Lecture 9 part 2 Flashcards
In powder bed fusion how do the materials come?
Powders are manufactured in different ways and come in different shapes and sizes (morphology)
Spherical powders are the best as they flow/deposit well
Spherical powders also increase packing density
Metal powders do not degrade as easily as polymers
what are the pros and cons of finer powder particles in powder bed fusion?
Pros…
Requires less energy to melt and can reduce surface roughness
Increases powder packing
Cons…
Risk of inhalation
Risk of dust explosion
what are the pros and cons of larger powder particles in powder bed fusion?
Pros…
Safer to handle
Cons…
increased surface roughness
More energy required to melt
How is the powder made for powder bed fusion and what are some materials that it can be made from?
The powder manufacturing process is Gas Atomisation
Some materials that can be used are
- Steel alloys
- Titanium, commercially pure and alloys
- Aluminium alloys
- Nickel alloys
- Cobalt chrome alloys
Laser and Electron beam systems in powder bed fusion
What are general characteristics of the process?
- Materials are melted by absorbing laser energy, typically fibre lasers are used, lasers are versatile, accurate and have a high energy density
- Systems are known as selective laser melting (SLM) or direct metal laser sintering (DMLS)
Laser and Electron beam systems in powder bed fusion
What is the general process for laser based systems?
- Powder layer of 20 - 40 micro meters depositied
- Laser operates at 50W-1kW
- Builds at 30 - 50 cm3/h
- Processes most metals
- Chamber purged with inert gas
- Uses galvo mirrors to direct laser energy and “draw” part geometric
Laser and Electron beam systems in powder bed fusion
What are general characteristics laser based systems?
Typical system (EOS M280)
- Buld volume: up to 250x250x300 mm
- up to 400 W Yb fibre laser
- Spot size: 100 micro meters
- Build speed up to 32.4 cm3/h
- up to 200C powder bed pre-heat
- Minimum wall thickness/feature size 0.04mm
- Accuracy +/- 0.2mm
Laser and Electron beam systems in powder bed fusion
What is the general process for electron beam based systems?
- Materials are melted by transfer of kinetic energy from incoming electrons
- Process known as Electron Beam Melting (EBM) by Arcam
- No moving mechanical part to deflect electron beam, therefore extremely quick scanning.
- High energy density beam that can be split into multiple beams
- 55-80cm3/h build speed
- Can only process conductive materials
- Operates in vacuum, pressure <x10-4mbar></x10-4mbar>
</x10-4mbar>
Laser and Electron beam systems in powder bed fusion
What are general characteristics electron beam based systems?
Typical System (Arcam M280)
Key system characteristics
– Build volume: up to 250x250x350mm
– 3.5kW electron gun
– Spot size: 200-1000μm
– Layer thickness: 50μm to 200μm
– Scan speed 8000m/s
– Build speed Up to 55-80 cm3/h
– 1-100 spots
– ~700C pre-heat (can reduce residual stress build-up)
Part Surface finish
– As built: Ra~25/35μm
- Minimum wall thickness / feature size 0.1mm
- Accuracy – +/- 0.05mm
Laser vs Electron PBF systems - what are the pros and cons (10 sections)
What are some commercial PBF systems (9 examples)
Renishaw SLM (UK) £185-250K
EOS DMLS (Germany) £120-260K
Concept laser Lasercusing (Germany)£100-930K
SLM Solutions (Germany) £125-430K
Realizer SLM (Germany) £80-310K
Phenix Systems SLM (France) £100-300K
Matsuura Luminex SLM (Japan) £500K
LaserCore SLM Beijing Long Yuan (China) £60-130K
Arcam EBM (Sweden) £310-380K
- purchase price is generally proportional to build volume
PBF vs CNC machining (8 Points)
1) CNC machining can remove material quicker than PBF can add, however if setup times are included for a new part, PBF may build parts quicker
2) CNC needs block of material as big as part
CNC Lots of material wastage
3) CNC Cannot machine certain intricate geometries
4) CNC can produce a better surface finish
5) CNC has better accuracy and resolution
6) PBF generates more residual stress than CNC
7) PBF cost independent of geometric complexity
8) PBF may require supporting structures
PBF vs Casting (13 points)
PBF has shorter solidification times, finer microstructure with chemical elements less likely to segregate
Casting has longer solidification times, chemical segregation is more likely to occur effecting mechanical properties
PBF parts has a more uniform microstructure
PBF tends to have better ultimate tensile strengths but worse fatigue life compared to cast
Difficult to remove pores from Ti64 Casts, therefore aerospace does not use for certain applications
PBF has higher surface roughness than cast
Geometric complexity is more costly in casting
PBF better suited to small complex geometries in small to medium volumes
Casting has high setup costs for each new mold and limited materials (require material with high fluidity)
Casting better for large components (e.g door frames)
PBF better for customisation (e.g dental or medical implants)
What are the similarities/differences between metal and polymer powder bed systems systems
Similar benefits for part creations (e.g geometric complexity, low material wastage etc.)
Similar techniques/methods used to build polymer parts
Similar accuracy to polymer systems
Higher melt temperature than polymers, may require higher energy density to join layers
Larger temperature gradient, generally need to be built onto a substrate plate and may require supports
Generally lower build speeds due to requirement for a higher energy density
Generally more expensive systems than polymers due to hardware requirement for processing and handling of metal
