| 3D printing has been praised as the core technology of “the third industrial revolution”,but the production of metal powders needed has become a bottleneck of this technology development.Traditional powder-making technologies have limitations.Hybrid atomization is a new atomization techique which combine s free-fall gas atomization with centrifugal atomization.This process can produce powders with good sphericity,low oxygen levels,small average particle sizes and narrow size distributions,which can meet with the requirements of 3D printing.The process principles and laws of hybrid atomization was studied in this work,which is aimed at providing a new powder-making for 3D printing.In order to ensure gas atomization run smoothly,the hybrid atomization device adopts double-nozzle free-fall atomizer.Simulation experiments were carried out to explore the characteristics of gas flow field.The pressure between the tip of delivery tube and centrifugal disc has two negative pressure peak value and a positive pressure peak value.To avoid the nozzle clogging,the appropriate main atomizing gas pressure and auxiliary atomizing gas pressure can be set by simulation experiments.Hybrid atomization process and melt disintegration modes were investigated through a combination of theory analysis and experiments.Metallic melt keep as liquid column between the main atomizer and the auxiliary atomizer.The liquid column is broken into droplets under the main gas atomizer,which converge into liquid film on the rotating disc.There is a critical radius rc.When r is smaller than rc,the liquid film is in a turbule nt flow state.When r is greater than rc,the liquid film is in a laminar flow state.In order to obtain a thin and stable film,the size of the rotating disc can be designed according to the critical radius rc.At the edge of the rotating disc the liquid film does not have a single disintegration mode.A mathematical model is established to describe the in-flight dynamics and thermal history of AlSi10 Mg and C u-6.5 wt%Sn alloy droplets as a function of flying distance during hybrid atomization.It is also developed to simulate the influence of main processing parameters such as melt superheat and disc rotating speed on flight and solidification of droplets.Velocity and thermal history of hybrid atomized droplets is affected remarkably by droplet diameter.With decreasing droplet diameter,the velocity of droplets changes quickly and the flight distance become short during solidification of droplets.Melt superheat has a weak effect on cooling rate and solidification.However,large superheat would delay the solidification process of doplets,which is not good for the design of atomization chamber.Rotating speed has a slightly greater influence on the droplet thermal history than superheat.A higher rotating speed can shorten the flight distance during solidification.The cooling rates of powders were calculated by measuring the secondary dendrite spacing,which ranges from 103 to 106 K/s and is consistant with numerical calculation results.Cu-6.5 wt%Sn alloy powders prepared by hybrid atomization equipment were evaluated.The average particle size is 21.89 ?m.The geometric standard deviation is 1.42.The particle sizes of most powders are in the range of 14.4 ? m and 35.2 ? m.The powder mobility is 18.4 g/s and the apparent density is 4.80 g/cm3,which meets with the requirements of 3D printing.The SLM samples obtained by orthogonal tests mainly include Cu-Sn solid solution and intermetallic compounds Cu13.7Sn.Density is affected by scanning speed mostly,followed by hatching space and laser power.The main defects contain holes,cracks and ball.The surface roughness can be improved by setting the appropriate specific energy input.Fracture mode is a type of brittle-ductile mixture.Besides optimising SLM processing parameters,the 3D printing formability of atomized powders could be improved by reducing the level of impurities,and increasing the bulk density and the sphericity. |
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