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@PHDTHESIS{Schniedenharn:807057,
author = {Schniedenharn, Maximilian},
othercontributors = {Schleifenbaum, Johannes Henrich and Witt, Gerd},
title = {{E}influss von {F}okusshift und {P}rozessnebenprodukten auf
den {L}aser {P}owder {B}ed {F}usion {P}rozess; 1. {A}uflage},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {Apprimus Verlag},
reportid = {RWTH-2020-11079},
isbn = {978-3-86359-910-2},
series = {Ergebnisse aus der Lasertechnik},
pages = {1 Online-Ressource (vi, 180 Seiten) : Illustrationen,
Diagramme},
year = {2020},
note = {Weitere Reihe: Edition Wissenschaft Apprimus; Dissertation,
RWTH Aachen University, 2020},
abstract = {Using the additive manufacturing process Laser Powder Bed
Fusion (L-PBF), workpieces with an almost unlimited freedom
of geometry are produced layer by layer from a fine metallic
powder. The resulting properties of the workpieces are
thereby to a certain extend determined by the process
itself. Defects like pores, cracks and lack-of fusion can
for example deteriorate the mechanical properties. As of
today, the formation of such defects is not fully
understood, making their occurrence seem random. The scope
of this work is the investigation of mechanisms which can
lead to the formation of such defects. The objectives of
this work are investigations on the influence of focal shift
(1) and the interaction of laser radiation with process
by-products (metal condensate, splashes, ejected and
entrained powder particles) (2) on the L-PBF process.
Moreover, the influence of the shielding gas flow is
investigated (3) since the flow is supposed to remove
process by-products from the laserpowder interaction zone
and is therefore a main influencing factor on the
aforementioned interaction. Focal shift in L-PBF systems can
be induced by the physical properties of the optics and by
additional external influences. As for external influences,
the contamination of the laser window from process
by-products is investigated. For both influences the focal
shift is measured using a non-moving laser beam and
correlated with the time and space dependent relative
density of test specimen made from 1.4404. While an
influence of the focal shift induced by the optics itself
can be proven, no correlation between the contamination and
the relative density can be found. The investigations on the
interaction of laser radiation with process by-products are
conducted separately for metal condensate and
splashes/ejected and entrained powder particles (alien
particles) using the material Ti6Al4V. First, the general
correlation of the emission of process by-products with
laser power, scan speed and beam diameter is investigated
using a high speed camera. The experiments show that the
higher the laser´s intensity, the more shallow the ejection
angle of the by-products. The influence of the interaction
of metal condensate with laser radiation on the relative
density of test specimen is studied using Ti6Al4V and
Inconel 718. It can be shown that the relative density
decreases when the chance of interaction increases. A laser
scanner is used to determine the size, amount and position
of alien particles on the surface of test specimen before
and after exposure. These values are correlated with μCT
scans as well as the relative density. Finally, the
influence of the shielding gas flow on the process is
discussed. To do so, the space resolved flow is measured
using a thermal anemometer. The retrieved flow profiles are
correlated with the relative density at each location using
the materials Ti6Al4V and Inconel 718. An improved shielding
gas flow is proposed considering the previously gained
results.},
cin = {421510},
ddc = {620},
cid = {$I:(DE-82)421510_20170406$},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
doi = {10.18154/RWTH-2020-11079},
url = {https://publications.rwth-aachen.de/record/807057},
}
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