| The Ni-based superalloy powder is the main material for the turbine disc of military and civil aviation engines.Due to the advantages of vacuum induction melting gas atomization(Vacuum Induction Melting Gas Atomization,VIGA)in the preparation of Ni-based superalloy powder,such as good controllability,large yield,high sphericity and fine powder,the Ni-based superalloy powder used for engine turbine discs is mainly prepared by VIGA technology.The core technical component of VIGA process is the close-coupled nozzle.With the continuous improvement of the requirements for the powder yield,nozzle clogging problems occur frequently in the industrial production process.Once nozzle clogging occurs,the atomization process will be terminated,and the remaining alloy melt will be detained and solidified in the medium frequency furnace and tundish,which will lead to alloy waste,crucible reconstruction and other problems,which will have a greater iMPact on production efficiency and production benefits.At present,the research on VIGA technology is mostly focused on improving atomization efficiency and improving fine powder yield,while the research on nozzle clogging is less.In order to further explore the causes and laws of nozzle clogging in industrial production,under the conditions of VIGA atomizing equipment and nozzle in industrial production,this paper conducts in-depth research on nozzle clogging technology from the theoretical and experimental perspectives through numerical simulation and field industrial test methods.The specific research contents and conclusions are as follows:Due to the instability of gas pressure in industrial atomization pulverizing,nozzle clogging often occurs.In order to improve the nozzle clogging problem,the numerical simulation model of gas atomization of close-coupled nozzle disc was established by using Computational Fluid Dynamics(CFD)numerical simulation software.The influence of gas pressure at the end of the delivery-tube on the primary atomization of alloy melt was studied,and the formation rules of nozzle clogging under different gas pressures were analyzed,which was verified by field industrial test.The results show that when the gas pressure is not matched with the inner diameter of the delivery-tube,the alloy melt droplets after primary atomization will not only block the nozzle but also burn the spray disc.When the inner diameter of the delivery-tube is 4 mm,the probability of nozzle clogging decreases with the increase of gas pressure,but when the gas pressure is too large(4.5 MPa),the reverse injection of alloy melt will occur.When the inner diameter of the delivery-tube is 5 mm,the nozzle clogging phenomenon appears at the low pressure(1.5 MPa),and the atomization process is continuous at the high pressure.Based on the established finite element model,within the reasonable gas pressure range,the effects of the coupling length of the delivery-tuber and the nozzle on the turbulent kinetic energy of the gas-flow field,the negative pressure at the end of the delivery-tube and the primary atomization of the alloy melt during the atomization process were studied respectively,and the collision and bonding process of the alloy melt at the end of the delivery-tube when the coupling length does not match the structure of the nozzle was analyzed.The results show that the negative pressure decreases with the increase of coupling length(from positive pressure to negative pressure).The position of turbulent kinetic energy under the delivery-tube moves downward with the increase of coupling length.When the coupling length is small(0 mm),the reflow of melt droplets is obvious,which increases the probability of nozzle clogging;With the increase of coupling length(more than 3 mm),the alloy melt hanging at the end of the delivery-tube gradually decreases,and the nozzle changes from semi clogging to non clogging.The influence of coupling length on nozzle plugging remains unchanged through numerical simulation calculation with appropriate increase of gas pressure,and the accuracy of numerical model prediction is verified through water simulation test and field industrial test.Aiming at the nozzle clogging problem caused by the fluidity of alloy melt in the atomization process,based on the research results of the nozzle clogging caused by gas pressure and coupling length,the flow rate of alloy melt with different viscosity in the delivery-tube and the primary atomization crushing state at the end of the deli very-tube were simulated and studied,and the reason for the nozzle clogging caused by the alloy melt viscosity was analyzed.The results show that when the viscosity of alloy melt is too high,it will not only hang up at the end of the delivery-tube,but also stick to the inner wall of the delivery-tube to block the nozzle.The flow velocity of alloy melt in the delivery-tube increases with the decrease of melt viscosity.When the flow velocity of alloy melt is large,there is less heat transfer with the inner wall of the delivery-tube,which will reduce the risk of alloy melt sticking to the inner wall of the delivery-tube.With the decrease of alloy melt viscosity,the alloy melt hanging at the end of the delivery-tube disappears first,and the alloy melt bonded to the inner wall of the delivery-tube disappears later.According to the field industrial test results,when the alloy melt viscosity is 5.45～5.24 MPa·s,the alloy melt has good fluidity,can flow out of the delivery-tube smoothly for atomization,and there is no nozzle clogging.Based on the research results of coupling length on nozzle clogging,reducing coupling length appropriately can improve the strength of gas-liquid interaction,but will increase the probability of nozzle clogging.In order to avoid nozzle clogging,the influence of different modification angles of the end of the delivery-tube on the primary atomization of the alloy melt was simulated and analyzed.The results show that the gas velocity near the inner wall of the end of the delivery-tube increases with the increase of the modification angle,and the probability of the alloy melt droplets collision the small platform at the end of the delivery-tube decreases with the increase of the modification angle.Through the water simulation results,it is found that the atomization efficiency of the alloy melt increases with the increase of the modification angle.According to the industrial test and numerical simulation results,when the modification angle is 40～45°,the width of the small platform at the end of the deli very-tube is reduced,and the alloy melt droplets after the primary atomization rarely touch the wall,and the atomization process is continuous.In this paper,the formation mechanism of nozzle clogging in VIGA technology is deeply studied by combining numerical simulation and industrial test.By studying the effects of gas atomization pressure,coupling length of delivery-tube and nozzle disc,alloy melt viscosity and end modification of delivery-tube,certain theoretical and practical guidance is provided for solving nozzle clogging problem in atomization production process. |
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