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@PHDTHESIS{Munk:994774,
author = {Munk, Juri},
othercontributors = {Requena, Guillermo Carlos and Krupp, Ulrich},
title = {{E}influss der {G}eometrie auf die {E}igenschaften der
mittels pulverbettbasierten {L}aserschmelzen gefertigten
{L}egierung {T}i-6{A}l-4{V}},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-09537},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2024},
abstract = {Laser Powder Bed Fusion of Ti-6Al-4V allows manufacturing
of components with complex geometric shapes from various
metallic alloys, especially for lightweight applications. In
LPBF, the geometry of the component defines the thermal
history during the build process and therefore the
microstructure and the mechanical properties. Geometrically
complex shapes lead to locally different thermal histories
over the component, resulting in inhomogeneous
microstructures. This is in contrast to the fact that
microstructural and mechanical characterization of LPBF-made
material is carried out with standardized sample geometries
that do not represent the geometric complexity and
inhomogeneous thermal history. In the present work, the
influence of the geometry on the material properties of
Ti-6Al-4V was investigated. For measuring the impact of the
geometry effect, the β phase fraction was selected as it
depends on the thermal history on the one hand and defines
the mechanical properties on the other hand. Furthermore, α
lamellae width, tensile strength and fatigue life was
characterized. In this work, two mechanisms were described
that lead to heterogenous microstructures: Firstly,
intrinsic heat treatment (IHT) that represents the thermal
influence of all following layers above the point of
interest. Secondly, reduced cooling rate during
solidification of the point of interest and the thermal
influence of the few directly following layers. The
mechanisms were investigated separately by samples that have
been built under conditions solely with one mechanism. The
resulting morphology of the β phase matched well with
literature and confirmed the described mechanisms. In order
to evaluate the thermal histories of the different sample
geometries finite element simulations have been carried out.
In detail, super layer approach was applied which means that
multiple real LPBF layers were summarized in a lumped
package of one super layer. Two characteristic values were
extracted from the simulated thermal histories: Holding time
of the intrinsic heat treatment $Δt_IHT$ and cooling
constant k for describing the reduced cooling rate. It was
shown that for the majority of the investigated sample
geometries $Δt_IHT$ is capable of predicting the intrinsic
heat treatment. For evaluation of the second mechanism
reduced cooling rate the characteristic value k worked for
all investigated sample geometries. The time between two
subsequent layers in the LPBF process is called inter layer
time. It has been shown that the influence of the geometry
increases by reducing the inter layer time. The mechanism of
reduced cooling rate was only observed for inter layer time
below 28 s but intrinsic heat treatment remained present
even at the highest investigated inter layer time of 45 s
for some sample geometries. For investigation of the thermal
influence of directly following layers above the point of
interest, sample geometries with varied number of following
layers were analyzed. The first local maximum of the β
phase fraction and the α lamellae width were both observed
on samples that have only one single following layer above
the point of interest. As a possible explanation it was
assumed that the cooling rate in the relevant range from the
β transus temperature is the most reduced in this specific
condition. The observed morphology of the α lamellae
confirmed the more dominant role of the mechanism of reduced
cooling rate. For a built-up thickness of 0,6 mm above the
point of interest, representing 10 following layers, the
next increase of α lamellae width and β phase fraction was
observed and could be explained by the mechanism of
intrinsic heat treatment. The influence of geometry on the
mechanical properties was also identified. Samples with fast
increase of cross-sectional area in build direction were
characterized by a reduced strength, both under static
(ultimate tensile strength) and dynamic (fatigue life)
condition. The influence of the geometry on ultimate tensile
strength was observed to be present even at highest inter
layer times of more than 75 s. To make the geometry
influence predictable, a multiple linear regression model
was established to provide a correlation of simulated
thermal history with β phase fraction and tensile strength,
respectively. Discretization of the thermal histories was
done by use of multiple partitions of the integrated
time-temperature-curve. For prediction of the β phase
fraction and the ultimate tensile strength the adjusted
coefficient of determination was 99.8 $\%$ and 89.6 $\%$
respectively, showing the potential of the approach. To
summarize, the present work provides a better understanding
of the mechanisms that occur during LPBF of Ti-6Al-4V and
lead to inhomogeneous material properties, depending on the
component geometry. For the first time it has been shown how
simulation methods that are applicable on component level
can be utilized to predict application-relevant material
properties.},
cin = {521420 / 522110 / 520000},
ddc = {620},
cid = {$I:(DE-82)521420_20160125$ / $I:(DE-82)522110_20180901$ /
$I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-09537},
url = {https://publications.rwth-aachen.de/record/994774},
}
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