What Are The Requirements For The Performance Of Metal Powder In 3D Printing

       

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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