What conditions should metal powder meet to meet the material requirements of 3D printing? We should consider purity, powder particle size distribution, powder morphology, powder fluidity and bulk density.
1. Purity
Ceramic inclusions will significantly reduce the properties of the final product, and these inclusions generally have a high melting point and are difficult to sinter, so there must be no ceramic inclusions in the powder.
In addition, the content of oxygen and nitrogen also needs to be strictly controlled. At present, the powder preparation technology for metal 3D printing is mainly atomization method. The powder has large specific surface area and is easy to oxidize. In special application fields such as aerospace, customers have more strict requirements for this index, such as the oxygen content of superalloy powder is 0.006% - 0.018%, titanium alloy powder is 0.007% - 0.013%, and stainless steel powder is 0.010% - 0.025%.
2. Powder particle size distribution
Different 3D printing equipment and forming process have different requirements for powder particle size distribution. At present, the commonly used powder particle size range for metal 3D printing is 15-53 μ M (fine powder), 53-105 μ M (coarse powder), which can be relaxed to 105-150 in some cases μ M (coarse powder).
The selection of metal powder particle size for 3D printing is mainly divided according to metal printers with different energy sources. Printers with laser as energy source are suitable for 15-53 because of their fine focusing spot and easy melting of fine powder μ M powder is used as consumables, and the powder supply method is powder laying layer by layer.
The powder laying printer with electron beam as energy source has a slightly thicker focusing spot, which is more suitable for melting coarse powder, and is suitable for 53-105 μ M coarse powder; For coaxial powder feeding printer, the particle size can be 105-150 μ M powder as consumables.
3. Powder morphology
The morphology of powder is closely related to the preparation method of powder. Generally, when the metal is changed from gaseous or molten liquid to powder, the shape of the powder particles tends to be spherical. When the solid state is changed to powder, the powder particles are mostly irregular, while most of the powders prepared by aqueous solution electrolysis are dendritic.
Generally speaking, the higher the sphericity, the better the fluidity of powder particles. The sphericity of 3D printing metal powder is required to be more than 98%, so it is easier to lay and send powder during printing.
4. Powder fluidity and bulk density
Powder fluidity directly affects the uniformity of powder spreading and the stability of powder feeding in the printing process.
The fluidity is related to the powder morphology, particle size distribution and loose density. The larger the powder particle, the more regular the particle shape and the smaller the proportion of very fine powder in the particle size composition, the better the fluidity; The particle density remains unchanged, the relative density increases, and the powder fluidity increases. In addition, the adsorption of water and gas on the particle surface will reduce the fluidity of the powder.
Loose density refers to the powder mass per unit volume when the powder sample is naturally filled with the specified container. Generally, the coarser the powder particle size, the greater the loose density. The powder with thickness matching can obtain higher loose density. The influence of loose density on the density of the final metal printing product is uncertain, but the increase of loose density can improve the fluidity of the powder.
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