Research On Aluminum Powder Atomization Process For 3D Printing Based On Novel Discrete Nozzle Atomizer

The gas atomization method is the main preparation method for aluminum powder used in 3D printing.However,the powders produced using this method have defects such as satellite powders and hollow powders,and the particle size uniformity of the powder needs to be improved.The gas atomization process is closely related to the properties of aluminum powder,and the structure of atomizer has a great influence on the gas atomization process.Therefore,a novel discrete nozzle atomizer was designed,and the multiphase flow and heat transfer behavior during the molten aluminum gas atomization process were systematically studied by numerical simulation and experimental research,which aimed to lay the foundation for optimizing the gas atomization process.The main contents include the following aspects:Based on the atomizer used by a certain aluminum powder manufacturer,a new efficient and energy-saving annular porous atomizer was designed.The influence of atomization parameters on the flow field structure of atomized gas was studied based on the Realizable k-ε turbulence model.The results showed that when the width-to-thickness ratio of the nozzle hole was 12.8,the velocity and flow rate of the atomizing gas were maximized,which preliminarily confirmed that the novel discrete nozzle atomizer can achieve higher velocity and kinetic energy.And the optimal atomization parameters were determined as follows: atomizing gas pressure of 3 MPa,atomizing gas temperature of 900 K,nozzle divergence angle of 30°,extension length of the guide tube of 0.2 mm,and nozzle width-to-thickness ratio of 12.8.Based on large eddy simulation(LES)and the volume of fluid(VOF)model,a gas atomization mathematical model suitable for studying the molten aluminum breakup mechanism during high-speed gas atomization processes was established.The molten aluminum breakup processes of the delivery tube end non-expansion type and expansion type atomizers(NET and ET atomizers)were compared,and the formation mechanism of surface waves,liquid core,liquid film,liquid ligaments,and droplets during molten aluminum gas atomization was revealed.The results showed that for the NET atomizer,the molten aluminum completed primary atomization under the action of the recirculation zone,and the primary droplets were transported to the main nitrogen jet by the recirculation zone to complete the secondary atomization.For the ET atomizer,the “coneshaped” liquid core was gradually formed under the action of the atomizing gas,and a large number of primary droplets were separated from the liquid core and underwent secondary atomization by the nitrogen jet.The liquid ligaments were formed by gas shear stretching the liquid film and the expansion of the liquid hole.The droplets were formed by the rupture of the liquid ligaments at the liquid film front and the shearing and stretching of the liquid core surface.Based on the gas atomization mathematical model,the influence of atomization parameters on the atomization process was studied.The results showed that with the increase of the extension length of the delivery tube,the shape of the liquid core changed gradually from “cone-shaped” to “dumbbell-shaped”,resulting in larger droplet sizes.When the width-to-thickness ratio of nozzle hole was greater than 10,the sizes of atomized droplets was smaller,indicating that the novel discrete nozzle atomizer can produce droplet with smaller sizes.Compared to the ET atomizer,the NET atomizer produced smaller droplets during gas atomization.A mathematical model coupling the k-ε turbulence model,volume of fluid(VOF)model,and solidification/melting model was established to solve the cooling and solidification process of flying melt droplet.The deformation and breakup behavior of flying melt droplet were analyzed,and the formation mechanism of hollow powder and satellite powder was revealed.The results showed that the higher the initial temperature of the melt droplet,the later the time when solidification begins,but the cooling rate remained relatively constant.An increase in gas velocity led to an accelerated cooling rate of melt droplet.An increase in the diameter of melt droplet and the temperature of the gas would result in a decrease in the cooling rate.The gas was entrained into the melt droplet and eventually enveloped by the solidified particle,resulting in the formation of hollow powder.In the high-speed gas flow,a smaller solidified particle with higher velocities collided with the windward surface of a large solidifying melt droplet.The small particle adhered closely to the surface of the large droplet,and the satellite powder was formed after the large droplet solidified.The atomization performance of the novel discrete nozzle atomizer was tested through water model experiments and industrial tests.The results showed that the droplets/powders produced by the novel discrete nozzle atomizer were finer.