Study On The Process And Numerical Simulation Of Spherical Powders Prepared By Radio Frequency Plasma

Radio-frequency (RF) plasma has the advantages of high temperature (up to10000K), large-volume plasma torch, high energy density, fast heat transfer and cooling speed, no electrodes pollution and controlled atmosphere, which has broad application prospects in the field of materials processing area. In this paper, the spheroidization of refractory metals and ceramic powders were studied by using the RF plasma processing system with vacuum system. At the same time, the new short preparaing processes of spherical TiC/Fe metal ceramic composite particle, fine titanium powder and mangano-manganic oxide powder were developed. In addition, numerical simulation of flow field, temperature field and velocity field of the plasma torch are completed by using finite element analysis software. The influence of operating parameters on the plasma torch was further analyzed. The particle size distribution and collecting rate of powders were studied through researching the trajectory of particles.Aiming at solving the oxidation of metal powders, the design and transformation of vacuum system which based on the existing100KW plasma powder processing system were carried out in this paper. The limiting vacuum of plasma system reaches1.0×10-3Pa after rebuilding. Through adjusting the process parameters of plasma operation, this system can continuous operating near30h in the case of the operation power of50KW, center gas flow rate of28L/min, sheath gas flow rate of85L/min and the system reactor pressure of85KPa.The experimental parameters of RF plasma treatment were optimized for obtaining spherical refractory metal powders (Nb, Ta) and ceramic powders (SiO2, Al2O3). The results show that the as-prepared spherical powder have good dispersity and smooth surfaces, and spheroidization ratio is almost100%. The loose density, tap density and powder flowability significantly increases after the treatment. The loose density increases from1.33g/cm3to4.35g/cm3and tap density increases from1.95g/cm3to5.61g/cm3. The powder flowbility increases to12.51s/(50g). With increasing the feeding rate, the spheroidization ratio of niobium powders drops gradually. The raw powders with smaller grain size are more likely to get higher spheroidization efficiency under the same technological conditions. In addition, the carrier gas flow rate and plasma operation power also have important influence on spheroidization efficiency of powders. The plasma prepared powders show better performance than raw powders. Compared with ordinary silica powder, the spherical silica powder has a higher filling level. Under otherwise equal conditions, epoxy molding compound prepared by spherical amorphous SiO2powder shows lowviscosity and good liquidity. The thermal expansion coefficient of spherical silica/epoxy resin composite is10.1×10-6/℃with the content of75%spherical silica. Under the same plasma spraying process conditions, the performance of alumina coating made by self-made spherical Al2O3powder is better than those made by the angular Al2O3and granulated Al2O3powders. The coating features are high density, less defect and high hardness of HV0.3=957.6.Spherical iron matrix composite powder with high volume fraction (82vol.%) of fine TiC reinforcement was produced by a novel process, which combines in situ and plasma techniques. The composite powder has good sphericity and dense structure, and the fine sub-micron TiC particles exhibit homogeneous distribution in the α-Fe matrix. Using the as-prepared spherical composite powder as raw material, the TiC/Fe cermet prepared by powder metallurgy technique at low sintering temperature shows a hardness of HRA88.5and flexural strength of1360MPa.Using large size irregular shape titanium hydride powder as raw material, the short process preparation of fine spherical powder with low oxygen content was realized by combined the RF plasma spheroidization technology with hydrogen decrepitation technique. The average particle size of TiH2powder reduced from180.17μm to21.28μm after plasma and subsequent dehydrogenation treatment. The prepared spherical titanium powder has high sphericity, good fluidity and low oxygen content of0.21wt.%.Using the large size of the MnCO3as raw materials, fine spherical Mn3O4powder was successfully prepared by combined the radio frequency plasma spheroidization technology with thermal decomposition technology, which offers a new production technique for the fine Mn3O4powder.A numerical simulation model of plasma spheroidization process was established by using CFD software. The flow field, temperature field and velocity field of plasma torch were successfully calculated. The centeral temperature of plasma torch is up to10100K. The working gas flow and gas flow rate have significant effect on the temperature distribution of plasma torch, which agree with the experimental results. The simulation results of particle size distribution of spheroidizing powder agree with the results of experiment, so the preparation of specific size spherical powder could estimate the necessary size range of raw powder by numerical simulation, which would greatly reduce the workload. Through the discussion of powder collecting rate, a new scheme is developed to improve powder collecting rate, which provids feasible guidance for the improvement of the equipment.