| Electrode Induction Melting Gas Atomization(EIGA)technology is an emerging gas atomization technology.The EIGA process completely avoids contact with refractories during smelting and atomization,eliminating the pollution sources of inclusions,and is often used to prepare ultra-clean metal powders which tend to react with crucible materials.After more than 30 years of rapid development,EIGA has become the main method for preparing alloy powders for turbine disks of aerospace engines.In this paper,a systematic study was carried out to improve the fine powder yield and powder fluidity of the EIGA process.The relevant parameters affecting the fine powder yield and powder fluidity were studied in depth by combining numerical simulation with industrial tests,providing a theoretical basis and reference method for improving the production efficiency and technical improvement of EIGA technology.The actual production showed that the continuous melting of superalloy in the EIGA process would achieve higher fine powder yields,but the mechanism was not clear.Therefore,this paper studied the influence mechanism of the continuity of alloy melt flow on the EIGA fine powder yields.The numerical simulation software FLUENT was used to simulate the continuous or non-continuous atomization of the metal melt during the EIGA process.The results showed that the droplet temperature of the continuous melt flow after the impact of the recirculation zone was above 1900 K,which was higher than the melting point,the primary atomization crushing effect was good,and the powder D50 after secondary atomization was 70 μm.The temperature of partial droplets dropped below 1550 K when the primary atomization of the non-continuous melt flow was done,which was lower than the melting point,the primary and secondary atomization crushing effect was poor,resulting in the generation of large-size particles,and the powder D50 was 100 μm.The results of the EIGA industrial experiment showed that the powder D50 obtained by continuous melt flow atomization was 67.6 μm,while the powder D50 of non-continuous melt flow was 97.6 μm,which was consistent with the numerical simulation results.The necessity of realizing continuous melt flow in the EIGA process was verified,providing a theoretical and experimental basis for the follow-up research.The fine powder yield of the EIGA process is lower than that of the vacuum induction melting gas atomization(VIGA)process.To make up for this deficiency,the effects of different atomization parameters(atomization pressure,nozzle angle,nozzle diameter)on fine powder yield were studied.Numerical simulations and industrial experiments were conducted with four different atomization pressures of 2,3,4,and 5 MPa,four different nozzle angles of 30°,35°,40°,and 45°,five different nozzle diameters of 20,25,30,35 and 40 mm.The results of different atomization pressures showed that with an increase in atomization pressure,the strength of the gas jet and its intersection recirculation zone increased,and the primary atomization and secondary atomization crushing efficiency of the metal melt increased.When the atomization pressure was from 2 MPa to 4 MPa,the D50 powder of EIGA experiments was 95 μm,70 μm,and 60 μm,respectively,and when the atomization pressure increased to 5 MPa,the reverse spray phenomenon occurred.It was found that an increase in atomization pressure can improve the fine powder yield in the EIGA process,but high-pressure atomization might affect production efficiency.The research results of different nozzle angles showed that when the nozzle angle was too large,the position and strength of the intersection recirculation zone were too high,and the melt droplets appeared obvious reverse spray phenomenon.When the nozzle angle was too small,the position of the intersection recirculation zone was lower than the nozzle position,the superheat of the metal melt in the primary atomization stage was insufficient,and the crushing was not complete,which decreased the fine powder yield.For the free-fall nozzle used in this study,the nozzle angle of 35° was found to ensure continuous production and high fine powder yield.The research results of different nozzle diameters showed that the nozzle diameter greatly affects powder production.When the nozzle diameter was too small(<30 mm),the turbulent kinetic energy in the intersection recirculation zone was concentrated,and the primary atomization position was too close to the melt inlet position,which led to the reverse spray phenomenon.With the increase in nozzle diameter,the strength,range,and turbulent kinetic energy of the intersection recirculation zone decreased,the primary atomization fragmentation decreased,and the fine powder yield decreased.For the free-fall nozzle used in this study,the fine powder yield was the highest when the nozzle diameter was 30 mm,and the powder D50 was 50 μm.Powder testing revealed the presence of satellites in the superalloy powders prepared by EIGA,and the powder fluidity still needed to be improved.Therefore,this paper studied the influence of atomization process parameters on powder fluidity,mainly exploring the formation mechanism of satellites,the relationship between powder fluidity and atomization pressure,and the application of rectification measures to suppress satellites.A numerical simulation of particle trajectory verified the formation of satellites,and the simulation results were consistent with the experimental photographs.Moreover,the decreasing trend of particle temperature in the numerical simulation was verified by the particle cooling equation,and the accuracy of the simulation work was confirmed.Numerical simulation and several EIGA experiments with different argon pressures(2.5-4.0 MPa)were designed to observe the powder morphology and properties.With the atomization pressure increased,the centric position gradually approached the primary atomization area,and the recirculation velocities increased from 11.7 to 15 m/s,which provided a theoretical basis for the increase in satellite content in highpressure atomization.The powder detection under different pressures proved the decrease in powder fluidity prepared under high pressure.The Carr compressibility and Hausner’s ratio increased from 11.1%to 17.7%and from 1.13 to 1.22,respectively,and the Hall flow rate increased from 13.3 to 16.8 s/(50 g),which indicated that the powder fluidity decreased with the pressure increased.The numerical simulation showed that the recirculation zone of the original argon flow field could be effectively suppressed by auxiliary argon flow,but considering the economic cost,the flow of the auxiliary flow needed to be controlled.A modified Laval nozzle was designed to achieve a balance between fine powder yield and powder fluidity in the preparation of EIGA superalloy powder.Numerical simulations showed that the double arc edge design of the modified nozzle weakened the impact force of the argon on the nozzle wall and allowed the gas to expand ahead of the nozzle outlet.The shock wave number of the modified nozzle at the outlet was lower than that of the original nozzle,resulting in a reduced gas energy loss rate.Seven groups of optimized nozzles with a throat diameter of 0.5mm,0.32mm,and 0.19mm,and nozzle angles of 35°,40°,45°,50°,and 55°were simulated,and it was found that the gas jet strength of the modified nozzle with a throat diameter of 0.19 mm at an angle of 50° exceeded that of the original nozzle,and the strength of the device recirculation zone was weak,which inhibited the formation of satellites.Comparisons of EIGA industrial experiments showed that the powder D50 prepared by the original nozzle and the modified nozzle was 67.6 μm and 48.9 μm,respectively.The fine powder yield(<50 μm)was 35.3%and 51.1%,respectively.The powder fluidity index was 76.5 and 92,respectively.The modified nozzle improved the low yield of fine powder in the EIGA process and ensured the high fluidity of the EIGA powder,thus achieving a balance between the fine powder yield and the powder fluidity and meeting the needs of preparing high-quality superalloy powder.In this paper,the necessity of continuous melt flow in EIGA technology for atomization production was studied through numerical simulation and industrial tests.The effects of different atomization parameters on fine powder yield and powder fluidity were investigated.A modified Laval nozzle was designed to enhance the fine powder yield and powder fluidity of the EIGA process,which provided technical guidance and experience for the improvement of the EIGA technology. |
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