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@PHDTHESIS{Ukwungwu:1016145,
author = {Ukwungwu, David},
othercontributors = {Hameyer, Kay and Reisgen, Uwe},
title = {{E}lectromagnetic evaluation and quantification of welding
process for packaging of electrical steel sheets; 1.
{A}uflage},
volume = {63},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Düren},
publisher = {Shaker Verlag},
reportid = {RWTH-2025-06772},
isbn = {3819101381},
series = {Aachener Schriftenreihe zur Elektromagnetischen
Energiewandlung},
pages = {xi, 155 Seiten : 26 Illustrationen},
year = {2025},
note = {Dissertation, RWTH Aachen University, 2025},
abstract = {In order to ensure a mechanically stable magnetic core for
electromagnetic energy converters such as electrical ac
machines and transformers, it is essential that the
electrical steel sheets are securely packaged together. The
welding process is one of the most utilized packaging
technologies nowadays. This is due to the ease of its
integration into the manufacturing process. The complex
interaction between the thermal degradation and the
deterioration of the electromagnetic properties of welded
cores necessitates the need to characterize the influence of
weld-packaging on the electromagnetic properties of
non-oriented (NO) electrical steel lamination with the view
of developing a locally varying material model. The study on
the impact of weld-packaging process parameters on the
electromagnetic properties of packaged cores highlights the
benefits of using low powered lasers to minimize the
deterioration effect of weld-packaging. This is because, a
general decrease in the magnetizability (relative
permeability) and increase in specific iron loss is observed
with increasing laser power due to decreases in temperature
gradient. An increase in the laser focal position resulted
in less deterioration, as it reduces the depth of thermal
energy penetration (heat affected zone) through its impact
on the resulting temperature gradient. Higher working
pressure is observed to affect the electromagnetic property
deterioration positively (reduction) through its impact on
the reduction of the energy penetration depth. However, its
overall influence on electromagnetic deterioration is
dependent on the formation of weld defects resulting from
the high energy densities. Furthermore, the analysis of the
impact of different laser topologies revealed that high
powered lasers with increased fibre diameter, leading to low
power density, resulted in less deterioration of the
electromagnetic properties of the weld-packaged cores in
comparison to lasers with low fibre diameters. This is
attributed in part to the reduced effective heat affected
zone (penetration depth), resulting in low residual stress
accruing from the low temperature gradient of the process,
and partly due to decreased possibility of the weld defects
formation because of the reduced thermal energy density. The
quantification of the impact of applied stress using a
series of electromagnetic characterization methods shows,
that whereas, an improvement effect of low stress on the
electromagnetic properties due to the reduced resistances to
domain wall mobility resulting from the effect of stress on
the free energy of the domain is seen, increased
deterioration of the annealed samples is observed with
increasing stress value. The deterioration behavior of the
samples annealed at different temperatures is seen to be
frequency dependent. This is because of the relationship
between the increased average grain size and the different
iron loss components. Conclusively, the determined advantage
of using low laser power, increased (positive or negative)
laser focal position and a determined optimal working
pressure on the degradation effect of weld-packaging on the
electromagnetic properties due to lower temperature gradient
associated with the parameters validates the second
hypothesis of this work, which states that a reduction in
residual (internal) stresses and average grain size can be
achieved with the modification of the laser welding process
parameters. Characterization of the annealed samples shows
that the effects of residual stress due to temperature
changes are higher than the influences of increased grain
size on the electromagnetic properties of the material. This
is seen in the characterized deteriorations at low
frequencies, which is due to Villari-reversal that leads to
deteriorations. This validates the first hypothesis of this
work, which states that the effects of micro-structural and
residual stress changes on the magnetization and iron loss
of electrical steel material can be quantified. In order to
analyze and assess the impact of weld-packaging using
simulation, a locally varying material model that accounts
for the changes in the electromagnetic properties of the
welded core due to the micro-structural degradation is
developed. It maps the local variations in electromagnetic
properties (macroscopic) caused by packaging effects with
the changes in micro-structure and residual stress
(microscopic). The simulated results show an overall
decrease in the magnetizability and increase in specific
iron loss of the core due to increased residual stress
(reduced domain wall mobility) associated with
weld-packaging. The reduced mobility is due to the
obstructed domain wall movements resulting from the domain
dislocations (increased residual stress) around the grain
boundaries because of the domain restructuring aimed at
minimizing the magnetic free energy. Although the overall
impact of welding on excess loss components is generally
positive due to increased domain wall smoothness, the
increases in non-linear loss are due to increased domain
shape deterioration, which hinders domain mobility at
saturation results. The simulation results also verify the
third working hypothesis, that a combination of experimental
tests and simulations enables a separate consideration of
the impacts of weld-packaging on the electromagnetic
properties of the core.},
cin = {614410},
ddc = {621.3},
cid = {$I:(DE-82)614410_20140620$},
pnm = {DFG project G:(GEPRIS)432930813 - Elektromagnetische
Bewertung und Quantifizierung von Schweißprozessen zur
Paketierung von Elektroblechen (432930813)},
pid = {G:(GEPRIS)432930813},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
url = {https://publications.rwth-aachen.de/record/1016145},
}
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