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@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},
}