Laser based powder bed fusion systems for metals

Powder bed fusion systems for metal can be divided in two groups.
Most common are systems using a laser beam as an energy source.
The other systems are using an electron beam to apply the energy.


How does the process work?

The process is a cycle of three steps:

Step 1
Metal powder is spread in a thin layer on the build platform.

Step 2
The laser scans the cross-section of the part to be build.
The powder grains fully melt and solidify as dense material.
This first layer bond onto the build platform.
(Note: this is not sintering this is laser welding)

Step 3
The build platform drops by one layer thickness and the next powder layer is applied.

What’s the next step?
It starts over with Step 1, scanning & fusing the next cross-section
of the part. This layer bonds with the layer underneath and the part grows.
The cycle is repeated until the part is finished.

 

Resolution
The resolution of laser based powder fusion systems for metal can vary. The positioning
accuracy of the scanner is very high and not to be considered as an influence. It is more the
focal diameter, laser power and processing speed influencing the build resolution.
These factors affect the melt pool size and so the width of every molten track.
The resolution in the z-axis is affected by the layer thickness set but also on the scan strategy.
Depending on which track is molten at what stage of the process it can affect the amount of
powder particles sticking, just partially molten, to the surface.

 

Build speed
Build speed of laser based powder bed fusion systems for metal can not be generalized.
It is highly depended on material, laser power, spot sized, scan speed and layer thickness.
The limiting factors are usually not the machine set-up but the materials properties affecting
absorption of the energy and transmission of thermal energy.
The thickness of the powder layers usually ranges between 20 and 120 microns but there
are exemptions.


Materials
The materials used for Selective Laser Melting/Direct Metal Sintering / LaserCusing are usually
gas atomized powders. The grain size ranges from about 5 – 70 microns depending on machine
and material. The powders need to have certain grain size distribution to ensure flow ability,
the right tab density for energy absorption and melting behavior.
All kind of metals can be used as long they can be gas atomized can be welded by laser.
This makes the process very interesting for processing materials  which are difficult to
machine with traditional methods. Titanium, Stainless steels, Cobalt chrome alloys and
Nickel based super alloys are commonly processed. But also Aluminium and
precious metals such as Gold, Platinum and Palladium can be processed.
Even trials with  Magnesium were successful.

 

The machine set-up
Laser based powder bed fusion systems are all working very similar.
ost commonly a fiber laser in constant wave mode (cw) is used as an energy source.
This is usually a Ytterbium fiber laser, emitting radiation near the infrared spectral range (1060-1080 nm).
The laser power ranges from about 50 Watts as the lower end up to 1kw as the upper end.
Medium sized machine are typically equipped with 200 400 Watts lasers. The radiation is guided through
a fiber optic entering a system to modify the beam diameter. A galvano scanner is used to direct the beam
in the X&Y axis. 
To compensate for the focal height in the z-axis a flat plane / F-Theta lens used.

Alternatively a beam expander can be used to adjusting the beam diameter and with
this also the focal length actively during the process. Due to the high laser power and
energy transmitted through the optical components special measurements needs to be taken.
Lenses are made of special materials and may have special coatings to reduce energy absorption.
Some components are actively cooled with liquid and gas made to prevent them from warming up
and changing their optical behavior. To avoid absorption through dust and contamination on the
components the optical bench is sealed and sometimes pressurized.

The build plate is usually made of the same or similar material as the build material. It is mounted on
the platform/lift. A zero-point-clamping system might be used to ease post machining. The build plate
might be heated from underneath to reduce mechanical stress.
The re-coating system may vary. Powder can be fed coming from top or pumped up from underneath.
All systems have in common a recoater. This is a rubber lip, a metal or ceramic rake or a carbon fiber brush
to spread the powder and create the next layer.
To prevent the material from oxidation during the process a shielding gas is used. Most commonly Argon
and Nitrogen are used.  The atmosphere of the process chamber is circulated in a closed loop cycle. A filter
is used to extract evaporated material to keep the gas stream over the powder bed clear. This is necessary
to avoid residue on the laser window creating absorption and loss of energy for the process.

 

What are the applications?

Applications
Especially the ability to create free-form shapes from materials difficult to process
with subtractive machining makes SLM/DMLS interesting for various industries.

Dental replacements
Selective Laser Melting / Direct Metal Laser Sintering found one of its’ first
applications in dental. Dental replacements such as bridges and copings
are generated layer by layer in cobalt chrome alloys on an full industrial scale.
Patient specific parts, always one-offs in a material that is hard to machine
with CNC milling makes SLM/DMLS ideal for this application.
-> Read more about this application here

Aero parts
Aero engine manufacturer MTU states they are using the DMLS process for production parts.
Borescope bosses for the PurePower® PW1100G-JM, the engine powering the A320neo but
also parts for their geared Turbofan engine are made using DMLS technology.
MTU recently announced a corporation with DMLS machine manufacturer Electro Optical Systems / EOS
to develop together process monitoring technology for the DMLS process. This is a further step

Screen Shot 2015-03-06 at 16
This component is a part of MTUs  Geared Turbofan™ engine.
Source:http://www.mtu.de/fileadmin/EN/7_News_Media/2_Media/2_MTU_Report/PDFs/Report_2014_2.pdf

 

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Electron Beam based powder bed fusion systems

Electron Beam Melting machines (EBM) are not as common as
laser based systems as they are currently build only by ARCAM AB.
The build envelope and the whole powder feeding mechanism is in a
a vacuum chamber.  An electron gun with power up to 10 kW of power
emits an electron beam which is directed by magnetic fields.
The technique is very similar to an old TV set but the electron beam used
is by far more powerful. Instead of the the screen, the metal powder is
scanned to fuse it locally together. Due to the very high speed of the
electron beam it can also be used to temperate the part during the process.
This is beneficial for the metallurgical crystal structure and results in lower
levels of residual stress compared to systems using a laser beam as an energy source.
The high scan speeds and making the process very productiv.

The resolution of electron beam melting systems is dependent on the
processing speed and the layer thickness. It is also influenced by the grain size
of metal powder used. In general it can be said that the parts have a higher
surface roughness compared parts being made on laser based systems.

The materials used on EBM machines are little more limited compared to
laser based systems. ARCAM initially offered only Titanium and Titanium
alloys as build materials. Today also Aluminide and Inconels are available.
However it is possible to develop process parameters for more materials.
Powders for EBM have a grain size typically between 50-100 μm. This range
of powder grains is usually cheaper compared to the finer powders used for
laser based powder bed fusion. This gives EBM beside the higher productivity
another cost advantage.

Applications for electron beam based powder fusion system:
…coming soon…

 

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