Which steps are repeated in powder bed fusion processes?
Apply powder layer (recoater)
exposure with layser/ electron beam
Lowering building platform & recoat new powder
What are the two variants of the powder bed fusion of metals process? How do they differ?
PBF-EB/M: Electron Beam Melting
PBF-LB/M: Laser Beam Melting
They use the same procedure and workflow, but differ in the type of energy source (Electron beam, and laser beam)
So they use different types of powders
So they have different individual optimized scanning strategies
So they use different shielding gases: Inert Gases (Ar, He) used by PBF-LB/M, and vacuum used by PBF-EB/M
What limits the process window in powder bed fusion?
mainly given by Scanning speed and Laserpower are defects which limitate the process window:
occuring errors are:
to high scan speed & to low laser power
Lack of fusion:
Balling:
no metling
sintering
to high laser power combined with low scan speed:
keyhole formation
Gas pores
What physical phenomena are mainly responsible for melt pool dynamics?
Maragoni Effect:
heat gradient causes heat flow leading to a gradient of surface tension
Liquid metal flows along the surface interface from regions of low surface tension (hot center) to high surface tension (colder)
Recoil pressure:
caused by vaporization at the surface of the melt pool
escaping metal vapor pushes back onto the liquid melt pool, creating a downward pressure called recoil pressure
leads to keyhole generation
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