Powder Bed Fusion Flashcards
What is powder bed fusion?
Thermal energy selectively fuses regions of a powder bed
What are the most common forms of energy used in powder bed fusion? And what does it do?
Laser or electron beam
The irradiated energy is absorbed by the powder and converted into localised heat zones which fuse the particles of the powder bed
What is the process for general powder bed fusion?
Fine powder layer is applied to a preheated platform.
Sliced file data is fed to the laser system and slides image onto powder surface.
Material is partially fused or fully molten and a new layer of powder is rolled onto the surface.
Once slices are complete, part is allowed to cook and stabilise.
Part removed and unbonded powder is brushed off and reused
What is SLS?
Selective laser sintering
Powder bed fusion process for polymers and ceramics
What is SLM?
Selective laser melting
Powder bed fusion process for metals
What is EBM?
Electron beam melting
Another powder bed fusion process for metal alloys
What is the SLM process?
Print chamber is filled with inert gas like argon and heated to a temp below the metals melting point
Particles are close to spherical but in a heated state
An additional heat source (infrared laser) causes the metal powder to melt and flow
Rapid cooling occurs and grain boundaries are formed, the powder is fused
Why is the print chamber filled with inert gas in SLM?
Avoids oxidation and contamination
To avoid the oxidation of the material after melting and to remove spatter, metal vapour and plasma plumes
Common SLM materials…
Metals and metal alloys Steels (including stainless) Titanium Aluminium Copper Precious metals
Why is the SLM print chamber heated?
By heating the chamber, you are also heating he metallic powder. You will need less energy from the laser
Why can metals high reflectivity be challenging to process?
The particles will be absorbing far less energy than a less reflective particle
Why do SLM parts sometimes require support structures?
Due to the thermal conductivity of metals, residual stress occurs as the print layers increase. Support structure is generated by printing fine structure of the same powder
What are the 3 SLM post processing options?
Mechanically removing support structures
Sand blasting to reduce surface roughness
Thermal treatment to alter the mechanical properties
What are the advantages of SLM?
Common materials that are easy to access
Capable of producing dense metallic components
Un fused powder is fully recyclable
Parts can be heat treated as standard metallic engineered components
Highly complex bespoke geometries are achievable
What are the disadvantages of SLM?
Requires support structure to combat residual stresses which require time and cost to remove
Some heat treatment (annealing) may be required to rectify residual stresses
Higher overall costs due to the need for powder material generation, inert atmosphere, higher temps and energy source
What is EBM?
Electron beam melting
An AM process which utilises an electron beam to selectively fuse regions of a metal powder bed
What is the electron beam melting process? and the benefits of it?
key component of high power electron beams
operates in a vacuum
This high power enables pre-heating of powder bed and chamber to temperatures close to the melting point of the powder
This consistent temperature profile means less residual stress across the built part
This minimises the need for support structure
As a result more parts can be stacked in the build chamber
Similar set of materials to SLM
What does powder bed fusion allow that vat polymerisation and metal extrusion does not?
Enables metal 3D printing without additives
but metal powder is more hazardous than bulk metal
Benefits of SLM compared to EBM?
Material selection is broader
Provides slightly higher precision
Benefits of EBM over SLM?
Allows for high throughput due to the stacking process
Higher part densities
What are some examples of PBF applications?
Lightweight metal parts
Customisable metal parts
Aerospace
Medical implants
What is sintering
The process of compacting and forming a solid mass of material by heat and/or pressure without reacting a fully molten phase
What is SLS?
Selective laser sintering
An AM process which utilises a laser beam to selectively fuse regions of polymer powder bed.
What is the print chamber heated to in SLS?
It is heated to a temperature just below the melting point of the polymer powder
Advantages of SLS?
Powders are easy to produce and readily available
Any unfused powder acts as a support mechanism, meaning that separate support material is not required
Unused material is recyclable (to a degree)
Stacking is possible
Very little post processing required
Disadvantages of SLS?
Compensation for distortion is required
Higher energy demands that result in higher costs
Dust and particle control is required
Powders are dangerous due to toxicity and combustible state
Powder can become trapped within smaller hollowed features
What is flowability of the powder governed by?
Size and shape of the particles
What materials are used in SLS?
Polymers and their composites
Polyamide (PA) (Nylons) is the most commonly used material in SLS are it is lightweight and has good strength and durability
Polyether ether ketone (PEEK) is also used and has good thermal and chemical stability
Composites of PA and PEEK with glass and carbon fibre also used
Why are support structures not necessary for SLS?
Consistent temperature profile in the chamber means that there is less residual stress across the built part which eliminates the need for support structures.
As a result more parts can be stacked in the build chamber and it allows for moving parts to be built without the need for assembly
What does the post processing of SLS involve?
Not much, it is mainly to remove excess powder
Jet stream of air and liquid or sand blasting could be used to reduce surface roughness
How does the porosity of SLS parts compare to SLM and EBM parts?
SLS printed parts have intrinsic porosity - higher than SLM and EBM
How is high speed sintering achieved?
Patterns of an infrared absorbing ink are inkjet printed on the powder bed.
An infrared lamp scans the entire powder bed
The inkjet printed patterns absorb the infrared irradiation and enable localised heating and sintering
Why is the powder bed fusion of ceramics limited?
Ceramics have very broad optical and thermal properties
Low thermal conductivity compared to metals
Very high melting points compared to polymers
How do ceramics have to be processed by powder bed fusion due to their complicated properties?
Due to their high melting point, they are often coated in polymer to initiate sintering and produce green parts
Additives can be added to ceramics to increased their laser absorbance
They exhibit sintering or melting behaviour based on the type of ceramic and the print parameters used
What is a common example of a coated ceramic used for sintering?
Alumina powder coated in polyvinyl alcohol (PVA) then mixed with an epoxy resin to form spherical powders
Explain binder sintering for ceramics
Polymer coated particles are sintered in the standard way which can result in a final part
This green part is then placed in a furnace where the binder is burned off
The part is then infiltrated with a lower temperature to complete the part
What kind of process uses a certain type of beam to process ceramics?
Electron beam selectively fuses regions of a powder bed but it must be operated in a vacuum
How does PBF compare to vat polymerisation and material extrusion?
PBF enables the printing of ceramics and glasses with minimal additives but it is still at an early research stage
What are some application areas of SLS?
Aerospace Rapid prototypes Healthcare (vascular stents) Consumer goods (mascara brushes) Fashion garments
