Ch.5 Powder Bed Fusion Flashcards
Why is the powder preheated in PBF? How does it affect the build process & quality?
minimize thermally induced stresses and laser power requirements
reduces warping, curling, non-uniformity, and contraction
What characteristics should a polymer have to be well-suited for PBF?
thermoplastics with:
low melting temp
low balling tendency
low thermal conductivity
Which type of thermoplastic is better for PBF? Why?
Crystalline and semi-crystalline
(regular molecular structures)
broad supercooling window
T_deterioration»_space; T_melt
well-defined melting temp
Which type of thermoplastic is not suitable for PBF?
Amorphous: highly porous, melts over a range of temps so no distinct onset of cooling or melting
Discuss three ways solid-state fusion affects PBF
1.) Loose powder agglomerates each time the material is recycled
- alters spreading properties
- reduces laser requirement, warping, and internal stresses
2.) Part growth due to conduction heating of loose particles around the part
- causes low density, high porosity further out
3.) Rapid fusion makes it difficult to achieve 100% dense part
- can be solved with sintering + high-temperature + grain growth
Why is chemically induced sintering not widely used in commercial PBF?
the process creates part porosity
post-processing/high-temperature furnace sinter/infiltration needed to achieve desired properties
–> increases cost & time
What are the characteristics of the two scanning modes in PBF?
Contour lines: outline cross-section for accuracy and surface finish
Fill mode: rastering (polymers), squares, and stripes used to reduce residual stresses (metals)
What are the 4 variations of binder and structural material in liquid phase sintering (LPS)
Separate particles: mixture
Coated particles: structural coated with binder
Composite particles: each powder particle = binder + structural
Indistinct binder and structural particles: partial melting
Advantages and disadvantages of separate particles?
Advantage:
- binder smaller than structural –> more efficient packing –> minimizes shrinkage
Disadvantage:
- insufficient binding time –> high porosity –> separation when handling
- post-process needed
Advantages and disadvantages of Composite particles
Advantage:
- high-density part –> better surface finish
Disadvantage:
- structural material selected to be more beneficial for enhancing binder properties
Advantages and disadvantages of Coated particles
Advantage:
- better laser absorption –> structural binds better
- better flow properties
Disadvantage:
- if structural melts before binder –> microstructure differs
Advantages and disadvantages of indistinct binder and structural particles
Advantage:
- melt smaller particles or outer regions without melting the entire structure
Disadvantage:
- only a portion of alloy melts –> regions with high concentrations of low melt temp metals melt first
What are the requirements for powder handling? (4)
- large powder reservoir volume to meet max build height
- a sufficient layer of powder transferred each time to platform
- spread layers of repeatable thickness, thin, smooth
- minimize disturbance of shear force caused by spreading
What are the characteristics of powder handling? (3)
- particle size sufficient for the powder to be flowable
- enclosed environment because larger SA:Vol ratio make powder more reactive
- minimize the presence of airborne particles
What characteristics are required of materials used in EBM? What detrimental effects occur without this characteristic?
- high conductivity
effects:
- rapid expulsion (repelling neighboring negative charged particles)
- diffuse beam (repelling incoming negative charged particles)
Why are there microstructural differences between metals produced with mLS vs. EBM?
mLS:
- less than 80% energy to beam eergy conversion
- lower temperature than EBM
- laser scan line distinguishable
- rapid cooling –> smaller grain size
EBM:
- most of the energy converted
- powder bed maintained at a higher temperature
- contiguous grain pattern with less porosity
What are the 4 powder bed fusion mechanisms?
- solid- state
- chemically induced
- liquid phase sintering
- full melting
What would be the resulting part quality in these situations:
laser power/bed temp
- high/high
- low/low
- high/low
HH: dense part –> part growth
LL: better dimensional accuracy –> low density, delamination
HL: part curling, increased residual stress
What is the process of indirect part fabrication?
Loose powder –> Green Part –> Brown part –> Finished part
Design considerations for SLS printing: wall thickness, supports?
Wall thickness material dependent:
PA12-0.8mm
Carbon polyamide- 2.0mm
no supports required
Design considerations for SLS printing: edge radius, holes, drainage holes?
Edge radius: 0.4mm
Holes: diameter > 1.5mm
Drainage holes: diameter > 3.5mm
Design considerations for SLS printing: tolerance, feature size, and embossed/engraved details?
Tolerance: 0.3mm or 0.05mm
minimum feature size: 0.8mm
Embossed/engraved: 1mm depth & height
How to minimize shrinkage and warping in SLS prints?
increase overall print dimensions by 3.5%
check orientation or add ribs to increase stiffness
Design considerations in SLS printing applications for the following:
- axles
- tanks
- threads
- interlocking parts
- integrated hinges
Axles: clearance 0.3mm
Tanks: designed with drainage with walls >1mm thick
Threads: not good
Interlocking parts: 0.2mm tolerance
Integrated hinges: 0.2-0.3mm
Why are supports needed in laser metal PBF?
- dissipate heat to prevent residual stresses
Design considerations in laser metal PBF for the following:
- wall thickness
- hole size
- drainage holes
- tolerance
- aspect ratio
- unsupported overhangs
wall thickness: 0.4mm
holes: min diameter 0.5mm, support not needed if diameter >6mm
drainage holes: 2-5mm
tolerance: 0.127mm
Aspect ratio: 8:1
Unsupported overhangs: >45 deg
How to avoid the stair-stepping effect in laser metal PBF to create a better surface finish?
surface angled at 20 degrees or greater
