| In the past three decades,additive manufacturing technology has developed rapidly in aerospace,medical,energy,automotive and other fields,especially providing new ideas for the manufacturing of large size amorphous alloy components.However,as an important raw material,the metal powder characteristics,including particle size distribution,microstructure,and powder shape,etc.,are put forward higher requirements.Therefore,the preparation of metal powders with small particle size,narrow size distribution,high amorphous content and good sphericity has a crucial impact on the structure and properties of large-size amorphous alloy components prepared by additive manufacturing(especially selective laser melting(SLM)).As one of the important methods of preparation of amorphous alloy powder,gas atomization has advantages of low cost,high efficiency and high purity,but also obvious micro powder of low yield,complicated structure,regulation and control of spherical degree,difficult,etc.In particular,the gas atomization process presents significant temperature and velocity gradients,and the intermediate process is often difficult to observe and record in real time.To solve above three problems,this paper chooses has strong glass forming ability of Fe-base amorphous alloy as the base,the finite element numerical simulation,detailed has carried out in the process of gas atomization of single-phase gas-liquid two phase field simulation calculation,through regulating the atomizing gas pressure,reveals the condition of closed vortex order double facies evolution law of pressure in the flow field and velocity field,The characteristics of primary crushing,secondary crushing and cooling solidification of melt during atomization were analyzed.The particle size distribution and shape of Febased amorphous alloy powders were measured and verified by high-speed camera.This work has important theoretical and practical value for designing and preparing high-performance amorphous alloy powders and promoting their engineering applications in additive manufacturing and other fields.For the three problems including powder size and distribution,phase composition and powder morphology,the main research results are as follows:(1)In the study of powder size and size distribution,this paper,by using volume of fluid method(VOF)and the coupling of discrete phase model(DPM)to calculate,analyze the atomizing gas pressure range of 5 to 8 MPa,closed vortex under the condition of the gas phase flow field and gas-liquid two phase flow field of fluid dynamics,set up the relationship between the high-pressure gas and powder particle characteristics.The position of the reflux zone and the shape of the Mach disk are analyzed in detail.It is found that the eddy current at the front of Mach disk is a new method to slow down the airflow in the gas phase flow field.In the gas-liquid twophase flow field,the shape of the Mach disk changes from bow to "S" shape and then to "Z" shape due to the impact of the molten droplets,and finally it is thrust away from the middle to form a backflow zone on both sides of the axis.With the increase of gas pressure,the cross-sectional area of the secondary reflux area increases,and the particle size gradually decreases,and the distribution becomes more concentrated.The powder size,frequency distribution and cumulative distribution of particle size distribution are in good agreement with the experimental results of Fe-based amorphous alloy powder.(2)The microstructure analysis results show that the amorphous content and nucleation mode of the powders are sensitive to the atomizing gas pressure.It is found that with the increase of the pressure,an interesting phenomenon appears:the amorphous content of the powder increases first and then decreases,and the extreme value exists at 7 MPa.Under this pressure,the nucleation mode of the crystal is obviously different,which mainly shows the inhomogeneous nucleation in the powder rather than on the surface.Based on the heat transfer model of the droplet,it is found that this phenomenon is related to the cooling rate of the droplet.The calculation results of droplet cooling process show that the gas-liquid pair velocity in the cooling and solidification stages of atomization process reflects this difference.The microstructure characterization results show that the amorphous alloy underwent {Fe,Ni} and Fe2P,α-Fe and M23B6 three-order reactions during the crystallization process,respectively.Based on the kinetic analysis of droplet cooling rate and phase selection competition,the amorphous formation characteristics of powders under different pressures were clarified.(3)The melt crushing process was simulated by the method of VOF-to-DPM.When the high-speed airflow impinging on the liquid column,the cross-sectional area of the liquid column first decreases,and then the neck shrinks and breaks off.At this time,the first breakage occurs in three locations.However,there is only one place in the stable flow field,namely the edge of the liquid guide pipe.At this time,the liquid flow at the bottom of the guide pipe oscillates with a frequency of about 833 Hz.This phenomenon was confirmed by a high-speed camera,and the atomization oscillation frequency was measured to be about 805 Hz.Different from the traditional concept of the droplet secondary crushing process,the simulation results show that the droplet secondary crushing process is not one broken into many,but one broken into two.In this process,if the cooling rate is too fast,you will get ellipsoidal or spindle shaped droplets.When the superheat of the melt decreases,the secondary crushing process of the droplet is difficult to proceed due to the increase of the viscosity,resulting in the formation of elongated nanowires with diameters of tens to hundreds of nanometers.Even if the two droplets break apart successfully,the droplet or tadpole shape will be formed because the solidification time is less than the spheroidization time.According to the length-diameter ratio formula,the temperature dependence of the drawing properties of the Fe-based amorphous alloy was obtained,and the optimum melt atomization temperature was calculated as 1462 K. |
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