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@PHDTHESIS{Komerla:808705,
author = {Komerla, Krishna Chaitanya},
othercontributors = {Bleck, Wolfgang and Reisgen, Uwe},
title = {{A}n investigation of microstructure and mechanical
properties of low carbon steels subjected to welding},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2020-12241},
pages = {1 Online-Ressource},
year = {2020},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2021; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2020, Kumulative Dissertation},
abstract = {Welding is a complex thermo-mechanical process in which,
under transient and non-equilibrium conditions multiple
metallurgical phenomena can occur simultaneously. The weld
thermal cycles introduce significant alterations to the
microstructure of material and thereby, affect the
mechanical properties of the weldments. In order to create a
sound joint, it is essential to understand the impact of
various process parameters on the weld microstructure and
weld mechanical properties. Beyond conventional process
parameters like, acceleration voltage, amperage, welding
speed and applied external pressure, a deeper understanding
of the influence of welding parameters like tool offset,
beam oscillations and oscillation trajectories is necessary
to produce long lasting and sustainable joints. In this
study, the effect of such uncommon process parameters on the
quality of welds is presented. To that end, the two
following welding methods have been investigated 1.Fusion
welding - High energy beam welding 2.Pressure welding -
Dissimilar friction stir welding. Low carbon steels like the
automotive dual phase steel (DP1000) and deep drawable mild
steel DC04 each possessing 1 mm thickness, along with a 5 mm
thick structural steel S235JR were investigated.
Additionally, for dissimilar friction stir welding, a 1.12
mm thick solution treated and aged aluminum alloy AL6016-T4
was also studied. Results indicate that, for the same weld
heat input, the application of uncommon process parameters
can yield a range of different weld microstructures that
exhibit improved weld mechanical properties. In the case of
high energy beam welding, beam oscillations were applied to
create a dynamic distribution of power around the stationary
position of the beam that enhanced the flow of heat inside
the keyhole and created wider fusion and heat-affected
zones. A reduction in the size of welding crowns could be
achieved by simply oscillating the energy source. As a
consequence of this dynamic power distribution, the weld
microstructure exhibited large columnar grains in the fusion
zone and equiaxed grains of varying size in the
heat-affected zone. This variation in grain size across the
weld joint could be attributed to the steep temperature
gradient produced during electron beam welding. Typically
high hardness is observed in welds due to the occurrence of
quenched martensite during conventional welding. However,
weld samples fabricated using beam oscillations possessed
not only lower hardness but also exhibited good tensile
strength, lower residual stress and minimal distortion in
comparison to the joints produced by stationary beam
welding. This decrease in hardness arose from enhanced grain
growth and additional indirect tempering like effect caused
by beam oscillations. While in the case of friction stir
welding, the tool offset resulted in a mode of welding
where, minimal shear strain was accumulated in steel and
complete dynamic recrystallization of microstructure in
aluminum was achieved. Additionally, macroscopic defect free
and inter-metallic compound free joints were obtained, with
only a small fraction of Fe chips embedded in the aluminum
matrix. Through plastic deformation and thermal effects,
significant differences in grain size and micro-hardness
were created in the weld joint. High temperatures and
extreme strain rates inherent to the welding process,
activated the dynamic recrystallization of microstructure in
the aluminum alloy. The kernel average misorientation map
revealed the lack of stored deformation energy, indicating
the presence of fully recrystallized grains. However, due to
its high thermal stability, steel did not recrystallize but,
exhibited marginal grain growth in both thermo-mechanically
affected zone and heat-affected zone. It also contained a
highly refined grain structure in the weld zone. The texture
maps plotted for the weld zone of Fe alloy showed weak shear
texture indicating minimal shear strain accumulation. Thus,
by adopting such process parameters and understanding their
impact on the microstructure, joints with enhanced life can
be produced at low costs. However, experimental research of
such parameters is both time consuming and expensive. Hence,
there is a strong need for numerical models that predict the
final mechanical state of the weldments. Therefore, such an
attempt is also made towards the development of a
metallurgical framework in this project to study and analyse
the process of welding, its metallurgy and microstructure. A
sequentially coupled finite element formulation was adopted
to simulate the fusion welding process. To accurately
predict the sharp temperature gradients across the different
weld zones, a moving volumetric heat source model, combining
a spherical and a conical thermal flux distribution was
developed. The temperature profiles computed with this heat
source model were found to be in excellent agreement with
the experimentally measured data. Additionally, a comparison
of the optical micrographs with the simulated weld
geometries further strengthened the basis of the developed
framework.},
cin = {520000 / 522110},
ddc = {620},
cid = {$I:(DE-82)520000_20140620$ / $I:(DE-82)522110_20180901$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2020-12241},
url = {https://publications.rwth-aachen.de/record/808705},
}
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