A laser powder bed fusion process operating at elevated temperatures is introduced capable of fabricating crack-free and dense intermetallic titanium aluminide alloy specimens as well as demonstrator components using a base plate heating up to 900 °C and a unique heating system of the uppermost powder bed layer up to 1200 °C. Two so-called 4th generation alloys, TNM and TNM+, were used for this study. The microstructure and its evolution during subsequent heat treatments were investigated and explained by employing scanning electron microscopy, hardness testing, X-ray diffraction, differential scanning calorimetry and thermodynamic equilibrium calculation. Selected specimens were subjected to creep tests at 750 °C. The microstructures after processing consist of extraordinarily fine lamellar γ-TiAl/α2-Ti3Al-colonies with globular γ and βo-TiAl grains for both the TNM and TNM+ alloy, exhibiting a microstructure gradient from the last consolidated powder layer down to the starting layer due to cellular reaction, which increases the amount of globular γ and βo at the boundaries of the γ/α2-colonies. During annealing in proximity to the γ-solvus temperature, banded microstructures might form, as the α-grain size is only partially controlled by heterogeneously distributed γ/β-phase, which stems from the process-related Al loss. Additionally, the occurrence of thermally-induced porosity is investigated. Optimizing the microstructure to a homogenized, almost fully lamellar microstructure, involved annealing in the β-single phase field region and led to improved creep properties. Finally, TNM demonstrator components with complex geometries, such as aero engine blades and turbocharger turbine wheels, are fabricated by employing the novel laser powder bed fusion process.
Zusammenfassung: Intermetallische Titanaluminide werden bei erhöhten Temperaturen als Turbinenschaufeln in Flugzeugtriebwerken, als Laufräder in Turboladern sowie als Ventile im Rennsport eingesetzt. Um neue Herstellungswege zu nutzen und endkonturnahe 3D gedruckte Teile produzieren zu können, müssen geeignete Pulver für additive Fertigungsprozesse verfügbar sein. Dazu wurden zwei vielversprechende Legierungsvarianten herangezogen und entsprechende Pulver hergestellt. Anschließend wurden die Pulver heißisostatisch verdichtet. Um die Qualität der Pulver sicherzustellen und mögliche mikrostrukturelle Inhomogenitäten des Materials auszuschließen, erfolgte eine Defektanalyse mittels Lichtmikroskopie in Kombination mit Rasterelektronenmikroskopie. Dazu wurden einzelne Defekte im Lichtmikroskop mittels Härteeindrücken markiert und deren lokale chemische Information über detaillierte Flächenscans im Rasterelektronenmikroskop ermittelt. Die Ergebnisse dieser Untersuchung können dazu verwendet werden, Fehlerquellen bei der Herstellung sensibler Titanaluminid-Pulver zu vermeiden und so die Qualität der erzeugten Bauteile zu steigern.
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