| In recent years,spherical refractory powder used in additive manufacturing,e.g.,tungsten,tantalum,molybdenum,alumina,silicon oxide,zirconia,has attracted increasing interest.As the spherical refractory powder has high strength,wear resistance,high-temperature resistance,oxidation resistance,and other advantages,it has been widely used in aerospace,biomedical and other fields.However,the existing powder production technology is difficult to prepare spherical refractory powder.As the triple-cathode plasma torch has the advantages of high efficiency and effectiveness for the spheroidization of powders with a wide particle size distribution,it will be an effective tool for the spheroidization of refractory powder.However,most of the existing triple-cathode plasma torches were designed for the preparation of coatings.No attention has been paid to the use of triple-cathode plasma torches for the preparation of spherical refractory powder.Besides,the interaction process of powder-jet is different from that of single-beam jet,which leads to the different spheroidization simulation.Therefore,there is a lack of experiments of a triple-cathode plasma torch for the preparation of spherical refractory powder and related simulation research.To overcome the above shortcomings,the simulation and the experiments of spheroidization of refractory powder based on a triple-cathode plasma torch were studied.The main research contents and conclusions were as follows:(1)The simulation model of the triple-cathode plasma jet and the calculation model of spheroidization rate were established.The effects of gas flow rate and arc current on the temperature and velocity of plasma jet were studied by multi-physics coupling analysis.Then,the process of heat and mass transfer was studied by the thermodynamic model of the interaction between the powder and the plasma jet.The results exhibit the following characteristics: 1)the temperature of the plasma jet increases with the increasing arc current and gas flow rate.The velocity of the plasma jet increases with the increasing arc current and gas flow rate.2)The spheroidization rate increases with the increasing arc current and decreases with the increasing gas flow rate.(2)A model for the steady and equilibrium state of the triple-cathode plasma torch was established,and the influence of equilibrium state on the spheroidization rate was studied.According to the arc voltage of the triple-cathode plasma torch,the stability of the plasma jet was studied and the equilibrium index of the triple-cathode plasma torch was defined.Besides,numerical simulation was used to study the effect of working parameters on the equilibrium state of the triple-cathode plasma jet.The results exhibit the following characteristics: 1)The triple-cathode plasma torch can easily be maintained at a steady-state and arc voltage fluctuation within 10%.2)The equilibrium index is an effective index to judge the equilibrium state of the plasma torches.Experiments showed that the triple-cathode plasma torch was easy to be maintained at the equilibrium state with the equilibrium index of the three plasma jets within 3.5 V.3)The numerical simulation results show that the equilibrium index is negatively correlated with the spheroidization rate under the same input power.(3)The experiments of spheroidization of refractory powder were studied.According to the plasma spheroidization model,the reasonable working parameters for the spheroidization of refractory powder were selected,and the effects of the powder size,the arc current and the gas flow rate on the spheroidization rate of plasma were studied.The results exhibit the following characteristics: 1)The triple-cathode plasma torch can effectively spheroidize refractory powder.2)The spheroidization rate of plasma spheroidization decreases with the increasing original particle size.The spheroidization rate of plasma spheroidization increases with the increasing arc current and gas flow rate. |
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