the requirements of 3D printing for the performance of metal powder

       

In order to meet the material requirements
of 3D printing, what conditions should metal powder meet?

1. Purity.

Ceramic inclusions will significantly
reduce the performance of the final product, and these inclusions generally
have a high melting point, so it is difficult to sinter, so there must be no
ceramic inclusions in the powder.

Besides, the content of oxygen and nitrogen
should 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 be oxidized. In aerospace and other
special applications, customers have more stringent requirements for this
index, such as oxygen content of superalloy powder is 0.006% - 0.018%, oxygen
content of titanium alloy powder is 0.007% - 0.013%, and oxygen content of
stainless steel powder is 0.010% - 0.025%.

2. Powder size distribution

Different 3D printing equipment and forming
process have different requirements on powder particle size distribution. At
present, the particle size range of metal 3D printing powder is 15-53 μ m (fine
powder) and 53-105 μ m (coarse powder), which can be extended to 105-150 μ m
(coarse powder) in some cases.

The choice of the particle size of metal
powder for 3D printing is mainly based on the metal printers with different
energy sources. The printer with laser as the energy source is suitable to use
15-53 μ m powder as consumables because of its fine focusing spot and easy to
melt fine powder. The powder supply mode is layer by layer powder laying.

For the printer with electron beam as the
energy source, the focusing spot is slightly thicker, which is more suitable
for melting coarse powder, and the coarse powder of 53-105 μ m is suitable; for
the printer with coaxial powder feeding, the powder with particle size of
105-150 μ M can be used as consumables.

3. Powder morphology

The morphology of powder is closely related
to the preparation method of powder. Generally, the shape of powder particles
tends to be spherical when the metal is transformed from gaseous or molten
liquid to powder. When the metal is transformed from solid to powder, the
powder particles are mostly irregular, while the powder prepared by aqueous
solution electrolysis is mostly dendritic.

Generally speaking, the higher the
sphericity, the better the fluidity of powder particles. The sphericity of
metal powder for 3D printing should be more than 98%, which makes it easier to
lay and send powder.

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.

Fluidity is related to powder morphology,
particle size distribution and bulk density. The larger the particle size is,
the more regular the particle shape is, and the smaller the proportion of very
fine powder in the particle size composition is, the better the fluidity is;
the relative density increases while the particle density remains unchanged,
but the fluidity of powder increases. In addition, the adsorption of water and
gas on the particle surface will reduce the powder fluidity.

Bulk density is the mass of powder per unit
volume when the powder sample is naturally filled with the specified container.
Generally, the coarser the powder particle size is, the higher the bulk density
is. The powder with the combination of thickness and fineness can obtain higher
bulk density. The impact of bulk density on the density of the final product of
metal printing is still uncertain, but the increase of bulk density can improve
the fluidity of the powder.