When using powder bed fusion methods of printing, the fresh layer of powder at the top is typically deposited before being evened out with rollers or a wiper mechanism.
While it is fine for printing with singular metal powders, any attempts to add additional metal powders to the same layer can result in poor powder distribution, and consequently, unpredictable alloy formations.
ESPS
A team of researchers has devised a potential solution to this issue by developing a means of spreading the top layer out using an electrostatic system. This means that it should, in principle, be able to add extra metal powders to the same layer, while retaining some degree of control over the mix.
The process has been designed and tested by researchers at the Lawrence Livermore National Laboratory.
The technique has been named Electrostatic Powder Spreading, or ESPS for short.
Take a look at the incredibly neat diagram below and let’s get into the details.
The ESPS method deposits metal powder layers by use of electrostatic field, which is generated by the charged powder container and from a counter electrode placed above the bed, as you can see in the graphic. When the system is energized, the powder particles bounce inbetween the counter electrode and the powder container, and eventually through the mesh and onto the powder bed.
The electrode angle seen in the graphic can be varied to better guide the particles towards the target. The researchers experimented with angles between -5 and +5 degrees.
By switching the electrical field in the electrode on and off, the researchers were also able to produce patterns in the freshly deposited powder bed layer.
Additional parameters were controlled by varying the strength of the electric field. These parameters include deposition rate and layer thickness. All of these tunable parameters combined with the switching contribute to the ability to create gradients of powders with different metals.
By use of this method, the research team were able to create printed coupons of over 99.8% density by adding the ESPS system to a commercially available Laser Powder Bed Fusion system.
The underlying principle of the ESPS system is known as Electrophoresis, which is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field.
Previous Efforts
Previous attempts to deposit multiple powders in the same layer have been met with mixed results.
One previously attempted method involves removing powder from specific areas with a vacuum nozzle, and then refilling the area with the second powder. However, while it does work (more or less), the resolution is limited by the nozzle size and the strength of the vacuum.
Other methods tried in the past have made use of laying down so-called “voxels” of the secondary powder of a specific size. This size is also a bottleneck for resolution.
Functionally graded materials such as those printed with the ESPS method can have tailored properties enabling the tuning of heat transfer properties, density variations, and wear resistance.
You can read the pre-proof version paper titled “Electrostatic Powder Spreading for Metal Powder Bed Fusion Applications” in the Additive Manufacturing journal, over at this link.