Study On The Preparation Process And High Frequency Application Of High-Performance Fe-Based Amorphous Magnetic Powder By Gas-Water Combined Atomization

With the continuous development of the electronic and power industry,the trend of miniaturization,integration,and high efficiency has put forward higher requirements for the soft magnetic properties of materials.Conventional soft magnetic materials,such as silicon steel and ferrite,cannot combine low core loss under high frequency and high saturation magnetic induction.Amorphous soft magnetic materials,as a newly emerging metal material in recent years,have become an ideal choice for preparing a new generation of electronic components due to their excellent comprehensive soft magnetic properties such as high resistivity,low coercivity and high saturation magnetic induction.In recent years,the powder metallurgy industry has developed rapidly,and the yield of metal powders has increased year by year.The atomization method has the advantages of low cost,adjustable alloy composition,fast cooling rate,large production scale and excellent overall performance of as-produced powders,which is one of the ideal processes for producing fine amorphous powders.However,China’s current atomization process is not yet mature,and high-end powder products such as fine amorphous magnetic powders still rely on imports,leaving a gap with Japan and South Korea.Therefore,the research of efficient preparation techniques for amorphous soft magnetic powder is of great significance for overcoming the difficulties in the preparation of high-performance amorphous soft magnetic powder cores and improving the international competitiveness of key basic electronic components in China.In view of this,this paper mainly adopts theoretical calculations,numerical simulations and relevant industrial experiments to investigate the key issues in the preparation and application of high-performance Fe-based amorphous magnetic powder by gas-water combined atomization,including the thermodynamics and kinetics of crystallization reactions,for amorphous system,the regulation of atomization production compliance and efficiency,the breakup mechanism of primary and secondary atomization,the cooling of particles and the improvement of properties for powder materials,which is expected to provide a theoretical and practical basis for large-scale production of fine amorphous soft magnetic powder and the conquest of high-performance magnetic powder core technology in China.Firstly,a pre-experimental platform of the gas-water combined atomization was set up,i.e.a rotating water stream droven by a stirring paddle was used to cool the metal droplets after gas-atomization in order to provide adequate thermodynamic conditions for particle cooling.Then,completely amorphous FeSiBCCr fine amorphous powders were screened from the raw powders produced under different working conditions,and the thermodynamics/kinetics behaviours of the crystallization process of the amorphous powders were investigated based on differential scanning calorimetry and X-ray diffraction.The results showed that the high Si content in the amorphous system and the eutectic reaction at the initial stage of crystallization leads to the transition between Fe-Si phase A2(α-Fe(Si)),B2(FeSi),and D03(Fe3Si)with ordered structures,results in three crystallization exothermic peak on DSC curves.The crystallization process of this amorphous material is as follows:(a)Amorphous;(b)Amorphous+Fe3B+a-Fe(Si)+Fe3Si+Fe5Si3+FeSi;(c)Amorphous+Fe3B+Fe2B+α-Fe(Si)+Fe3Si+Fe5Si3+FeSi;(d)Fe2B+α-Fe(Si)+Fe3Si+Fe5Si3+FeSi;(e)Fe2B+α-Fe(Si)+Fe3Si+Fe5Si3.The undercooled liquid region of amorphous powder is 52 K,the activation energy of glass transition is 396.73 kJ·mol-1,the activation energy of the first crystallization peak is 292.64 kJ·mol-1,and the brittleness value is 27,which exhibits excellent amorphous forming ability and thermal stability and is suitable for the main component of the gas-water combined atomization process in this study..Then,the influence of atomization parameters on the atomization capacity of Laval nozzles was investigated based on the computational fluid dynamics method,and the effect of the assisted gas nozzles on the gas-water combined atomization was discussed.In order to analyse the correlation between droplet and particle shape in the atomization process,the VOF(Volume of fluid)model and dynamic adaptive mesh were used to simulate the primary atomization process.Based on the results of the primary atomization,the DPM(Discrete phase model)was then used to calculate the secondary atomization process,thus realizing the coupled simulation of the whole atomization process.The results showed that the gas velocity can be effectively increased by increasing the atomization pressure,but suction pressure at the outlet of the delivery tube will also be increased,and the optimum atomization pressure is 2.0 MPa.Excessively low atomization pressure below 1.0 MPa leads to backflow behavior of droplets,while excessively high atomization pressure above 3.0 MPa leads to the generation of defective powder.For the atomization process analyzed in this study,it is found that the gas to melt ratio(GMR)≥4.4 leads to the liquid film breakup that is prone to steel plugging,while the GMR ≤4.3 cause"fountain" breakup,indicating that GMR can be used as a simple standard for predicting the primary atomization breakup mode in industrial production.The calculation results of secondary atomization show that the d50 decreases with the increase of GMR;the standard deviation of particle size distribution—d84/d50 decreases with the decrease of the melt mass flow rate and reaches the minimum value at the atomization pressure of 2.0 MPa;the average cooling rate of particles increases with the increase of atomization pressure and the decrease of melt mass flow rate.In addition,after the application of the assitsted gas nozzles with the inlet velocity of 200 m·s-1,the droplet velocity in the breakup and solidification regions can be increased by 40 m·s-1 under the acceleration of auxiliary airflow,and the maximum cooling rate can be increased by more than 20%,which is conducive to improving the yield of fine amorphous powder.Gas heating technology can effectively improve the particle cooling rate and refine the powder particle size.However,the gas temperature must be matched with the atomization process,and the powder circularity will deteriorate under improper gas temperature.Based on the above pre-experiment results and simulation analysis,an industrial gas-water combined atomization platform was built to determine the influence of different process parameters on particle size,circularity and amorphous fraction by studying the regulation mechanism of each process parameter on atomization efficiency,and then establish the correlation between each process parameter and soft magnetic properties of powder.The research showed that the FeSiBCCr spherical amorphous powders with d50=25～30 μm can be produced under the optimized process with the delivery tube of 2 mm,melting temperature of 1673 K,congealer inclination of 5°,atomization pressure of 2 MPa,and assisted gas nozzles of 10°.The circularity,saturation magnetization strength and coercivity of the powders are on the same level with foreign powders of the same composition,indicating that the gas-water combined atomization developed in this study has successfully realized the large-scale preparation of amorphous fine magnetic powder and broken through the foreign technological monopoly.Finally,the FeSiBCCr amorphous powder prepared by the gas-water combined atomization was used as raw material,and the optimization of the preparation process of FeSiBCCr amorphous magnetic powder cores,the preparation of FeSiBCCr/carbonyl iron powder hybrid magnetic powder cores and the preparation of bulk amorphous/nanocrystalline by spark plasma sintering(SPS)were systematically investigated.The results indicated that although the DC bias performance of the magnetic powder cores can be increased by excess phosphoric acid,the permeability,density and core loss deteriorate;the density of the magnetic powder cores can be improved by increasing the compaction pressure,but the permeability and core loss characteristics deteriorate under too high pressure;the permeability and core loss can be significantly improved by increasing the annealing temperature below the crystallization temperature due to the reduction of internal defects,although the DC bias characteristics are reduced.Meanwhile,the proper addition of carbonyl iron powder can fill the air gaps between the amorphous powder and improve the compressibility,thus significantly improving the overall performance of the magnetic powder cores;while the excessive addition of carbonyl iron powder will introduce a large number of micro air gaps and crystal defects,which will deteriorate the performance of the magnetic powder cores.In addition,bulk amorphous/nanocrystalline materials can be successfully prepared by SPS technique with sintering above 773 K.The densitiy of compacts increases with increasing temperature and the amorphous fraction of compacts decreases with increasing SPS temperature.