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@PHDTHESIS{Szczepaniak:661160,
author = {Szczepaniak, Agnieszka},
othercontributors = {Raabe, Dierk and Schneider, Jochen M.},
title = {{I}nvestigation of intermetallic layer formation in
dependence of process parameters during the thermal joining
of aluminium with steel},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2016-05763},
pages = {1 Online-Ressource (viii, 163 Seiten) : Illustrationen,
Diagramme},
year = {2016},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2016},
abstract = {This thesis examines the formation of intermetallic layers
at the interface between aluminium and low-carbon steel
during thermal joining using state of the art electron
microscopy techniques applied both ex-and in-situ. To sample
a wide range of intermetallic layer formation conditions,
the well-established coating method of hot-dip aluminising,
as well as more modern welding techniques such as laser beam
welding (in conduction mode) and friction stir welding, are
applied. The intermetallic layer always consists of brittle
phases, mainly Fe2Al5and Fe4Al13, which makes it a
detrimental factor for mechanical properties of the
assembly. Therefore, the possibility to control the
thickness and morphology of this layer is of great
importance for industrial processes. This requires in-depth
knowledge about the influence of the joining parameters on
the conditions of intermetallic layer formation, the factors
affecting its growth, and finally, the resulting
microstructure; all of which are discussed in this thesis.
Hot-dip aluminising experiments show that a significant
reduction of the intermetallic layer thickness can be
obtained when the immersion process is performed at
sufficiently high temperatures (above 900 °C for 30 s of
aluminising). Thus counterintuitively, a high process
temperature, even for extended holding times, is an
effective alternative to the conventional case of simply
limiting growth conditions to reduce the intermetallic layer
thickness, owing to increased dissolution of the
intermetallic layer in the melt. It also was shown that the
intermetallic layer experiences significant growth under
insufficiently rapid cooling from high temperatures.
Furthermore, evidence of a carbon buildup in front of the
growing layer ($\kappa$-phase, pearlite, martensite/acicular
$\alpha$-Fe) is presented and discussed in terms of
producing a beneficial hardness gradient. The specific
influence of the parameters related to the energy density
delivered to the assembly (laser power and welding speed) as
well as the parameters affecting mainly the heat dissipation
in the weld microstructure (overlap size and lateral spot
position) was established systematically. Additionally, a
correlation with finite element method simulations of
thermal cycles, performed for selected points at the
aluminium/steel interface, allow determining that mainly the
peak temperature and related cooling rate differ between
particular welds. Moreover, the laser beam intensity
distribution results in a variation of the intermetallic
layer thickness and morphology along the weld interface.
Regardless of the comparatively faster kinetics induced by
the welding process, the intermetallic layer constitution is
nearly the same as in case of aluminising: Fe2Al5and
Fe4Al13. Transmission electron microscopy shows that
formation of the Fe-richer phases (Fe3Al, FeAl and
eutectoid: FeAl + FeAl2) occurs only in the region where the
joining temperature approached the melting point of the
Fe2Al5phase. The carbon build-up effect is not observed in
the welds. Electron back-scatter diffraction, applied in a
systematic study of this problem for the first time in this
thesis, reveals how the ii width of Fe2Al5grains is strongly
affected by the temperature and time of the joining process,
leading to vanishing of the typical tongue-like morphology.
In case of laser beam welding, the maximum width of
Fe2Al5grains can reach up to 500 μm with the corresponding
height of about 50 μm in comparison to few microns in the
initial case. For the first time, using a novel
in-situheating transmission electron microscopysetup, a
characterisation of the FexAlyphase formation at the weld
interface of friction-stir welded samples, used as model
microstructures, is attempted in real time. It is shown that
Fe4Al13is the first stable phase that forms under annealing,
in agreement to the Walser-Bene model. In addition, a new
method of the site-specific sample preparation and lift-out
process for in-situheating measurements was proposed.
Throughout this thesis, the literature data is discussed and
compared to the experimentally obtained microstructures,
which are characterised in detail by electron back-scatter
diffraction techniques and transmission electron microscopy.
Practical applications and potential microstructure
optimisation for industrial processing is also discussed.
Examples are the reduction ofthe intermetallic layer
thickness through enhanced dissolution and combination of
high temperature and quenching rates, as well as the
tailoring of layer morphology.},
cin = {523110 / 520000},
ddc = {620},
cid = {$I:(DE-82)523110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2016-057635},
url = {https://publications.rwth-aachen.de/record/661160},
}
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