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)
