% IMPORTANT: The following is UTF-8 encoded. This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.
@PHDTHESIS{Liu:224260,
author = {Liu, Jie},
title = {{P}hase transformations and stress evolution during laser
beamwelding and post heat treatment of {T}i{A}l-alloys},
school = {Technische Universität Hamburg-Harburg},
type = {Dr.},
reportid = {PUBDB-2015-03465},
pages = {1-91},
year = {2015},
note = {Technische Universität Hamburg-Harburg, Diss., 2015},
abstract = {γ-TiAl material is an attractive alternative candidate for
aerospace and automotiveapplications because of its low
density, high specific yield strength and high creep
andoxidation resistance. It has been used successfully in
the manufacture of low-pressureturbine blades in the GEnxTM
engine, which represents a major advance in
propulsionefficiency, with a $20\%$ reduction in fuel
consumption, a $50\%$ reduction in noise, and an
$80\%reduction$ in NOx emissions compared with prior engines
of the same class. In future, γ-TiAlwill most likely be
applied as a structural material in the combustion turbine
of aircraft; forthis purpose, the challenge of determining a
method for the proper joining of this materialmust be
met.Laser beam welding is considered to be a promising
joining method. However, because ofthe low ductility and
fracture toughness of γ-TiAl alloys at ambient temperature,
cracks arefrequently observed in welding seams. The high
cooling rate further reduces the weldabilityof the alloy, as
a large amount of brittle phase and high residual stresses
are formed. Thus,there are four problems that must be
solved: (1) How can a crack-free TiAl butt joint beproduced?
(2) How can the microstructural and mechanical properties of
the weld beimproved? (3) How do the phases transform during
welding? (4) How to perform a fast andreliable welding?The
objective of this thesis is to methodically address these
challenges. First, because of theintrinsic brittleness of
the alloy, several heat treatments using a furnace and a
defocusedlaser are applied to heat the alloys above the
brittle-to-ductile transition temperature andreduce the
cooling rate. The butt joint welds are investigated via
radiography and found tobe free of cracks.Second, the task
is the modification of the microstructural and mechanical
properties of thewelds. The grain refinement induced by
borides, which is widely exploited during casting,
issuppressed by the high cooling rate. Additionally, a large
amount of α2 phase and residualstress are detrimental to
the weld. Thus, post-weld heat treatments are applied to the
asweldedspecimens to refine the grain size, modify the
microstructure and relieve the residualstress. Tensile tests
are also conducted to evaluate the mechanical properties of
the asweldedand heat-treated specimens.Third, an in situ
investigation of the phase transformation as a function of
the heating rate isperformed at the HZG beamline HEMS at
DESY. The phase transformation is plotted overtime, and the
transformation mechanism is explained. A new concept of
grain refinementwith the assistance of superheated particles
is proposed and applied to the optimisation ofthe laser beam
welding parameters and the mechanical properties of the
welds.Fourth, besides solving the scientific questions,
attention is paid to produce fast and reliablewelds. Several
heating methods are developed and applied, such as
resistance furnaceheating, induction furnace heating,
dual-laser-beam heating and FLEXILAS experiment.Both the
advantages and disadvantages of these methods are discussed.
It is found out thatthe FLEXILAS set-up is able to heat the
specimen homogeneously and fast. It is proved tobe potential
for industrial application.In conclusion, it is
demonstrated, on the laboratory scale, that laser beam
welding has thepotential to be employed for the joining of
γ-TiAl alloys used as structural materials. Thewelding
approach developed here is capable of producing a sound
joint with an optimisedmicrostructure, low residual stress
and good mechanical properties. Moreover, the newlygained
understanding of the phase transformation during laser beam
welding has revealed anovel grain-refinement mechanism that
can be further employed to improve the mechanicalperformance
of the final welds.},
cin = {HZG},
cid = {I:(DE-H253)HZG-20120731},
pnm = {6G3 - PETRA III (POF3-622)},
pid = {G:(DE-HGF)POF3-6G3},
experiment = {EXP:(DE-H253)P-P07-20150101},
typ = {PUB:(DE-HGF)11},
url = {https://bib-pubdb1.desy.de/record/224260},
}