Satellite-particle control technique based on gas-flow-regulation during gas atomization process
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摘要: 金属熔体气体雾化法是制备增材制造专用金属粉末的重要方法。然而，气体雾化工艺制得的粉末中通常混有大量卫星粉，对金属增材制造工艺产生不利影响。本文通过施加辅助气流并采用阶梯状雾化室结构等气体整流措施抑制回流区中的粉尘回旋，进而控制卫星粉的形成。利用计算流体力学软件ANSYS Fluent进行数值模拟，研究施加辅助气流或采用阶梯状雾化室结构时，雾化室内宏观流场特征以及颗粒运动轨迹的变化规律。结果表明，在雾化室顶部距雾化室中心R/2（R为雾化室半径）处施加辅雾比（辅助气流与雾化气流的流量比）大于0.8的辅助气流时能够有效抑制回流区中的粉尘回旋；采用阶梯宽为300 mm、高为575～600 mm的雾化室结构能够有效抑制回流区中的粉尘回旋。根据数值模拟结果，采用气体整流措施制备TC4钛合金粉末，并检测粉末的粒径分布、球形度、赘生物指数等指标，发现与不采用气体整流措施制备的粉末相比，赘生物指数降低约45%。Abstract: Gas atomization (GA) is an essential method for the preparation of metal powders specifically used in the additive manufacturing. However, there are plenty of “satellites” among the metal powders prepared by GA. The existence of these satellite particles negatively affects the metal additive manufacturing process. In this paper, two gas-flow-regulation strategies, i.e., the introduction of ancillary gas flow and the design of step-shape atomization chamber, were employed to prevent the fine particle entrainment in gas recirculation zone, thus restricting the formation of satellite particles. The evolution of macro flow patterns and particle trajectories were studied based on the numerical simulation by the commercial computational fluid dynamics software ANSYS Fluent. The results show that, the fine particle entrainment in gas recirculation zone can be effectively prevented when the ancillary gas flow is introduced at the position of R/2 (R is the radius of atomization chamber) away from the chamber center with the AAR (the ratio of ancillary flow rate to atomization flow rate) larger than 0.8, or when a step-shaped atomization chamber is employed with a step width of 300 mm and a step height of around 575～600 mm, respectively. Some TC4 titanium alloy powders were prepared by applying the above gas-flow-regulation strategies while their size distribution, sphericity and outgrowth rate were characterized, respectively. The results show that, the outgrowth rate of the powders has been reduced by about 45%, compared with those prepared without the gas-flow-regulation strategies.
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图 1 雾化气体喷嘴模型：（a）压力入口；（b）速度入口
Figure 1. Atomization gas nozzle model: (a) pressure inlet; (b) velocity inlet
图 3 雾化室网格划分：（a）全局；（b）局部
Figure 3. Mesh generation of the atomization chamber: (a) global; (b) local
图 4 湍流模型适用性对比：（a）Standard k‒ε模型；（b）SST k‒ω模型
Figure 4. Comparison of the two turbulence models: (a) Standard k‒ε; (b) SST k‒ω
图 5 宏观流场结构随雾化压力变化情况：（a）1.0 MPa；（b）2.0 MPa；（c）3.0 MPa；（d）4.0 MPa；（e）5.0 MPa
Figure 5. Variation of the macro flow pattern with the atomization pressure: (a) 1.0 MPa; (b) 2.0 MPa; (c) 3.0 MPa; (d) 4.0 MPa; (e) 5.0 MPa
图 6 过回流区中心水平方向的轴向速度分布
Figure 6. Distribution of the axial velocity in the horizontal direction through the center of gas recirculation zone
图 8 在1/4R处施加辅助气流时宏观流场结构随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 8. Variation of the macro flow pattern with AAR at 1/4R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 9 在1/4R处施加辅助气流时颗粒轨迹随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 9. Variation of the particle trajectory with AAR at 1/4R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 10 在3/8R处施加辅助气流时宏观流场结构随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 10. Variation of the macro flow pattern with AAR at 3/8R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 11 在3/8R处施加辅助气流时颗粒轨迹随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 11. Variation of the particle trajectory with AAR at 3/8R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 12 在1/2R处施加辅助气流时宏观流场结构随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 12. Variation of the macro flow pattern with AAR at 1/2R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 13 在1/2R处施加辅助气流时颗粒轨迹随辅雾比变化情况：（a）0.6；（b）0.8；（c）1.0；（d）1.2；（e）1.4
Figure 13. Variation of the particle trajectory with AAR at 1/2R: (a) 0.6; (b) 0.8; (c) 1.0; (d) 1.2; (e) 1.4
图 14 不同位置施加辅助气流时颗粒的运动轨迹：（a）无辅助气流；（b）1/4R；（c）3/8R；（d）1/2R
Figure 14. Variation of the particle trajectory with injection position of ancillary gas flow: (a) no ancillary gas flow; (b) 1/4R; (c) 3/8R; (d) 1/2R
图 15 辅助气流参数对颗粒碰撞区轴向位置的影响
Figure 15. Influence of the ancillary gas flow parameter on the axial position in the particle collision region
图 17 阶梯状雾化室结构参数设置：（a）D=400 mm；（b）D=300 mm；（c）D=200 mm
Figure 17. Structural parameter setting of the step-shaped atomization chamber: (a) D=400 mm; (b) D=300 mm; (c) D=200 mm
图 18 阶梯宽400 mm时宏观流场结构随阶梯高变化情况：（a）ΔH=500 mm；（b）ΔH=600 mm；（c）ΔH=700 mm；（d）ΔH=800 mm
Figure 18. Variation of the macro flow pattern with the step height at the step width of 400 mm: (a) ΔH=500 mm; (b) ΔH=600 mm; (c) ΔH=700 mm; (d) ΔH=800 mm
图 19 阶梯宽300 mm时宏观流场结构随阶梯高变化情况：（a）ΔH=500 mm；（b）ΔH=600 mm；（c）ΔH=700 mm；（d）ΔH=800 mm
Figure 19. Variation of the macro flow pattern with the step height at the step width of 300 mm: (a) ΔH=500 mm; (b) ΔH=600 mm; (c) ΔH=700 mm; (d) ΔH=800 mm
图 20 阶梯宽200 mm时宏观流场结构随阶梯高变化情况：（a）ΔH=500 mm；（b）ΔH=600 mm；（c）ΔH=700 mm；（d）ΔH=800 mm
Figure 20. Variation of macro flow pattern with the step height at the step width of 200 mm: (a) ΔH=500 mm; (b) ΔH=600 mm; (c) ΔH=700 mm; (d) ΔH=800 mm
图 21 有效抑制粉尘回旋的阶梯尺寸：（a）D=400 mm, ΔH=725～750 mm；（b）D=300 mm, ΔH=575～600 mm；（c）D=200 mm, ΔH=500～525 mm
Figure 21. Step dimensions that effectively restrict the fine particle entrainment: (a) D=400 mm, ΔH=725～750 mm; (b) D=300 mm, ΔH=575～600 mm; (c) D=200 mm, ΔH=500～525 mm
图 22 粉末样品粒度分布：（a）未施加气体整流（TC4-1）；（b）施加气体整流（TC4-2）
Figure 22. Particle size distribution of the powder samples: (a) without gas-flow-regulation (TC4-1), (b) with gas-flow-regulation (TC4-2)
图 23 不同视场下粉末样品的表面形貌及相应的图像处理结果：（a）、（d）100 ×；（b）、（e） 200 ×；（c）、（f） 500 ×
Figure 23. Surface morphology and the corresponding image processing results of the powder samples under the different magnifications: (a), (d) 100 ×; (b), (e) 200 ×; (c), (f) 500 ×
图 24 不同视场下粉末样品TC4-1的电镜图片：（a）200 ×；（b）200 ×；（c）400 ×；（d）400 ×
Figure 24. SEM images of TC4-1 under the different magnifications: (a) 200 ×; (b) 200 ×; (c) 400 ×; (d) 400 ×
图 25 不同视场下粉末样品TC4-2的电镜图片：（a）200 ×；（b）200 ×；（c）400 ×；（d）400 ×
Figure 25. SEM images of TC4-2 under the different magnifications: (a) 200 ×; (b) 200 ×; (c) 400 ×; (d) 400 ×
表 1 环缝辅助气流流量
Table 1. Flow rate of annular ancillary gas flow
雾化气流流量 / (kg·s‒1) 辅助气流流量 / (kg·s‒1) 辅雾比 0.40 0.24 0.6 0.32 0.8 0.40 1.0 0.48 1.2 0.56 1.4 
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表 2 优化的阶梯尺寸与阶梯角的关系
Table 2. Relationship between the optimized step size and the step angle
阶梯宽，D / mm 阶梯高，ΔH / mm 阶梯角，θ 400 725~750 30°50′~29°50′ 300 575~600 29°14′~28°04′ 200 500~525 22°36′~21°30′
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表 3 两组粉末样品的形貌特征
Table 3. Morphological characteristics of the powder samples
粉末样品 球形度 赘生物指数 TC4-1 0.9278±0.0311 0.489±0.062 TC4-2 0.9427±0.0165 0.270±0.027
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