Impact of gas pressure on particle feature in Fe-based amorphous alloy powders via gas atomization: Simulation and experiment

doi:10.1016/j.jmst.2021.06.075

Abstract

Abstract:

Gas atomization is now an important production technique for Fe-based amorphous alloy powders used in additive manufacturing, particularly selective laser melting, fabricating large-sized Fe-based bulk metallic glasses. Using the realizable k-ε model and discrete phase model theory, the flow dynamics of the gas phase and gas-melt two-phase flow fields in the close-wake condition were investigated to establish the correlation between high gas pressure and powder particle characteristics. The locations of the recirculation zones and the shapes of Mach disks were analyzed in detail for the type of discrete-jet closed-coupled gas atomization nozzle. In the gas-phase flow field, the vortexes, closed to the Mach disk, are found to be a new deceleration method. In the two-phase flow field, the shape of Mach disk changes from “S”-shape to “Z”-shape under the impact of the droplet flow. As predicted by the wave model, with the elevation of gas pressure, the size of the particle is found to gradually decrease and its distribution becomes more concentrated. Simulation results were compliant with the Fe-based amorphous alloy powder preparation tests. This study deepens the understanding of the gas pressure impacting particle features via gas atomization, and contributes to technological applications.

Key words: Gas atomization, Fe-based amorphous powder, Closed-wake, Gas-melt flow, Break-up, Particle size distribution

Yutong Shi, Weiyan Lu, Wenhai Sun, Suode Zhang, Baijun Yang, Jianqiang Wang. Impact of gas pressure on particle feature in Fe-based amorphous alloy powders via gas atomization: Simulation and experiment[J]. J. Mater. Sci. Technol., 2022, 105: 203-213.

Figures/Tables 16

Fig. 1. (a) Schematic drawing of the atomization process, (b) schematic diagram of the atomizer and the computational field (shaded dotted area).

Table 1. The main thermodynamic parameters of Fe-based glassy alloy.

Density (kg/m³)	Specific heat (J/(K mol)) [25]	Viscosity (Pa s) [26]	Surface tension (N/m) [27]
7820	44.09	5.8 × 10^-5	0.8

Fig. 2. CFD model of the high-pressure gas atomization nozzle at gas pressures ranging from 5 to 8 MPa in the closed-wake condition. The velocity magnitude increases with decreasing shades of darkness in the CFD figures. (a) The primary and the second recirculation zone at 5 MPa; (b) the primary and the enlarged second recirculation zone at 6 MPa; (c) the primary, the second and third recirculation zone at 7 MPa; (d) the primary, the second the enlarged third recirculation zone at 8 MPa.

Fig. 3. The velocity profiles along the geometric centerline of the gas atomization nozzle at gas atomization pressures ranging from 5 to 8 MPa. The blue arrow points to the stagnation front in the primary recirculation zone, the black arrow points to the Mach disk, the red arrows point to the stagnation points in the second recirculation zone, and the green arrows point to the stagnation points in the third recirculation zone.

Fig. 4. The static pressure profiles along the geometric centerline of the gas atomization nozzle at gas atomization pressures ranging from 5 to 8 MPa. The blue arrow points to the stagnation front in the primary recirculation zone, the black arrow points to the Mach disk, the red arrows point to the stagnation points in the second recirculation zone, and the green arrows point to the stagnation points in the third recirculation zone.

Fig. 5. Temporal evolution of the gas-melt flow at 8 MPa gas pressure. (a) A little melt at the orifice; (b) a thin circular sheet at the bottom of the melt delivery tube; (c) the initial break-up of the melt sheet at the edge of the tube; (d) the flowing liquid droplets to the centerline; (e) collision at the centerline; (f) flowing downstream along the centerline; (g) fine droplets after the second recirculation zone.

Fig. 6. CFD model of the gas atomization nozzle at gas atomization pressures ranging from 5 to 8 MPa in the closed-wake condition with multiple phase flows (gas and melt). The velocity magnitude increases - shades of darkness decreases. (a) “S”-shaped Mach disk; (b) “Z”-shaped Mach disk; (c) “Z”-shaped Mach disk; (d) vortex off the central line.

Fig. 7. The velocity profiles along the geometric centerline of the gas atomization nozzle respectively at gas atomization pressures ranging from 5 to 8 MPa with multiple phase flows (gas and melt). The blue arrow points to the stagnation front in the primary recirculation zone, the black arrow points to the mach disk, and the red arrows point to the stagnation points in the second recirculation zone.

Fig. 8. The static pressure profiles along the geometric centerline of the gas atomization nozzle, respectively at gas atomization pressures ranging from 5 to 8 MPa with multiple phase flows (gas and melt). The blue arrow points to the stagnation front in the primary recirculation zone, the black arrow points to the Mach disk, and the red arrows point to the stagnation points in the second recirculation zone.

Fig. 9. The particle diameter (a) and the particle velocity (b) in the gas atomization process simulated via the WAVE model at gas atomization pressure of 8 MPa. Region A: the forming and accelerating droplets. Region B: the converging of droplets in the centerline with lower velocities. Region C: the second break-up process.

Fig. 10. The frequency density distribution obtained via the atomization simulation and actual gas atomization technology: (a) 5, (b) 6, (c) 7, (d) 8 MPa.

Table 2. Comparison and relative errors of the mean values (μG) and standard deviations (σG) of experimental and simulated frequency density distribution.

Pressure (MPa)		Experiment (μm)	Simulation (μm)	Error (%)
5	μ_G	78.47 ± 1.02	76.52 ± 1.22	2.49
5	σ_G	60.56 ± 2.02	64.80 ± 2.55	7.03
6	μ_G	60.30 ± 0.58	62.14 ± 0.51	3.05
6	σ_G	41.10 ± 1.15	44.21 ± 1.26	7.57
7	μ_G	52.86 ± 0.48	56.02 ± 1.11	5.79
7	σ_G	33.07 ± 0.94	34.41 ± 2.55	4.05
8	μ_G	39.91 ± 0.28	36.79 ± 0.45	7.82
8	σ_G	25.23 ± 0.55	28.48 ± 0.99	9.23

Fig. 11. SEM images of Fe-based amorphous powders ranging from 25 to 45 μm. (a) 5; (b) 6; (c) 7; (d) 8 MPa.

Fig. 12. XRD patterns of the Fe-based amorphous powders ranging from 25 to 45 μm at gas atomization pressures ranging from 5 to 8 MPa.

Fig. 13. Partial velocity vector and the mirroring schematics of the gas flow structures in the second recirculation zone at 5 MPa. Left-vortex downstream; right-vortex upstream.

Fig. 14. (a) The droplet break-up process. (b) The injection of melt into the chamber; (c) the deformed melt under the upstream gas; (d) a thin sheet of the melt; (e) forming the large droplets; (f) the collision at the centerline; (g) the second break-up.

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