Phase transformations and stress evolution during laser beamwelding and post heat treatment of TiAl-alloys

Liu, Jie

Dissertation / PhD Thesis

PUBDB-2015-03465

Phase transformations and stress evolution during laser beamwelding and post heat treatment of TiAl-alloys

Liu, J. (Corresponding author)>Extern*

2015

1-91 (2015) = 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.

Note: Technische Universität Hamburg-Harburg, Diss., 2015

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