Research On Flow And Heat Transfer Mechanism Of Liquid Metals In Melt Delivery Nozzle During Gas Atomization Process

Metal additive manufacturing gains a promising expectation in the entire 3D printing system.Compared with traditional manufacturing techniques,metal 3D printing has unique advantages in preparing complex shaped components and functionally gradient materials,and has broad application prospects in medical,aerospace,automobile manufacturing and other fields.Powder is the most commonly used feedstock form for metal 3D printing.However,the raw powders used in traditional processes,such as powder metallurgy,thermal spraying and other processes,cannot be directly applied to metal 3D printing.Generally speaking,metal additive manufacturing has higher requirements on the particle size,shape and impurity content of feedstock powders than traditional processes.For example,the selective laser melting(SLM)process prefers metal powders which are in a size range of 15-53 ?m,spherical or nearly spherical,and of low impurity.Therefore,the production method of metal powders for metal additive manufacturing is supposed to be improved accordingly.Currently,gas atomization is an important method to produce metal powders for 3D printing.Most of the previous research has focused on the numerical simulation and experimental observation of atomization gas flow field,atomization and spray process.However,little attention has been paid to the process of metal melt flowing and heat transfer in melt delivery nozzle(MDN).This process is the fundamental part of the whole gas atomization production,and it is essential for the stability and continuity of the subsequent atomization process.The investigation on the mechanism of metal melt flowing and heat transfer in the MDN can provide theoretical support and technical reference for all the gas atomization technique using the MDN for melt delivering.Therefrom,the main work of this thesis is as follows:1)The Volume of Fluid(Vo F)two-phase flow model and Shear-Stress Transport k-?(SST k-?)turbulence model which have been implemented in the ANSYS Fluent computational fluid dynamics software were employed to investigate the transient development process of the metal melt in the MDN,with an emphasis on the influence of the MDN parameters on the flow of metal melt(taking molten aluminum as testing material).The effects of the following parameters,including contact angle,roughness height,MDN diameter,MDN length,on the axial velocity distribution of the metal melt on the axis and the radial distribution of the axial velocity at the position of the 1/2 length of the MDN were analyzed and discussed.2)Three different turbulence models,i.e.,the SST k-? turbulence model,the modified SST k-? turbulence model with the low Reynolds number correction option and the Spalart-Allmaras(SA)turbulence model,were employed to calculate the flow resistance of liquid metals(molten aluminum,iron,and nickel)in the MDN under different nozzle diameters.The numerical results based on different turbulence models were analyzed and compared with those derived from empirical formulas.Taking molten aluminum as an example,the SST k-? turbulence model was used to study the flow resistance under different mean velocities,and the effects of the MDN diameter and mean velocity on the flow resistance were carefully compared.3)The SST k-? turbulence model and the Solidification / Melting model were employed to systematically investigate the heat transfer process of molten aluminum,iron,and nickel in the MDN.The numerical results and analysis focused on the temperature field cloud distribution,the axial temperature distribution and the radial temperature distribution at the position of 1/2 length of the MDN under different MDN diameters.Taking molten aluminum as an example,the effects of the MDN diameter,inlet velocity and wall temperature on the heat transfer of liquid metal were analyzed and compared.4)Based on the similarity of fluid flow,water and alcohol were selected as experimental materials in the simulation verification device to validate the numerical simulation results on the flow process of the metal melt in the MDN.Additionally,the solidified metal melt in the MDN was collected and the metallographic structure was analyzed to verify the heat transfer process.5)Based on the numerical results,the experimental MDN parameters were optimized for gas atomization process to produce three typical powders of aluminum,iron,and nickel alloys for metal additive manufacturing.The standardized testing was subsequently conducted on these metal powders.The calculation of melt flow resistance and the analysis of melt heat transfer process in the MDN are of theoretical and practical significance for avoiding the clogging of the MDN and optimizing the design of the MDN.This is helpful to increase the yield of powders with suitable size range for metal additive manufacturing.