Preparation And Characterization Of Specific Metal Powders For 3D Printing

As 3D printing technique develops rapidly,it is highly desired for the production of metal powders with high quality for 3D printing.At present,there are still some limitations in preparing metal powders for 3D printing.Most of the existing preparation methods in China are transplantation and imitation of foreign countries' techniques.Therefore it is demanded to develop innovative preparation technology for metal powders that meets the requirements for 3D printing process.On the other hand,the density and mechanical properties of printed metallic components are affected by both powder features and printing parameters.However,there is no systematic study on the effects of the properties of metal powder materials and the printing parameters on the structure and properties of printed components.Based on the above-mentioned background,the technical route combined nano-agglomeration granulation and heat treatment was adopted in this study to explore the preparation methods of several special metal powders with certain features.The preparation methods and regulation mechanism of various metal powders with high sphericity,high fluidity,high purity and low oxygen content were investigated.Transition metals,high active pure metals and refractory metals were studied.Particle size distribution features such as single peak distribution,double peak distribution,small particle size and narrow particle size distribution were realized.The effects of printing parameters,powder sub-structure,specific particle size distribution and particle density on the microstructure and properties of printed components were studied.The main research contents and results of this study are summarized as following.Firstly,a series of transition metal nano-powder(such as Fe,Co and Ni)and highly active metal nano-hydride powder(taking Ti as an example)with high purity and uniform particle size were prepared by optimizing the process parameters of evaporation-condensation.Using the prepared metal nano-powder and the purchased refractory metal W nano-powder as raw materials,a series of transition metals and highly active metals spherical powders were prepared by adjusting the process parameters of spray drying and heat treatment.The powders have high purity,low oxygen content,high fluidity,high sphericity and micro-nano sub-structure.In order to improve the density and production yield of powder particles,the granulation process for different initial average particle sizes was studied,and the preparation method for spherical granulated powder with bimodal particle size distribution was determined.The peak size distribution can be accurately controlled in a series of spherical powders with bimodal distribution.These powders have high purity,high sphericity and much higher apparent density than the powders with single peak size distribution.It is found that the micron granulation powder prepared by the initial powder with average particle size around 1 micron has higher density than the granulation powder with initial powder in nanometer scale after spray drying and heat treatment.However,it is difficult to obtain high sphericity granulated powder when the initial particle size is too large.In terms of selective laser melting process,the effects of the 3D printing parameters for spherical powders with micro-nano substructure on the density of printed components was studied,and the mechanism was elucidated.It is found that the density of printed components is mainly affected by the coupling of laser power and scanning rate.Taking the prepared Ni powder as an example,the results demonstrate that the powder with higher surface roughness and larger specific surface area is beneficial to laser reflection and absorption.The laser absorption rate of the prepared powder is more than twice that of commercial Ni powder prepared by gas atomization with smooth surface.The relative density of the printed components prepared by the spray-dried Ni powder is as high as 99.2%,which is obviously higher than that of the printed components prepared by the gas-atomized Ni powder.It is due to the surface temperature is higher when the Ni powder prepared by spray drying method is used for laser printing,which leads to larger melting volume,and decreasing in surface tension and liquid bridge capillary force.Thus the spheroidization phenomenon on the surface of the printed component can be greatly suppressed.The microstructure and mechanical properties of the printed components using the prepared Ni-powder produced by the spray-drying method and the commercial Ni powder produced by gas-atomization method were tested and analyzed.It is found that the prints are composed of fine columnar crystals and cell crystal structures,and the columnar crystals could across the interlayer.A fish scale-shaped molten pool is formed on the forming surface parallel to the printing direction,and the printed component prepared by the spray-dried Ni powder has a wider melting channel and a larger molten pool depth during the printing process than the printed component prepared by the gas-atomized Ni powder.The forming surface perpendicular to the printing direction has a stronger(101)texture,and the texture distribution intensity increases as the laser power increases.The printed component prepared by the spray-dried Ni powder has obviously improvedmicro hardness,and its hardness value is twice that of as-cast samples.The micro-hardness of the printed part prepared by the spray-dried Ni powder is 26.8% higher than that of the printed part prepared by the gas-atomized Ni powder.The printing parameters for spherical powder with special particle size distribution and its influence on the microstructure and mechanical properties of the printed components were studied.Efforts were made to elucidate the mechanism.Taking the Ni powder with a bimodal distribution as an example,the powder has a larger specific surface area due to the larger amount of small-sized particles,and it can absorb higher laser energy.The apparent density of the powder is higher than that of the powder with single peak distribution,which is beneficial to heat transfer in the material.Therefore,the surface of the printed component prepared by the Ni powder with bimodal particle size distribution is smoother and denser,and the relative density is as high as 99.8%.The hardness is higher and more stable,and the plasticity is increased by 30% compared to the printed component prepared by the Ni powder with single-peak particle size distribution.The relationships among the density of powder particle and printing parameters,the density and mechanical properties of printed components were studied.Taking the refractory metal W as an example,studies have shown that the formability is mainly affected by the ratio of the laser energy density to the scanning rate.When the ratio of the energy density to the scanning rate does not exceed 2,a formed print can be obtained.The main defects of the printed components are irregular pores and cracks with a large size scale.As the laser power increases,the porosity and the crack density are decreased,and the density of the printed component is increased.The powder prepared by spray drying has lower density,lower thermal conductivity and higher surface tension compared to the powder prepared by gas-atomization.Thus,the density and micro-hardness of the printed components are lower than those prepared by gas-atomized powder.Increasing the heat treatment holding time of the W powder prepared by the spray drying method can increase the density of the granulated W powder particles,which is beneficial to the improvement of the density and mechanical properties of the printed components.