Laserauftragschweißen von γ-Titanaluminiden als Verfahren der additiven Fertigung
Aachen / Thyme 2 Reed Dissertationsverlag (2020) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (IV, 147, xxii Seiten) : Illustrationen, Diagramme
Due to their high specific strength and good high-temperature properties, titanium aluminides are a promising material for use in turbomachinery construction. The most important industrial application is currently found in aviation in the form of low-pressure turbine blades. Due to their low ductility and fracture toughness at room temperature, conventional manufacturing processes are complex and costly, so that industry's interest in tool-free additive manufacturing is growing. With Laser Material Deposition (LMD), components can be built up using additives as well as repaired close to the final contour and (re)put into service. With in-creasing use of γ-TiAl, the demand for repair or modification solutions independent of the original manufacturing route will also increase. A fundamental and systematic research on the processing of γ-TiAl alloys by means of LMD has not yet been carried out. The aim of the present work is to extend the state of the art by the identification and quantification of essential influencing and result variables in the LMD of intermetallic γ-TiAl materials in order to evaluate the suitability of LMD as a process for processing γ-TiAl. Special challenges also exist in additive processing in the high brittleness and oxygen sensitivity of γ-TiAl. The alloys TNM-B1, GE4822 and 4522XD are being investigated. In addition to the basic feasibility of pro-cessing with LMD, influences of process parameters and process boundary conditions on oxygen absorption and aluminum burn-off and properties of resulting microstructures (phase fractions, grain sizes, and hardness) are considered. The solidification conditions determined by LMD simulation and thermographic measurements are included. The influence of subsequent heat treatment on the materials is investigated. The compressive strength at 700°C is tested as a comparative characteristic value of LMD specimens with each other and with specimens made of materials produced by other methods (forging, casting, laser powder bed fusion (LPBF)). The production of dense (>99.9%), crack-free samples is possible by preheating the pro-cessing plane to 900°C. The oxygen uptake is reduced by a global argon shielding gas atmosphere and is also measurably influenced by the selected process parameters and the particle size of used powders. A correlation between aluminum burn-off and process control is not proven. The fineness of the microstructure is largely determined by the solidification conditions and thus by the parameter-dependent cooling rate. Influences of different initial microstructures and chemical composition (oxygen, aluminum) on microstructure, hardness and compressive strength at 700°C can no longer be significantly measured after heat treatment. The greatest differences between materials produced with different processes are found in the phase distribution. The hardness and HT compressive strength of additive manufactured LMD samples made of TNM-B1 do not differ after heat treatment from those of comparable LPBF, cast and forged samples. The creep resistance of TNM-B1 processed with LA is within the range of known literature values for γ-TiAl. Thus, the properties of γ-TiAl produced with LA correspond to those achieved with the processes used for the production of components made of γ-TiAl. LMD is therefore suitable for processing γ-TiAl. However, the oxygen content is still above the limit value of 1000 ppm, which is regarded as critical, which reduces the ductility, especially at room temperature, and thus makes reworking and assembly of components even more difficult.
Reisgen, Uwe Kaspar