Inconel 718横截面凝固组织动态演化过程
Figure 6. Dynamic evolution process of solidification microstructure in cross-section of molten pool for laser additive manufacturing Inconel 718
图 8 激光增材制造Inconel 718纵截面凝固组织动态演化过程
Figure 8. Dynamic evolution process of solidification microstructure in longitudinal-section of molten pool for laser additive manufacturing Inconel 718
表 1 激光直接能量沉积Inconel 718工艺参数
Table 1. The processing parameters for DED-L of Inconel 718
Processing parameters Value laser power, P/W 600 laser scan speed, V/(mm·s−1) 6 laser beam diameter, dL/mm 1 powder feed rate, F/(g·min−1) 9 shielding gas (Ar), sg/(L·min−1) 6 delivering gas (Ar), ng/(L·min−1) 15 defocus distance, n/mm +19 
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Variables Value liquidus temperature[21], $ {T}_{l} $/K 1609 solidus temperature[21], $ {T}_{m} $/K 1533 dendsity, $ \rho $/(kg·m−3) 8190 partition coefficient[33], $ k $ 0.48 alloy composition, $ {c}_{\infty } $/wt% 5.08 Gibbs−Thomson coefficient[33], $ \varGamma $/(K·m) 3.65 × 10−7 anisotropy[33], $ \varepsilon $ 0.02 liquidus slope[33], $ m $/(K·wt%) −10.5 thermal conductivity of solid, $ {k}_{s} $/(J·m−1·s−1·K−1) 11.4 thermal conductivity of liquid, $ {k}_{l} $/(J·m−1·s−1·K−1) 28.3 specific heat, $ {c}_{p} $/(J·kg−1·K−1) 435/720 latent heat[33], $ L $/(J·kg−1) 2.95 × 105 chemical capillary length[33], $ {d}_{0} $/m 6.4 × 10−9 liquid diffusion coefficient[33], $ {D}_{L} $/(m2·s−1) 3 × 10−9 laser absorption rate[21], $ {A}_{\alpha } $ 0.26
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