Laser-based powder bed fusion systems for metals
Powder bed fusion systems for metal can be divided into 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:
The metal powder is spread in a thin layer on the build platform.
The laser scans the cross-section of the part to be built.
The powder grains fully melt and solidify to dense material.
This first layer bond onto the build platform.
(Note: this is not sintering this is laser welding)
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.
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 to the surface.
Build speed of laser-based powder bed fusion systems for metal can not be generalised.
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
The materials used for Selective Laser Melting/Direct Metal Sintering / LaserCusing are usually
gas atomised 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 flowability,
the right tab density for energy absorption and melting behaviour.
All kind of metals can be used as long they can be gas atomised 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 being processed. But also Aluminium and precious metals such as Gold, Platinum and Palladium can be processed.
The machine set-up
Laser-based powder bed fusion systems are all working very similar.
Most commonly used as the energy source is a fibre laser in constant wave mode (CW). This is usually a Ytterbium fibre 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 equipped typically with 200 400 Watts lasers. The radiation is guided through a fibre optic entering a system to modify the beam diameter. A galvo 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 materials with high transmission/low absorption rates and may have special coatings to reduce energy absorption.
Some components are cooled actively with liquid and or gas to prevent them from warming up and changing their optical behaviour. To avoid absorption through dust and contamination on the components the optical bench is sealed and sometimes pressurised.
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. The powder can be fed coming from top or be ‘pumped’ up from underneath. All systems have in common a recoater. This is a rubber lip, a metal or ceramic rake or a carbon fibre 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?
Especially the ability to create free-form shapes from materials difficult to process
with subtractive machining makes SLM/DMLS interesting for various industries.
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 a 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-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 GmbH / EOS
to develop together process monitoring technology for the DMLS process.
This component is a part of MTUs Geared Turbofan™ engine.
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 commercially only by ARCAM AB.
The build envelope and the whole powder feeding mechanism is in
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 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 highly productive.
The resolution (so the level of detail achievable and surface finish) 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.
Materials used on EBM machines are not as diverse compared to
laser based systems. ARCAM initially offered only Titanium and Titanium alloys as build materials.
Today also Titanium-Aluminide , Aluminium and Nickel-base-alloys 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 systems
Manufacturing of implants, standardised hip-cups and patient specific solutions
Components for turbines like turbine blades
Special components for oil&gas and marine
Would you like to know more about Direct Metal Technologies and its’ applications?
Do you nee to determine which process is better suitable for your product?
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